Piriformis and obturator internus morphology: a cadaveric study.

Preservation of piriformis during exposure of the hip joint via a posterior approach may result in a lower rate of dislocation following total hip arthroplasty. The aim of this study was to identify anatomical variations in the tendons of piriformis and obturator internus which could inform piriformis sparing approaches to the hip. Twenty-nine proximal femora from 15 cadavers, 5 male and 10 female aged 65-79 years, were examined. Tendon crossing angles, location and mode of insertion to the greater trochanter and the extent of fusion between tendons prior to insertion were noted. The mean (and associated standard deviation) of the vertical and horizontal widths of the piriformis tendon were 4.78 ± 1.31 mm and 7.35 ± 1.74 mm, respectively. The mean (and associated standard deviation) of the vertical and horizontal widths of the obturator internus tendon were 6.87 ± 1.61 mm and 5.72 ± 1.38 mm, respectively. The mean distances of the anterior and posterior margins of the piriformis tendon insertion from the posterior limit of the greater trochanter, defined as a percentage of the anteroposterior length of the greater trochanter, were 63.3% ± 9.4% and 43.0% ± 9.4%, respectively. Equivalent mean distances for the obturator internus insertion were 73.0% ± 6.6% and 55.9% ± 7.0%, respectively. On the basis of the relationship between the piriformis and obturator internus tendons in terms of the angle and point at which they cross, in addition to any degree of tendon fusion, four classifications were identified. This study shows that the most posterior margins of the piriformis and obturator internus attachments are located more than one-third of the way along the greater trochanter, suggesting that current osteotomies would not include these external rotators in the majority of cases.