Effect of defined varus deformities of the proximal humerus on the elevation forces of the arm. A biomechanical study on human shoulder specimens

Objectives. This study investigated the hypothesis that different varus deformities of the humeral head decrease the efficiency of the M. supraspinatus (SSP) and increase the deltoid elevation forces. Methods. A varus deformity model of the proximal humerus was developed with an intact rotator cuff and deltoid muscle in human specimens.Three groups were differentiated per random distribution: group I (n=8): 45 degrees varus deformity, group II (n=8): 20 degrees varus deformity, and a control group (n=8). The effect of different varus malunions (20 degrees and 45 degrees varus) on the SSP efficiency and on the arm elevation forces was analyzed with a robot -assisted shoulder simulator and a force controlled hydraulic system in three defined phases of elevation: 0-30 degrees, 30-60 degrees, and 60-90 degrees. Results. The SSP efficiency (i.e., the degree of elevation per unit muscle force) was 0.12 +/- 0.03 degrees/N in group 1,0.18 +/- 0.05 degrees/N in group II, and 0.24 +/- 0.10 degrees/N in the control group and was significantly lesser in group I than in group II (p=0.036) and in the control group (p=0.039).Under physiological loading of the rotator cuff, the deltoid elevation force per elevation angle was significantly greater in groups I and II compared to the control group in the elevation phases between 0-30 degrees and 60-90 degrees. In case of an unloaded SSP (i.e., simulation of a SSP tear), the elevation forc-es were significantly greater in group I than in group II (p=0.040), and in the control group (p=0.004) in the elevation phase between 60 degrees and 90 degrees. Conclusion. Varus deformities of the humeral head significantly decreased the SSP efficiency (45 degrees varus), and significantly increased the arm elevation forces (>= 20 degrees varus in the elevation phases 0-30 degrees and 60-90 degrees). The hypothesis of our study could be confirmed.