Assessment of Posterior Subluxation of the Humeral Head Using Biplanar Imaging in the Upright Position in Patients Scheduled for Total Shoulder Arthroplasty

Total shoulder replacement is now recognized as a reliable and effective therapeutic option for the treatment of various degenerative joint conditions. Over time, clinical outcomes have significantly improved, both in terms of patient satisfaction and pain relief and functional recovery. Anatomical total shoulder replacement (TSR) has become the gold standard for pathologies such as glenohumeral osteoarthritis with an intact rotator cuff, with an estimated incidence of 12.5 cases per 100,000 patients, while reverse total shoulder replacement, intended primarily for pathologies of massive, irreparable rotator cuff tears, has an estimated incidence of 19.3 cases per 100,000 patients. These surgical approaches restore shoulder mobility, particularly during elevation movements, while significantly reducing pain.

However, loosening of the glenoid implant remains one of the main complications observed after total shoulder arthroplasty (TSA), often associated with increased pain, impaired function, and decreased overall patient satisfaction. Analysis of the explanted glenoid components frequently reveals posterior wear of the glenoid, which is closely correlated with the phenomenon of loosening. Among the factors likely to contribute to this complication, the rate of preoperative posterior subluxation of the humeral head has been identified as a major determinant of clinical outcomes in acetabular impingement (AIM), due to the progressive postoperative recurrence of this subluxation and its deleterious mechanical effects on the glenoid. Furthermore, the rate of posterior subluxation of the humeral head wa historically calculated from 2D CT scans obtained preoperatively. More recently, the limitations of the 2D approach have been highlighted, thus giving way to quantification using 3D reconstructions from the preoperative CT scan. However, whether for the 2D or 3D approach, the CT scan is performed while the patient is lying on their back. Although they did not directly quantify the posterior subluxation of the humeral head, showed, in healthy individuals, that the kinematics of the shoulder (i.e., glenohumeral angle) and more specifically the translation of the humeral head relative to the glenoid were different depending on whether the participant was lying down or standing. Considering that the majority of daily human activities are performed with the torso upright, it is crucial to determine whether the estimated rate of posterior subluxation of the humeral head and shoulder kinematics (i.e., glenohumeral angles) measured in the supine position accurately reflects the actual measurements when the patient is standing. One reason for this discrepancy could be the difficulty in measuring the rate of posterior subluxation of the humeral head and shoulder kinematics in the upright position. Standard motion analysis methods using video systems are not precise enough to measure these parameters, particularly due to soft tissue artifacts. Conversely, biplanar fluoroscopy or low-dose imaging (EOS®) are the gold standard for measuring shoulder kinematics with accuracy on the order of degrees and millimeters. However, no study to date has quantified the rate of posterior humeral head subluxation and shoulder kinematics in patients in an upright position.

The main objective of this study is to evaluate the influence of position, namely supine versus upright, on the quantification of posterior subluxation of the humeral head in patients scheduled for total shoulder replacement.