Clinical Application of the “Glenoid Track” Concept for Defining Humeral Head Engagement in Anterior Shoulder Instability

Background: The optimal treatment of Hill-Sachs injuries is difficult to determine and is potentiated by the finding that a Hill-Sachs injury becomes more important in the setting of glenoid bone loss, making engagement of the humeral head on the glenoid inherently easier. The “glenoid track” concept was developed to biomechanically quantify the effects of a combined glenoid and humeral head bony defects on instability. Purpose: To clinically evaluate humeral head engagement on the glenoid by utilizing glenoid track measurements of both humeral head and glenoid bone loss. Study Design: Retrospective cohort. Methods: A total of 205 patients with recurrent anterior shoulder instability were evaluated, and of these, 140 patients (68%; 9 females [6%] and 131 males [94%]) with a Hill-Sachs lesion and a mean age of 27.6 years (range, 15-47 years; standard error of mean [SEM], 0.59) were included in the final magnetic resonance angiogram [MRA]) analysis. Bipolar bone loss measures of glenoid bone loss (sagittal oblique MRA) and multiple size measures of the Hill-Sachs injury (coronal, axial, and sagittal MRA) were recorded. Based on the extent of the bipolar lesion, patients were classified with glenoid track as either outside and engaging of the glenoid on the humeral head (OUT-E) or inside and nonengaging (IN-NE). The 2 groups were then compared with clinical evidence of engagement on examination under anesthesia (EUA) using video arthroscopy, number of dislocations, length of instability, and patient age. Results: The mean glenoid bone loss was 7.6% (range, 0%-29%; SEM, 1.20%), and 31 of 140 (22%) patients demonstrated clinical engagement on EUA. Radiographically, 19 (13.4%) patients were determined to be OUT-E, while 121 (86.6%) were IN-NE and not expected to engage. Of those 19 patients with suggested radiographic engagement (OUT-E), 16 (84.5%) had clinical evidence of engagement versus only 12.4% that clinically engaged (15/121) without radiographic evidence of engagement (IN-NE) (P < .001). Younger age and a greater number of recurrence events were jointly predictive of a patient being classified as OUT-E (11.8 vs 6.4 dislocations; P = .015). Conclusion: This study demonstrates that glenohumeral engagement was well predicted based on preoperative glenoid and humeral head bone loss measurements using the glenoid track method. In addition, younger age and a greater number of recurrences were predictive of engagement. The glenoid track concept may be important to fully assess the overall risk for engagement prior to surgery and may help guide surgical decision making such as bony augmentation procedures.