Arthroscopy: The Journal of Arthroscopic & Related Surgery
Original ArticleThe Cam Effect of the Proximal Humerus: Its Role in the Production of Relative Capsular Redundancy of the Shoulder
Section snippets
Methods
In this study 5 normal shoulders from 5 different fresh-frozen cadavers were examined. The shoulders came from 3 men and 2 women, with a mean age of 53.7 ± 4.2 years (range, 44 to 65 years). The shoulders were dissected free of all soft tissue except for the inferior glenohumeral ligament complex, composed of the anterior and posterior bands, as well as the capsular hammock between the 2 bands (Fig 2). The cam effect could be observed in each specimen as the shoulder was brought into a position
Results
Measurements of the topographic distance between the humeral and glenoid insertions of the anterior band of the inferior glenohumeral ligament (TABD) were performed as detailed in the “Methods” section. The results of repeated-measures ANOVA indicated significant differences among the mean TABD values for the 3 positions (P < .010).
When the shoulder was brought from the neutral anatomic position (position 1) to the 90°-90° position without shift (position 2), we observed the cam effect of the
Discussion
The senior author has described a “tethered shoulder” model of the disabled throwing shoulder that is influenced by glenohumeral internal rotation deficit, the peel-back mechanism, hyper–external rotation, and the cam effect.5 SLAP lesions can create an anteroinferior pseudolaxity, manifested arthroscopically by a positive drive-through sign, by virtue of the posterosuperior break in the labral ring.9, 10 This pseudolaxity occurs in the absence of the usual arthroscopic signs of instability
Conclusions
The proximal humerus produces a significant cam effect in the 90°-90° position, and a reduction in the cam effect as a result of a posterosuperior shift of the glenohumeral contact point causes a relative redundancy of the anteroinferior capsule. It follows that the relative capsular redundancy that results from a reduction in the cam effect, coupled with the pseudolaxity that occurs with SLAP lesions, can produce a degree of apparent anterior laxity that is independent of any true
Acknowledgment
The authors thank John Schoolfield, M.S., for his assistance with the statistical analysis of the data in this study.
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Cited by (16)
Arthroscopic findings of the glenohumeral joint in symptomatic anterior instabilities: comparison between overhead throwing disorders and traumatic shoulder dislocation
2023, Journal of Shoulder and Elbow SurgeryIs early trunk rotation really hazardous for shoulder biomechanics in baseball throwing?
2022, Journal of Shoulder and Elbow SurgeryCitation Excerpt :For the throwers with shoulder Hadd lag (ETR group), a more pronounced peel-back effect may occur, and they tend to have a higher incidence of SLAP lesions. In addition, excessive repetitive hyperhorizontal abduction and subsequent anterior stretching of shoulder can lead to symptomatic anterior instability.3,20,25 In this study, the ETR group had a greater maximal shoulder posterior force and inferior force than did the non-ETR group.
The Thrower's Shoulder
2014, Orthopedic Clinics of North AmericaCitation Excerpt :With the concomitant posterior shift in humeral head contact, an obligatory anterior laxity is thought to occur, owing to the cam-shaped structure of the humerus. Burkhart and Lo40 describe this as pseudolaxity and has shown that posterior shift in contact point does indeed introduce laxity of the anterior capsule. The same laxity allows the thrower to hyperrotate, promoting even more contact between the posterior labrum and cuff.
Arthroscopic management of posterior instability
2010, Orthopedic Clinics of North AmericaCitation Excerpt :The humeral head possesses an oblong, or cam, shape due to the presence of the greater and lesser tuberosities. In the ABER position, the anterior band of the IGHL becomes taut as it is draped over the anteroinferior aspect of the eccentrically positioned humeral head, providing a check-rein against excessive external rotation in that position.27,28 In pitchers, increased glenohumeral external rotation is favored at the expense of internal rotation in an effort to prime the late cocking phase of throwing, primarily because an enhanced late cocking phase leads to more rapid internal humeral rotation in follow-through and increased pitch velocity.
Biomechanics of the Throwing Shoulder
2021, Orthopaedic Biomechanics in Sports Medicine
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S.S.B. and I.K.Y.L. are both consultants for and receive royalties from Arthrex, Naples, Florida.