BIOMECHANICAL OUTCOMES FOR ADLS POST-RTSA ARE AFFECTED BY SCAPULAR MORPHOLOGY VARIATION: A SIMULATION STUDY

Reverse Total Shoulder Arthroplasty (RTSA) has become a common method of restoring shoulder function in patients with arthropathy. Despite RTSA accounting for 30 to 62% of shoulder replacements the biomechanical implications and functional outcomes of these operations are still to be fully understood, in part because the effects of patient bone morphology remain unknown. It is impractical to examine the effect of anatomical variability on functional outcomes experimentally or clinically, however computational anatomy, modelling and simulation methods can provide biomechanical information to help identify causal relationships across a patient population. The aim of our study was to evaluate the effect of scapular morphology on RTSA biomechanics with musculoskeletal models and predictive simulations. We hypothesised that morphological changes would strongly affect joint loading, muscle moment arms and muscle coordination during functional motions. A previously developed statistical shape model generated scapular morphologies by independently varying six modes of variation (Table 1) with four levels for each mode and the population average. Modes were chosen that varied the glenoid fossa, acromion process, scapular spine and coracoid process as these are believed to most strongly affect glenohumeral biomechanics. Glenospheres of 39 mm diameter were orientated to correct for superior glenoid inclination and positioned with a 7 mm inferior overhang and 2 mm lateralisation from the inferior most point of the glenoid following guidelines provided by an expert orthopaedic surgeon. The newly defined glenohumeral joint geometries resulting from the digital surgery were used to redefine a previously validated musculoskeletal shoulder model. Predictive simulations (OpenSim 4.3) were completed that required the model to achieve a lateral and upward reaching tasks in 3D space by minimising muscular effort and time taken. Optimal solutions were computed for the lateral and upward reaching tasks across all morphologies. Morphological variation affected muscle length and moment arms across the range of motion that resulted in changes to moment generation and joint loading (Figure 1). Inferior translation of the glenoid relative to the acromion seen in Modes 4 & 5 resulted in greater resultant glenohumeral joint loads in early abduction (0–40°) whereas anterior and medial translations seen in Modes 5 & 6 reduced joint loading in later stages (40–80°) of the lateral motion but not the upward reaching motion. This study demonstrates that existing generic RTSA construct placement guidelines will not yield the same biomechanical outcomes across a population due to the wide range of anatomical variations that affect muscle force, moment generating properties and joint loads. This is the first musculoskeletal modeling study to systematically investigate the effects of morphological variation. Wider scale studies such as these using validated models and simulation methods are needed to understand the interactions between morphological variations and RTSA construct configurations. This study provides foundational evidence that morphological factors should be considered when conducting preoperative RTSA planning. For any figures or tables, please contact the authors directly.