Study involved the fifty dry adult cadaveric femora non-fractured without any deformity in anatomy museum and biomechanical laboratory. Selected femora 24 were of Right side and 26 Left side. Dissection of four lower extremities of formalin fixed adult Indian cadavers with mobile hip joints that allowed 60 degrees of hip flexion and without any scars indicating former hip surgery was undertaken to localize and describe the anatomical relationship of piriformis fossa with the surrounding structures.

The dissection of the specimens of human cadaveric hip showed piriformis muscle as a key structure in deep gluteal region. The muscle arises from the front of middle three sacral vertebrae. Its rounded tendon inserted into the medial part of the tip of the greater trochanter of femur. It divides Greater sciatic foramen into superior and an inferior part; the nerve and the vessels that emerge superior to the piriformis muscle are the superior gluteal nerve and vessels and inferiorly named as inferior gluteal nerve and vessels. Sciatic nerve also emerges inferiorly to this muscle [Table/Fig-1,2].

[Table/Fig-1]:

Showing location and relationship of muscles in posterior dissection of cadaveric hip specimen

[Table/Fig-2]:

Showing locations of piriformis and obturator externus muscles

Obturator externus muscle is a flat, triangular muscle originates from the anteromedial two-thirds of the external surface of the obturator membrane and from the adjacent part of the pubis and ischial rami and was inserted on the floor of a fossa located on medial surface of greater trochanter.

Cadaveric dry femoral bone showed the greater trochanter which is large and quadrangular projection at the junction of the neck and shaft. Rough area at tip of greater trochanter gives the insertion to piriformis tendon. Its medial surface has deep depression which gives the insertion to obturator externus muscle. This fossa was also identified during dissection of cadevaric hip. For anatomical and safe entry site of femur antegrade nailing direction of femoral shaft and location of this fossa was determined. In 12 (24%) dry cadaveric femora, fossa lies in the direction of shaft. In 34 (68%) alignment of the femoral shaft was lateral to the fossa in the region of tip of greater trochanter. In 4 (8%) femoral shaft was aligned medial to fossa in the direction of femoral neck [Table/Fig-3,4]. Piriformis muscle was consistently attached to tip of greater trochanter and fossa provides the insertion site to the obturator externus.

[Table/Fig-3]:

Showing location of the trochanteric fossa (site of insertion of the obturator externus) and tip of greater trochanter (site of insertion of the piriformis)

[Table/Fig-4]:

Skiagram of the femora verifying radiological location of the trochentric fossa (Piriformis fossa) and greater trochanter tip

The piriformis [5,6] (L. piriformis = “pear shaped”) is a muscle in the gluteal region of the lower limb. It was first named by Spigelius, a professor from the University of Padua in the 16^th century. However, anatomical dissection of this region and the study of the proximal end of the dry femoral bones suggested that so called “Piriformis fossa” is neither piriform in shape nor piriformis muscle is being inserted in to it.

The insertion of piriformis muscle represents a small, area on the tip of the greater trochanter whereas the trochanteric fossa is a deep depression on the inner surface of the greater trochanter. This fossa is not the insertion site for piriformis muscle rather obturator externus muscle was found to be inserted in the fossa. The obturator externus is actually located anterior to the remaining all lateral rotators just behind the neck of femur and can’t be seen easily from posterior aspect of hip during surgical exposure while, piriformis can be easily identified in surgical exposure, hence probably it has been taken as synonymous with the fossa. Piriformis muscle and “Piriformis fossa” are two unrelated entities. Fact was also been studied by Papadakis SA et al., [7] and considered as drawing mismatch or terminology error. Current study further analyses and describes anatomical location and description of both the terms.

Piriformis fossa can be felt during surgical exposure and can be seen in the skiagram. In orthopedic literature [8,9], it has been described as standard entry site for femoral nail insertion and considered to lie in the direction of femoral canal. In an study by Ricci WM et al., [10], trochanteric versus piriformis fossa entry portal for treatment of femoral shaft fracture and anatomical study by Wolfgang Grechenig et al., [11] it was found that piriformis fossa is not the anatomical site for femoral nail insertion. Clinical [12–13] and anatomical studies [14] described the various entry points for femoral nail insertion. In the present study we found that the femoral shaft did not always correspond to the piriformis fossa. In only 24% dry cadaveric femora, this fossa lies in the direction of shaft, in 68% femora alignment of the shaft was lateral to the fossa in the region of tip of greater trochanter and in 8% femora, the femoral shaft was aligned medial to fossa in the direction of femoral neck.

So called Piriformis fossa should be termed as Trochateric or Obturator fossa for clear and correct anatomical description. Proper entry site for femoral nailing is variable and cannot be fixed universally, due to variable alignment of femoral canal, trochateric fossa cannot be used as universal entry site; rather it should be determined by proximal femoral anatomy and direction of shaft of femur to the proximal corresponding area on femur.