Piriformis Muscle Essentials
The ability to recognize and comprehend the nature of piriformis syndrome requires understanding of the anatomical structure and physiological function of the piriformis muscle and its relationship to the sciatic nerve.
Piriformis Muscle and the Sacrum
The sacrum is a shield-shaped bony structure found between the hip bones and the lower area of the lumbar spine. It is a part of the spine in the gluteal region. The sacrum is connected to the pelvis bones to form the sacroiliac joint. There are two sacroiliac joints in the human body.
The piriformis muscle is described as a flat, triangular-shaped, and oblique muscle. It begins at the sacrum, between S2 and S4. This opening is considered as the main passageway for the piriformis muscle entering or leaving the pelvis. The piriformis muscle exits through the larger sciatic foramen, occupying most of the passage. After passing through the greater sciatic eminence, the piriformis connects to the upper middle part of the large and bumpy bone on the top side of the hip. The attachment of the piriformis muscle to the greater trochanter is secured by a round tendon. (Boyajian-O’Neill, McClain, Coleman and Thomas, 2008). To be visualize it more clearly, the piriformis muscle runs from the sacrum to the hip joint.
The piriformis muscle is supplied by spinal nerves S1 and S2 and at times, by the spinal nerve L5.
The piriformis muscle acts as an external rotator of the hip, a hip abductor, and hip flexor. During hip extension, the act of bringing the hips backward, the piriformis functions as the main external rotator. In this range of motion, the piriformis muscle assists the thigh in turning outward. During hip flexion, the act of moving the hips forward, the piriformis acts as a hip abductor. Hip abduction is the movement of the leg away from the midline of the body.
Overall, the actions of the piriformis muscle provide postural stability during standing and walking.
Piriformis Muscle and the Sciatic Nerve
The sciatic nerve, similar to most other nerves, performs two essential functions. First, it sends signals to the muscles from the brain, and second, it gathers all the sensory information in the lower extremities and sends it back to the brain. Any abnormal occurrence affecting the sciatic nerve will alter these normal functions. It can be manifested by pain or weakness.
The sciatic nerve is recognized as the largest, widest, and longest nerve in the human body. In an average adult, this nerve has a diameter of approximately 16 to 20 millimeters at its origin near the sacral plexus (Alian and Zhang, 2008). Its diameter is about the size of an adult thumb. It exits the pelvis through the greater sciatic foramen, the same exit point as that of the piriformis muscle. The sciatic nerve then descends between the femur’s greater trochanter and the ischial tuberosity, or the sitting bone. To put simply, the sciatic nerve runs from the lower spine to the buttock and hip area and down the back of each leg.
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Rick Kaselj, MS