The effect of hip position upon the location of the sciatic nerve: an MRI Study.

ABSTRACT

BACKGROUND: Allowance for the positional changes of the sciatic nerve is important when considering the safest position of the leg to perform hip operations, specifically the ischial osteotomy during a pelvic triple or periacetabular osteotomy. As for its proximity to the osteotomy site the sciatic nerve can be injured during these operations with the consequence of severe functional impairment. This is the first in-vivo study that demonstrates the effect of hip position upon the location of the sciatic nerve. METHODS: We determined how altering the position of the hip moves the nerve toward or away from the infracotyloid groove, the desired starting point of the ischial osteotomy site just inferior to the acetabulum when performing a pelvic triple or periacetabular osteotomy. Magnetic resonance imaging scans of the left hip in 3 different positions (neutral/supine, 30 to 45 degrees flexion, 30 to 45 degrees flexion/abduction/external rotation) were performed in 11 healthy children (5 boys and 6 girls, age 7 to 17 y) without prior hip surgery. The distance between the sciatic nerve and the infracotyloid groove was measured on the magnetic resonance images. Distance ratios based on the neutral position were calculated for flexion and flexion/abduction/external rotation for each of the participants. RESULTS: The sciatic nerve moves toward the ischium osteotomy site in hip flexion without abduction (mean flexion neutral ratio 0.79, P<0.01). However the nerve moves away from the osteotomy site when the hip is 30 to 45 degrees flexed, abducted, and externally rotated (mean flexion/abduction/external rotation neutral ratio 1.34), meaning the distance from nerve to infracotyloid groove increases significantly (P<0.01). The mean distances were 14.8 mm (11 to 20 mm) in neutral, 11.8 mm (9 to 16 mm) in flexion, and 20.0 mm (9 to 30 mm) in flexion/abduction/external rotation. CONCLUSIONS: The likely safest position of the hip/leg to perform the ischium osteotomy as part of a pelvic triple or periacetabular osteotomy is in flexion, abduction, and external rotation. In this position the osteotomy can be performed via a medial or anterior approach with the nerve the furthest away from the osteotomy site. LEVEL OF EVIDENCE Level II.