Piriformis muscle with three bellies and a sacrotuberous-ligament belly: cadaveric case report and belly-count classification proposal

The piriformis muscle (PM) contributes to hip stabilisation and external rotation. Given its proximity to the sciatic nerve, PM variability is clinically relevant within deep gluteal syndrome, which encompasses non-discogenic entrapments in the subgluteal space. Although numerous variants have been described, the number of discrete bellies is inconsistently reported across studies. During routine dissection of a 79-year-old female cadaver, we identified a three-bellied PM. The superior belly arose from the gluteal surface of the ilium near the greater sciatic notch, the middle belly from the anterior sacrum (S2–S4), and the inferior belly from the sacrotuberous ligament. All three bellies converged into a single tendon inserting on the superior surface of the greater trochanter. Morphometry demonstrated distinct dimensions for each belly and for the common tendon. The contralateral side displayed a typical single-bellied PM. This report documents a three-bellied configuration of the PM. To standardise concise reporting, we propose a belly-count scheme with four categories: Type 0 absent, Type 1 single belly, Type 2 double belly, and Type 3 multiple bellies (≥ 3). Both the present case and the triple-bellied configuration reported by Koziol et al. fall under Type 3. This scheme is reporting-oriented and preliminary and does not imply prevalence or clinical risk without confirmation in larger series.