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Background: Glenohumeral joint contact loading before and after glenoid bone grafting for recurrent anterior instability remains poorly understood. Purpose: To develop a computational model to evaluate the influence of glenoid bone loss and graft positioning on graft and cartilage contact pressures after the Latarjet procedure. Study Design: Controlled laboratory study. Methods: A finite element model of the shoulder was developed using kinematics, muscle and glenohumeral joint loading of 6 male participants. Muscle and joint forces at 90° of abduction and external rotation were calculated and employed in simulations of the native shoulder, as well as the shoulder with a Bankart lesion, 10% and 25% glenoid bone loss, and after the Latarjet procedure. Results: A Bankart lesion as well as glenoid bone loss of 10% and 25% significantly increased glenoid and humeral cartilage contact pressures compared with the native shoulder (P <.05). The Latarjet procedure did not significantly increase glenoid cartilage contact pressure. With 25% glenoid bone loss, the Latarjet procedure with a graft flush with the glenoid and the humerus positioned at the glenoid half-width resulted in significantly increased humeral cartilage contact pressure compared with that preoperatively (P =.023). Under the same condition, medializing the graft by 1 mm resulted in humeral cartilage contact pressure comparable with that preoperatively (P =.097). Graft lateralization by 1 mm resulted in significantly increased humeral cartilage contact pressure in both glenoid bone loss conditions (P <.05). Conclusion: This modeling study showed that labral damage and greater glenoid bone loss significantly increased glenoid and humeral cartilage contact pressures in the shoulder. The Latarjet procedure may mitigate this to an extent, although glenoid and humeral contact loading was sensitive to graft placement. Clinical Relevance: The Latarjet procedure with a correctly positioned graft should not lead to increased glenohumeral joint contact loading. The present study suggests that lateral graft overhang should be avoided, and in the situation of large glenoid bone defects, slight medialization (ie, 1 mm) of the graft may help to mitigate glenohumeral joint contact overloading. [ABSTRACT FROM AUTHOR]

Rotator cuff tears are known to affect clinical outcome of reverse total shoulder arthroplasty (RSA). This study aimed to use computational modelling to quantify the effect of rotator cuff tear severity on muscle and joint forces after RSA, as well as stresses at the glenosphere, base‐plate, fixation screws, scapula, and humeral components. A multi‐body musculoskeletal model of the glenohumeral joint was developed comprising the scapula, humerus and nine major upper limb muscles. Simulations of abduction and flexion after RSA were performed in the case of the intact rotator cuff and tears to (i) supraspinatus; (ii) supraspinatus and infraspinatus, and (iii) supraspinatus, infraspinatus and subscapularis. The intact and supraspinatus deficient rotator cuff resulted in the largest calculated muscle forces, glenohumeral joint contact forces and implant stresses. Peak glenohumeral joint forces during flexion were lower than those during abduction in all cases; however, substantially more posterior joint shear force was generated during flexion than abduction. A tear involving the supraspinatus and infraspinatus reduced glenohumeral joint forces by a factor of 8.7 during abduction (603.1 N) and 7.1 during flexion (520.7 N) compared to those in the supraspinatus deficient shoulder. RSA with an intact or supraspinatus deficient rotator cuff produces large glenohumeral joint forces that may increase base‐plate failure risk, particularly during flexion when posterior shear forces are largest. Infraspinatus tears after RSA greatly reduce glenohumeral joint compression and may ultimately reduce joint stability. Future research ought to focus on experimental validation of subject‐specific muscle recruitment strategies and joint loading after RSA. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res [ABSTRACT FROM AUTHOR]