THE RELATIONSHIP BETWEEN HUMERAL HEAD MIGRATION AND GLENOID ARTICULAR CONTACT IN ANATOMIC TOTAL SHOULDER ARTHROPLASTY

Loosening and wear of polyethylene glenoid components used for anatomic total shoulder arthroplasty (TSA) are ongoing clinical concerns [1]. Load transfer through the glenoid component has been investigated using both experimental and computational methods. It is well established that the glenoid experiences significant variations in the magnitude and direction of the glenohumeral joint reaction force during activities of daily living [2]. Movement of the humeral head contributes to this range of joint reaction force. However, the relationship between articular loading and humeral head position is not well-defined. The purpose of this study is to evaluate contact mechanics at the glenohumeral joint compared to the position of the humeral head under various loads using finite element analysis. 3D scapula models were developed from the CT scans of six patients with glenohumeral osteoarthritis. Each model was fitted with a standard 4-peg UHMWPE glenoid component, which was positioned to have full contact between the backside geometry and the bone. A 1-mm PMMA cement mantle was modelled around the pegs [3]. Cortical bone was assigned a Young's Modulus of 20,000 MPa and Poisson's ratio of 0.3. Trabecular bone was assigned material properties based on CT attenuation. The humeral head was modelled as cobalt chrome. Abaqus CAE was used to simulate the application of loads of equal magnitude in various directions. The load was applied in compression and then at five-degree increments in the superior and inferior directions up to 25° from the horizontal in both directions. The position of the center of the humeral head and the center of articular contact in each loading state was extracted. The relationship between humeral head position and articular contact position was found to be significant. The results of this study establish the direct relationship between humeral head position and articular contact position. These findings also demonstrate that even a small shift in the position of the humeral head leads to large eccentric loading, which is known to be a contributing factor in producing the rocking horse effect [1], [4]. Future studies will investigate further the effect of bone material properties on contact mechanics.