Previous work has demonstrated vulnerability of the femoral nerve to damage by anterior acetabular retractors during THA. The aim of this study was to quantify the proximity of the femoral nerve to the anterior acetabulum, on cadaveric material and MRI studies.

A standard posterior approach to the hip was carried out in 6 fresh frozen cadaveric hemipelves. Following dislocation and removal of the femoral head, measurements were taken from the anterior acetabular lip to the posterior aspect of the femoral nerve as it passed over this point. 14 MRI studies of the hip were obtained from the local PACS database (7 male, 7 female; mean age 58 (range 32–80)). T1 weighted axial scans were reviewed. Measurements were obtained from the anterior acetabular lip to the posterior surface of the femoral nerve and artery, and the cross-sectional area of iliopsoas was calculated.

There was no significant difference between the mean distances to the femoral nerve in the cadaveric (24 mm) and MRI groups (25.3mm) (p=0.7). On MRI images, the distance between the acetabular wall and both the femoral artery (p=0.003) and femoral nerve (p=0.007) was significantly larger in men. The femoral artery is strikingly close to the acetabulum in females, passing a mean distance of 14.8 mm, whereas in males this was 23.9 mm. The mean femoral nerve distance was 28.7 mm in males and 21.9 mm in females. The cross-sectional area of iliopsoas was significantly smaller in women (5.97 cm² compared to 11.37 cm², p<0.001).

Both the femoral artery and nerve run in close proximity to the anterior acetabular lip. Care should be taken when placing instruments in this area to avoid neurovascular injury. The increased incidence of femoral nerve damage in women following THA may be due to the significantly smaller bulk of iliopsoas.

Acetabular retractors have been implicated in damage to the femoral and obturator nerves during total hip arthroplasty (THA). Despite this association, the anatomical relationship between retractor and nerve has not been elucidated.

A posterior approach to the hip was carried out in 6 fresh frozen cadaveric hemi- pelvises. Large Hohmann acetabular retractors were placed anteriorly over the acetabular rim, and inferiorly, as per routine practice in THA. The femoral and obturator nerves were identified through dissection and their relationship to the retractors was examined.

If contact with bone was not maintained during retractor placement, the tip of the anterior retractor had the potential to compress the femoral nerve, by passing either superficial to, or through the bulk of the iliopsoas muscle. If pressure was removed from the anterior retractor, the tip pivoted on the anterior acetabular lip, and passed superficial to iliopsoas, overlying and compressing the femoral nerve, when pressure was reapplied. The inferior retractor pierced the obturator membrane, medial to the obturator foramen in all specimens. Subsequent retraction resulted in the tip moving laterally to contact the obturator nerve.

Both the femoral and obturator nerves are vulnerable to injury around the acetabulum through the routine placement of retractors in THA. The femoral nerve is vulnerable where it passes over the anterior acetabulum. Iliopsoas can only offer protection if the retractor passes deep to the muscle bulk. If pressure is removed from the anterior retractor intra-operatively it should be reinserted. The obturator nerve is vulnerable as it exits the pelvis through the obturator foramen. Vigorous movement of the inferior retractor should be avoided. Awareness of the anatomy around the acetabulum is essential when placing retractors.