GLENOID COMPONENT POSITIONING IN REVERSE SHOULDER ARTHROPLASTY IS IMPORTANT FOR REDUCING CONJOINT TENDON IMPINGEMENT

Humeral impingement against the conjoint tendon during internal rotation may cause pain and lead to reduced internal rotation in the setting of reverse shoulder arthroplasty (RSA). Glenoid component configuration and positioning may influence the allowable glenohumeral internal rotation prior to impingement with the conjoint tendon. Therefore, the aim of this study was to determine the impact of glenoid component lateralization, glenosphere diameter, and glenoid baseplate position on conjoint tendon impingement. It was hypothesized that greater glenoid lateralization and glenosphere size, in addition to anteriorly and superiorly placed glenoid baseplates would increase the risk for earlier conjoint tendon impingement. The conjoint tendon path was digitized using an optical tracking system in twelve upper extremity cadavers. This data was transposed onto three-dimensionally derived models of each scapula and humerus. Each model was then virtually planned for RSA using a generic implant design. A baseline implant configuration was first established with a 36mm glenosphere and a glenoid baseplate with 100% backside seating positioned flush to the inferior glenoid rim. The baseline humeral configuration included a 0mm offset polyethylene insert and was virtually implanted at a neck shaft angle of 135° and retroversion of 20°. Glenoid parameters assessed were lateralization (0mm, 2.5mm, 5mm, 7.5mm, 10mm, 12.5mm), glenosphere diameter (36mm, 39mm, 42mm), glenoid baseplate superior-inferior position (flush to inferior glenoid rim and centered on the glenoid), and glenoid baseplate anterior-posterior position (0mm and ±2.5mm of translation). The allowable internal rotation prior to conjoint tendon impingement was then evaluated with each implant configuration using a custom motion software. Humeral internal rotation was simulated in 0°, 30°, and 60° of glenohumeral forward elevation. The software modelled the muscle path of both the coracobrachialis and short head of the biceps throughout internal rotation with the arm at the side. The influence of the various implant design parameters on impingement free internal rotation was evaluated using a repeated measures analysis of variance (P < 0 .05). Glenosphere size, in addition to glenoid baseplate anterior-posterior and superior-inferior position significantly influenced conjoint tendon impingement (P 0.293) (Figure 1). Larger glenosphere sizes and anterosuperior glenoid baseplate positioning led to earlier conjoint tendon impingement (Figure 1). Several glenoid-sided implant parameters in RSA have been identified that can significantly increase the risk of early conjoint tendon impingement during internal rotation. These parameters included a larger glenosphere and anterosuperior positioning of the glenoid baseplate and glenosphere. It is likely that these variables lead to earlier conjoint tendon impingement as they reduce the distance between the conjoint tendon origin and proximal humerus, therefore reducing the internal rotation needed to achieve impingement. These results may be considered by surgeons preoperatively to reduce the risk of post-operative conjoint tendon impingement. For any figures or tables, please contact the authors directly.