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The effect of component positioning on intrinsic stability of the reverse shoulder arthroplasty Favre, P; Sussmann, P S; Gerber, C Favre, P; Sussmann, P S; Gerber, C (2010). AbstractBackground: Anterior instability is one of the most common complications in reverse

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Shoulders with rotator cuff tears (RCT) tears are associated with significantly larger critical shoulder angles (CSA) (RCT CSA ¼ 38.2˚) than shoulders without RCT (CSA ¼ 32.9˚). We hypothesized that larger CSAs increase the ratio of glenohumeral joint shear to joint compression forces, requiring substantially increased compensatory supraspinatus loads to stabilize the arm in abduction. A previously established three dimensional (3D) finite element (FE) model was used. Two acromion shapes mimicked the mean CSA of 38.2˚found in patients with RCT and that of a normal CSA (32.9˚). In a first step, the moment arms for each muscle segment were obtained for 21 different thoracohumeral abduction angles to simulate a quasi-static abduction in the scapular plane. In a second step, the muscle forces were calculated by minimizing the range of muscle stresses able to compensate an external joint moment caused by the arm weight. If the joint became unstable, additional force was applied by the rotator cuff muscles to restore joint stability. The model showed a higher joint shear to joint compressive force for the RCT CSA (38.2˚) for thoracohumeral abduction angles between 40å nd 90˚with a peak difference of 23% at 50˚of abduction. To achieve stability in this case additional rotator cuff forces exceeding physiological values were required. Our results document that a higher CSA tends to destabilize the glenohumeral joint such that higher than normal supraspinatus forces are required to maintain modeled stability during active abduction. This lends strong support to the concept that a high CSA can induce supraspinatus (SSP) overload. The etiology of rotator cuff tears (RCT) is considered multifactorial and the contribution of an excessive load on the rotator cuff (RC) is a subject of ongoing debate. Recent clinical studies have reported a correlation between scapular geometry and the prevalence of RCT 1-4 indicating that anatomy related biomechanical factors may play an important role in the development of RCT. The lateral extension of the acromion (acromion index 2 ), and the inclination of the glenoid 1 have both emerged as important factors of interest. The critical shoulder angle (CSA) considers both the lateral extension of the acromion and the inclination of the glenoid, and is defined as the angle between the glenoid surface and a line connecting the inferior rim of the glenoid and the lateral tip of the acromion (Fig. 1). It has recently been identified as the strongest known anatomical predictor for the development of RCT 5 . The intuitive biomechanical basis of this relationship is that the lateral extension of the acromion leads to a more vertical line of action of the deltoid and therefore, promotes the tendency for a superior dislocation of the humeral head.2,3 More recently, this hypothesis has been experimentally supported using a robotic shoulder simulator.6 However, this experimental simulator setup required reduction of the problem to a two dimensional (2D) load case in which potentially important c...