Hypertrophic teres minor restores shoulder strength and range of external rotation in posterosuperior rotator cuff tears.

BACKGROUND: In posterosuperior rotator cuff tears (PS-RCT), the progression of infraspinatus (ISP) muscle atrophy seems to induce compensatory hypertrophy of the teres minor (TM) muscles. However, the effect of these changes on shoulder strength and range of external rotation (ER) remains unclear. This study determined the strength and range of ER in patients with PS-RCT with atrophic ISP and hypertrophic TM and compared this with patients with PS-RCT and normal or deficient TM. METHODS: We investigated 35 patients with PS-RCT and atrophic ISP. TM muscles were classified as hypertrophic (type A) in 17, normal (type B) in 10, or deficient (type C) in 8. The strength ratio of the affected shoulder to the healthy contralateral shoulder was calculated, and the active range of motion was measured for both shoulders. RESULTS: The strength ratios of ER in types A, B, and C were 60%, 33%, and 7% (P < .01) with the patient's arm at the side and were 60%, 35%, and 5% (P < .001) at 90° abduction, respectively. The average ranges of ER in types A, B, and C were 22.6°, 15.0°, and -12.5° (P < .001) with the patient's arm at the side and were 71.6°, 44.5°, and 21.9° at 90° abduction (P < .01), respectively. The differences between shoulder types in other measures of strength or ER range were not significant. CONCLUSIONS: In patients with PS-RCT and atrophic ISP, shoulders with compensatory hypertrophy of the TM had greater strength and range of ER than shoulders with normal or atrophic TM.