Topographic variations of the relationship of the sciatic nerve and the piriformis muscle and its relevance to palsy after total hip arthroplasty

Abstract

The aim of this paper was to study the anatomical relationship between the piriformis muscle and the sciatic nerve with regard to the possibility of neurological deficit after THA. The incidence of anatomical variation of both structures is 15-30% in the literature. The authors studied 91 cadavers and found an atypical relationship in 19 cases (20.9%). In this study individual variations were found with the following frequency: The sciatic nerve exits below the piriformis muscle in 79.1% of the cases. The sciatic nerve separates into two divisions above the piriformis, one branch passing through the muscle, the other below it (14.3%). An unsplit nerve passes through the piriformis muscle in 2.2%. The nerve separates into two divisions above the piriformis, one branch exiting above the muscle and passing along its dorsal aspect, the second exiting distally below the muscle in 4.4%. The most common reasons for sciatic nerve injury in surgery of the hip joint are direct injuries, ischemia of the nerve tissue, compression or excessive distraction of the nerve, compression by bone cement, thermal damage during cement polymerization, injury during THA dislocation, compression by hematoma, bone prominence or an implanted acetabular component. According to the presented anatomical study, overstretching of the nerve itself or its branches in the area of the pelvitrochanteric muscles after their release from their origin can be another mechanism. Such overstretching can appear in the presence of some of the aforementioned anatomical variants.

The sciatic nerve in human cadavers — high division or low formation?

“…Pokorny et al [17] customised the Beaton and Anson classification and stated that the type 1 variation, undivided nerve below undivided muscle, was seen in 79.1% of their specimens [16]. In our study we also noted that in 19 specimens the SN was dividing in the popliteal fossa.…”

“…Many previous studies on the varied relationship between the piriformis and SN, have found variations in 15-30% of cases [17,20].…”

“…The incidence of high division of SN, that is, the division of SN above its usual site varies considerably. For instance, the reported incidences of SN division before its exit in the gluteal region vary from 4.0% [23], to 20.9% [17] to 48.0% [7].…”

Variaciones del Nervio Isquiático en Relación al Músculo Piriforme

“…La primera, tipo II de estos autores, es una división alta del nervio isquiático, donde el nervio fibular común pasa a través del músculo piriforme (Fig. 1B) siendo ésta la más frecuente, entre 7,9 % y 16% (Gabrielli et al, 1997;Güvençer et al;Moore & Dalley;Machado et al, 2003;Pokorny et al;Ogeng'o et al), disposición a la que corresponde el caso descrito en este estudio en el lado derecho. En la tipo III de Beaton & Anson, el nervio fibular común pasa superiormente al mús-culo piriforme y el nervio tibial desciende bajo el mismo (Fig.…”

Arterial Supply of Sciatic Nerve and Its Impact on Clinical Practice

“…The sciatic nerve enters the gluteal region posterior to obturator internus and the underlying hip joint capsule. As it leaves the pelvis, the relationship of the sciatic nerve to piriformis is variable, some configurations of which may lead to compression of the nerve, particularly during total hip arthroplasty (2). Higher division of the sciatic nerve is the most common variation where the TN and CPN may leave the pelvis through different routes.…”