The obturator internus muscle plays an important yet often overlooked role in the development of a range of musculoskeletal disorders, including pelvic floor and sciatic pain. This article provides a brief overview of obturator internus anatomy and discusses the important role the obturator internus muscle plays in the development of sciatica. While sciatica can be due to a wide range of conditions, such as herniated discs, spinal stenosis, piriformis syndrome, and/or trauma, contracture of the obturator internus muscle is rarely considered as a differential diagnosis.
The obturator internus muscle is unique among the body’s 6 hip external rotators as its entire muscle belly is located within the pelvic bowl, forming the lateral portion of the pelvic floor. When tight, this muscle is a common source of pain, primarily because
its tendon angles more than 90° forward after passing beneath the ischial spine before inserting on the greater trochanter. This abrupt angulation creates a considerable compressive force where the tendon crosses the ischium, and it is common for a protective boomerang-shaped bursa to form at this point. Unfortunately, even though the bursa reduces friction at this location, the chronically inflamed bursa can become a source of long-lasting pain, which often resolves with aggressive obturator internus stretching. According to Mumma, with or without bursitis, a tight obturator internus is a frequent cause of pelvic floor pain. Researchers from the University of Washington recently demonstrated that 45% of patients with pelvic floor pain report significant discomfort when the obturator internus is manually palpated.
Along with the other external rotators, obturator internus functions to stabilize the femoral head in a manner similar to how the rotator cuff muscles stabilize the humeral head. In addition to producing hip external rotation, the obturator internus muscle can also produce hip abduction, particularly when the hip is flexed 90°. Because it is such an important stabilizer of the femoral head, a common mechanism for injury occurs when there is a rapid change in direction on the weight- bearing leg and/or while losing balance while kicking a ball. These 2 mechanisms explain why this injury is so common in soccer players. As with most muscle injuries, obturator internus is more likely to be hurt if there is underlying weakness. Once injured, the muscle stiffens, greatly increasing the compressive force of the obturator internus tendon against the ischium. Performing stretches and exercises that specifically target the obturator internus is essential to avoid chronic injury.
Putting aside the potential for developing ischial bursitis and/or pelvic floor pain, a tight obturator internus is highly likely to cause recalcitrant sciatic pain. More than 20 years ago, Meknas et al. performed Lasegue tests during exploratory operations on patients as they were being treated for sciatica that was believed to be due to piriformis tightness.
To their surprise, it was not the piriformis muscle that was compressing the sciatic nerve; rather, the sciatic nerve was being tractioned as it ran over the obturator internus muscle. The authors described 6 surgical cases in which tension on the sciatic nerve was relieved by sectioning the obturator internus. More recently, Balius et al. performed a detailed study on 6 fresh cadavers and 31 healthy volunteers to determine the exact mechanism in which obturator internus can cause sciatica. They performed meticulous dissections on the cadavers and then used ultrasonography to evaluate movement of the sciatic nerve relative to the obturator internus as the hips were internally and externally rotated. The cadaveric dissections were especially interesting as they discovered a connective tissue anchor between the sciatic nerve and the obturator internus tendon. They theorized that this connective tissue anchor stabilizes the sciatic nerve against excessive back-and-forth movements associated with upper and/or lower body movement. The extent of this fibrous anchorage varied from individual to individual but was present in all eight of the cadaveric specimens.
Although the fibrous bridge between the obturator internus tendon and the sciatic nerve helps to stabilize the sciatic nerve in the hip, it can also be problematic when the hip internally rotates excessively and/or when obturator internus is tight. Using ultrasonography to evaluate movement of the sciatic nerve, the authors found that during passive internal hip rotation in both cadavers and healthy subjects, the tendon of the obturator internus is pulled down and forward, displacing the corresponding section of the sciatic nerve. When the obturator internus muscle is relaxed with external hip rotation, displacement of the sciatic nerve is reduced, allowing the sciatic nerve to assume its naturally straightened position.
This finding explains the connection between obturator internus tightness and sciatica: when the obturator internus muscle is supple and the hip is internally rotated, the muscle itself absorbs some displacement that would otherwise cause the tendon to shift with hip internal rotation. The tighter the muscle belly, the more the obturator internus tendon will pull on the sciatic nerve, potentially leading to chronic sciatic pain.
In many situations, the obturator internus tendon is bound to the sciatic nerve by a small connective tissue anchor that is capable of significantly displacing the sciatic nerve. Sciatica secondar y to obturator internus contracture can be diagnosed with a modified thigh thrust test, in which the hip is flexed 90°, horizontally adducted, and slightly internally rotated prior to producing long axis compression of the femur. When present, obturator internus related sciatica responds well to specific muscle energy mobilizations, stretches, and strengthening exercises.