- Obturator internus is a fan shaped muscle that passes through lesser sciatic notch. The muscle lies within the pelvis and the tendon lies posterior to the hip joint.
- Origin and insertion: The obturator internus arises from the medial surface of the obturator membrane, ischium, and the rim of pubis.
- The obturator internus muscle ends in several tendinous bands that are found in the deep surface of the muscle between the ischial tuberosity and ischial spine.
- After passing through the lesser sciatic notch, the bands of the muscle then unite into a single tendon and passes over the capsule of the hip joint.
- The obturator internus makes a right angled bend over the grooved surface of the ischium between the ischial spine and the tuberosity.
- It then passes horizontally across and posterior to the hip joint.
- The tendon inserts into the superior posterior part of the medial aspect of the greater trochanter of femur in conjunction with the superior and inferior gemellus tendons.
- A bursa lies between the tendon of the obturator internus and the grooved surface of the ischium.
- Ischial bursitis gives vague symptoms and may create myofascial irritability and when it becomes infected, it may be confused with infection of hip joint.
- Ischial bursitis may also be confused with piriformis syndrome. Part of the obturator internus is within the lesser pelvis and part of it is in the back of hip joint. This makes diagnosis of problems involving the muscle and tendon a challenging problem.
- Innervation: the nerve to obturator internus supplies the obturator internus and superior gemellus muscle (L5-S1 branch of the sacral plexus)
- Function: The obturator internus muscle rotates the thigh laterally and also helps to abduct the thigh when it is flexed.
- When doing the posterior approach to the hip, care should be given not to cause injury to the medial femoral circumflex artery.
- The deep branch of the MFCA crosses the tendon of obturator internus. As this branch travels superiorly ,it crosses anterior to conjoint tendon of gemellus inferior, obturator internus and gemellus superior .
- It then perforates the joint capsule at the level of gemellus superior and distal to the tendon of piriformis. This deep branch can be damaged during division of the short external rotators in posterior approaches to the hip, interfering with perfusion of the hip.
- Short external rotators can be resected at least 1cm away from their femoral attachment to avoid injury of the deep branch of the MFCA.
- Ultrasound guided injection into the obturator internus tendon or bursa can be done and may play a role in the diagnosis and treatment of patients with retrotrochanteric and gluteal pain.
- Note that sciatic nerve is anterior to piriformis muscle and posterior to the obturator internus muscle.
Courtesy : Prof Nabile Ebraheim, University of Toledo, Ohio, USA
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