Three-Dimensional (3D) Animation and Calculation for the Assessment of Engaging Hill–Sachs Lesions With Computed Tomography 3D Reconstruction

PURPOSE: To dynamically assess for Hill–Sachs engagement with animated 3-dimensional (3D) shoulder models. METHODS: We created 3D shoulder models from reconstructed computed tomography (CT) images from a consecutive series of patients with recurrent anterior dislocation. They were divided into 2 groups based on the perceived Hill–Sachs severity. For our cohort of 14 patients with recurrent anterior dislocation, 4 patients had undergone osteoarticular allografting of Hill–Sachs lesions and 10 control patients had undergone CT scanning to quantify bone loss but no treatment for bony pathology. A biomechanical analysis was performed to rotate each 3D model using local coordinate systems to the classical vulnerable position of the shoulder (abduction = 90°, external rotation = 0-135°) and through a functional range. A Hill–Sachs lesion was considered “dynamically” engaging if the angle between the lesion’s long axis and anterior glenoid was parallel. Results: In the vulnerable position of the shoulder, none of the Hill–Sachs lesions aligned with the anterior glenoid in any of our patients. However, in our simulated physiological shoulder range, all allograft patients and 70% of controls had positions producing alignment. CONCLUSIONS: The technique offers a visual representation of an engaging Hill–Sachs using 3D-animated reconstructions with open-source software and CT images. In our series of patients, we found multiple shoulder positions that align the Hill–Sachs and glenoid axes that do not necessarily meet the traditional definition of engagement. Identifying all shoulder positions at risk of “engaging,” in a broader physiological range, may have critical implications toward selecting the appropriate surgical management of bony defects. LEVEL OF EVIDENCE: level III, case-control study.