A larger critical shoulder angle requires more rotator cuff activity to preserve joint stability

Abstract

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...

Critical shoulder angle combined with age predict five shoulder pathologies: a retrospective analysis of 1000 cases

“…Previous biomechanical findings showed a more superiorly directed deltoid force vector in patients with wide lateral acromion and a concomitant increased requirement of supraspinatus force to stabilize the humeral head [4, 8, 12]; whereas a shorter acromion extension produced higher glenohumeral joint reaction forces potentially leading to OA [13]. A wider lateral extended acromion was expected in CTA accompanying with superior migration of the humeral head.…”

Critical shoulder angle: Acromial coverage is more relevant than glenoid inclination

“…Therefore, different anatomical predispositions have been suspected to be linked with these two pathologies. Evidence for anatomical variants predisposing to either disease could only be demonstrated by the introduction of the Critical Shoulder Angle (CSA) in 2013 and the understanding of its biomechanical implications . The CSA is a radiographic parameter and is measured in antero‐posterior (A/P) radiographs as an angle between a line from the lower to the upper glenoid rim (glenoid plane) and a second line from the lower glenoid rim to the lateral edge of the acromion roof .…”

“…Evidence for anatomical variants predisposing to either disease could only be demonstrated by the introduction of the Critical Shoulder Angle (CSA) in 2013 1 and the understanding of its biomechanical implications. [4][5][6] The CSA is a radiographic parameter and is measured in antero-posterior (A/P) radiographs as an angle between a line from the lower to the upper glenoid rim (glenoid plane) and a second line from the lower glenoid rim to the lateral edge of the acromion roof. 1 High angles (>35-38˚) was associated with RCT and biomechanical analyzation could show an increased joint instability.…”

Editorial Commentary: Angles, Ratios, and Rotation—Is It Worth Splitting Hairs Over the Critical Shoulder Angle?

“…The authors were also correct in pointing out that specific published reports have obtained nonstandard AP radiographs while calculating these established indices. 10,11 Do we then question the conclusions of these published reports? Or do we assume that minor rotations do not make any significant difference?…”