Background: It has been speculated that a shift of the throwing arc
commonly develops in athletes who perform overhead activities, resulting in
greater external rotation and decreased internal rotation caused by anterior
capsular laxity and posterior capsular contracture, respectively. Osseous
adaptation in the form of increased humeral and glenoid retroversion may
provide a protective function in the asymptomatic athlete but cannot explain
the pathological changes seen in the shoulder of the throwing athlete.
Therefore, the objective of the present study was to examine the biomechanical
effects of capsular changes in a cadaveric model.
Methods: Ten cadaveric shoulders were tested with a custom
shoulder-testing device. Humeral rotational range of motion, the position of
the humerus in maximum external rotation, and glenohumeral translations in the
anterior, posterior, superior, and inferior directions were measured with the
shoulder in 90° of abduction. Translations were measured with the humerus
secured in 90° of external rotation. To simulate anterior laxity due to
posterior capsular contracture, the capsule was nondestructively stretched 30%
beyond maximum external rotation with the shoulder in 90° of abduction.
This was followed by the creation of a 10-mm posterior capsular contracture.
Rotational, humeral shift, and translational tests were performed for the
intact normal shoulder, after anterior capsular stretching, and after
simulated posterior capsular contracture.
Results: Nondestructive capsular stretching resulted in a
significant increase in external rotation (average increase, 18.2°
± 2.1°; p < 0.001), and subsequent simulated posterior capsular
contracture resulted in a significant decrease in internal rotation (average
decrease, 8.8° ± 2.3°; p = 0.02). There also was a significant
increase in anterior translation with the application of a 20-N anterior
translational force after nondestructive capsular stretching (average
increase, 1.7 ± 0.3 mm, p = 0.0006). The humeral head translated
posteroinferiorly when the humerus was rotated from neutral to maximum
external rotation. This did not change significantly in association with
anterior capsular stretching. Following simulated posterior capsular
contracture, there was a trend toward a more posterosuperior position of the
humeral head with the humerus in maximum external rotation in comparison with
the position in the stretched conditions, although these differences were not
Conclusions: A posterior capsular contracture with decreased
internal rotation does not allow the humerus to externally rotate into its
normal posteroinferior position in the cocking phase of throwing. Instead, the
humeral head is forced posterosuperiorly, which may explain the etiology of
Type-II superior labrum anterior-to-posterior lesions in overhead
Clinical Relevance: Understanding the biomechanical effects of
capsular changes in a cadaveric model of the throwing shoulder may confirm
clinical observations and provide insight into the pathological changes often
seen in the shoulder of throwing athletes.