What structures pass through both the greater and lesser sciatic foramina?

Anatomical Structures Passing Through Both Greater and Lesser Sciatic Foramina

The pudendal nerve and internal pudendal vessels (artery and vein) are the primary structures that pass through both the greater and lesser sciatic foramina.

Anatomical Pathway

The anatomical course of these structures follows a specific path:

1. Exit from Pelvis: These structures initially exit the pelvis through the greater sciatic foramen, typically below the piriformis muscle 1

2. Hook Around the Ischial Spine: After exiting the greater sciatic foramen, they course around the sacrospinous ligament and ischial spine 2

3. Re-entry to Perineum: They then enter the perineum by passing through the lesser sciatic foramen 1

Detailed Anatomy of the Structures

Pudendal Nerve

• Originates from sacral nerve roots S2-S4
• Exits the pelvis through the greater sciatic foramen, typically in zone A (medial to the ischial spine, entirely on sacrospinous ligament) or zone B (on the ischial spine) in 97.6% of individuals 2
• Hooks around the ischial spine and sacrospinous ligament
• Re-enters the perineum through the lesser sciatic foramen
• Provides sensory and motor innervation to the external genitalia and perineum

Internal Pudendal Vessels

• The internal pudendal artery (branch of the internal iliac artery) and accompanying vein
• Follow the same path as the pudendal nerve
• Exit through greater sciatic foramen
• Course around the ischial spine
• Enter the perineum through the lesser sciatic foramen
• Supply blood to the external genitalia and perineum

Clinical Significance

This anatomical arrangement has several important clinical implications:

1. Transgluteal Approach for Pelvic Procedures: The route through the greater sciatic foramen can be used for drainage of pelvic abscesses and fluid collections. Ideal catheter placement should traverse the lower portion of the greater sciatic foramen at the level of the sacrospinous ligament to avoid vascular and neural elements located more cephalad 3

2. Pudendal Nerve Entrapment: The pudendal nerve is vulnerable to compression as it passes between the two foramina, particularly at the level of the ischial spine, which can result in chronic perineal pain 2

3. Surgical Considerations: During transgluteal approaches to the pelvis, care must be taken to avoid injury to these structures. The transgluteal route through the greater sciatic foramen should be below the piriformis muscle to avoid the sciatic nerve and other important neurovascular structures 1

4. Tumor Spread: Tumors can spread between the pelvis and gluteal region via these foramina, potentially having both intrapelvic and extrapelvic components 4

Anatomical Variations

It's important to note that anatomical variations exist:

• The relationship between the pudendal nerve and ischial spine can vary, with the nerve passing either medial to the spine (entirely on sacrospinous ligament) or directly over the spine 2

• The sciatic nerve normally exits only through the greater sciatic foramen, but variations in its relationship to the piriformis muscle and superior gemellus muscle have been reported 5

Understanding these anatomical relationships is crucial for clinicians performing procedures in this region, particularly for interventional radiologists performing transgluteal drainage procedures and surgeons approaching tumors with components in both the pelvis and gluteal region.