The anteroinferior labrum helps center the humeral head on the glenoid.

Previous work suggests that the labrum helps center the humeral head. We hypothesized that detachment of the labrum alone would shift the head from its centered position toward the detachment, and repair would restore its centered position. Five young shoulders were used, and glenoids were potted with the articular surfaces oriented horizontally. Unconstrained humeral heads were subjected to 30-N compressive loads and no displacing force. Using a technique with 10-micrometer resolution, we quantified head and glenoid positions before and after anteroinferior labral detachment and after three types of repair. Detachment was associated with humeral head shift toward the labral lesion in all specimens, averaging 0.74 mm (range, 0.51-1.00 mm) (P <.005). Repair to the lip restored the labrum's centering effect variably. Repair with suture anchors on the glenoid face over-reduced the humeral head, shifting it posterosuperiorly by a mean of 3.47 mm (range, 0.71-6.7 mm) (P <.05). The labrum is important for humeral head centering, even without displacing loads.