The clinical tolerance of rotator cuff tears is extremely variable, so the question is, what is the role of the deltoid in the shoulder stability? First of all, ex vivo experiments are necessary to analyse its effect. The aims of this study were: (1) to propose a testing protocol to measure the glenohumeral joint kinematics during the abduction motion by pulling on the deltoid without constraining the humerus and (2) to evaluate the repeatability of the 3D measurements. Six fresh-frozen anatomic specimens were tested. The kinematics follow-up of the osseous parts was carried out using an optoelectronic system (Polaris((R)), NDI, Canada). The abduction motion is realized by the pulling on anterior and medium fibers of the deltoid. For a 25 mm displacement, the range of motion: for the abduction was 24 degrees to 30.5 degrees , for the flexion was 1.5 degrees to -30.5 degrees (extension), for the medio-lateral rotation was 12 degrees (lateral rotation) to -5 degrees (medial rotation). For a displacement of the whole acromion-clavicle between 0 and 25 mm, the three humeral head translations were less than 5 mm. The three rotations and three translations were (with SD 95%): abduction: 0.5 degrees , flexion: 1 degrees , medio-lateral rotation: 1.5 degrees , three translations: 0.5 mm. The results showed a very high repeatability of the values. Results suggest that the deltoid alone can realize a motion of lateral elevation with a good stability in the glenohumeral joint as shown by the slight translation motion of the head and the value reproducibility. The protocol can be used to validate a finite element model of the glenohumeral joint.
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