Preservation of the short external rotator muscles of the hip is effective in preventing dislocation after total hip arthroplasty. The external rotation torque exerted by the short external rotator muscles can resist internal rotation of the hip joint;​ however, the absence of quantitative data on external rotation torque makes it difficult to determine which short external rotator muscles should be preserved. We compared the external rotation torque of individual short external rotator muscles in hip flexion. The external rotation torque of the piriformis, obturator internus, conjoined tendon, and obturator externus was evaluated in 15 fresh-frozen cadaveric hips from 0° to 105° of hip flexion, by applying muscle force based on physiological cross-sectional area to the muscle-string model constructed along the anatomical course of each muscle on the pelvis. External rotation torque of the piriformis, obturator internus, and conjoined tendon peaked at 15° hip flexion, and then decreased as the hip flexed. External rotation torque of the obturator externus increased as the hip flexed. At 75° of flexion or more in deep flexion, ranges associated with a high risk of dislocation, the obturator externus had the highest external rotation torque. External rotation torque of the obturator externus was, on average, 2.18 times greater than that of the conjoined tendon at 90° of hip flexion, and 3.80 times greater at 105° flexion. These findings, which include the element of muscle force, suggest that preservation of the obturator externus is the most effective in resisting dislocation among the short external rotator muscles.

Clinical Relevance:​ No study has evaluated dislocation resistance based on external rotation torque, including both a force component and muscle course. In this study, we estimated the muscle force of the short external rotator muscles from the PCSA, and quantitatively evaluated how that muscle force changes as external rotation torque during hip flexion. While approaches to hip replacement surgery and the preservation of tissues vary, our results suggest criteria for which muscles should be effectively preserved to resist dislocation in selective dissection of the posterior soft tissues of the hip joint. In future, surgeons will be able to advocate surgical approaches that are less invasive and more resistant to dislocation, which will require clinical evaluation.