Defining the greater trochanter-ischial space: a potential source of extra-articular impingement in the posterior hip region.

The purpose of this study was to describe greater trochanteric-ischial impingement and the relative position of the hip joint where impingement occurs. Twenty-three hips from 13 embalmed cadavers (seven males and six females) with a lifespan ranging between 46 and 91 years were used for this study. The pelvic region of each cadaver was skeletonized leaving only the hip capsule and the sciatic nerve. From 90° of flexion, the hip was extended while maintaining a position of 30° abduction and 60° external rotation. The position of hip flexion was recorded when there was contact between the greater trochanter and the ischium. The procedure was repeated in 0° abduction. A Flexion-Abduction-External Rotation (FABER) test was then performed on all specimens with a positive finding defined as contact between the greater trochanter and the ischium. In 30° abduction, contact of the ischium and the greater trochanter occurred in 87% (20/23) of the hips at an average of 47° of flexion (SD 10; range 20-60°). In 0° abduction, a positive finding was noted in 39% (9/23) of hips at an average of 59° flexion (SD 6; range 52-70°). A positive finding in the FABER test position was noted in 96% (22/23) of hips. The greater trochanter can impinge on the ischium when the hip is extended from 90° flexion in a 60° externally rotated position. This impingement occurred more commonly when the hip was in 30° abduction compared with neutral abduction. The FABER test position consistently created greater trochanteric-ischial impingement.