Purpose of the study: The aim of this biomechanical study was to assess the performance of the deltoid muscle in the absence of a rotator cuff using different models for shoulder prosthesis.

Material and methods: A computer model reproducing the three dimensions of the glenohumeral joint was use to analyze the force of the deltoid muscle during abduction movements in shoulders devoid of a rotator cuff. The three heads of the deltoid were analyzed in order to determine the most effective level of muscle tension. The lever arm of the deltoid was measured from 0–90° abduction. Using this 3D model, we simulated implantation of six different models of reversed prostheses in order to assess the biomechanical situation which would be the most favorable for the deltoid. Performance of the normal deltoid was compared with the performance of the deltoid after implantation of an anatomic prosthesis and after implantation of an reversed prosthesis. Several variables were studied: medial offset of the center of rotation, lateral offset of the humerus, lengthening of the deltoid muscle.

Results: Optimal deltoid performance (especially from 60–90° abduction) was observed if the center of rotation was offset medially and the humerus was offset laterally and lowered. A 10% increase in the length of the muscle fibres increased muscle performance 18%. Exaggerated lateral offest of the humerus increased deltoid performance between 30 and 60° abduction but lost its beneficial effect at 90° abduction. From 15° abduction, a scapular notch appeared when the humerus was off set medially. This could be avoided if the humerus was offset laterally with a less medial center of rotation. Beyond 150° abduction, superior impingement appeared irrespective of the type of prosthesis.

Discussion and conclusion: This biomechanical study proved the superiority of reversed prostheses compared with anatomic prostheses for massive rotator cuff tears. Medial offset of the center of rotation reduced shear forces on the glenoid. Lateral offset of the humerus increase via a pulley effect the lowering force of the deltoid. Lowering the humerus pulled on the muscle fibers of the deltoid and increased their performance. Dosing these three variables with an appropriate («ideal») design for the reversed prosthesis would optimize deltoid performance in patients with deficient rotator cuffs.