Anatomy, Bony Pelvis and Lower Limb, Piriformis Muscle

Article Author:
Carol Chang
Article Editor:
Steve Bhimji
Updated:
9/13/2018 12:47:56 AM
PubMed Link:
Anatomy, Bony Pelvis and Lower Limb, Piriformis Muscle

Introduction

The piriformis is a flat, oval-shaped muscle located in the gluteal region of the upper thigh. It is one of the six muscles in the lateral rotator group that lie parallel to the posterior margin of the gluteus medius. The piriformis muscle has origins from several anatomical locations including the anterior sacrum, the spinal region of the gluteal muscles, and the superior edge of the sciatic notch. The muscle passes through the greater sciatic foramen and terminates on the greater trochanter of the femur. The tendons of the piriformis muscle connect with tendons of the obturator internus and the inferior and superior gemellus before inserting on the femur. The muscle abuts the posterior wall of the pelvis and the posterior wall of the hip joint.

Piriformis syndrome is a neurological disorder that often presents with hip and buttock pain. The syndrome is often misdiagnosed and undertreated.

Structure and Function

The piriformis muscle is a lateral rotator of the hip along with the superior and inferior gemellus, quadratus femoris, and obturator internus and externus. The piriformis muscle rotates the femur during extension of the hip and abducts the femur during flexion of the hip. The abduction of the femur is critical during walking as it shifts the body weight to the opposite side, which prevents one from falling.The piriformis muscle also serves as a landmark in the gluteal region. As it passes through the greater sciatic foramen, it divides it into a superior and inferior segment. This anatomy also helps name the nerves and vessels of the region. The superior gluteal nerve and artery exit superior to the piriformis. The inferior gluteal nerve and artery exit inferiorly. The sciatic nerve also travels inferior to the piriformis.

Embryology

The limb buds appear at four weeks as tiny masses on the ventrolateral body wall. Reciprocal induction of ectoderm and mesoderm forms the limb buds. Each bud is initially composed of a mass of mesenchymal cells covered by ectoderm. As the limb bud grows, it receives innervation from the last four and first three sacral metameres. As the distal limb end flattens into foot plates, grooves appear in the limb. Rotation of the foot plate occurs so that it aligns with the fibula and tibia by week 9 to 12. At the same time, the muscles start to grow on the flexor and extensor surfaces of the limb.

Blood Supply and Lymphatics

The piriformis muscle receives its blood supply from the superior and inferior gluteal arteries. It also receives several small branches from the pudendal artery.

Nerves

In at least a fifth of the population, the piriformis is pierced in different parts by the sciatic nerve. However, in some cases, it may be penetrated by the common peroneal nerve. In a few people, the muscle may integrate with the gluteus medius and minimus muscles. The piriformis may also have one or two attachments to the sacrum or capsule of the hip joint.

Physiologic Variants

In about 20% of the population, the sciatic nerve or its branches innervate the piriformis muscle. There are many nerve variations, but in about 80% of people, the common peroneal nerve penetrates the muscle.

Sometimes the anatomy of the piriformis muscle may not be clearly defined. This lack of definition occurs because the muscle may have merged with the gluteus medius or gluteus minimus.

Surgical Considerations

Surgery for the treatment of piriformis syndrome should be the last choice. It should only be contemplated when all other therapies have failed. Surgery usually entails a reduction of any tension on the piriformis muscle or releasing adhesions on the sciatic nerve. The outcome after surgery depends on the chronicity of the condition, but results are not optimal. Patients continue to complain of pain after the procedure.

All patients with piriformis syndrome should enroll in a physical therapy program. The patient should undergo stretching and motion exercises to help relieve the adhesions on the nerve. The goal should be to eliminate symptoms and improve function.

Clinical Significance

Piriformis syndrome is said to occur when the muscle irritates the sciatic nerve, which enters the gluteal region just below the muscle.

The condition is often misdiagnosed and undertreated. Buttock pain is often confused with sciatica, sacroiliitis, lumbar radiculopathy, or trochanteric bursitis. The number of patients with piriformis syndrome has dramatically increased over time. The condition accounts for many cases of partial or total disability. When there is a delay in diagnosis, it leads to chronic pain, hyperesthesia, paresthesias, and muscle weakness.

Piriformis syndrome is associated with pain in the buttocks with sporadic, referred pain along the distribution of the sciatic nerve. Individuals in whom the sciatic nerve pierces the piriformis muscle are prone to sciatica. Sciatica will present with tingling, numbness, or pain deep in the buttock area and along the sciatic nerve. Prolonged sitting, climbing stairs, stretching, and performing squatting can also worsen the pain. The patient's history and clinical exam make the diagnosis of piriformis syndrome. MRI and nerve conduction studies are done to exclude other pathologies. Once diagnosed, piriformis syndrome is treated with physical therapy and stretching exercises. Rarely, corticosteroids or botulinum toxin may be injected into the piriformis muscle. Surgery for nerve decompression is the last resort for the treatment of piriformis syndrome.

Other Issues

Pharmacological therapy is often the first therapy in patients with piriformis syndrome. Both NSAIDs and prescription analgesics are used to treat the condition with varying results. Overall, some studies indicate that NSAIDs are preferred to opiates and tend to work well. 

Muscle relaxants have also been used to manage patients with piriformis syndrome. While these agents do work, they are also associated with many serious adverse effects like drowsiness, dry mouth, and dizziness.

Osteopathic manipulation is often recommended for patients with piriformis syndrome. Both direct and indirect osteopathic manipulative treatments are used to facilitate a normal range of motion and relieve pain. 

Steroid injections have also been used to treat piriformis syndrome. The steroids are thought to act by decreasing the inflammation around the nerve. However, evidence to support the use of steroid injections in chronic cases is lacking. A common complication of steroid injections in many studies has been an infection.