Muscle efficiency during total shoulder arthroplasty: Dependence on position of the humeral head and function of the rotator cuff

Modern shoulder prostheses permit an anatomic reconstruction of the joint, although the biomechanical. advantages are not proven. The goal of this stu dy was to investigate the relationship between position of the humeral head and function of the shoulder prosthesis (muscle efficiency). Shoulder elev ation-motion and rotator cuff defects were simulated in vitro in a robot-as sisted shoulder simulator. The EPOCA Custom Offset shoulder prosthesis (Arg omedical AG, Cham, CH) was implanted in seven normal shoulders (77 +/- 20 k g, 55 +/- 14 years). Active elevation was simulated by hydraulic cylinders, and scapulothoratic motion by a specially programmed industrial robot. Mus cle efficiency (elevation-angle/muscle-force of the deltoid muscle) was mea sured in anatomic (ANA), medialised (MED) and lateralised (LAT) positions o f the humeral head, with or without rotator cuff muscle deficiency. Mediali sation increased efficiency by 0.03 +/- 0.04 deg/N (p = 0.022), lateralisat ion decreased it by 0.04 +/- 0.06 deg/N (p = 0.009). Supraspinatus muscle d eficiency increased the deltoid force required to elevate the arm, and thus decreased efficiency (ANA p = 0.091, MED p = 0.018, LAT p = 0.028). The da ta confirm that the position of the humeral head affects the mechanics of t otal shoulder arthroplasty. Medialisation increases efficiency of the delto id muscle and may prove useful in compensating isolated supraspinatus muscl e deficiency. Lateralisation, in contrast, leads to an unfavorable situatio n.