Anatomy, Abdomen and Pelvis, Obturator Muscles

Structure and Function

The obturator externus and internus are bilateral-triangular shaped muscles originating in the pelvis and inserting onto the greater trochanter of the femur. The obturator muscles work synergistically with surrounding muscles to provide hip mobility. Their primary function is external rotation of the hip. In addition, the obturator externus contributes to adduction of the flexed hip, while the obturator internus plays a role in the abduction of the flexed hip.  

The obturator externus originates primarily from the bony margin of the obturator foramen, although it has been shown that some fibers originate from the obturator membrane.[3] The muscle inserts as a cylindrical tendon onto the greater trochanter of the femur. A cadaveric study highlighted that the obturator externus forms a musculotendinous junction at the level of the femoral neck and that the fibers at the inferior margin of the acetabulum act like a sling for the inferior portion of the femoral neck.[3] Additionally, when placing the hip in abduction and external rotation, it was noted that the obturator externus was brought closer to the caudal border of the acetabulum.[3]

Together, these findings suggest that the obturator externus may provide some reinforcement to the hip joint. The location of the obturator externus has also been shown to protect the medial circumflex femoral artery (MCFA). One study performed demonstrated that the obturator externus protected the deep branch of the MCFA from being damaged during hip dislocation in any direction.[4]

The obturator internus has two points of origin. It originates from both the posterior surface of the obturator membrane and the rami surrounding the obturator foramen and inserts onto the greater trochanter of the femur.[5] In addition to contributing to external rotation and abduction of the hip, a critical function of this muscle is the stabilization of the hip joint. The obturator internus shortens as the hip moves from flexion to extension, which indicates that it provides support to the hip joint during weight-bearing and propulsion activities.[6] 

It has been well established that the obturator internus is a “postural muscle” that holds the femoral head in the acetabulum.[7] Yoo et al. state that due to its role in stabilization, it is frequently monitored in gait analysis and muscle rehabilitation after an injury to the hip joint.

Embryology

The development of the musculoskeletal system begins during the fourth week of development with the formation of a limb bud.[8] Between weeks four through eight, bone and muscle further develop [8]. Skeletal muscle, in general, is derived from somites which are transient structures that come from the embryologic paraxial mesoderm.[8]

Each somite contains a myotome from which the muscles derive. Myotomes can be further divided into epaxial or hypaxial portions. Innervation is used to classify a muscle as either epaxial or hypaxial. Epaxial muscles receive innervation from the dorsal roots of the spinal nerves, and hypaxial muscles receive innervation from the ventral roots of the spinal nerves.[9] The obturator muscles both stem from the sacral hypaxial myotome.[10]

Unlike cardiac and smooth muscle, skeletal muscle is unique since it is under voluntary control by the organism. Myogenesis of skeletal muscle requires coordination with the lateral plate mesoderm, which gives rise to the neural, vascular, and connective tissue of the extremities.

Blood Supply and Lymphatics

The obturator externus muscle receives blood supply from branches of the obturator and the femoral artery. The anterior branch of the obturator artery travels along the pubic ramus and supplies the obturator externus. The MCFA is the branch off of the femoral artery and also provides vascularization to the muscle. These two branches, as well as the posterior branch of the obturator artery, anastomose with one another near the obturator externus muscle.[11] 

It should be noted that there is some variability in the supply of the anterior branch of the obturator and the MCFA; therefore, the obturator externus can receive vascularization from either one or both of these arteries.[12][13] 

The obturator internus muscle receives blood supply from branches of the obturator artery, the gluteal artery, and the pudendal artery. Branches of the obturator artery include the muscular branches which supply the iliacus and iliopsoas muscles, in addition to the obturator internus.[11] The muscular branches of the inferior gluteal artery also provide blood supply to the muscle.[14] Lastly, the gemellar branch and inferior rectal branch of the pudendal artery supply the lateral portion of the obturator internus.[15] 

One study by Fronhofer et al. found significant variation in the branching pattern of the inferior rectal branch to the obturator internus muscle. Their analysis concluded that 31% of cases had one branch to the obturator internus, 43% had two branches, 11% had three branches, and 15% lacked a branch from the inferior rectal artery altogether.[15]

Nerves

The posterior branch of the obturator nerve innervates the obturator externus muscle. The obturator nerve is a branch of the lumbar plexus and originates from the L2-L4 spinal nerves. Near the obturator foramen, the obturator nerve branches into posterior and anterior divisions.[16] The anterior and posterior divisions provide motor innervation to muscles in the medial compartment of the thigh, which makes the obturator nerve essential for the adduction of the thigh.[17] The anterior division eventually becomes a cutaneous nerve and provides sensory innervation to the upper medial portion of the thigh and parts of the knee and hip.[17]

The obturator internus muscle is innervated by the nerve to the obturator internus. This is a branch of the sacral plexus and originates from the L5-S2 spinal nerves. This nerve also innervates the superior gemellus muscle. However, some variation in the innervation of the superior gemellus muscle has been identified. It was shown that 60.4% of the time, the superior gemellus muscle received dual innervation from the nerve to the obturator internus and the nerve to the quadratus femoris.[18] Additional testing confirmed that the nerve to the obturator internus and the nerve to the quadratus femoris are two independent nerves since they take different courses and lack communication.[18]

Muscles

The obturator muscles are considered part of the short external rotators of the hip, along with the gemellus superior and inferior, piriformis, and quadratus femoris. These muscles are innervated by nerves of the sacral plexus (L5-S2), with the exception of the obturator externus, which receives innervation from the lumbar plexus. Similarly to the obturator muscles, the short external rotators of the hip originate from the pelvic region and insert onto the greater trochanter of the femur. The muscles in this group sit posteriorly to the pelvic bone. During contraction, the muscles shorten and work synergistically to rotate the femur laterally. 

The medial rotators of the hip act as antagonists to the short external rotators. The medial rotators include the gluteus medius and minimus, tensor fasciae latae, and the hip adductors. 

While the main function of the obturator externus is external rotation, it can provide a small amount of assistance with adduction of the flexed hip. With the exception of the obturator externus, the hip adductors originate from the pubis or ischium and insert onto the medial posterior part of the femur bone. The hip adductors include obturator externus, pectineus, gracilis, and adductor brevis, longus, magnus, and minimus. 

When the hip is flexed, the obturator internus muscle acts as an antagonist to the obturator externus since it plays a role in abduction. The primary hip abductors include the gluteus medius and minimus and the tensor fasciae latae.

Physiologic Variants

The literature has only reported a few physiological variations of the obturator muscles. One study by Pine et al. examined variations in the relationship between the piriformis and obturator internus to better inform piriformis preservation during posterior hip arthroplasty. The tendon crossing angles, location of insertion onto the greater trochanter and the fusion between tendons prior to insertion were analyzed. It was found that in 10% of cases, the obturator internus and piriformis tendons combined before insertion on the femur.[19]

Somewhat related to anatomical variations, it has also been noted that the architectural design of the obturator internus is particularly affected by aging. The literature demonstrates that age-related changes in the structure of the obturator internus led to a decrease in force-generating capability and an increase in muscle fibrosis.[20]

Surgical Considerations

There are three main approaches to total hip arthroplasty: the anterior approach, the anterolateral approach, and the posterior approach. During the posterior approach, the short external rotators, such as the obturator internus, often have to be dissected so that the surgeon can access the hip joint. Before dissection occurs, the sciatic nerve should be identified since it is in close proximity to the short external rotators. The piriformis muscle is used most reliably to locate the sciatic nerve since it typically exits near the inferior edge of the piriformis muscle belly.

After identification of the nerve, the short external rotators can be detached and reflected medially. Medial reflection protects the sciatic nerve from injury during the operation; however, if there are variations in the course of the sciatic nerve or variations in the musculotendinous confluence of the piriformis and obturator internus then a false sense of security can be created.

Once dissection is complete, the short external rotators are tagged with a suture and repaired at the end of the operation. The literature shows that by performing a soft tissue repair of the short external rotators during closure, postoperative posterior dislocation rates dropped from 4.5% to 0.5%.[21][22][23] However, it was also noted that the soft tissue repair increased the atrophy of the short external rotator muscles.[21][22]

Another surgical consideration related to the obturator externus is the damage to the nerve. Due to its location within the pelvis, the obturator nerve has the potential to become damaged during gynecological surgery, specifically lymphadenectomy for gynecological malignancies.[24] 

This could potentially weaken the obturator externus since this muscle is innervated by the posterior branch of the obturator nerve. The obturator internus would not be affected by this surgical complication since it is innervated by the nerve to the obturator internus.

Clinical Significance

Due to their close proximity, the obturator muscles are clinically relevant when there is an injury to the hip, such as dislocation. Posterior hip dislocations are the most common type of hip dislocation. They are typically caused when a posteriorly directed force is applied to a flexed knee while the hip is flexed and adducted. This drives the femoral head posteriorly and causes dislocation of the hip. During dislocation, gluteal muscles and short external rotator muscles, such as the obturator internus, can be injured. However, the most common complications from posterior hip dislocation are avascular necrosis and sciatic nerve injury. The deep branch of the MCFA is the primary blood supply of the femoral head. Damage to this vessel dramatically increases the likelihood of avascular necrosis. Studies have shown that the obturator externus plays a vital role in the protection of the deep branch of the MCFA during hip dislocation in any direction.[4] After posterior hip dislocation, hip stabilizer muscles are monitored in gait analysis and muscle rehabilitation.[7]

In addition to becoming damaged as a result of hip dislocations, the obturator internus can also become strained from sports that involve kicking, tackling, and falling.[2] These strains are extremely rare, but when they do occur, it is often seen in young male athletes.[2] 

One study by Byrne et al. examined two cases of obturator internus strains and found that this strain can happen both acutely or as a result of overuse. Additionally, obturator internus strains can occur in association with adductor longus strains or independently.[2]

The obturator externus bursa also has involvement in pathological conditions. A study performed by Robinson et al. analyzed eight cadaver hemipelvises and ten clinical patients with obturator externus bursa enlargement and associated intraarticular hip abnormality to better understand the anatomy of this bursa and its clinical significance.[25] They found that in all ten patients, the enlarged bursa displaced the obturator externus muscle inferiorly. Additionally, they noted that this bursa could act as a posteroinferior communication of the hip joint capsule and could be a site of disease spread.[25]

Lastly, deep gluteal syndrome is a term used to describe the compression of the sciatic or pudendal nerve by a combination of gemelli-obturator syndrome, piriformis syndrome, ischiofemoral impingement syndrome, and proximal hamstring syndrome.[26] Symptoms include pain in the hip, buttock, and posterior thigh due to nerve entrapment.[26] 

Gemelli-obturator syndrome occurs when the sciatic nerve becomes compressed between the gemellus muscles and obturator internus during passive hip rotation.[27] Treatment for this condition can include rest, non-steroidal anti-inflammatory drugs, gabapentin, muscle relaxants, and physical therapy.[26]