Effect of humeral head rotation on bony glenohumeral stability

The humeral head and glenoid cavity are not perfectly spherical, nor do they have matching radii of curvature. We hypothesized that glenohumeral stability is dependent on axial humeral rotation.

Seven cadaveric shoulders were investigated. For each test, the humeral head was translated relative to the glenoid in 2 directions (starting from neutral), anterior and anteroinferior. Contact forces and lateral humeral displacement were recorded. Joint stability was quantified using the stability ratio and energy to dislocation. The humerus was set in 60° of abduction for all tests. Testing was performed in neutral rotation and 60° of external rotation.

The force displacement curves differed between rotations. In both displacement directions, the peak translational force occurred with less displacement in neutral rotation than in external rotation. The stability ratio and energy to dislocation in the anteroinferior direction were greater than in the anterior direction for both rotation positions. While there were no significant differences in the stability ratio or energy to dislocation between rotation conditions at complete dislocation, the energy required to move the humeral head 10% of the glenoid width was significantly greater with the arm in neutral rotation.

The energy to dislocation, a new parameter of dislocation risk, and the stability ratio, indicate that the glenohumeral joint is more stable in the anteroinferior direction than the anterior direction. During initial displacement, axial rotation of the humeral head contributes to glenohumeral geometrical stability. However, humeral head rotation does not have a significant effect when looking at complete dislocation.