Contributions of Muscle Forces to Stability and Mobility in Radiography-Driven Models of Total Shoulder Arthroplasty

Several complications are associated with total shoulder arthroplasty (TSA), including glenoid loosening and joint instability. These complications are linked to imbalances in glenohumeral muscle forces, which cause eccentric loading that occurs on the outer edge of the glenoid component, rather than in the center of the glenoid, and can be damaging to the implant and destabilizing the joint. The purpose of this study was to develop musculoskeletal models of TSA patients using their skeletal geometry acquired from CT and their kinematics acquired with high-speed stereo radiography (HSSR) to predict glenohumeral joint loading and muscle forces for abduction and flexion and describe how these loads differ from their native contralateral side. CT data was collected for both shoulders of 6 TSA patients (three anatomic and three reverse). Models were developed using CT reconstructions of each patient's scapula and humerus, and placed implant geometry with CT-derived alignment. Mesh morphing was used to create personalized muscle attachment sites on the scapula. Simulation of abduction and flexion to quantify muscle and joint forces was driven with limb-specific HSSR kinematics. At 90 deg abduction, reverse TSA increased superior shear force by 11.2% bodyweight (BW) and decreased medial compressive force by 27.5%BW compared to each patient's contralateral shoulder. Lateral deltoid elevation moment arms and stability ratios increased 9.61 mm and 0.58, respectively. At 90 deg abduction, anatomic TSA patients increased humeral internal rotation by 52.7 deg and decreased rotator cuff force by 9.4%BW. Patient-specific musculoskeletal models quantified differences in glenohumeral joint mechanics. This study revealed muscle and kinematic mechanisms that could lead to edge loading and glenoid loosening in TSA and can inform personalized surgical planning for implant alignment with the goal of improving patient outcomes.