How does a varus deformity of the humeral head affect elevation forces and shoulder function? A biomechanical study with human shoulder specimens.

OBJECTIVES

A biomechanical study was performed to test the hypothesis that a varus deformity of the humeral head decreases supraspinatus (SSP) efficiency and increases deltoid elevation forces in human specimens.

METHODS

Twenty-four fresh-frozen human shoulder specimens were prepared by preserving the rotator cuff and deltoid. A defined, medial closed-wedge osteotomy was performed and lateral locked plate applied to simulate a varus deformity of 45° in Group I (n = 8) and 20° in Group II (n = 8). The control group (n = 8) was not osteotomized. The effect of the deformities on arm elevation forces was measured in a robot-assisted shoulder simulator under a physiologically loaded rotator cuff during three elevation phases. Phase 1 encompassed 0° to 30°, Phase 2 was from 30° to 60°, and Phase 3 included 60° to 90° of shoulder elevation.

RESULTS

SSP efficiency, defined as the degree of elevation attained per unit muscle force, was significantly less in Group I compared with Group II (P = 0.036) and the control group (P = 0.039) (Group I = 0.12 ± 0.03°/N, Group II 0.18 ± 0.05°/N, and control group 0.24 ± 0.10°/N). Under physiological loading of the rotator cuff, the deltoid (DELT) elevation forces were significantly greater in Group I (Pphase 1 = 0.015, Pphase 3 = 0.001) and Group II (Pphase 1 = 0.015, Pphase 3 = 0.006) compared with the control group in elevation Phase 1 (Group I: 3.20 ± 1.04 N/°, Group II: 3.03 ± 0.96 N/°, control group: 2.01 ± 0.53 N/°) and Phase 3 (Group I: 2.50 ± 0.85 N/°, Group II: 1.55 ± 0.28 N/°, control group: 1.21 ± 0.18 N/°). When the SSP was unloaded, the DELT elevation forces were significantly greater in Group l than in Group II (P = 0.040) and the control group (P = 0.004) during elevation Phase 3 (Group I: 2.12 ± 0.60 N/°, Group II: 1.47 ± 0.34 N/°, control group: 1.24 ± 0.32 N/°).

CONCLUSIONS

A varus deformity of the humeral head changes the pretension of the rotator cuff and results in a significantly decreased SSP efficiency (45° varus) and significantly higher arm elevation forces (20° varus). Clinically, the study's findings are relevant because they indicate that varus deformities of more than 20° should not be accepted intraoperatively and might indicate the need for surgical correction in case of subsequent symptoms.