Arthroscopy: The Journal of Arthroscopic and Related Surgery 9(6):611-616 Published by Raven Press, Ltd. © 1993 Arthroscopy Association of North America

The Rotator Crescent and Rotator Cable: An Anatomic Description of the Shoulder's "Suspension Bridge"

S t e p h e n S. B u r k h a r t , M . D . , J a m e s C . E s c h , M . D . , a n d R. S c o t t J o l s o n , M . D .

Summary: Twenty fresh frozen cadaver shoulders were dissected in order to study the rotator cable-crescent complex. The rotator crescent is a term that we have used to describe the thin, crescent-shaped sheet of rotator cuff com- prising the distal portions of the supraspinatus and infraspinatus insertions. The crescent was found to be bounded on its proximal margin by a thick bundle of fibers that we have called the rotator cable. This cable-crescent configura- tion was found to consistently span the insertions of supraspinatus and in- ffaspinatus tendons. The dimensions of the rotator cable and crescent were measured by a digital micrometer. The rotator cable was found to be a very substantial structure, averaging 2.59 times the thickness of the rotator crescent that it surrounded. This anatomic study supports the concepts of stress- shielding of the rotator crescent by the stout rotator cable and stress transfer by this loaded cable system. Key Words: Rotator crescent--Rotator cable-- Rotator cuff--Suspension bridge--Rotator cuff tear.

Ar throscopy of the shoulder joint shows a con- sistently identifiable crescent configurat ion to a port ion of the distal rotator cuff (1-3) (Fig. I). This crescent, which we refer to as the rotator crescent , comprises supraspinatus and infraspinatus inser- tions that are contained within the avascular zone. On ar throscopic examination the margin of the cres- cent has thick bundles of fibers that are perpendic- ular to the axis of the supraspinatus tendon, and arch anteriorly and posteriorly to attach onto the humerus. This thick bundle of fibers resembles the cable o f a suspension bridge, and a biomechanical model of the rotator cuff tear as a suspension bridge (loaded cable) has been developed (1) (Fig. 2). We refer to this anatomic structure as the rotator cable. Our purpose in this study is twofold: to describe

From the University of Texas Health Science Center, San Antonio, Texas (S.S.B.); University of California at San Diego and Tri-City Orthopedics, Oeeanside, California (J.C.E.); and Freiberg Orthopedic Group, Inc., Cincinnati, Ohio (R.S.J.), U.S.A.

Address correspondence and reprint requests to Dr. Stephen S. Burkhart, San Antonio Orthopaedic Group, P.A., 540 Madi- son Oak Drive, Suite 620, San Antonio, TX 78258-3913, U.S.A.

and thereby define the ro ta tor crescent as a consis- tent anatomic structure and to anatomically validate the arthroscopic impression that the margin of the crescent forms a cablelike structure (the rotator ca- ble) consistent with the suspension bridge model.

MATERIALS AND METHODS

Twen ty fresh f rozen c a d a v e r shoulders w e r e thawed and dissected. The age at the time of death ranged from 60 to 85 years. None of the shoulders had rotator cuff tears by gross examination. In each specimen, the rotator cuff insertions were dissected sharply from the humerus, maintaining the muscu- lotendinous units on the scapular side so as to be able to relate the span of the crescent to specific rotator cuff muscles (Fig. 3).

The dimensions of the rota tor crescent and the dimensions of the rotator cable were measured with an electronic digital caliper (Digimatic series 500; Mitutoyo Corp., Tokyo, Japan). The results are tab- ulated in Tables 1 and 2 (see also Figs. 4 and 5). In addition, the thickness of the rotator cable and the

611

612 S. S. B U R K H A R T ET AL .

FIG. 1. Arthroscopic view of crescent configuration of the ro- tator cuff. The cable surrounds the crescent. This is a right shoul- der viewed through a posterior portal.

thickness of the rotator crescent tissues were mea- sured and compared for each shoulder (Fig. 6). The thickness of the cable was measured at its midpoint. The thickness of the crescent was measured 3 mm lateral to the lateral border of the rotator cable at its midpoint. In this way, the crescent measurement was taken close to the cable in order to avoid any areas of artifactual thinning where the cuff was dis- sected from the humerus. The specific diameter measurements were taken to define the size of the rotator crescent subtended by the rotator cable as well as to define the dimensions of the cable.

~ . ~.-~!~)i'. ~i ': "z '" ::" : ~: '':' ~;:" . . . . '~i

RESULTS

The average size of the rotator crescent was 41.35 mm (greatest anteroposterior dimension) by 14.08 mm (greatest mediolateral dimension). The average width of the rotator cable was 12.05 mm, and its average thickness was 4.72 mm. By comparison, the average thickness of the crescent (the tissue sur- rounded by the cable) was 1.82 mm. The average ratio of cable thickness to crescent thickness was 2.59 (i.e., the cable was 2,59 times the thickness of the crescent tissue).

In each specimen, the outer border of the cable surrounding the crescent extended anteriorly to the biceps and posteriorly to the inferior border of the infraspinatus, thereby spanning the supraspinatus and infraspinatus insertions (Fig. 4).

Because the purpose of the study was to define the gross anatomic characteristics of the rotator crescent and cable, histological examination was not dor/e.

DISCUSSION

We have noted arthroscopically that most rotator cuff tears that we have encountered, both partial and complete, occurred within the rotator crescent defined in this study. Independent confirmation of a crescentic thickening of the joint capsule beneath the infraspinatus and supraspinatus tendons has come from a detailed anatomic study by Clark and Harryman (3). Their study described a strip of fi- brous tissue 1 cm wide running posteriorly in a di- rection perpendicular to the fibers of the su- praspinatus tendon and extending to the posterior edge of the infraspinatus. They described this strip as a deep extension of the coracohumeral ligament. This strip of fibrous tissue corresponds in size and location to the cable that forms the margin of the rotator crescent in the present study.

L FIG. 2. A rotator cuff tear has a similar con- figuration to a suspension bridge and can be modeled after the loaded cable of the bridge. The free margin of the tear corresponds to the cable, and the anterior and posterior at- tachments of the tear correspond to the sup- ports at each end of the cable 's span.

Arthroscopy, Vol, 9, No. 6, 1993

ROTATOR CRESCENT AND CABLE 613

FIG. 3, Anatomical specimen of the ro- tator cuff after dissecting its insertions from the humerus. H, humeral attach- ment of rotator cuff; C, rotator cable; Cr, rotator crescent; A, anterior attachment of rotator cable; P, posterior attachment of rotator cable.

One of us (S.S.B.) has previously presented a biomechanical model of a rotator cuff tear as a loaded cable or suspension bridge (1). Certainly the thick fibrous margin of a chronic rotator cuff tear resembles the cable of a suspension bridge.

Free-body analysis of a tear involving the su- praspinatus plus half the infraspinatus can be per- formed (see Appendix). This analysis predicts a maximal axial force of 169.79 N and maximal shear force of 124.65 N a t the margins of the tear. We have recently completed an experimental study (4)

TABLE 1. Crescen t d imens ions

AP diameter Mediolateral Specimen (mm) diameter (ram)

1 46.57 19.22 2 30.89 15.24 3 52.20 21.86 4 44.15 15.59 5 42.60 8.30 6 35.23 17.59 7 42.08 14.92 8 44.33 19.24 9 32.06 8.41

10 37.90 15.14 11 37.52 8.19 12 36.86 11.17 13 38.92 9.46 14 39.41 12.76 15 44.33 12.07 16 49.58 13.70 17 39.16 12.70 18 48.24 17.17 19 45132 12.82 20 40.56 16.14

on seven cadaver shoulders in which we created the above tear (supraspinatus plus half of infraspinatus) and symmetrically loaded the tear to a maximum of 684.0 N, with an average of 497.6 N ultimate load on each cuff tear (by means of an MTS Model 810; Materials Testing System, Minneapolis, MN). In this way, we were able to isolate a distributed load to the suspension bridge. These experimental loads

TABLE 2. Thickness o f ro ta tor c re scen t and rotator cable

Crescent Cable Ratio of thickness thickness cable/crescent

Specimen (ram) (mm) thickness

t 2.20 3.92 1.78 2 2.45 5.55 2.27 3 1.10 3,20 2.91 4 1.10 4.90 4.45 5 1.20 3.35 2.79 6 1.86 3.19 1.72 7 1.90 5.50 2.89 8 1.49 4,64 3.11 9 3.49 4.31 1.23

10 1.47 4.12 2.80 11 1.28 4.92 3,84 12 2.49 5.87 2.36 13 1.54 3.47 2,25 14 2.45 6,78 2,77 15 2.54 4.68 1,84 16 1.54 5.95 3.86 17 2.12 5.20 2.45 18 1.82 3.38 1.86 19 1.32 5.73 4.34 20 1.00 5.81 5,81

Average 1.82 4.72 2,59

Arthroscopy, VoI. 9, No, 6, 1993

614 S. S. B U R K H A R T E T A L .

4A,B

~J FIG. 4. Superior and posterior projections of the rotator cable and crescent. The rota- tor cable extends from the biceps to the in- ferior margin of infraspinatus, spanning the supraspinatus and infraspinatus insertions. C, width of rotator cable; B, mediolateral diameter of rotator crescent; S, supraspina- tus; I, infraspinatus; TM, teres minor; BT, biceps tendon.

were greater than three times the predicted mar- ginal loads for a maximal contraction of the su- praspinatus and half of the infraspinatus, yet they caused no extension of the tear either anteriorly or posteriorly. Failure always occurred through the soft tissues of the suspension bridge rather than through the bone insertions of the rotator cuff fi- bers. Therefore, one would not expect unrepaired tears in the rotator crescent to propagate anteriorly or posteriorly on a mechanical basis alone. Signifi- cant biologic weakening would have to occur before fiber failure would be anticipated.

The marked difference in thickness of the rotator cable (4.72 ram) and thickness of the rotator cres- cent bordered by the cable (1.82 ram) was striking. This finding supports the concept of the rotator ca- ble as a functional cable system in which there is stress transfer from the cuff to the thick cable and stress-shielding of the thin capsular tissue distal to the cable and within the crescent (1). Preliminary biomechanical testing of the intact cable-crescent complex has been conducted in four cadaver shoul-

'-:i:!~ ~)i!ii::i::~s::~'-:'!::: .1" " " ' . . . . ". . . .- . . ~ . ~ %.!~...

FIG. 5. Dimensional measurements of rotator cable and rotator crescent. A: Anteroposterior diameter of rotator crescent. B: Mediolateral diameter of rotator crescent. C: Width of rotator cable.

ders using Hall-effect transducers to measure per- centage elongation of the crescent and cable under load (5). All four shoulders were from specimens >60 years of age and had thin crescent tissue. Un- der symmetric load, the cable underwent a greater percentage elongation than the crescent, confirming a mechanical stress-shielding of the crescent in these four shoulders.

A second group of cadaver shoulders was tested in a similar manner. In this study, percentage elon- gation was measured by differential variable reluc- tance transducers (Micro Strain Co., Burlington, VT) (5). In two younger shoulders with thick rotator

S

H G

FIG. 6. Coronal section of rotator crescent and cable. Note the thickened rotator cable in comparison with the thin rotator cres- cent. Inset: T, thickness of rotator cable; t, thickness of rotator crescent.

Arthroscopy, Vol. 9, No. 6, 1993

R O T A T O R C R E S C E N T A N D C A B L E 615

crescents in this study, the crescent was not stress- shielded by the cable. The older shoulders (>60 years of age) with thin crescent tissue again dem- onstrated stress-shielding of the crescent by the ca- ble. These preliminary findings suggest that there may be two different functional classes of rotator cuff based on the behavior of the cable-crescent complex under load. We have categorized these two functional classes of rotator cuff as cable dom- inant (in which the crescent is stress-shielded by the cable) and crescent dominant (in which there is no stress-shielding of the crescent by the cable). The fact that the crescent-dominant pattern was identi- fied in the younger specimens with thick crescents and the cable-dominant pattern was present in the older shoulders with thin crescents leads to an in- triguing hypothesis: that the aging shoulder devel- ops progressive thinning of the crescent and in- creasing reliance on the cable, eventually evolving to a cable-dominant cuff from a crescent-dominant cuff.

The arthroscopic view of the rotator cable and crescent often shows that the flimsy crescent tissue has a redundant invagination adjacent to the rotator cable, suggesting that the rotator crescent is not under tension (Fig. 7). These arthroscopic findings

lend additional support to the concept of stress- shielding of the rotator crescent by the rotator cable in some shoulders. One can then postulate that ro- tator cuff tears within that crescent are biomechan- ically inconsequential in cable-dominant shoulders. Therefore, a patient with an intact cable system and intact force couples in the transverse and coronal planes can have a rotator cuff that is biomechani- cally intact even though it is anatomically deficient (6). This hypothesis suggests that the location of a rotator cuff tear is much more important than the size of the tear in regard to its effect on shoulder function. That is, a tear that involves the rotator cable may be biomechanically much more signifi- cant than a tear that involves only the rotator cres- cent.

The age of the specimen shoulders bears further mention. All specimens were 60-85 years of age at the time of death. We have noted that some younger individuals have a less well-defined cable-crescent complex on arthroscopic examination. It is possible that the crescent undergoes adaptive changes with age. Stress-shielding of the relatively avascular crescent might lead to progressive thinning with ad- vancing age. Even within this older age group, there was a wide range of cable-to-crescent thickness ra- tios. Despite this range, the overwhelming majority of the specimens had cables that were much thicker than their crescents (Table 3). Most of the speci- mens in this study could be considered cable dom- inant because of their large cable-to-crescent thick- ness ratios. However, those specimens with a rel-

FIG. 7. Arthroscopic view of rotator cable and crescent in a left shoulder. Redundant invagination adjacent to the rotator cable suggests that the rotator crescent is not under tension.

TABLE 3. R o t a t o r cable wid th

Specimen Rotator cable width (mm)

1 12.01 2 11.50 3 11.90 4 16.80 5 12.00 6 11.91 7 13.50 8 12.96 9 5.25

10 9.73 11 14.05 12 14.16 13 10.73 14 12.00 15 9.33 16 t3.03 17 11.60 18 11.41 19 10.68 20 16.54

Arthroscopy, Vol. 9, No. 6, 1993

616 S. S. B U R K H A R T ET AL.

atively thin cable may represent a subset of shoulders that are crescent-dominant. Again, one may speculate that progressive thinning of the rel- atively avascular crescent tissue is an inevitable consequence of aging and that hypertrophy of the cable gradually develops as the cable has to take on increasing loads with age.

The clinical significance of this biomechanical model is great. By considering the biomechanical integrity of the rotator cuff rather than its anatomic integrity, one can then follow specific criteria to select appropriate patients for arthroscopic rotator cuff debridement and decompression (7). Such cri- teria can provide guidelines for a reasonable arthro- scopic alternative to massive open procedures, with their associated morbidity, in selected older pa- tients.

Additionally, the concept of partial repair of a massive irreparable rotator cuff tear, to restore the rotator cable and the transverse plane force couple, makes biomechanical sense. One of us (S.S.B.) has used this technique successfully on several occa- sions. This technique avoids muscle transposition to cover the residual hole in the superior cuff that is frequently unrepairable, even after massive mobili- zation of cuff tissue.

We acknowledge that most rotator cuff tears should be treated by repair. The clinical application of our research into the cable-crescent complex awaits clarification by studies that are currently in progress.

By anatomically confirming the shoulder's "sus- pension bridge," the present study supports the hy- pothesis that the rotator cuff possesses a cable sys- tem capable of stress transfer and that tears within the relatively stress-shielded crescent may be bio- mechanically insignificant in some shoulders.

APPENDIX

Average cross-sectional area (8) of supraspinatus muscle is 5.5 cm 2.

Average cross-sectional area of infraspinatus muscle is 8.8 cm 2.

The portion of infraspinatus insertion enclosed within the cable is -50% muscle, so the effective cross-section of infraspinatus acting on the cable is 4.4 c m 2. Therefore, the total cross-sectional area of muscle subtending the crescent is 5.5 cm z + 4.4 c m 2 = 9 . 9 c m 2.

Maximal contraction of muscle per unit area is 3.5 kg/cm 2 (9).

Therefore, maximal contraction of the muscle subtending the crescent (supraspinatus plus half of infraspinatus) is 9.9 cm 2 x 3.5 kg/cm 2 = 34.65 kg = 339.57 newtons (nt).

Assuming maximal contraction, A y + By =

339.57 nt, and Ay = By = maximal force at tear margins. So, Ay = By = 339.57 nt/2 = 169.79 nt.

To calculate the maximal shear force at the mar- gins of the tear, one uses the following formula (10):

A x = B x = Wy Lz/8 h,

where A× = B x = horizontal component of tensile force at any point along cable (free margin of rotator cuff tear). Wy is the muscular force per unit distance along the margin of the tear, L is the length of the span of the tear, and h is the "sag" of the tear.

Assume a maximum tear diameter of the inside diameter of the crescent (average 4.135 cm in this series). Then Wy = force/length of tear = 339.57 nt/4.135.cm = 82.12 nt/cm and Ax = Bx = Wy LZ/8 h = 82.12 nt/cm × (4.135 cm)2/8 × 1.408 cm when h = 1.408 cm, the sag in a parabolic curve of this diameter, which corresponds to the mediolateral di- mension of the rotator crescent. Therefore, A x = B x = 124.65 nt.

REFERENCES

1. Burkhart SS. Fluoroscopic comparison of kinematic pat- terns in massive rotator cuff tears: a suspension bridge model. Clin Orthop 1992;284:144-52.

2. Clark J, Sidles JA, Matsen FA. The relationship of the gle- nohumeral joint capsule to the rotator cuff. Clin Orthop 1990;254:29-34.

3. Clark JM, Harryman DT Ii. Tendons, ligaments, and cap- sule of the rotator cuff. J Bone Joint Surg jAm] 1992;74:713- 25.

4. Burkhart SS, Cawley PW, Pflaster DS, Thomas TD, Esch JC, Nottage WM. The ultimate strength to failure of rotator cuff tears involving the rotator crescent. Unpublished data.

5. Burkhart SS, Cawley PW, Pilaster DS, Thomas TD, Esch JC, Nottage WM. Symmetric tensile loading of the rotator cable: Demonstration of a stress-shielding effect on the ro- tator crescent. Unpublished data.

6. Burkhart SS. Arthroscopic treatment of massive rotator cuff tears: clinical results and biomechanical rationale. Clin Or- thop 1991;267:45-56.

7. Burkhart SS. Arthroscopic debridement and decompression for selected rotator cufftears: clinical results, pathomechan- ics, and patient selection based on biomechanical parame- ters. Orthop Clin North Am 1993;24:111-23.

8. Bassett RW, Browne AD, Morrey BF, An KN. Glenohu- meral muscle force and mechanics in a position of shoulder instability. J Biomech 1990;23:405-15.

9. Ikai M, Fukunaga T. Calculation of muscle strength per unit of cross-sectional area of human muscle. Int Zeitschrift An- gewandte Physiol einschtiesslich Arbeitsphysiol 1968;26:26- 31.

10. Hibbeler RC. Engineering mechanics. 5th ed. New York: Macmillan, 1989:314.

Arthroscopy, Vol. 9, No. 6, 1993