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 degrees-90 degrees neutral shift position, with contact at the glenoid bare spot; and 90 degrees-90 degrees 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 degrees-90 degrees posterosuperior shift position from the TABD in the 90 degrees-90 degrees 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 degrees-90 degrees neutral shift position (P < .020), confirming the cam effect of the proximal humerus in the 90 degrees-90 degrees position. Furthermore, the TABD decreased significantly when the 90 degrees-90 degrees neutral shift position changed to the 90 degrees-90 degrees 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 degrees abduction and 90 degrees 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 degrees-90 degrees 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.