The Position of the Arm in Anterior Glenohumeral Joint Instability with a Hill Sachs Lesion

Background:
Patients suffering from anterior glenohumeral joint instability report dislocating their shoulder in different positions of the arm.

The purpose of this research is to study the position of the arm at dislocation in anterior glenohumeral joint instability.

Methods:
We studied 39 consecutive CT Scans from 3/2007 to 12/2011 of patients with anterior glenohumeral joint instability and Hill Sachs lesion (HSL).

Based on CT scan images processing, maximal depth and volume of the HSL volume and glenoid bone loss area were calculated.

3D models of the Glenoid and Humerus were printed from CT images. Longitudinal diameter of the humeral head, length and maximal width of the HSL, intersection of the HSL with surgical humeral neck were measured with a digital caliper.

Angulation of the HSL relative to the longitudinal axis of the humerus was measured as well as the position of the HSL relative to the equator of the humeral head. Relative length and width as a percentage of the head diameter were calculated.

The models were positioned in the dislocated position.

The amount of Flexion-Extension, Abduction-Adduction and rotation from a defined starting position were measured at the dislocated position.

Results:
The articular surfaces of the glenohumeral joint for each model lost contact in two positions, the arm being in an inferior position and the arm being in a superior position (below or above the horizontal scapula plane).

For the inferior position, in 15 cases (38%) the arm was anterior to the scapular coronal plane and in 24 (62%) posterior, when dislocated.

Statistical analysis showed that in larger humeral heads and highly positionned HSL the probability of anterior inferior dislocation was significantly higher.

Conclusion:
Patients suffering from anterior glenohumeral joint instability and a HSL should be advised to avoid the positions of dislocation according to the characteristics of the HSL.