What is Piriformis?
The piriformis is a muscle located deep in the buttock region that serves as an important anatomical landmark and functions primarily as a lateral rotator of the hip, though its action varies depending on hip position 1.
Anatomical Location and Structure
The piriformis is a biarticular muscle that forms a bridge anterior and inferior to the sacroiliac joint and posterior and superior to the hip joint 2. It originates from the anterior surface of the sacrum and inserts on the crest of the greater trochanter (not the trochanteric fossa as commonly misrepresented in surgical literature) 1. The tendon attachment encompasses the summit and medial aspect of the greater trochanter and has variable attachment to the hip joint capsule 1.
Functional Biomechanics
The piriformis functions differently based on hip position:
- Primary action: Lateral (external) rotation of the hip
- Secondary role: Hip extensor and abductor
- Position-dependent function: When the hip is flexed beyond 90°, the piriformis paradoxically functions as a medial rotator rather than lateral rotator 2
- At 90° hip flexion: The muscle lies directly posterior to the hip joint and may contribute to joint stability 1
Innervation
Contrary to traditional teaching of a single "nerve to piriformis," the muscle receives innervation from multiple sources in 80% of cases 3:
- Superior gluteal nerve (70% of specimens)
- S1 ventral ramus (85%)
- S2 ventral ramus (70%)
- Less commonly: inferior gluteal nerve or L5 ventral ramus
The term "nerve to piriformis" should be abandoned as it does not accurately reflect the anatomical reality 3.
Clinical Significance
The piriformis is clinically relevant in several contexts:
Piriformis Syndrome: A controversial condition involving compression of the sciatic nerve by the piriformis muscle, accounting for an estimated 5-8% of low back pain cases and up to 17% of chronic low back pain 4, 5. The sciatic nerve may pierce the piriformis muscle in approximately 16% of healthy individuals, though this anatomic variant does not correlate with syndrome development 4.
Surgical Landmark: The muscle serves as a critical landmark during posterior approach total hip replacement 6, 1. However, routine hip replacement can be performed without sacrificing the piriformis tendon, as its attachment site is superior to the entry point for femoral canal broaching 1.
Common Pitfalls
- Misidentification of attachment site: The "piriform fossa" used by surgeons typically refers to the trochanteric fossa and has no relation to the actual piriformis tendon attachment 1
- Overdiagnosis: Piriformis syndrome is frequently over-diagnosed when other causes of sciatica should be considered 7
- Anatomical misconceptions: The muscle's function as a medial rotator in hip flexion is often overlooked in clinical examination 2