Three-dimensional analysis of the locked position in patients with recurrent shoulder instability

Although recurrent anterior shoulder instability (RASI) is a common condition in young patients, no studies to date have measured the 3-dimensional (3D) locked position of the glenohumeral joint during an anterior dislocation. Therefore, our goal was to estimate it with 3D computed tomography (CT) scans.

Patients in this prospective observational study were separated in 3 groups: normal laxity, hyperlaxity, and epilepsy. They were characterized by questionnaires (Western Ontario Shoulder Instability Index, 11-item version of the Disabilities of the Arm, Shoulder and Hand, and Beighton Laxity Score), and a CT scan revealing bipolar bone defects. 3D models of the humeral head and the glenoid were reconstructed from the CT scan, and the rotations and displacements of the humerus relative to the glenoid, from initial to locked position, were calculated. Intraobserver and interobserver reliability by intraclass correlation coefficient (ICC), analysis of variance test, and the Pearson correlation were used to evaluate data.

This study involved 44 patients (46 shoulders): 18 with "normal" laxity, 18 with hyperlaxity and 8 (2 bilateral) with epilepsy. The mean locked position was of 12° of abduction, 90° of external rotation, and 21° of extension. The intraobserver and interobserver reliability was excellent for all the rotations and displacements (ICCs, 0.751-0.977) except the proximal-distal displacement (ICCs, 0.409-0.688). Significant differences were found for external rotation, anterior displacement, and medial displacement among the 3 groups of patients. Correlation was found between locked position and function.

This study produced highly reliable measurements, with abduction angles proving to be lower than expected. Future work should focus on the effect of this low abduction angle on Hill-Sachs lesion management.