Infra-Piriformis Structures of the Gluteal Region (Superficial to Deep)
The infra-piriformis foramen contains several neurovascular structures that exit the pelvis below the piriformis muscle, arranged from lateral (outward) to medial (inward):
Standard Anatomical Arrangement
The typical sequence from lateral to medial through the infra-piriformis foramen is:
- Sciatic nerve (most lateral/superficial)
- Inferior gluteal nerve
- Posterior femoral cutaneous nerve
- Inferior gluteal artery and vein
- Internal pudendal artery and vein
- Pudendal nerve (most medial/deep)
- Nerve to obturator internus (most medial)
Important Anatomical Context
The infra-piriformis foramen represents the space below the piriformis muscle through which these structures exit the greater sciatic foramen. The width of this foramen varies considerably (up to 2.0 cm in diameter), with female specimens demonstrating statistically greater width compared to males 1.
Key Fascial Relationships
The inferior gluteal vessels at the level of the sacrospinous ligament transition from the parietal layer of the pelvic fascia into the duplicature of the deeper layer of the gluteus maximus muscle. The inferior gluteal nerve, above the lower margin of the piriformis muscle, adjoins the vessels and enters through a fissure in the parietal layer of the pelvic fascia 2.
Critical Anatomical Variations
Anatomical variations occur in approximately 19.7% of specimens and must be considered during any gluteal region procedures 1:
- Type B variation: Common peroneal division passes through the piriformis muscle while tibial division exits below
- Type C variation: Entire sciatic nerve may pierce the piriformis muscle
- Type D variation: Complete division of sciatic nerve with components taking different routes
- Concurrent neural and vascular variations can occur simultaneously, including branches of the inferior gluteal nerve passing through the piriformis 3
Atypical Pudendal Structures
In rare cases, the internal pudendal vessels and pudendal nerve may pass over the sacrotuberous ligament rather than through the infra-piriformis foramen 4.
Clinical Significance
The distance between the sciatic nerve and the greater sciatic notch is statistically larger in Type A (normal) anatomy compared to Type B variants 1, which has direct implications for:
- Intramuscular injection safety
- Surgical approach planning in the gluteal region
- Understanding piriformis syndrome pathophysiology 5, 6
- Deep gluteal syndrome evaluation 7
The knowledge of these fascial canals and their connections between the subperitoneal level of the small pelvis and the gluteal region is essential for understanding potential pathways of infection spread and nerve compression syndromes 2.