This is" a reprint of an article in The Journal of Hand Surgery (American Volume) as it was published in itsoriginal foi’m. Articles in The Journal are not prepared for any company or distributor, and publication ofany article therein does not constitute an endorsement by the Publisher, Editor, or ASSH of any productdescribed therein.

Anatomic relationships of an endoscopic carpal¯ tunnel device to surrounding structures

Anatomic relationships of an endoscopic carpal tunnel device to surrounding soft tissue structuresalong the ring finger and the long-ring interspace axis were investigated in 28 adult cadaverhands. The average distance from the center of the device to the median nerve in the carpaltunnel averaged 3.3 mm in the ring finger axis and 2.5 mm in the long-ring interspace axis.The average distance from the distal edge of the transverse carpal ligament to the superficialpalmar arch was 4.8 mm in the ring finger axis and 5.5 mm in the long-ring interspace axis.These and other more subtle anatomic observations indicate the greater safety of using the ringfinger axis for endoscopic carpal tunnel release. (J HAND SURG 1993;18A:442-50.)

Mitchell B. Rotman, MD, and Paul R. Manske, MD, St. Louis, Mo.

The anatomy of the carpal tunnel has been

well described.13 With the recent development, in-creased popularity, and reports of complications ofendoscopic carpal tunnel surgery,4I° the relationshipsto adjacent soft tissue structures in the region of therelease is of extreme importance. The purpose of thepresent study was to define these anatomic relation-ships. ~

From the Division of Orthopedic Surgery, Washington UniversitySchool of Medicine, St. Louis, Mo.

Received for publication Aug. 23, 1991; accepted in revised formJuly 15, 1992.

No benefits in any form have been received or will be received froma commercial party related directly or indirectly to the subject ofthis article.

Reprint requests: Mitchell B. Rotman, MD, Division of OrthopedicSurgery, Washington University School of Medicine, 11300 WestPavilion, One Barrtes Hospital Ptaza, St. Louis, MO 63110.

3/1/42893

Materials and methods

Twenty-eight cadaver hands were studied. In 24paired adult cadaver hands, a transverse incision wasmade in the palmar wrist crease between the palmarislongus and the flexor carpi ulnaris tendons, identifyingthe anterior forearm fascia and the proximal edge ofthe transverse carpal ligament (TCL). Through an open-ing in the anterior forearm fascia, the carpal tunnel wasentered; the ulnar bursa, adjacent to. the radial borderof the hook of the hamate, was dissected off the over-lying TCL with a Freer elevator. A cannulated pathfin-der probe was passed through the carpal canal, alongthe direction of either the ring finger (RF) axis or thelong-ring interspace (LRI) axis. A guide wire was thenplaced through the probe, and the probe was removed.An Inside Job blade assembly housing (OrthopedicProducts Division, 3M Health Care, St. Paul, Minn.),with the tip removed, was placed ov~er the guide wire(Fig. 1).. The guide wire was removed, leaving the6 mm device in place.

442 THE JOLYRNAL OF HAND SURGERY

Vol. 18A, No. 3May t993 Endoscopic carpal tunnel device 443

Fig. 1. Blade assembly housing (below) and modification of blade assembly housing for cannulationwith guide wire (above).

Fig. 2. A, Device placed within carpal canal against radial border of hook of hamate and directedalong RF axis. Markings on the palm show the RF axis and the hook of hamate. B, Device placedalong LRI axis.

The specimens were divided into two groups. Ingroup A (14 specimens) the device was directed alongthe RF axis (Fig. 2, A); in group B (10 specimens),the device was directed along the LRI axis (Fig. 2, B).The prepared hands were frozen at - 70° C to maintainthe specific position of the device during sectioning.Four additional hands that had not been operated onwere prepared for composite sectioning.

The anatomic relationships investigated were (1) the

distance from the center of the device (correspondingto the position of the endoscopic blade) to the mediannerve (transverse sections), (2) the location of the rounding flexor tendons relative to the device (trans-verse sections), (3) the distance from the distal marginof the TCL to the superficial palmar arch (SPA) and thefat that invests the SPA and common digital nerves(sagittal sections), and (4) the interface between TCL and the more superficial palmar fascia and sub-

444 Rotman and Manske

The Journal ofHAND SURGERY

Fig. 3. Specimen transversely sectioned at 1 cm intervals, starting at palmar wrist crease, showingdevice in proximal carpal tunnel. The median nerve (circle) is seen just radial to the device. P,Pisiform; S, scaphoid.

Fig. 4. Specimen sagittally sectioned in line with RF axis and bisecting device.

cutaneous structures (composite and transverse sec-tions).

Transverse sections. Nine specimens (five in groupA, four in group B) were transversely sectioned in thefrozen state with a vertical band saw at 1 cm intervals,starting at the level of the palmar wrist crease and con-tinuing distally to divide the tunnel into proximal, mid-

dle, and distal regions (Fig. 3). Measurements to thenearest 0.5 mm were made in each of the three regionsunder x 6.3 to x 10 magnification (PhotomakroscopM-400, Wild Microscopes, Division of Leica, Inc.,Rockleigh, N.J.). The distance from the center of thedevice (i.e., the position of the endoscopic blade) the median nerve was recorded. Since the edge of the

Vol. 18A, No. 3May 1993 Endoscopic carpal tunnel device 445

Table I. Transverse sections

Distance (mm) from center of device to median nerve the carpal tunnel

Group A (RF axis)Proximal 3.8 -+ 0.5 (3.0 - 5.0)Middle 3.0 ± 0.0Distal 3.1 -+ 0.2 (3.0 - 3.5)

Group B (LRI axis)Proximal 3.1 --- 0.5 (2.5 - 4.0)Middle 2.5 ± 1.3 (0.0 - 4.0)Distal 1.8 ± 1.7 (-0.5 - 3.0)

Values are mean ~ standard deviation, with range in parentheses.

Table II. Sagittal sections

Distance (mm) from distal edge of TCL to SPAGroup A 4.8 ± 0.8 (3.0 - 6.0)Group B 5.5 ± 0.7 (5.0 - 6.0)

Distance (mm) from proximal extent of fat pad distal edge of TCLGroup A 2.8 ± 0.6 (2.0 - 3.5)Group B 0.8 ± 1.1 (0.0 - 3.0)

Values are mean +- standard deviation with range, in parentheses ......

device was 3 mm from the center, a measurement of<3 mm indicated that the nerve was between the deviceand the overlying TCL. A negative measurement indi-cated that the nerve extended ulnarly beyond the center

of the device (i.e., between the position of the bladeand the TCL).

Sagittal sections. Fifteen specimens (nine in groupA, six in group B) were sagittally sectioned along theRF or LRI axis, bisecting the device (Fig. 4). The

distances to the nearest 0.5 mm were measured fromthe distal margin of the TCL to the SPA, as well as tothe proximal extent of the investing fat pad.

Composite sections. A composite section of fourspecimens, including skin, the TCL, and the interposedsoft tissue structures, was obtained along the long finger(LF), LRI, and RF axes (Fig. 5). Longitudinal histo-logic sectionsn’ 12 were obtained along each axis. Thelongitudinal palmar fascia fibers were seen in the sag-ittal plane, whereas the TCL fibers were seen in thecoronal plane.

Statistical analysis. Statistical analysis was com-puted by means of PC analysis of variance statisticalanalysis software (Human Systems Dynamics, North-ridge, Calif.). Statistical significance was defined as p value <0.05. All p values were computed by meansof analysis of variance for a simple randomized designwith unweighted means.

Fig. 5. Hand specimen showing where composite section hasbeen obtained.

Results

Transverse sections (Table I). In group A (RF axis),the nerve was positioned along the side of the device(Fig. 6, A). The average distance from the center the device to the median nerve in the proximal carpaltunnel was 3.8 ram; in the midcarpal tunnel region thedistance was 3.0 mm; and in the distal carpal tunnelthe distance measured 3.1 mm. In no instance was thenerve found between the device and the TCL.

In group B (LRI axis), when the nerve was positionedalong the side of the device, it appeared flattened (Fig.6, B). The average distance from the center of the deviceto the nerve in the proximal carpal tunnel measured3.1 mm. In the midcarpal tunnel, the distance measured2.5 ram, and in the distal carpal tunnel the distancemeasured 1.8 mm; this placed the nerve between thedevice and the TCL. The nerve was found between thedevice and the TCL in 20% of the specimens; in oneinstance the nerve would have been under the blade.

446 Rotman and Manske

The Journal ofHAND SURGERY

Distance from Center of Device to

Median Nerve

CProximal Middle Distal

Region

Fig. 6. A, Transverse section of distal carpal tunnel along RF axis showing device along undersurfaceof TCL between hamate (H) and blackened median nerve (M). The nerve is directly alongside,but not pressed against, the device. B, Median nerve (M) pressed against device along LRI axis.(Original magnification x 6.3.) C, Distances from center of device to median nerve in RF and LRIaxes are compared. The differences are statistically significant in the proximal (p 0.03) and distal(p 0.04) carpal tunnel regions. D, In this specimen, the device was inadvertently rotated towardmedian nerve. Note how the device conforms to the curve of the lateral border of the hamate (H).The importance of placement of the device snugly against the palmar surface of the TCL is apparent.The nerve (M) is between the center of the device and the TCL. (Original magnification x 6.3.)

The differences at the proximal and distal regions aresignificantly different between the two axes as noted inFig. 6, C.

In one specimen the device was inadvertently rotatedtoward the median nerve (Fig. 6, D). (These measure-ments were not included in the Results.) Rotation to-ward the nerve causes the nerve to lie in the resultingspace between the device and the TCL.

In both groups A and B, flexor tendons were founddorsal or ulnar to the device but were never found be-tween the device and the median nerve or between thedevice and the TCL.

A separate fasciat layer was seen directly palmar tothe TCL in seven of nine specimens but could not beeasily separated from it. The layer was derived pri-marily from thenar muscle fascia, but it also receivedcontributions from the hypothenar muscle fascia andthe dorsal fascia of the palmaris brevis overlying Gu-yon’s canal. The thickness of this layer decreased as itwent from the radial side to the ulnar side, measuring3.5 to 0.5 mm in thickness (Fig. 7, A and B). It wasseen more clearly in the distal carpal canal sections inspecimens with more developed thenar musculature.

Sagittal sections (Table II). In group A (RF axis)

Vol. 18A, No. 3May 1993 Endoscopic carpal tunnel device 447

Fig. 7. A, Transverse section showing separate layer arising from thenar muscle fascia overlyingTCL. This layer is being held by forceps. M, Blackened median nerve. (Original magnificationx 6.3.). B, Diagram of transverse section taken from distal half of carpal tunnel. The separatefascial layer seen directly palmar to the TCL is derived primarily from thenar muscle fascia butalso receives contributions from hypothenar muscle fascia and dorsal fascia of the palmaris brevis.

the distance to the SPA averaged 4.8 mm distal to thedistal edge of the TCL. The proximal extent of the fatpad was 2.8 mm proximal to the distal edge of the TCL(Fig. 8, A).

In group B (LRI axis), the SPA averaged 5.5 distal to the distal edge of the TCL. The proximal extent

of the fat pad was located almost immediately at thedistal margin of the TCL, averaging only 0.8 mm prox-imal (Fig. 8, B). In this axis the distal edge of the TCLwas at times difficult to determine because of its junc-tion with the palmar fascia. Comparisons between thetwo axes are noted in Fig. 8, C and D; the differences

448 Rotman and ManskeThe Journal of

HAND SURGERY

Distance from TCL to SPA Distance from Fat Pad to TCL

Distance (ram)7.06.t)t

RFAxis

LRI

Distance (mm)4.0o

3.5-3.0-2.5-2.0-1.5-1.0-0.5-0.0

DLRI

Axis

Fig. 8. A, Sagittal section along RF axis showing distal edge of TCL (black arrow), SPA (asterisk),and proximal extent of the pad (white arrow). The distance between the distal edge of the TCLand the SPA is 6 mm in this specimen; the fat pad begins 3 mm proximal to the distal edge of theligament. B, Sagittal section along LR1 axis showing proximal extent of fat pad (white arrow)originating near distal edge of TCL. The SPA (asterisk) is noted on the left. (Original magnification× 10.) C, Distances from distal edge of TCL to SPA in RF and LRI axes are compared. Thedifference is not statistically significant. D, Distances from proximal extent of fat pad to distaledge of TCL in RF and LRI axes are compared. The difference is statistically significant betweenthe two (p 0.02).

related to the proximal extent of the fat pad are statis-tically significant between the two axes.

Composite sections. At the junction between thetransverse fibers of the TCL and the overlying struc-tures, the palmar fascia was found to overlie the TCLonly along the LF axis and the LRI axis, but not alongthe RF axis. The overlying fascial layer derived fromthe thenar and hypothenar muscles (described above)could not be differentiated from the transverse collagenTCL fibers in either of the axes.

In the anterior half of the ligament, TCL collagenfibers were interspersed with thenar and hypothenarmuscle fascicles. Adjacent structures found directly su-

perficial to the TCL in the RF axis consisted of sub-cutaneous tissue mixed with fibrous connective tissueand small blood vessels and small nerves.

Discussion

The results of this study underscore the importanceof understanding the anatomic relationships between anendoscopic device and .surrounding soft tissue structureswhen performing endoscopic carpal tunnel release. Al-though the results presented in this study apply directlyto the use of the Inside Job device, they may be ap-plicable to the use of other endoscopic devices6’ 8 in-asmuch as anatomic relationships are similar.

Vol. 18A, No. 3May 1993 Endoscopic carpal tunnel device 449

Median nerve and flexor tendons. The recom-mended surgical approach for both endoscopic and openrelease of the carpal tunnel is in the line of the RFaxis.4, 9, 13-16 With respect to open release of the carpaltunnel, this axis avoids damage to the palmar cutaneousbranches of both the median and ulnar nerves. 13.16 Dur-ing endoscopic carpal tunnel release, this recommendedaxis presumably avoids placement of the endoscopeblade directly beneath the median nerve. The resultsemphasize the importance of placing the device in theRF axis and the jeopardy of straying into the LRI axis.

In this small series, no ulnar branches of the mediannerve as described by Lanz~5 in 1% of carpal tunnelexplorations were observed. If this accessory branch orany other anatomic structure is found superficial to orpassing through the transverse carpal ligament, it couldbe injured by endoscopic carpal tunnel release. Fortu-nately, most anatomic variations of the median nerveor its branches are found radial or directly palmar tothe nerve and are not likely to be injured.

Injuries to either the main trunk of the median nerveor its common digital branches have been reported withthe Inside Job5,10 and the two-portal technique describedby Chow.9 It is not clear how these injuries were caused.We found that the device easily rotates toward the nerveduring insertion, inasmuch as it tends to conform to thecurve of the lateral border of the hamate. The surgeonhas to be careful to keep the device snugly abuttedagainst the undersurface of the ligament and not rotateit toward the nerve. There is also a tendency tO leverthe device against the hook of the hamate and point ittoward the LRI axis.

The flexor tendons were found to be either on theulnar side or dorsal to the Inside Job housing in allspecimens. There was no instance in which a flexortendon was found between the device and the nerve orbetween the device and the TCL. With dissection ofthe ulnar bursa off the overlying transverse carpal lig-ament, the flexor tendons are pushed away from thedevice and are protected from injury.

Dissection of the ulnar bursa allows visualization ofthe transverse fibers of the TCL. It is important thatthese fibers be clearly identified and visualized at thetime of ligament incision. Any anomalous structuresalong the undersurface of the TCL (e.g., palmaris pro-fundus)17 that prevent visualization of the transversefibers should caution the surgeon against proceedingwith endoscopic release.

SPA and fat pad. The TCL begins proximally as acontinuation of the antebrachial fascia and then endsdistally by blending with the fibers of the midpalmar

fascia. The ligament varies in length from 26 to 34mm. 18 In this study the distal edge of the TCL was moreclearly defined along the RF axis by its junction witha layer of loose, cellular connective tissue mixed withfat. Kaplan and Milford~ described this fat pad as theproximal aspect of the fatty tissue that continues distallyand invests the superficial palmar arterial arch, the com-mon digital nerves, and the synovial bursa of the flexortendons. Although Kaplan and Milford do not indicatewhether this fat envelops the distal border of the TCL,our studies clearly show that it does so along the RFaxis. This fat pad consistently extended 2 to 3.5 mmproximal to the distal edge of the TCL in the RF axis.Failure to incise the ligament beyond the proximal ex-tent of this fat would result in an incomplete release ofthe distal TCL fibers.

In this study, the SPA was approximately 5 mm distalto the TCL. Okutsu et al. 8 indicated that the SPA was10 to 15 mm distal to the distal margin of the TCL butgave no details as to how the measurements were made.With careful endoscopic release along the RF axis, theSPA is not likely to be in jeopardy.

Superficial fascial structures. The improved short-term clinical results noted with endoscopic carpal tunnelrelease, as compared with open carpal tunnel release,are attributed in part to preservation of the continuityof the more superficial fascial structures palmar to theTCL; this continuity is thought to achieve stronger post-operative grip strength than standard carpal tunnel re-lease (personal communication from J.M. Agee andfrom J.C.Y. Chow). Clinically, during endoscopic re-lease, Agee and Chow have noted that the incised mar-gins of the TCL will separate but will leave transversecollagen fibers intact palmar to it. The identity of thesefibers has not previously been clarified. We do not be-lieve they represent fibers of the palmar fascia or pal-maris brevis. In composite histologic sections, longi-tudinal palmar fascial fibers were found only along theLF and LRI axes; there was no evidence ofpalmar fasciaoverlying the TCL in the RF axis. Furthermore, al-though transverse fibers of palmar fascia have beendescribed, they are located distal to the TCL.19.2o

. A separate fascial structure was noted immediatelypalmar to the TCL arising from the thenar musculatureand joining ulnarly with the hypothenar muscle fasciaand the dorsal or deep fascia of the palmaris brevis thatoverlies Guyon’s canal. This layer could not be me-chanically separated from the TCL; nor could it bedifferentiated histologically from TCL collagen fibers.We propose that the fascial layer that we have describedrepresents the intact transverse collagen fibers noted by

450 Rotman and ManskeThe Journal of

HAND SURGERY

Agee and Chow after endoscopic carpal tunnel re-lease.

Both thenar and hypothenar muscles take origin par-tially from the TCL. ~ These muscle fascicles were foundinterspersed within the transverse collagen fibers in the

anterior half of the TCL. These muscle fascicles, ifencountered during endoscopic release, should not be

confused with muscle fascicles originating from thepalmaris brevis muscle, which would be found palmarto the transverse carpal ligament.2~

The purpose of this study is not to advocate the useof an endoscopic device in carpal tunnel release. How-ever, if the surgeon chooses to use such a device, knowl-

edge of the anatomic relationships with regard to theposition of the endoscopic device during carpal tunnelrelease is of critical importance. Awareness of the an-atomic relationships presented in this study may enablethe surgeon to avoid potential complications. The sur-geon is advised that it is important to (1) dissect theulnar bursa away from the overlying transverse carpalligament, (2) position the device along the ring fingeraxis, (3) be aware that the distal margin of the TCLextends approximately 2.5 to 3.0 mm into the fat pad,and (4) be aware that the superficial palmar arch 5 mm distal to the TCL within this investing fat pad.

Inside Job blade housing assemblies were provided by theOrthopedic Products Division, 3M Health Care, St. Paul,Minn.

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