The piriformis fossa is a surgical term for the small depression on the medial side of the greater trochanter, where it meets the back of the femoral neck. Many texts treat it as the same as the trochanteric fossa, although strictly the trochanteric fossa receives the obturator externus tendon and the piriformis tendon attaches just above it on the upper border of the greater trochanter. It lies in line with the femoral shaft canal when viewed from the side, which is why surgeons use it as a nail entry point.
The piriformis fossa is a classic entry point for antegrade intramedullary nailing of femoral shaft fractures, because it is roughly in line with the medullary canal. An entry point placed too far forward or medially risks splitting the femoral neck or causing malalignment. The deep branch of the medial circumflex femoral artery, the main blood supply to the femoral head, runs close to this area, so piriformis entry nails are generally avoided in children and adolescents.
Damage to the deep branch of the medial circumflex femoral artery near the piriformis fossa can cut off blood supply to the femoral head. The risk is highest in skeletally immature patients, so trochanteric or lateral entry points are preferred in that group.
An entry point placed too anterior or too medial in the piriformis fossa can split the femoral neck during reaming or nail insertion, adding a hip fracture to the shaft fracture.
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