INVITED REVIEW

Tibial Plateau Leveling Osteotomy or Tibial Tuberosity

Advancement?

RANDY J. BOUDRIEAU, DVM, Diplomate ACVS & ECVS

Objective—To review the proposed biomechanical basis of the tibial plateau leveling osteotomy(TPLO) and tibial tuberosity advancement (TTA) and recommendations for these techniques.Study Design—Literature review.Methods—Literature search through Ovid Medline Plus, Pub Med, CAB Abstracts, and conferenceproceedings abstracts (August 1983 to March 2008).Results—TPLO and TTA stabilize the cranial cruciate ligament (CrCL) deficient stifle jointneutralizing tibiofemoral shear forces by altering the geometry of the proximal aspect of the tibia.Stability is attained by placing the joint in a functionally greater flexion angle so that the patellartendon angle (PTA) remains �901. Both procedures target slightly differing endpoints, thesignificance of which is unknown. Many of the biomechanical variables investigated appear to favorthe TTA; however, TPLO appears to have more clinical versatility. The clinical ramifications ofthese differences remain to be determined but the reported results for both procedures arecomparable. Only the early results of these techniques have been reported, which is reflected inthe relatively high number of complications associated with the early learning curve for bothprocedures.Conclusions—There are many similarities between TPLO and TTA although it remains to be fullyelucidated if either procedure is superior and under what conditions.Clinical Relevance—TPLO and TTA are effective at returning dogs with a CrCL-deficient stiflejoint to good limb function. Surgeon discretion and case selection drive selection of TPLO or TTAbased mostly on anecdotal evidence and personal experience.r Copyright 2009 by The American College of Veterinary Surgeons

INTRODUCTION

CRANIAL CRUCIATE ligament (CrCL) deficiencyresults in both translational and rotational instabil-

ity of the stifle joint that leads to the development ofosteoarthritis.1–3 Surgical techniques to restabilize thejoint, with either static or dynamic repairs, are performedto neutralize the tibiofemoral shear forces in a CrCL-deficient knee. One popular surgical technique used toachieve stifle joint stability that neutralizes the tibiofem-oral shear forces dynamically in a CrCL-deficient knee istibial plateau leveling osteotomy (TPLO).4,5 TPLO

has been reported to functionally stabilize the stiflejoint during weight bearing, neutralizing the cranialtibiofemoral shear force (cranial tibial thrust [CrTT])by reduction of tibial plateau angle (TPA). This isaccomplished by radial osteotomy of the proximal aspectof the tibia and rotation of the proximal fragment.5,6 Themechanics of TPLO have been validated in 2 experimen-tal models.7,8

Another popular surgical technique used to achievestifle joint stability that neutralizes the tibiofemoral shearforces dynamically in a CrCL-deficient knee is tibialtuberosity advancement (TTA).9–11 TTA has been

Address reprint requests to Dr. Randy J. Boudrieau, Department of Clinical Sciences, Cummings School of Veterinary Medicine at

Tufts University, 200 Westboro Road, North Grafton, MA 01536. E-mail: randy.boudrieau@tufts.edu.

Submitted March 2008; Accepted June 2008

From the Department of Clinical Sciences, Cummings School of Veterinary Medicine at Tufts University, North Grafton, MA.

r Copyright 2009 by The American College of Veterinary Surgeons

0161-3499/08

doi:10.1111/j.1532-950X.2008.00439.x

1

Veterinary Surgery

38:1–22, 2009

reported to functionally stabilize the stifle joint duringweight bearing by neutralizing the cranial tibiofemoralshear force (CrTT) by advancing the tibial tuberosity.This is accomplished by an osteotomy of the tuberosity inthe frontal plane with advancement of this bone frag-ment.9 The mechanics of TTA have also been validated in2 experimental models.12,13

Success rates with CrCL repair are reported to be490% regardless of the surgical technique used.14 His-torically, selection of a surgical technique has been basedprimarily on surgeon preference rather than definitiveevidence that one technique might be better thananother.15 Unfortunately, there is no study that demon-strates that TPLO is a better procedure than, forexample, the extra-capsular stabilization16—this despitemuch anecdotal opinion that the TPLO is a superiortechnique, especially for active, athletic dogs. Likewise,there are no data for functional evaluation of TTA otherthan similar anecdotal evidence that it is an effectivetechnique in similar types of affected dogs. There are nopeer-reviewed reports comparing the outcome of TPLOto TTA; however, anecdotal comments have beenpresented.17 Therefore, one selection criteria for TPLOversus TTA may simply continue to be surgeon prefer-ence, where perhaps one can admit to be captivated withthe latest and greatest newest technique(s).

The questions posed regarding these 2 techniquesare many and include what we definitely know, what weassume to be true, and what we definitely do not know.Many questions have been raised by proponents of oneor the other of these procedures for which a number ofongoing and future studies may help provide answers.The purpose of this review is to address the availabledata, discuss the pressing questions to be answered, andsuggest possible areas that should be investigated.

PROPOSED MECHANISM OF ACTION

A number of biomechanical studies in humans havedemonstrated an increase in translational knee jointinstability by variation with tibial plateau slope (TPS),18

axial loading,19 and flexion knee angle.20,21 It has been rea-sonably assumed that these findings are similar in the dog.Evaluation of this resultant translational stifle joint insta-bility because of a CrCL rupture in the dog can be clinicallydetected by the tibial compression test.20 Axial compressiveforces are thought to cause a cranial subluxation of the tibiarelative to the femur because of unopposed CrTT.

TPLO

Slocum proposed that the tibiofemoral shear force, orCrTT was an internally generated force that caused thetibia to translate cranially (and is opposed by

the CrCL).4,6 Slocum proposed that the CrTT in the nor-mal stifle joint was primarily controlled by the caudallydirected forces of the hamstring muscles.5,6 In this theory,the compressive forces across stifle joint were proposed tobe parallel to the tibial axis, but because of the caudallydirected TPS, compression between the joint surfaces re-sulted in cranial tibial translation.5,6 This theory was devel-oped as the ‘‘active model of the stifle’’5; CrTT is created bycompression between the femur and tibia, which actsthrough the functional axis of the tibia, and is dependentupon the amount of compression and the TPS.5

Axial compression of the limb is thought to generate acompressive force across the joint, and this resultant forcecan be reduced to 2 orthogonal components, one per-pendicular and one parallel to the tibial plateau, the latterrepresenting the CrTT (Fig 1).4 The CrTT is a result ofthe tibial plateau oriented at an angle to the axial com-pressive force. If the angle of the tibial plateau is reducedto zero, the joint compressive force and resultant forcebecome the same, as the CrTT becomes zero, as this forcevector is eliminated (Fig 1).4 Clinically, however, the

Fig 1. Schematic representation of the tibiofemoral forces in

the stifle joint, according to Slocum,5 before (A) and after (B)

tibial plateau leveling osteotomy (TPLO). The resultant com-

pressive force (large white arrow) across stifle joint is parallel

to the tibial axis. Using the tibial plateau slope (TPS) as the

baseline, whereby the femur can move along this surface if the

cranial cruciate ligament (CrCL) is deficient, the resultant

force can be broken down into its 2 orthogonal components

(small shaded arrows), one perpendicular and one parallel to

the tibial plateau. The latter represents the tibiofemoral shear

force (resulting in cranial tibial thrust [CrTT]). If the angle of

the tibial plateau is reduced to zero, the tibiofemoral shear

force vector becomes zero, and the joint compressive force and

resultant force become one and the same.

2 TPLO OR TTA?

plateau is not returned to 01, but has �51 of remainingslope.5,6,22 This concept relies on the hamstring muscles,which contribute to neutralizing this small remainingforce.5,6,22 The CrTT, therefore, can be eliminated bychanging the angle of the TPS, thereby enhancing theeffectiveness of the flexors of the stifle joint.5,6,22 Thetechnique performed to accomplish this was standardizedas a radial osteotomy of proximal aspect of the tibiaand rotation of proximal fragment, and was called TPLO(Fig 1; Seminars entitled Tibial Plateau Leveling Osteo-tomy for Cranial Cruciate Ligament Repair; SlocumEnterprises Inc., Eugene, OR).5,6

TTA

The premise of TTA was based on a biomechanicalmodel analysis of joint forces in the human knee.23 Thismodel demonstrated that the tibiofemoral compressiveforce was approximately of the same magnitude, andoriented in the same direction, as the patellar tendonforce, which resulted in a variable tibiofemoral shearforce.23 This force was either anteriorly or posteriorlydirected dependent upon the angle of knee joint extensionor flexion, respectively.23 The point of neutral tibiofem-oral shear force was termed the crossover point in thishuman mechanical model, which occurred at a patellartendon angle (angle between the tibial plateau andthe patellar tendon; PTA) of �1001.23 Therefore, itwas proposed that the direction and magnitude of thetibiofemoral shear force was determined by the PTA.23

The assumption was made that this same reasoning couldbe applied to the dog’s stifle joint, where the total jointforces were also believed to be approximately parallel tothe patellar tendon, although in the dog the crossoverpoint was suggested to be at �901 PTA.9–11 Similarly, itwas proposed that there is a cranially directed tibiofem-oral shear force at a PTA4901 and a caudally directedtibiofemoral shear force at a PTA o901.9–11 With thecrossover point at 901 it was proposed to move the tibialtuberosity cranially so that at full stifle joint extension,and any joint flexion during weight bearing ( �1351), thePTA was always �901, and therefore only a neutral orcaudally directed tibiofemoral shear force would remain,thus stabilizing the joint.9–11 The CrTT, thus, is neutral-ized by advancement of the tibial tuberosity.

The technique performed to accomplish the advance-ment of the tibial tuberosity was standardized byperforming an osteotomy of the tibial tuberosity in thefrontal plane and moving it far enough cranially suchthat the PTA is 901 in full extension (during weightbearing, �1351), and called the TTA.9

In this model, the total joint force is parallel to thepatellar tendon; therefore, if the compressive joint forcesare oriented as such, the resulting components can be

viewed as parallel and orthogonal to this force along thetibial plateau (Fig 2).10 The CrTT is a result of the tibialplateau oriented at an angle to the axial compressive force.If the tibial tuberosity is moved cranially, thus changing thePTA to 901, the joint compressive force becomes a singlecomponent, parallel to the axial compressive force, and theCrTT force vector is eliminated (Fig 2).10

The proof of concept for both TPLO and TTA havebeen demonstrated in experimental cadaver studies.7,8,12,13

Two of these studies used the same experimental limb-pressmodel.7,12 In 3 studies, it was shown that the caudal cruci-ate ligament (CaCL) becomes the primary stabilizer to thejoint as CrTT is converted to caudal tibial thrust.7,8,12

The primary difference between the 2 proposed mech-anisms is the direction of the tibiofemoral compressiveforce. With TPLO, the force is proposed to be parallelwith the tibial long axis (Fig 1), whereas with TTA it isproposed to be parallel to the patellar tendon (Fig 2).

Fig 2. Schematic representation in the stifle joint of the

tibiofemoral forces, according to Tepic et al,10,11 before (A)

and after (B) tibial tuberosity advancement (TTA). The resul-

tant compressive force (large white arrow) across stifle joint is

parallel to the patellar tendon. Using the tibial plateau slope

(TPS) as the baseline, whereby the femur can move along this

surface if the cranial cruciate ligament (CrCL) is deficient,

the resultant force can be broken down into its 2 orthogonal

components (small shaded arrows), one perpendicular and

one parallel to the tibial plateau. The latter represents the

tibiofemoral shear force (resulting in cranial tibial thrust

[CrTT]). If the angle of the tibial tuberosity is advanced

cranially until the patellar tendon angle (PTA, angle between

the tibial plateau and the patellar tendon) is reduced to 901, the

tibiofemoral shear force vector becomes zero, and the

joint compressive force and resultant force become one and

the same.

3BOUDRIEAU

Both theories are credible; however, in the proposal bySlocum (compressive forces through the joint parallel tothe long axis of the tibia), regardless of the assumptionsmade for this mechanical model of the knee, there are nostudies that document that the direction of the proposedforce is valid. Furthermore, the contribution to joint sta-bility because of the quadriceps muscle group is not fac-tored into the model.20,24

Without a functional quadriceps, there is no ability tobear weight. Despite this apparent deficiency, the idea ofleveling the plateau remains as a reputably plausibleexplanation with an analogy often used of a wagonparked on a hill (Seminars entitled Tibial PlateauLeveling Osteotomy for Cranial Cruciate LigamentRepair; Slocum Enterprises Inc.),6 demonstrating that ifthere is a slope the weight of the wagon will generate aforce that will pull it down the slope, which is counter-acted by the rope, or CrCL, securing it. This argumentillustrates that once the slope becomes level, the weight ofthe wagon no longer generates a force in either direction,and therefore the wagon no longer needs to be secured.One problem with this theory is that it remains a staticmodel that does not consider the contribution of thequadriceps muscle, the major muscular force generatorabout the stifle joint. Secondly, irrespective of thejoint flexion angle, the TPS remains constant and jointposition is not accounted for. Finally, regardless of theTPS being level, there is a considerably low coefficient offriction between the 2 joint surfaces (lower than standingon ice), and this surface is not flat, but convex, both ofwhich would indicate that other factors must play a rolein maintaining joint stability.

As noted, the TTA theory proposes that the compres-sive forces at the joint are approximately parallel tothe patellar tendon. This theory does account for thequadriceps muscle. Furthermore, there are studies thatdocument that the direction of this proposed force isvalid.20,23,25

Despite the differences in proposed mechanisms ofaction, clinical results appear comparable between TPLOand TTA.26–30 An argument could be made that theproposed mechanism of action for TTA can also explainthe mechanism for TPLO (Fig 3). This can be observedafter reduction of TPA via radial osteotomy and rotationof the proximal tibial fragment, as this also reorients thepatellar tendon to the angle of the patellar tendon, whichapproaches 901, although these exact measures are yet tobe determined. Obviously, different techniques are usedto approach this mechanism from 2 different perspec-tives: (1) a radial cut of the proximal tibial plateausegment for TPLO, and (2) an osteotomy of the tibialcrest performed in the frontal plane for TTA. For TPLO,the tibial plateau is moved to meet the force; alterna-tively, for TTA, the force is moved to meet the tibial

plateau (Fig 4). In either case, it appears that theend result is the same: the tibial plateau and the patel-lar tendon become oriented at � � 901 to each other.Specific documentation of this endpoint remains to bedemonstrated for TPLO; however, it has been demon-strated that at �901 of stifle joint flexion theCrCL-deficient joint does become stable, which againcorroborates the proposed theory that the flexionangle affects the direction of the joint reaction forcesand subsequent stability.31

Additionally, it can be argued that the quadricepsmechanism plays a primary role in stability of the stiflejoint, as without it there no longer remains a possibilityto weight-bear or extend this joint; therefore, thequadriceps, and more specifically the direction of thequadriceps muscle force,32,33 needs to be considered in

Fig 3. Schematic representation in the stifle joint of the ti-

biofemoral forces, according to Tepic et al,10,11 before (A) and

after (B) tibial plateau leveling osteotomy (TPLO). The re-

sultant compressive force (large white arrow) across stifle joint

is parallel to the patellar tendon. Using the tibial plateau slope

(TPS) as the baseline, whereby the femur can move along this

surface if the cranial cruciate ligament (CrCL) is deficient, this

resultant force can be broken down into its 2 orthogonal com-

ponents (small shaded arrows), one perpendicular and one par-

allel to the tibial plateau, the latter represents the tibiofemoral

shear force (resulting in cranial tibial thrust [CrTT]). If the

angle of the tibial plateau is reduced to zero, the tibiofemoral

shear force vector becomes negative, resulting in a negative

force vector (resulting in caudal tibial thrust). Compare with

Fig 1 to note that the tibiofemoral shear vector (parallel to the

tibial plateau slope [TPS]) is smaller with this proposal, and

leveling the tibial plateau results in a correspondingly more

negative tibiofemoral shear force for the same amount of ro-

tation.

4 TPLO OR TTA?

any mechanical analysis of the stifle joint. Reduction ofthe PTA, whereby the effect of the cranial pull of thequadriceps muscle is limited during extension may be thekey to the success of both these procedures. In humans, ithas been demonstrated that the angle of the patellartendon insertion onto the tibia is a key determinant as towhether there is an effect of quadriceps contractionaggravating anterior tibial translation in an anteriorcruciate deficient knee.33 This would suggest that anytechnique that reduces the PTA would protect the jointfrom CrTT. Nonetheless, it must also be realized that anybiomedical modeling of the stifle joint is a crude approx-imation, or simplification, of that which exists in vivo. Inboth models, there is no consideration of magnitude ofthe many redundant forces attributed to the hamstringmuscle group, which certainly must play a role. The roleof the hamstring muscle group can be demonstratedwith the many surgical techniques used in humans toadvance these muscles, and the significant role of aggres-sive physical therapy in strengthening these muscles.34–36

The impact of these muscles, and the concept ofco-contraction, may be recognized in a theoretical3-dimensional, 3-segment mathematical model of the

stifle joint that demonstrated an elimination of the cranialtibiofemoral shear forces at an angle of 01 after TPLO,but not at 51.37 Subsequently, it was concluded that 51might be insufficient to neutralize the cranial tibiofemoralshear forces.37 Alternatively, one in vitro experimentalstudy showed a neutralization of the cranial tibiofemoralshear force at an angle of 6.5 � 0.91 after TPLO.7 Thelatter appears to support the results seen clinically wherethe target angle has been 51 (Seminars entitled TibialPlateau Leveling Osteotomy for Cranial Cruciate Liga-ment Repair; Slocum Enterprises Inc.).6 Furthermore,there are reports that postoperative TPS angles can be ashigh as 141 after TPLO with good results.38–42 Therefore,it is reasonable to assume that other factors appear to beacting at the stifle joint not accounted for in the mod-eling, such as proximal tibial conformation, articularsurface geometry, meniscal integrity, and weight-bearingangle.

For these reasons, any model’s assumptions may bequestioned. As a prerequisite, any biomedical modelingapproach must meet certain criteria: (1) any unknownparameter may be calculated using reasonable approxi-mations, mechanical laws, and experimental measure-ments; (2) it must be realistic and its validity studied bycomparing experimental measurements with results fromthe model.25 The model of the human knee as putforward by Nisell and colleagues,21,23,25 in which thetibiofemoral shear forces were proposed to be eitheranterior or posterior depending upon the knee jointflexion angle, and where the compressive forces were ap-proximately parallel to the patellar tendon, can be shownto meet these criteria. It is not an exaggeration to thenapply this reasoning to the stifle joint of the dog, asproposed by Tepic et al.10 Furthermore, there isexperimental support for this model in the dog, as hasbeen demonstrated in an in vitro cadaver model.12

Based upon the absence of a similar reasonable pro-posal supported by some published scientific literaturethat the direction of the proposed force for the mecha-nism of the TPLO is parallel to the tibial axis, it can beargued that the mechanism of action is identical, i.e., thatthe forces around the canine stifle joint may be approx-imated to being parallel to the patellar tendon (Fig 3).Furthermore, in both TPLO and TTA, the neutralizationof the tibiofemoral shear forces occurs as a result ofmoving the cross-over point such that the cranial tibio-femoral shear force is eliminated during weight bearing(the point at which the patellar tendon force is perpen-dicular to the tibiofemoral shear force, defined along theTPS), and becomes negative (caudal tibiofemoral shearforce) once the stifle joint flexes. The aforementioned invitro studies certainly support this concept as an elimi-nation of cranial tibiofemoral shear force was observedwith either technique; additionally, it was observed that

Fig 4. Schematic representation of the stifle joint of the tibial

plateau leveling osteotomy (TPLO) and tibial tuberosity

advancement (TTA) with respect to patellar tendon angle

(PTA). With the TPLO, the tibial plateau is moved to meet the

force (parallel to the patellar tendon). With the TTA, the force

is moved to meet the tibial plateau. In both cases, the end-point

sought is for the PTA to become oriented at approximately 901

or less.

5BOUDRIEAU

this force became caudally directed with either increasedrotation (TPLO)7 or increased advancement (TTA).12

The latter correspond to the PTA obtained, similar toadditional stifle joint flexion, and in both cases then relyupon the CaCL becoming the primary stabilizer of thejoint.7,8,12

THEORETICAL ADVANTAGES OF TPLO

VERSUS TTA

The question that is raised is whether one or the otherof these techniques (TPLO or TTA) might be a betterchoice for repair of the CrCL-deficient stifle joint, and ifso, under what conditions would one choice perhaps bebetter than the other.

Total Joint Force

The first issue may have to do with the proposedmechanism. If one accepts that with either technique,tibiofemoral shear force is dependent upon PTA, thenaltering the direction of the patellar tendon force is themechanism for obtaining dynamic joint stability. Then,based upon the proposed directions of the total jointforce, either parallel to the patellar tendon (TTA) or tothe functional axis of the tibia (TPLO), the differencecould account for as much as 10–151 difference in end-point after surgery (compare Figs 1 and 2, redirection ofthe resultant vector forces, a �10–151 difference). Basedon this argument, it would appear that the TPLO over-corrects for the cross-over point compared with TTA, ascan be observed with the caudally directed component ofthe tibiofemoral shear force with similar postoperativerotation of the proximal aspect of the tibia in TPLO(compare Figs 1 and 3). It is worth considering, however,that this proposed difference between the 2 techniquesmight be o10–151 because of the anatomic configurationof the joint, i.e., the craniocaudal translation of thefemoral condyles and the contact point with the tibiaduring flexion/extension (Fig 5). During extension, thefemorotibial contact point moves cranially; similarly,during flexion, it moves caudally; whereas the TPS re-mains constant. The effect of this varied positioning is yetto be evaluated, but a suggestion has been made that itshould be considered when assessing the joint forces.43

The issue has been raised as to the determination of thebest method for calculating the amount of tuberosityadvancement to perform with TTA (Boudrieau RJ. TibialTuberosity Advancement [TTA]: Tibial plateau slope orcommon tangent? KYON 2007 Symposium: Innovationsin Veterinary Orthopedics and Trauma. Boston, MA;April 21, 2007).44 If, instead of determining the amountof advancement necessary with the TPS, it is performedwith the common tangent between the tibial and femoral

surfaces at the contact point (Fig 6), then although asmaller amount of advancement is performed with TTA(Fig 7), the same method results in a smaller change ofthe TPA with a TPLO (the TPLO does not permit thesame degree of stifle joint extension because the joint isplaced into greater flexion by rotation of the tibial pla-teau). These anatomic changes further minimize thedifference between the calculated endpoints between the 2techniques, perhaps only 5–101. This issue, consideringthe common tangent, is discussed further below.

The difference in endpoints after surgery raises thequestion as to whether it has any potential adverseeffects. Because the primary stabilizer of the joint be-comes the CaCL after either TPLO or TTA,7,8,12 couldthis difference put the CaCL at greater risk for subse-quent injury with TPLO? Does this �5–151 differencemake a measurable difference in the strain experienced by

Fig 5. Schematic representation of stifle joint flexion with

respect to patellar tendon angle (PTA). In full extension, the

PTA is 4901 and in full flexion the PTA is o901. There is a

point such that the PTA is 901, which is termed the ‘‘crossover’’

point. At this point, there is neither cranial nor caudal

tibiofemoral shear force present. The premise with respect to

the tibiofemoral force, according to Tepic et al,10,11 is that with

any geometric alteration of the proximal tibia with either the

tibial plateau leveling osteotomy (TPLO) or tibial tuberosity

advancement (TTA) (see Fig 4) the cranially directed shear

force as the stifle joint in fully extended during weight-

bearing position is eliminated. The PTA is thus maintained

� 901 throughout the stifle joint range of motion during weight

bearing.

6 TPLO OR TTA?

the CaCL? Furthermore, if there is a difference, is itenough to show a measurable effect on the ligament? Ithas been reported that the CaCL undergoes markedmorphologic changes in CrCL-deficient joints, which mayresult in compromised material properties.45 Regardlessof it, there is no clinical evidence that the CaCL is at riskfor failure after either TPLO or TTA. There is onlyanecdotal information as to the risk to the CaCL withover-rotation of the tibial plateau with TPLO.5 From atheoretical standpoint, TTA would be correcting thetibiofemoral shear force closer to a neutral tibiofemoralshear force at full extension (�1351) duringweight bearing, thus there may be less stress placed onthe CaCL.

A well-designed biomechanical study could determineif such a difference in endpoints exists between the 2surgical techniques. Similarly, there is no documentationas to the ideal point at which tibiofemoral shear isneutralized with either TPLO or TTA, and which of thesetechniques comes closer to this theoretical point. Moreinformation is needed about the possible risks to theCaCL over time, especially in its new role as the primary

stabilizer to the joint. No clinical studies have beenperformed that evaluate the stifle joint after either TPLOor TTA, much less clinical function, long-term.

Anatomic Configuration

Another point to consider is whether or not ananatomic alteration of the tibial plateau (i.e. changingthe TPS with a TPLO) makes a functional difference. Thetibial plateau remains unaltered with TTA, whereas withTPLO, the tibial plateau is effectively placing the joint in�15–201 of increased flexion. This raises 2 immediatequestions. (1) Is the gait altered with perhaps a dimin-ished range of motion in full flexion? (2) Is there anyadditional stress placed upon the menisci because of thischange in joint orientation?

Gait. Based upon kinematic gait analysis, stifle andhock mechanics remain unaltered with TPLO duringweight bearing; however, some changes can be seen in theswing phase of the gait.46 Therefore, gait seems to beunaltered with TPLO, assuming that the alterationsobserved in the swing phase have no functional or clinicalramifications. This assumption appears to be reasonablebased upon the absence of weight bearing during thistime frame. It has been theorized that because TTA doesnot alter the orientation of the articular surfaces, therewould be no effect on gait; however, to date there hasbeen no similar evaluation of gait with kinematics. Anyalteration of the point of insertion of the patellar tendoninto the tibia and any effect on gait mechanics remain tobe investigated.

Femorotibial Pressure Distribution. After TPLO, thefemoral and tibial articular surfaces are placed in arelatively increased flexed position (although the stiflejoint flexion angle remains unchanged). Altered flexion ofthese surfaces, or change in geometry, during weightbearing results in changes in the pressure distribution tothe caudal compartments of the joint (medial greater thanlateral),31 possibly affecting the menisci (especially themedial meniscus); this altered positioning may alsoreduce the space for the meniscus. Both factors mayplace the meniscus at a greater risk for injury. The ab-normal pressure distribution may either place the menis-cus at risk (potential increased risk of tear) and/orcontribute to articular cartilage changes (degenerationcaused by abnormal loading).31 The original descriptionfor TPLO recommended that a medial meniscal releasebe performed to spare the immobile caudal pole of thisstructure from subsequent injury in a cruciate-deficientstifle.5,6,47 However, the rationale presented was someremaining functional laxity with the anatomic alterationof the tibial plateau, and the lack of compensation bythe active muscular forces around the joint (hamstringmuscles), which could subsequently damage the immobile

Fig 6. Lateral radiographs of the stifle joint before (A) and

after (B) the common tangents are drawn to outline its position

in the joint. The anatomic shape of the femoral condyles in the

stifle joint changes the position of femoral–tibial joint contact

during stifle joint flexion and extension (see Fig 5). Therefore,

an alternate orientation of the tibiofemoral contact point is

used as the baseline to define the tibiofemoral shear force,

whereby the femur can move along this surface if the cranial

cruciate ligament (CrCL) is deficient. This line is determined

by circumscribing a circle over both the articular surfaces of

the femoral condyle and the tibial plateau (defined by the limits

of the menisci). The tibiofemoral shear force is then defined by

a line perpendicular to a line drawn through the center of these

2 circles at their contact point.

7BOUDRIEAU

caudal pole of the medial meniscus with tibiofemoralsubluxation.5 This was the rationale to perform a medialmeniscal release in these cases, and was taught at theTPLO courses (Seminars entitled Tibial Plateau LevelingOsteotomy for Cranial Cruciate Ligament Repair;Slocum Enterprises Inc.).5 More recently, it also hasbeen shown that performing the meniscal release, or cau-dal pole hemimeniscectomy, in a CrCL-deficient stiflejoint with a TPLO further changes and increases thepressure distribution in the medial compartment ofthe joint, an argument that would perhaps favor leavingthe meniscus intact.31 There are no similar evaluationsreported of medial meniscectomy after TTA.

Although there is no evidence that the increased stiflejoint flexion predisposes the menisci to damage, it wasproposed that TTA might provide less risk for suchdamage because of the unaltered joint position.48 This ledto the original recommendation to leave the intact men-isci in situ when performing TTA.9

Femorotibial contact pressure and location have beenevaluated in vitro with both TPLO and TTA.49 In aCrCL-deficient stifle joint, there is a �40% decrease incontact area with an associated 100% increase in peakpressure; furthermore, the positioning of the peak

pressure is found to shift caudally.49 TTA appears torestore the normal femorotibial contact and pressuredistributions, whereas TPLO results in a continueddecrease (12%) of contact area and caudal positioningof peak pressure distribution.49 These latter effects maypredispose the caudal pole of the intact medial meniscusto trauma after TPLO, and apparently spare themeniscus after TTA. These studies seem to suggest thatthe caudal pole of the medial meniscus is at risk fortrauma after TPLO, whereas not after TTA. This studyalso suggests that TPLO radically changes the biome-chanics of the stifle and potentially predisposes to furtherprogression of osteoarthritis; because TTA does notchange the geometry of the joint and the pressuredistributions essentially remain unchanged, there maybe less development of osteoarthritis. This issue may bemore relevant that the potential risk of meniscal injuryincreases after TPLO.

It has been proposed that after TTA there couldcontinue to be the possibility of meniscal damage29,30;thus, the varying recommendations as to whether or notto perform a meniscal release clinically (InstructionalCourse for Tibial Tuberosity Advancement [TTA] forCranial Cruciate Ligament Deficient Stifle Joints in Dogs;

Fig 7. Schematic representation of the tibiofemoral forces in the stifle joint, according to Tepic et al,10,11 before (A) and after (B)

tibial tuberosity advancement (TTA). The resultant compressive force (large white arrow) across the stifle joint is parallel to the

patellar tendon. Using the common tangent at the tibiofemoral contact point (see Fig 6) as the baseline (solid line), whereby the

femur can move along this surface if the cranial cruciate ligament (CrCL) is deficient, the resultant force can be broken down into

its 2 orthogonal components (small shaded arrows), one perpendicular and one parallel to the tibial plateau. The latter represents

the tibiofemoral shear force. If the angle of the tibial tuberosity is advanced cranially until the patellar tendon angle (PTA) is

reduced to 901, the tibiofemoral shear force vector becomes zero, and the joint compressive force and resultant force become one and

the same. Notice that the cranial tibiofemoral shear force is smaller than that depicted with the PTA using the tibial plateau slope

(TPS), which is indicated by the dotted line (compare with Fig 2). Notice that the common tangent and TPS are similar in stifle joint

flexion (C). Insets clarify the force vectors depicted on the schematic bone models.

8 TPLO OR TTA?

Kyon Veterinary Surgical Products, Denver, CO). Thespeculation is that despite the results of the experimentalstudies, a continued passive laxity not adequately bal-anced by muscular forces around the joint potentiallyremains after TTA (similar to that postulated afterTPLO). This instability and the resultant remainingtibiofemoral shear forces cause a tibiofemoral subluxat-ion injuring the menisci. Passive laxity has beendemonstrated in vitro with both TPLO and TTA in aCrCL-deficient stifle joint.50 It has been suggested thatthere is a difference in degree of passive laxity in thejoint between TPLO (less laxity) compared with TTA,although the impact of this laxity in clinical cases has notbeen determined.50 This concept suggesting a possiblemechanism for subsequent meniscal damage remains tobe investigated.

Meniscus. Whatever the opposing mechanisms pro-posed regarding the meniscus, debate remains as towhether or not to perform a medial meniscal release inboth TPLO and TTA. It appears there are 2 opposingopinions regarding the necessity of meniscal release atthis time with TPLO.51 Similar anecdotal information hasbeen presented for TTA.52

One argument to preserve the menisci has to do withthe important biomechanical functions of the meniscusand the role of this structure to stabilize the joint. Theopposing argument to perform a meniscal release relatesto the costs, both in terms of surgery to the patient andeconomic impact to the owner, for additional surgeryshould the preserved meniscus subsequently be damagedversus the clinical ramifications of a joint with a meniscalrelease performed, which essentially is a meniscectomy.The meniscus functions as a load-bearing structure todistribute the femoral condyle forces more uniformlyover the tibial plateau. Meniscal release eliminates thisfunction and results in increased areas of localized stressto the articular cartilage.53,54 As previously noted, there isexperimental evidence that preserving the meniscus witheither TPLO or TTA better preserves joint mechanics inthe CrCL-deficient stifle joint, and perhaps there is amore normal distribution after TTA.49,53 These studiessuggest leaving the meniscus intact if it is uninjured.

Alternatively, as the meniscus acts as a secondary jointstabilizer in the CrCL-deficient joint, with any persistentpassive laxity the caudal pole of the meniscus may bemore easily injured with a failure to fully neutralize thetibiofemoral shear forces.53,54 Sparing this structure frominjury by performing a meniscal release has been sug-gested.55 However, it has also been noted that performinga meniscal release does not completely eliminate thepossibility of subsequent meniscal tears.56

It has been reported that after TPLO with meniscalrelease, there is cartilage damage present on the tibialplateau and medial femoral condyle, confirming that

subsequent articular cartilage damage does occur.51,57,58

However, it has also been stated that there is littleoutward evidence of clinical dysfunction as a resultof these changes,51,56 which is consistent with mostclinical impressions. Despite the progressive effects ofosteoarthritis in these dogs, clinical dysfunction isconsidered minor as opposed to that occurring with aninjured meniscus; therefore, this argument supports themeniscal release in favor of possible future meniscalimpingement.51

The question is the unknown frequency of meniscalinjury at initial surgery, and the number of undetectedtears at that time as opposed to tears occurring aftersurgery.56 These meniscal injuries have been defined aseither latent (undetected) or postliminary (subsequent)tears.59 Failure to detect a medial meniscal tear is thoughtto be a result of incomplete inspection and probing of theposterior horn.59,60

Patellar Tendon. There is the question as to whetherthe altered anatomy created by either of the procedurescould result in other anatomic or functional changeswithin the joint. For example, there have been a numberof reports of patellar tendon thickening, or even patellartendinitis after TPLO.27,28,61 It has been proposed that byrotating the tibial plateau, greater stress is placed on thepatellar tendon compared with decreased stress to thisstructure after performing a TTA (Instructional Coursefor Tibial Tuberosity Advancement (TTA) for CranialCruciate Ligament Deficient Stifle Joints in Dogs; KyonVeterinary Surgical Products).61,62

The proposed increased stress (TPLO) versus de-creased stress (TTA) on the patellar tendon can theoret-ically be explained by the change in lever-arm lengths tothe patellar tendon after the osteotomies. If one considersthat the CaCL becomes the primary stabilizer to the jointafter either TPLO or TTA, the lever arm to the patellartendon is the distance between the femorotibial contactpoint to the point of attachment of the patellar tendon tothe tibial tuberosity (Figs 8 and 9). For TPLO (Fig 8), it isthought that this lever arm can shorten by as much as10% compared with the intact joint, whereas, with TTA(Fig 9) this lever arm can lengthen by as much as 10%(Instructional Course for Tibial Tuberosity Advancement(TTA) for Cranial Cruciate Ligament Deficient StifleJoints in Dogs; Kyon Veterinary Surgical Products).These measurements have not been demonstratedexperimentally other than to note that both saw bladekerf and osteotomy position will influence the femoro-tibial contact point position, as both will induce a cranialtibial long axis shift shortening the lever arm (Fig 8).62,63

A shorter lever arm requires more force to move anobject the same distance; conversely, a longer lever armrequires less force to similarly move an object. In thiscase, the object is the tibial tuberosity and the force is the

9BOUDRIEAU

quadriceps muscle pull on the patellar tendon. In otherwords, more force is required to extend the joint with aTPLO, and less force is required with a TTA. Patellar

tendon thickening after TPLO has been reported in 1–5%of cases.27,28 Anecdotally, this change is a much morecommon finding in stifle joints after TPLO, and is foundin 450% of cases, despite this low reported occurrence.In fact, this percentage of stifle joints with patellar tendonthickening is consistent with one study that specificallyevaluated the radiographic changes of the patellar tendonafter TPLO, and which reported moderate to severepatellar tendon thickening at 2 months postoperatively in79.8% of dogs.61 The proposed increased forces requiredto extend the stifle joint may be responsible for thisinflammation. There are no published reports of patellartendinitis with TTA at this time. Again, the decreasedforces proposed as a result of this lengthened lever armmay be the explanation. This proposed cause remains tobe investigated, both clinically and experimentally.

Regardless, there are no known clinical ramificationsin dogs with mild to moderate patellar tendon thicken-ing after TPLO, although a small percentage of dogswith severe patellar thickening have clinical signs con-sistent with patellar tendinitis.61 It remains to be deter-mined if TTA has fewer of such problems and if theproposed theory can be validated clinically. Regardless,there are no specific studies that have investigatedthis issue, clinically or experimentally, much less com-paring the different techniques and their potentialramifications.

Fig 8. Schematic representation of the lever arm in the stifle joint before (A) and after (B and C) tibial plateau leveling osteotomy

(TPLO). (A) Patellar tendon lever arm (bending moment) in the stifle joint extends from the patellar tendon to the femorotibial

contact point (heavy dotted line). (B) After TPLO, the patellar tendon lever arm (heavy dotted line) shortens marginally provided

that the osteotomy is centered on the intercondylar tubercles (because of the loss of bone removed by the kerf of the saw blade). (C)

Positioning of the osteotomy more often is centered just below the joint surface and caudal to the medial collateral ligament;

furthermore, any additional placement of the osteotomy more distal of the tibia results in further shortening of the patellar tendon

lever arm with rotation of the tibial plateau (heavy dotted line; compare with part [B]).

Fig 9. Schematic representation of the lever arm in the stifle

joint before (A) and after (B) tibial tuberosity advancement

(TTA). (A) Patellar tendon lever arm (bending moment) in the

stifle joint extends from the patellar tendon to the femorotibial

contact point (heavy dotted line). (B) After TTA, the patellar

tendon lever arm (heavy dotted line) lengthens.

10 TPLO OR TTA?

Retropatellar Pressure. Another result of the alteredpatellar tendon position has been the proposed alteration(decrease) of retropatellar force with TTA. Theoretically,this diminished force can protect the articular cartilage ofboth the patella and the femur from subsequent damage(P.M Montavon; personal communication, 2008). Thisclaim needs to be substantiated. As previously noted,there are isolated reports of articular cartilage damage tothe tibial plateau and femoral condyle after TPLO withsecond look arthroscopy.51,57

Decreased retropatellar pressure was the stimulus toperform TTA in humans to treat cases of severe patello-femoral arthritis.64 Recently, a 3-dimensional nonlinearjoint finite element model reconstruction based on ahuman cadaver knee joint confirmed not only thisprocedure’s effectiveness in decreasing patellofemoralcontact forces, but also femorotibial contact forces withstifle joint extension.65 This same model, however,showed an increase in these forces at larger flexionangles.65 The question remains as to whether or notthis same effect occurs in the dog, and if it has anysignificant effect.

SURGICAL TECHNIQUE

Both the TPLO and TTA are complex procedures,requiring appropriate preplanning and accurate execu-tion of the details of the procedure.

Preoperative Planning

TPLO. Preoperative planning must include a properassessment of the angle of the TPS, such that precisesurgical execution results in the desired postoperativeTPA of 51 (Seminars entitled Tibial Plateau LevelingOsteotomy for Cranial Cruciate Ligament Repair;Slocum Enterprises Inc.).6 The measurement techniquefor TPLO uses the TPS, and for ease of positioning, theconvention is to place the stifle joint at 901 to the tibia.These measurements are obtained from a true lateralradiographic projection of the stifle joint, centered on thestifle joint (perfect positioning confirmed by superimpo-sition of both femoral and tibial condyles).66 It has beendemonstrated that accurate preoperative radiographicpositioning is important to obtain precise and reproduc-ible measurements.66,67 In addition, there is someinter- and intraobserver variability when making thesemeasurements.67–69 Despite these attempts at obtainingprecise preoperative measurements, there have beena wide variety of postoperative TPAs reported, from0–141.38–42 Regardless of this postoperative variation,excellent clinical outcomes have been reported.38–42

Force plate analysis appears to indicate a wide range ofacceptable postoperative TPAs (0–141).42 This variation

indicates that the ideal clinical postoperative TPA isunknown, despite the recommendation for a postopera-tive TPA of 51. One in vitro experimental study showsneutralization of the cranial tibiofemoral shear force atan angle of 6.5� 0.91.7

TTA. Preoperative planning must also include aproper assessment to ensure that advancement of thetibial tuberosity results in a PTA of 901.29,30 A standardlateral radiographic projection of the affected stifle jointis obtained preoperatively to assess the joint. The lateralprojection is centered on the stifle joint (perfect position-ing confirmed by superimposition of both femoralcondyles) at a stifle joint angle of 1351 using the longaxes of the femur and tibia (the entire femur is includedto determine the appropriate femoral long axis). Thisreference point is made based upon some clinical andexperimental observations of the canine stifle joint duringweight bearing. The choice of this angle for assessment ofPTA in the preplanning for the surgical technique reflectsthe midstance phase angle of the gait cycle as determinedby kinematic analysis.70–72

Because the patellar tendon is part of this measure-ment, the PTA also is dependent upon the position of thefemur (angle of stifle joint flexion/extension; Fig 7). Inaddition, the joint must be positioned so that there is nocranial tibial translocation. A standardized TTA trans-parency (Kyon; Zurich, Switzerland) is used to determinethe amount of TTA required to position the patellartendon perpendicular to the tibial plateau in a standingposition (1351 stifle joint extension) and the size of plateto cover the entire extent of the tibial crest.30

No reported studies have investigated inter- and intra-observer variabilities when making these measurements.Similarly, it is unknown if any postoperative variation inthe amount of advancement actually obtained has aneffect on the outcome, and the degree to which sucherrors might affect postoperative results.

Despite these recommended measurement guidelinesfor the amount of TTA distance, the evaluation methodbased upon the TPS recently has been questioned(Boudrieau RJ. Tibial Tuberosity Advancement [TTA]:Tibial plateau slope or common tangent? KYON 2007Symposium: Innovations in Veterinary Orthopedics andTrauma. Boston, MA; April 21, 2007). In some cases, theproposed advancement appears to be excessive for thesize of the dog, which indicates that there is some differ-ence among dogs related to stifle joint anatomy. An al-ternative procedure has been recommended to obtainthese measurements with the tangent method, i.e., a lineperpendicular to the contact point between the femoraland tibial joint surfaces, again at 1351 of stifle jointextension at the point of contact between the femur andtibia (2007 Veterinary Symposium—The Surgical Sum-mit; Pre-Symposium Laboratories: TTA Laboratory;

11BOUDRIEAU

Chicago, IL; Fig 6). It has been suggested that this mea-surement is a more anatomically representative referencepoint as identified by the desired crossover point toobtain a neutral tibiofemoral shear force (Fig 7).43 Theresults of this study also suggest that the angle betweenthe patellar ligament and the tibial plateau decreaseslinearly with increasing stifle joint flexion in dogs, whichis similar to the findings in knee joints of humans.43

Furthermore, both inter- and intraobserver variabilitiesappear to be minimal with this technique.43

In clinical usage, both measurement techniques resultin similar amounts of calculated TTA in most cases,although the tangent method consistently is a slightlylower angle compared with the TPS. The assumption isthat this small difference is inconsequential in these cases,but this has not been documented (Fig 10). Of greaterpotential concern, however, is that the anatomic config-uration in some cases shows a large difference betweenthese measurements (Fig 10), as previously noted.

It is suggested that there is some variability in thepostoperative outcomes with the TTA despite an attemptto obtain precise preoperative measurements.29 No ob-jective studies, such as force plate analysis, have beenpublished to date to assess the results of the TTA, muchless the effect of any such variability. Similar toTPLO, there probably is some amount of postoperativevariability that will be tolerated, but the ideal clinical

postoperative PTA is unknown, despite the recommen-dation for a postoperative PTA of 901, using either theTPS or common tangent as the origin for the angle of thepatellar tendon.

There has been no validation for the newer proposal tomeasure PTA using the common tangent with TTA;however, the same could be said of the measurementtechnique for TPLO using the TPS. Despite the recom-mendations to rotate the TPA to 51, there is no evidencewhich supports this as the ideal target angle other than asingle in vitro cadaver study where 6.51 is suggested,7 aspreviously noted. The only presently published work withTTA is based upon the TPS similar to the method of theTPLO, and the data support the desired outcome of aPTA (oriented along the TPS and PT) of 90.3� 9.01.12

We are currently evaluating the tangential method in anin vitro experimental study. Regardless, in our currentclinical cases, the measurements being performed use thetangent method. This method is also being taught inthe Kyon TTA courses (2007 Veterinary Symposium—The Surgical Summit; Pre-Symposium Laboratories:TTA Laboratory) Another question to be raised iswhether the common tangent should be used instead ofthe TPS for determination of the amount of planned ro-tation and postoperative evaluation of rotation with theTPLO. The suggestion is that using the common tangentwould demonstrate less postoperative variability in thefinal joint position obtained, and perhaps be closer to the901 PTA proposed endpoint. If true, the apparent differ-ences between the techniques may not be so different af-ter all. This concept is yet to be evaluated.

Surgical Considerations

TPLO. As originally described, TPLO is a relativelyinvasive procedure where there is abundant circumfer-ential dissection of the entire proximal aspect of the tibia,and a greater potential for injuring some vital structuresaround the joint. The substantial soft tissue dissectionand limited coverage in the area of the proximal tibiamay contribute to dead space and may predispose dogs toincisional complications. In a retrospective evaluation of281 dogs at the Tufts Cummings School of VeterinaryMedicine, there was an infection rate of 1.8% for dogsthat had a lateral fabellotibial suture compared with6.1% with TPLO.73 More recently, however, a moreconservative approach has been recommended with theTPLO whereby there is an absence of lateral dissectionand limited caudal dissection. Even so, the location ofthe saw cut with the TPLO has greater potentialfor iatrogenic damage to vital structures comparedwith TTA. Nonetheless, there do not appear to be sig-nificant clinical ramifications as a result of such iatrogenicdamage.

Fig 10. Lateral stifle joint radiographs demonstrating differ-

ences in the anatomic shape of the femoral condyles resulting in

a difference in the tibial plateau slope (TPS) (solid line) and the

common tangent (dotted line) at the femorotibial contact point.

(A) A difference of approximately 101 between these measure-

ments in this example; the TPS is consistently observed at a

higher tibial plateau angle (TPA). (B) Essentially no difference

is observed between these measurements in this example. Com-

pare TPS (solid line) and the common tangent (dotted line) at

the femorotibial contact point and femoral condylar anatomy

between parts (A) and (B). Notice the different anatomic con-

formations between the femoral condyles.

12 TPLO OR TTA?

There are a number of surgical technical errors thatcan occur with TPLO. Placing the osteotomy too farcranial and/or distal will result in a higher than expectedTPA postoperatively, and inadequate neutralization ofthe CrTT.62,63 Such malposition of the osteotomy alsoresults in a small tibial tuberosity fragment, creating thepossibility of a tibial tuberosity fracture. Similarly, plac-ing the temporary holding pins (after initial rotation ofthe tibial plateau) too far distally into the tibial tuberosityresults in a possible stress-riser in this fragment, againpredisposing this bone to fracture. Fibular fractures canresult in excessive stresses placed on this structure duringthe rotation, or may occur as a result of stress-risers fromdrill holes into this structure. Other issues may includefractured drill bits, intra-articular screw placement, andmedial tibial cortical damage when placing osteotomymarks for the rotation. Additional miscellaneous itemsmay include excessive osteotomy gaps, sponges left atthe surgery site, intra-articular placement of jig pins,long digital extensor tendon damage, CaCL injury,broken holding pins, and screws placed within theosteotomy site. It can also be argued that the TPLO isa fairly complex procedure with perhaps some potentiallyunrecognized pitfalls where iatrogenic errors can result ina change in limb angulation and rotation.74 Theseerrors can be readily demonstrated in a carpentryanalogy where an angled cut results in a translation withrotation (Fig 11). Therefore, it is not unreasonableto consider the TPLO as a procedure with a steeplearning curve.

TTA. Anecdotally, TTA is considered by many to bea much simpler procedure than the TPLO. Similar com-ments regarding soft tissue dissection and limited cover-age in the area of the proximal tibia may be made forTTA; however, the surgical dissection is confined tocranial portion of the bone. There is very limited possi-bility for iatrogenic surgical injury with TTA, but damageto the long digital extensor tendon is one potentialpitfall.29 There are a number of surgical technical errorsthat can occur with TTA. These issues include too smallof an osteotomy fragment, shifting the patella distally, orpredisposing to a patellar luxation with improper tibialtuberosity repositioning. A small osteotomy fragmentdoes not allow adequate fork purchase as there isinsufficient bone available. Another potential error ispatellar malposition. Patella baja results if the tibialtuberosity is not allowed to shift proximally to maintainthe patellar position, i.e., a distal tibial crest centeredrotation rather than the desired patella-centeredrotation.30 Patellar luxation can result if attention is notpaid to the plate contouring, ensuring that the tibialtuberosity is advanced cranially without changing theorientation within the sagittal plane. Any shift eithermedially or laterally could result in a malalignment of

the quadriceps mechanism, and thus the resultantpatellar luxation.

Other issues may include poor plate position, eitherrostrally along the tibial crest or distally along the tibialdiaphysis. Additional miscellaneous items may includecage malposition and also intra-articular screw place-ment. There appear to be fewer technical issues with TTAcompared with TPLO, but there are no published reportsto document this. Finally, the difference in implantscould be considered: commercially pure titanium (cpTi) isthe norm with TTA versus stainless steel with TPLO.Pure titanium is touted as a more biocompatible implantwith tissues as compared with stainless steel.75–77 Fur-thermore, the plate profile with TTA is very thin andprovides less overall bulk in a position on the limb wheresoft tissue covering is limited to a thin muscle layer andskin. This may play a role in limiting soft tissue compli-cations. It may be argued that despite TTA also being arelatively complex procedure, there are fewer unrecog-nized pitfalls that may result in postoperative problems.Therefore, it is not unreasonable to consider TTA as aprocedure with a short learning curve. Regardless, muchof this is supposition as there are no reports of objectivecomparisons of the 2 surgical techniques comparing thesefactors and their possible effects.

Fig 11. Schematic representation of the differences in axial

positioning after a perpendicular cut to the axis (top) of a

straight beam versus an angled cut (middle) with rotation

(bottom); curved (circular) arrows indicate axis of rotation. An

angled cut results in a translation of the end of the beam after

rotation (single curved arrow). Similar malalignment of the

tibial osteotomy in a tibial plateau leveling osteotomy (TPLO)

will result in iatrogenic rotational and angular deformities after

rotation of the tibial plateau.

13BOUDRIEAU

OUTCOME AND COMPLICATIONS

TPLO

In general use for at least 9–10 years, TPLO has a‘‘track record,’’ even if much of it is regarded as mostlyanecdotal. Various published reports on outcome are theearly experiences of a number of surgeons.26–28 Thesereports, therefore, are biased with the respective learningcurves for these individuals.

In the 3 studies reporting complications in CrCL-deficient stifle joints with a TPLO, the overall complica-tion rate was 18.8–28%.26–28 A similar overall rate ofcomplications (31.1%) of another initial case series hasbeen reported recently.73 Averaging the complication ratein these 4 reports (1772 cases) results in an overallcomplication rate of 26.3%. A reoperation rate wasreported in � 5%27 to 9%73 of the cases. Thesecomplications, presented in order of frequency, were asfollows: incisional complications, infection, bone-relatedcomplications, intraoperative problems, and implant-related problems. Incisional complications (edema,swelling, seroma, dehiscence) were most commonlyreported and occurred in 2–16% (mean 9.3%).26–28,73

Infection was next most commonly reported in 5.9–7.1%(mean 6.0%),26,27,73 followed closely by bone-relatedproblems (tibial tuberosity, fibular, or tibial fracture) in4.9–7.1% (mean 5.4%).26–28,73 Intraoperative problems(hemorrhage, broken drill bit, damage to medial tibialcortex, intra-articular screw placement, etc.) werereported in 0.9–6.7% (mean 2.8%).26–28,73 Implant-relatedproblems (screw loosening, breakage) occurred in 1.1–5.2% (mean 2.4%).26–28,73 A number of these complica-tions can be considered technical failures associated withthe learning curve in performing this surgical procedure.

Other complications related to anatomic changes afterTPLO include pivot shift, patellar tendon thickening, andpatellar tendinitis. The pivot shift, as described incommon usage with this technique, is a sudden internalrotation of the tibia with lateralization of the hock, and asudden lateral change in direction of the stifle joint duringweight bearing (in humans it is characterized by anteriorsubluxation of the tibial plateau from beneath the lateralfemoral condyle).78 Although there are anecdotalcomments from a number of surgeons regarding thisphenomenon in the dog after TPLO, the actual frequencyis unknown. Furthermore, the reason for its occurrence isalso unknown, but is thought to be a result of insufficientcorrection of tibial torsion or angular deformity. Patellartendon thickening has been reported in 1–80% of casesafter TPLO27,28,61; this issue was previously discussed.

Healing of the osteotomy was not evaluated in thesereports, and in only 1 report was the overall outcomereported (owner satisfaction in 93% of the cases).26

TTA

A relatively new procedure, TTA has been in generaluse for o5 years. Similar to TPLO, there are anecdotalreports of good to excellent results, and some earlyclinical results published.29,30 The latter again are theearly experiences of a number of surgeons, which alsoremain biased with the respective learning curves forthese individuals. The 2 studies (179 cases) reportingcomplications in CrCL-deficient stifle joints repairedusing TTA, report an overall complication rate of31.6–59%.29,30 A number of minor complications werereported in both studies, including postoperative swellingand bruising accounting for 19.3–21% of these compli-cations.29,30 The major complications accounted for12.3–38%.29,30 A reoperation rate of 11.3% wasreported.30 Combining the data, these complications, inorder of frequency, were as follows: meniscal tears(7.2%), infection (3.9%), medial patellar luxation(1.1%), tibial fractures (1.1%), and catastrophic implantfailure (1.1%).29,30 The primary discrepancies betweenthese reports were the frequency of meniscal tears in one(21.7%)30 and technical failures in the other (22%).29

Radiographic healing was reported to be ‘‘partiallycomplete by � 7–8 weeks postoperatively and fully com-plete as early as 8–10 weeks postoperatively’’ in onereport,29 and at a mean of 9.4 weeks postoperatively inthe other report.30 Overall function (outcome and lame-ness) of the dogs postoperatively reported to be good toexcellent in 490% of the dogs.29,30

In both studies, 2 major points are discussed: earlytechnical errors with the procedure and meniscalinjury.29,30 It is important to note that with the elimina-tion of technical errors, also associated with the learningcurve in performing this surgical procedure, as with theTPLO, would significantly reduce the number of majorcomplications. The issue with the meniscus is moreconfounding, as there is much controversy as to the bestmethod of approach: meniscal release or no meniscalrelease. TTA was originally performed without ameniscal release, in contrast to the recommendation forTPLO; therefore, the disparity between reports (TPLOversus TTA) most likely reflects this bias. It appears thatmost complications occurring postoperatively in oneTTA study were because of meniscal tears, eitherthose that may have been missed at the time of theoriginal surgery or those that subsequently occurred.30

A suggestion was made that meniscal release mayeliminate this issue.30

Comparison of TPLO and TTA

The study designs with these reports, both TPLO andTTA, cannot be directly compared because of the many

14 TPLO OR TTA?

differences in study variables that are reported. The onlyunifying similarity in all studies is that they representearly experience with both techniques. Although there areno published reports of a comparison between these 2techniques, some initial data has been presented.17,79 Inone retrospective evaluation, the first 85 cases using eachtechnique were compared (the 2 series of cases were per-formed by the same surgeon � 6 years apart).17 Similarage, weight, and breeds were seen in both groups; fewshort-term (o2 months) and long-term (46 months)complications were reported.17 Outcomes were notpresented, other than noting that both techniques wereclinically effective in restoring good function and bothhad a similar frequency of postoperative complications.17

In the other report,79 TPLO and TTA outcomes werecompared over a 6-month time frame after surgery. Noobvious limb-loading differences, as determined by forceplate analysis, were observed between the techniques,although some minor early differences were noted(slightly quicker loading and clinical function seen withTTA, and greater patellar tendon thickening observedwith the TPLO).79

At this time, review of the early results and complica-tions of both the TPLO and TTA indicate that their out-comes are very similar. There are no published clinicalreports that compare the techniques, much less the com-plications and outcomes with experienced surgeons thatmight allow a direct comparison of the techniques and theiroutcome. Based upon anecdotal comments, it is assumedthat with both procedures the results currently obtained arebetter than that reported, but this is only a supposition.

CASE SELECTION

Some factors specific to the anatomic configuration ofthe limbs being operated need to be considered, from thestandpoint of conformational issues that would make oneprocedure perhaps the better choice than the other. Theseinclude angular and torsional limb deformities, patellarluxation, and excessive TPS. Furthermore, the size of thedog may also play a factor.

Low Versus High Insertion Point

When performing a TPLO, rotation of the tibialplateau segment only to the level of the patellar tendoninsertion on the tibial tuberosity has been suggested toensure its role as a buttress support for the tibial tubero-sity segment.80 Therefore, dogs with high patellar tendoninsertion point would run the risk of the rotation of thetibial plateau segment to a point below the patellartendon insertion, thus potentially leaving the tibialtuberosity more prone to fracture because of an absenceof buttress support (Fig 12). Alternatively, in dogs with a

low patellar tendon insertion point, much greater rota-tion could be obtained while continuing to preserve thecaudal buttress behind the tibial tuberosity. In contrast,the tibial tuberosity may be at more risk for possiblefracture with TTA in cases with a low patellar tendoninsertion point, as a smaller plate is applied to the tibialcrest and the usual position of the interspersed cage isabove the most proximal position of the plate with littlebone present for support. In dogs with a high insertionpoint, a larger TTA plate can be applied to the tibial crest,and the interspersed cage is placed within the gap, whichremains buttressed with adequate bone and a larger platethat disperses all the forces to the tibial crest (Fig 12).

It is suggested that cases with a high patellar tendoninsertion point are more conducive to a TTA, whereascases with a low patellar tendon insertion point arebetter suited for a TPLO. In any case, there are noexperimental or clinical studies reported that supportthese assumptions.

Excessive TPS

Cases where there is excessive TPS are not conduciveto TTA, because the procedure requires that theadvancement produce a PTA of 901, which likely would

Fig 12. Lateral stifle joint radiographs demonstrating differ-

ences in the anatomic shape of the proximal tibia and tibial

tuberosity with a low (A) versus high (B) insertion point of the

patellar tendon (PT; arrows). The dotted and solid lines rep-

resent the positions of the tibial plateau leveling osteotomy

(TPLO) and tibial tuberosity advancement (TTA) osteotomies,

respectively. Rotation of the tibial plateau (TPLO) maintains

continued buttress support behind the tibial tuberosity for a

larger distance of rotation with a low PT insertion point (com-

pare part [A] versus part [B]). Similarly, enhanced plate place-

ment (larger area that enables a plate with additional points of

contact/larger number of fork tines) and buttress support of the

tibial tuberosity after a frontal plane osteotomy with advance-

ment (TTA) is observed with a high PT insertion point (com-

pare part [A] versus part [B]).

15BOUDRIEAU

result in a required amount of advancement beyond thatobtained with the currently available implants (maximalcage width for the osteotomy gap of 12mm). Addition-ally, there is a conformational deformity of the joint withexcessive TPS that places it in a relative angle of hyper-extension despite the limb itself not being in the extendedposition (Fig 13). TTA does not address this malforma-tion, whereas the TPLO can correct it. Even with theTPLO, however, a straightforward correction by rotationof the TPS only may not provide sufficient correction ofthe TPA; it has been recommended that a postoperativeTPA of �141 be performed.81 It has also been recom-mended that the TPLO be combined with a closing wedgeosteotomy (CCWO) to obtain more uniform postopera-tive corrections of TPA to � 51.80 It has been suggestedthat the tibial tuberosity is at risk of fracture if the tibialtuberosity is rotated beyond the ‘‘safe point’’ where itremains buttressed by the rotated proximal tibial frag-ment.80,81 Regardless, it has also been suggested thatadditional implants be used to secure the osteotomy withlarge amounts of rotation with or without an additionalosteotomy.80,81 The question remains, however, as to themaximal angle of the TPS (or common tangent) thatshould be used as a guideline to consider whether to per-form a TTA. In one presentation,17 it was reported thatin a single case in which a dog had a TPS of 271 with aTTA, the dog continued to have a positive tibial com-pression test postoperatively, and subsequently was suc-cessfully reoperated at a later date with a TPLO.No TTAs were performed in dogs with a TPS 4271.17

Although no data has been published regarding the rangeof TPS in dogs with TTA, it has been presented that

successful procedures have been performed in dogs with aTPS of �301 and anecdotally proposed that angles4301 probably are not well suited for a TTA (2007 Vet-erinary Symposium—The Surgical Summit; Pre-Sympo-sium Laboratories: TTA Laboratory).

Similarly, despite the recommendation with a TPLOto maintain the rotation of the tibial plateau such that thetibial tuberosity remains buttressed caudally, there isevidence that this may not be necessary in all cases.81

Limits to the amount of rotation with a TPLO and theamount of advancement with a TTA to adequatelyneutralize the tibiofemoral shear forces in the clinicalpatient remain to be definitively defined.

Angular and Torsional Limb Deformities

Angular and torsional limb deformities may be treatedwith either TPLO or TTA; however, TPLO may be bettersuited to make these corrections simultaneous with therotational osteotomy, either alone or combined with aclosing wedge osteotomy, the latter procedure enhancingthe stability of the fixation. With minor angulation orrotation, these deformities can be addressed after rotationof the proximal tibial fragment, after it has been tempo-rarily secured with a pin or K-wire, and before platefixation. At this point in the procedure, an angularcorrection (stifle varus or valgus) can be performed byshifting the jig position along either the proximal or distaljig pin (a translation of the jig along the pin) so as toobtain limb realignment (Seminars entitled Tibial PlateauLeveling Osteotomy for Cranial Cruciate Ligament Re-pair; Slocum Enterprises Inc.).82 Similarly, a rotational

Fig 13. Lateral stifle joint radiographs demonstrating differences in the anatomic shape of the proximal tibia such that the tibial

plateau angle (TPA) is either considered excessive (A) at 431, or within normal limits (B) at 251. This conformational deformity of

the stifle joint angle in part (A) places the joint in a relatively hyperextended position (compare with part [B]). Advancing the tibial

tuberosity does not correct this hyperextension as the TPA remains unaltered. A tibial plateau leveling osteotomy (TPLO) corrects

this joint position by altering the TPA (either alone or combined with a cranial closing wedge osteotomy).

16 TPLO OR TTA?

correction can be performed by bending one of the jigpins (usually the distal pin) while the jig is securely fixedto these pins (Seminars entitled Tibial Plateau LevelingOsteotomy for Cranial Cruciate Ligament Repair;Slocum Enterprises Inc.).82 The osteotomy position,however, changes such that the opposing bone surfacesno longer remain in full contact. Instead, they tip respec-tive to one another, while maintaining cortical contacton the side opposite the distraction (Fig 14). The finalplate fixation spanning the osteotomy site, and gap, isthen applied.

Rotational and stifle varus deformities are amenableto repair by this technique as the gap is directly under theplate and cortical contact is obtained on the trans cortex.A cancellous graft may be applied to the gap. Smallcorrections can be performed in this manner, generallyo5–101, although no specific guidelines have beenpublished. Larger corrections could be performed, butthese result in large gaps at the osteotomy site, whichdecreases the rigidity of the fixation and potentiallyslows healing, the latter despite application of a cancel-lous graft. Additionally, larger corrections require anosteotomy of the fibula to allow the desired shift in thetibia, further decreasing the stability of the repair. Stiflevalgus deformities result in the gap occurring in the transcortex opposite the plate, which also is a more unstablesituation. The latter problem may be better handledwith a TPLO combined with a lateral closing wedgeosteotomy.

Treatment for larger deformities include a TPLOcombined with a single transverse osteotomy(rotational deformity) or medially or laterally based clos-ing wedge osteotomy (valgus/varus deformity), or a cu-neiform wedge osteotomy (valgus/varus deformity withexcessive TPS; Fig 15). Regardless of a TPLO performedalone or as in combination as indicated, all osteotomiesare spanned with plate fixation along the medialtibial surface. If a TTA was performed to correct theCrCL-deficient joint, a separate osteotomy, rotational,closing wedge or cuneiform, would still be required. Thedisadvantage is that the medial side of the bone alreadyhas the plate positioned for TTA in the proximal one-third of the medial tibial surface, which will interfere withsubsequent additional medial plate fixation. Although astandard plate could be applied over the thin TTA plate,this is far from ideal and generally not recommended.

Currently, there are no studies that evaluate any ofthese recommendations.

Patellar Luxation

Patellar luxation requiring tibial tuberosity transposi-tion, on the other hand, may be better suited for a TTA,as any desired transposition could be simultaneously

performed with the advancement. In this case, the TTAplate is slightly over-bent to conform to the new laterally(or medially) transposed tibial crest. The alteration in thesurgical technique occurs with cage application. Forexample, in a medial patellar luxation, where the tibialcrest is moved laterally, either the caudal ‘‘ear’’ ofthe cage must be recessed into the proximal tibia, or the

Fig 14. Craniocaudal preoperative (A) and postoperative (B)

radiographs illustrating the torsional correction performed

with the tibial plateau leveling osteotomy (TPLO) by rotating

the 2 tibial bone fragments relative to each other in the axis

parallel to the bone. The distal positioning of the medial side of

the calcaneus is seen to move such that it now intersects the

center of the talus postoperatively (straight arrows). Notice the

slight malalignment of the plated osteotomy in part (B) as a

result of this rotational correction. Lateral postoperative

radiograph (C) shows a slight gap at the osteotomy as a

result of this axial rotation; this gap is open along the medial

cortex only and spanned with the TPLO plate. The axial ro-

tation is schematically represented (D) to illustrate the opening

of the wedge with the rotation of the distal fragment (curved

arrow), and continues to maintain contact on the side opposite

the opening wedge (dotted line).

17BOUDRIEAU

cranial ‘‘ear’’ of the cage elevated above the surface of thetibial tuberosity by interposing some washers, or both.No additional fixation is required, although in an unusualcase some additional fixation may be required cranio-caudally. This fixation may be a K-wire or 2.4mm screwthrough the tibial tuberosity, and the spaces within thecage, into the proximal tibia. A figure-of-8 tension bandwire could also be applied, but generally is not thought tobe necessary. This technique has been briefly described(2007 Veterinary Symposium—The Surgical Summit;Pre-Symposium Laboratories: TTA Laboratory),52 andan abstract presented on a series of cases.83

With a TPLO in the presence of a patellar luxation,the preference is not to isolate the tibial tuberosity infront of the rotational osteotomy, although this could beperformed with pin and tension band wire fixation tech-niques. The limitation with the tibial tuberosity transpo-sition and TTA, however, would be cases of patellarluxation that also had a significant angular/torsional

deformity that also needed to be corrected at the sametime. In the latter instance, TPLO would again be thebetter choice. In this instance, if there was a simultaneousrotational deformity this correction could result inappropriate alignment of the tibial tuberosity afterthe repositioning. Alternatively, a single transverse osteo-tomy, or combined with a closing wedge or cuneiformosteotomy in the presence of an additional angulardeformity would allow the entire tibial tuberosity bonefragment to be translated and then secured as previouslydescribed (Fig 15).

Once again, there are no studies that evaluate anyof these recommendations. Furthermore, there have beenno comparisons performed to suggest whether TPLOor TTA provides a superior result, and no assessmentof frequency of the possible complications encountered.

Patient Size

Both TPLO and TTA have been performed in dogs assmall as 5 kg and as large as 92kg.26–30 Size limitation forthese techniques is dependent upon the availability of theappropriately sized implants. Both devices are producedin a variety of sizes such that they can accommodatealmost any sized dog. In some instances, in very largebreed dogs it has been suggested using 2 plates withTPLO; however, this is not a universally acceptedopinion. One current limitation of the TTA may be thelarge distance (412mm) of TTA that is required forsome large breeds of dogs (not necessarily the heavierdogs, but rather the taller dogs, e.g., Great Danes).44 Thewidest cage currently available to support the osteotomygap is 12mm. Whereas the cage can be moved furtherdistally to increase the width of the gap, this must bedone judiciously, as the tibial tuberosity above thecage may become prone to fracture because of alarge stress-riser created above the cage.44 One alterna-tive would be to buttress the tibial tuberosity above thecage with a cancellous bone block allograft, which hasbeen successfully performed in a few cases; however,adding this graft adds a considerable additional expenseto the procedure.44

Costs

The wide difference in implants not only encompassesa different plating concept, compression or neutralizationplating of the osteotomy with a TPLO versus atension-band plate with a supported gap (cage) with aTTA, but also a difference in costs. In general, for anaverage large breed dog, the implant costs for a TTA are� $250, whereas for a TPLO they are � $150–360depending upon which implant manufacturer is selected(Slocum

s

Enterprises; New Generation Devices, Glen

Fig 15. Schematic representation of the proximal tibia with a

tibial plateau leveling osteotomy (TPLO) and a second trans-

verse osteotomy coincident with the caudal margin of the

TPLO (A and B). Large rotational deformities may be

addressed by rotation around the transverse osteotomy (curved

arrows), allowing full contact and compression plate fixation to

be applied. In addition, this technique allows the tibial tubero-

sity fragment to be translated medially or laterally (large white

arrows) to adjust the position of the patellar tendon insertion

(tibial tuberosity transposition of this entire bone segment) in

order to realign the quadriceps muscle force in cases of patellar

luxation. Full bone contact is maintained between the bone

fragments such that compression plate fixation may be applied.

A further correction may be performed to address excessive

tibial plateau angle (TPA) or proximal tibial varus or valgus

by modifying the transverse osteotomy into a cranial, medial,

lateral (not shown), or cuneiform wedge (C) at this position

(double-headed arrow).

18 TPLO OR TTA?

Rock, NJ; Secuross

, Charlton, MA; Synthess

, Paoli, PA,etc.). For TPLO, using standard screw fixation, regard-less of manufacturer, costs are � $150–175. The additionof locked screws can increase the cost to $360 (e.g. Syn-thes

s

LC-DCP TPLO plate with 3 locked screws in thehead, and 3 standard screws in the body).

TTA also requires a graft for the osteotomy gap. Thiscan be filled either with an autogenous cancellous graft oran allograft using banked bone (corticocancellous chipsand demineralized bone matrix; Veterinary TransplantServices, Kent, WA). Adding a cancellous graft addssome operating time and a second surgical site to theprocedure, whereas adding the allograft (banked bone)adds a significant cost, � $200 for the average largebreed dog (�2mL). Another alternative to an auto- orallograft is to use an osteoconductive compound such asan injectable nanoparticle hydroxyapatite (Ostim

s

; Her-aeus Kulzer Inc., Armonk, NY). The latter has recentlybeen recommended by the original manufacturer of theTTA (Kyon); current costs for an average-sized largebreed dog (�2mL) are �$100. This new material hasrecently been investigated and was found to have min-eralization rates that were not significantly lower thanthose found with autogenous bone in the in vivo animalmodel investigated.84 This material is yet to be evaluatedin the dog, and more specifically, for use with the TTA.

CONCLUSIONS

There are a number of apparent advantages/disadvan-tages in both TPLO and TTA procedures. TTA maycorrect the tibiofemoral shear force closer to the neutralpoint compared with TPLO, which might protect theCaCL from additional stress as the primary joint stabi-lizer. Another advantage of the TTA is the unchangedjoint geometry and superior cartilage pressure distribu-tion compared with TPLO. TTA may also be a less in-vasive, simpler surgical procedure with fewer potentialtechnical issues with adverse effects. TTA may be moresuitable in cases of patellar luxation. On the other hand,TPLO is a more versatile procedure than TTA in caseswith excessive TPS, and in cases with a variety of angularand rotational limb deformities, including cases withconcurrent patellar luxation. Implant costs are margin-ally higher with the TTA, especially if a commercial allo-graft is used. Early clinical results and complicationsappear to be comparable; however, the TTA may have anadvantage as it appears that the technical aspects of theprocedure are fewer and it has a shorter learning curve. Inaddition, most complications appear to involve fewermajor issues, which appear to be more easily overcomewith adherence to the details of the procedure. As withmany surgical procedures, it is at the discretion of the

surgeon to select the technique that he/she believes is themost appropriate for the specific task at hand and theirown experience/expertise. This is not unlike the conceptof selecting from among the many different methodolo-gies, for example, with fracture fixation. Increasing ourarmamentarium simply allows a greater number ofoptions for a particular procedure, which hopefullyaddresses the specifics of the case presented moreideally. Lastly, it is recognized that in many cases,personal preference may be the overriding driving forceon technique selection.

There are a number of areas outlined where there is alack of documentation of many of the anecdotes pre-sented with both techniques. These include the proposedmechanisms and their ramifications, and indications fortheir use, much less any direct comparisons between the 2surgical techniques. At this time, there are only a fewstudies that present reasonable scientific support foreither procedure. A number of experimental and clinicalstudies are necessary to attempt to shed more light onthese repair methods. It is hoped that this summary hasstimulated some thought and will result in additional andcontinued investigations into these techniques, whichmay substantiate or contradict the present plethora ofanecdotal information that continues to be disseminated.

ACKNOWLEDGMENTS

The author thanks Drs. Michael P. Kowaleski and Leslie

L. Williams for their contributions; in addition, Tim Vojt

(Medical Illustration and Computer Graphics, Columbus,

OH) for the 3-dimensional computer graphics of the stifle

joint, and Beth Mellor (Graphic Design/Multimedia

Production, Cummings School of Veterinary Medicine at

Tufts University) for the line drawings.

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22 TPLO OR TTA?