2017 Volume 36 Issue 4 Pages 461-465
Introduction: Although posterior soft tissue repair reduces the incidence of dislocation after posterior-approach total hip arthroplasty (THA), little is known about the importance of tendon repair of the obturator externus muscle. This study makes a comparison for dislocation rates after the posterior-approach THA in two different procedures for short external rotators fixation.
Methods: We retrospectively reviewed 725 patients who underwent primary THAs via the posterior approach. In 541 patients we detached the piriformis muscle and the conjoined tendon from the greater trochanter backed with the capsule, and repaired to the posterior margin of the gluteus medius muscle (Group A). In this series the obturator externus muscle was cut with capsule incision far from the insertion at the greater trochanter, so we were unable to repair it effectively. In another 184 patients we released all external rotators (including the obturator externus muscle) from the insertion backed with the capsule in an L-shaped incision along the superior part of femoral neck and the trochanteric crest. The L-shaped flap was repaired back to the superior capsule and the posterior margin of the gluteus medius muscle (Group B). In each group we used two different head size (28 mm and 36 mm) according to the operation period and the cup size. We compared the dislocation rate between two groups at least one year after the operation. We also made a comparison for the dislocation rate in different head size within the two groups.
Results: The overall dislocation rate was 5.7% in Group A and 1.1% in Group B, respectively. The dislocation rate with the 28 mm head size was 6.9% and 3.1%, and with the 36 mm head size was 1.7% and 0%, for Groups A and B respectively.
Conclusions: The obturator externus muscle plays an important role in preventing posterior dislocation after THA. Anatomically it runs from the obturator foramen to the greater trochanter passing the posteroinferior part of the capsule like a sling. It gives dynamic pressure on the femoral head especially in flexion and during internal rotation in the natural hip. This function can be now be reproduced after THA as we have demonstrated, which gives low rates of dislocation.
