The Cam Effect of the Proximal Humerus: Its Role in the Production of Relative Capsular Redundancy of the Shoulder

Purpose: The purpose of this study was to investigate quantitatively the cam effect of the proximal humerus (whereby the anteroinferior capsule of the shoulder is tensioned in abduction–external rotation by virtue of asymmetric rotation of the eccentrically positioned humeral head), as well the extent to which a reduction in the cam effect would cause a relative capsular redundancy, in the cadaveric shoulder. Methods: Five fresh-frozen cadaveric shoulders were tested. K-wires were used to sequentially position the shoulder in 3 different positions: neutral anatomic position, with contact at the glenoid bare spot; 90°-90° neutral shift position, with contact at the glenoid bare spot; and 90°-90° posterosuperior shift position, with contact 4.3 mm posterosuperior to the glenoid bare spot. The topographic anterior band distance (TABD) was measured with the shoulder in each of the 3 positions. The relative redundancy that occurred by shifting the glenohumeral contact point was determined by subtracting the TABD in the 90°-90° posterosuperior shift position from the TABD in the 90°-90° neutral shift position. Results: The results of repeated-measures analysis of variance showed significant differences among the mean TABD values for the 3 positions (P < .010). There was a significant increase in TABD when the shoulder was brought from the neutral anatomic position to the 90°-90° neutral shift position (P < .020), confirming the cam effect of the proximal humerus in the 90°-90° position. Furthermore, the TABD decreased significantly when the 90°-90° neutral shift position changed to the 90°-90° posterosuperior shift position (P < .005), indicating a significant reduction in the cam effect with a concomitant relative redundancy in the anteroinferior capsuloligamentous complex. Conclusions: The proximal humerus produces a significant cam effect on the anteroinferior capsule when the shoulder is brought into a position of 90° abduction and 90° external rotation. A reduction in the cam effect as a result of a posterosuperior shift of the glenohumeral contact point with the shoulder in the 90°-90° position results in a relative redundancy of the anteroinferior capsuloligamentous complex. This relative capsular redundancy, coupled with the pseudolaxity that occurs with SLAP lesions, can produce a degree of apparent anterior laxity that is independent of any true translational anterior instability. Clinical Relevance: The relative redundancy in the anteroinferior shoulder capsule caused by a decrease in the cam effect of the proximal humerus may have clinical implications in the pathophysiology of the disabled throwing shoulder. This relative redundancy is a secondary pseudolaxity, with the primary pathology being a tight posteroinferior capsule. Appropriate treatment (stretching of the posteroinferior capsule) of the primary pathology is a more appropriate initial treatment than instability surgery.