Biomechanics of Reverse Total Shoulder Arthroplasty - PROJECT SUMMARY / ABSTRACT Reverse total shoulder arthroplasty transposes the glenohumeral ball and socket, relieving pain and restoring arm elevation. Yet many patients experience functional deficits in arm rotation, and up to 20% lose post-operative range of motion. Our long-term objective is to optimize patient-specific outcomes via procedural, implant design, and rehabilitation modifications by quantifying the biomechanical implications of the non-anatomic surgery. This proposal focuses on arm rotation capabilities related to patient-specific humeral anatomic twist between the shoulder and elbow (torsion) and the relative rotational orientation of the humeral implant (version). Humeral torsion varies by 80° in the general population. Implant version is set generically at 0-40° relative to the forearm, rarely referencing humeral anatomy. There is no consensus on optimal implant version, but retrospective clinical data suggests matching implant version to torsion may improve post-surgical outcomes. Differences between torsion and implant version are unknown during surgery, and this uncontrolled relationship may directly affect rotator cuff and deltoid muscle efficacy. We will quantify the impact of torsion and implant version on shoulder function in parallel studies of clinical outcomes and laboratory biomechanics. In Aim 1, data on 3D shoulder anatomy, range of motion, and patient reported outcomes will be collected in patients pre-operative to reverse shoulder arthroplasty using standard clinical assessments and imaging. Ultrasound will be used to determine its utility as a pre-operative planning tool to quantify humeral torsion specific to reverse shoulder arthroplasty. Surgeons will then perform the procedure per current standard of care without knowledge of patient-specific study metrics. After one year, patients will return for their standard clinical follow up and clinical metrics will again be assessed. A sub-cohort will then be recruited for additional data collection using dynamic stereoradiography, where 3D humerothoracic, scapulothoracic, and glenohumeral motion are quantified during functional activities. Humeral torsion will be analyzed with respect to implant version to determine how their relationships influence shoulder function and clinical outcomes. In Aim 2, cadaver shoulders will be used to again test the relationship between torsion and implant version, now adjusting implant version within each cadaver to quantify the discrete influence of implant version relative to torsion. Biomechanical metrics including passive and active range of motion, muscle force, joint reaction force, muscle length, and muscle moment arms will be captured on a biorobotic simulator and in computational models. These metrics will be analyzed to measure the relative effect size of torsion and implant version and determine if matching implant version to torsion improves shoulder function. Successful completion of both Aims will allow biomechanical factors measured in Aim 2 to be analyzed with respect to patient outcomes in Aim 1 to determine the origins of patient-specific functional deficiency. Ultimately, this research challenges the standard of care in reverse shoulder arthroplasty, with the objective to restore shoulder function in patients and set realistic and reliable expectations for the surgery.
