The nerve to obturator internus is a branch of the sacral plexus, specifically originating from the ventral rami of L5, S1, and S2 spinal nerves. It provides motor innervation to the obturator internus muscle and the superior gemellus muscle.
Location
The nerve to obturator internus is located in the pelvic region. It exits the pelvis through the greater sciatic foramen, curves around the ischial spine, and then re-enters the pelvis through the lesser sciatic foramen to reach the obturator internus muscle. It travels alongside other nerves and vessels in this region, such as the pudendal nerve and internal pudendal vessels.
Structure and Anatomy
Origin
The nerve to obturator internus arises from the ventral rami of the L5, S1, and S2 spinal nerves. It is a branch of the sacral plexus, which is formed by the convergence of nerves from the lower lumbar and sacral regions of the spine.
Course
After its origin from the sacral plexus, the nerve to obturator internus follows a distinct path through the pelvis and surrounding structures.
- Pelvic Exit: The nerve initially exits the pelvic cavity through the greater sciatic foramen, which is a large opening formed by the greater sciatic notch of the ilium, the sacrum, and the sacrotuberous ligament.
- Ischial Spine: Upon exiting the pelvis, the nerve runs inferiorly and posteriorly around the ischial spine, which serves as a bony landmark for the nerve’s path. It is here that the nerve to obturator internus briefly appears outside the pelvic cavity.
- Re-entry into the Pelvis: After curving around the ischial spine, the nerve re-enters the pelvic cavity by passing through the lesser sciatic foramen, a smaller opening located below the ischial spine. This movement allows the nerve to reach the obturator internus muscle, which lies deep within the pelvic cavity.
Relations
The nerve to obturator internus runs alongside important anatomical structures within the pelvis:
- Pudendal nerve: This nerve follows a similar course, exiting through the greater sciatic foramen and re-entering through the lesser sciatic foramen. The nerve to obturator internus and the pudendal nerve are in close proximity, particularly around the ischial spine.
- Internal pudendal vessels: These vessels accompany the pudendal nerve and the nerve to obturator internus in their path through the greater and lesser sciatic foramina.
Distribution
The nerve to obturator internus innervates the obturator internus muscle as it enters its deep surface. In some cases, it may also provide a branch to the superior gemellus muscle before reaching the obturator internus.
Anatomical Variations
While the typical course of the nerve to obturator internus follows the path described, there may be anatomical variations. These variations include:
- Variation in origin: The nerve may receive contributions from different spinal levels or have variations in the exact root contributions.
- Branches: Occasionally, the nerve may give off additional branches to surrounding muscles or have variations in its branching pattern before reaching the obturator internus muscle.
Vascular Supply
The vascular supply to the nerve to obturator internus comes from nearby vessels, particularly branches of the internal pudendal artery and associated veins. These vessels travel with the nerve, ensuring that the nerve has an adequate blood supply throughout its course.
Function
Motor Innervation
The primary function of the nerve to obturator internus is to provide motor innervation to the obturator internus muscle and the superior gemellus muscle. These muscles play critical roles in the movement of the hip joint.
Obturator Internus Muscle
The obturator internus muscle is a deep muscle located within the pelvis. It serves to laterally rotate the thigh at the hip joint and helps stabilize the head of the femur within the acetabulum during movements like walking or running.
- Lateral Rotation of the Thigh: The nerve to obturator internus activates the muscle to perform external (lateral) rotation of the thigh. This is crucial during actions that involve pivoting or rotating the leg outward, such as in certain athletic movements or while turning the leg.
- Hip Stabilization: Alongside the lateral rotator group, the obturator internus contributes to stabilizing the hip joint, ensuring that the femoral head remains properly seated within the acetabulum, especially during dynamic movements like walking, running, or climbing stairs.
Superior Gemellus Muscle
Although primarily associated with the obturator internus, the nerve to obturator internus may also innervate the superior gemellus muscle. This muscle is part of the deep gluteal group and assists the obturator internus in lateral rotation of the hip. Together, these muscles help stabilize the hip and control the leg’s external rotation.
Role in Gait and Posture
By innervating muscles involved in hip rotation and stabilization, the nerve to obturator internus contributes significantly to:
- Normal gait mechanics: Proper lateral rotation of the thigh is essential for walking and maintaining balance.
- Posture control: It helps stabilize the pelvis and lower limb during standing and dynamic movements.
Clinical Significance
The nerve to obturator internus is clinically significant because damage to this nerve can lead to issues with hip stability and mobility. Injury to the nerve may result from pelvic fractures, surgical procedures in the pelvic region, or compression due to adjacent structures like tumors or hematomas.
Symptoms of Injury
- Weakness in hip lateral rotation: Patients may have difficulty rotating the leg outward, affecting movements like walking or turning.
- Hip instability: Damage to the nerve can compromise hip joint stability, increasing the risk of dislocations or altered gait patterns.
Surgical Considerations
During procedures such as hip surgeries or pelvic reconstructive surgeries, care must be taken to avoid damaging the nerve to obturator internus, as this could impair hip function and post-operative recovery.
Pelvic and Hip Pain Syndromes
In cases of chronic pelvic pain or deep gluteal syndrome, the nerve may become involved, either through entrapment or inflammation, contributing to symptoms such as referred pain in the buttock, hip, or groin region.
