Piriformis

The piriformis is a flat, triangular muscle extending from the anterior surface of the sacrum to the proximal femur. It begins within the pelvis, passes laterally through the greater sciatic foramen, and enters the deep gluteal region before inserting on the greater trochanter of the femur.^[1][2]

Piriformis belongs to the group of short lateral rotators of the hip. It laterally rotates the extended thigh, assists abduction when the hip is flexed, and contributes to stabilization of the femoral head within the acetabulum.

The muscle is especially important as an anatomical landmark. It divides the greater sciatic foramen into spaces above and below the muscle. Major nerves and vessels entering the gluteal region are described according to whether they pass superior or inferior to piriformis.

Location

Piriformis occupies both the pelvic and gluteal regions. Its medial portion lies within the pelvis against the anterior surface of the sacrum, while its lateral portion lies deep within the buttock.

The muscle passes through the greater sciatic foramen and lies deep to gluteus maximus. In the gluteal region, it is positioned superior to the superior gemellus and obturator internus-gemelli complex.

Its course from the sacrum toward the greater trochanter places it posterior to the hip joint and makes it an important reference structure for the nerves and vessels emerging from the pelvis.

General Anatomy

Feature	Anatomy
Muscle	Piriformis
Location	Pelvis and deep gluteal region
Origin	Anterior surface of sacrum and adjacent structures
Insertion	Superior border and adjacent medial aspect of greater trochanter
Innervation	Nerve to piriformis
Root values	S1, S2
Principal actions	Lateral rotation of extended thigh, abduction of flexed thigh and stabilization of hip
Major landmark	Divides greater sciatic foramen into suprapiriform and infrapiriform spaces

Origin

Piriformis originates primarily from the anterior surface of the sacrum, typically from the region between the anterior sacral foramina.^[1][3]

Additional fibers may arise from the margin of the greater sciatic notch and associated ligamentous structures. The exact extent of these accessory attachments can vary between individuals.

The fibers converge laterally as the muscle approaches the greater sciatic foramen.

Insertion

The piriformis fibers converge into a tendon that inserts on the greater trochanter of the femur, typically along its superior border and adjacent medial surface.

The insertion lies close to those of other short lateral rotators but remains distinct from the obturator internus-gemelli complex and obturator externus.

Its attachment to the greater trochanter allows the muscle to influence rotation and abduction of the femur according to the position of the hip joint.

Course

From its sacral origin, piriformis passes laterally and anteriorly toward the greater sciatic foramen. The muscle then exits the pelvis through the foramen and enters the gluteal region.

After emerging from the pelvis, the muscle continues laterally toward the greater trochanter. Its tendon crosses posterior to the hip joint before reaching its femoral insertion.

The piriformis occupies much of the greater sciatic foramen but does not completely fill it. The remaining spaces above and below the muscle transmit major neurovascular structures.

Greater Sciatic Foramen

The greater sciatic foramen is formed when the greater sciatic notch is converted into a foramen by the sacrospinous and sacrotuberous ligaments.

Piriformis passes through this opening and divides it into two functionally important spaces:

• Suprapiriform space, above piriformis
• Infrapiriform space, below piriformis

This division provides one of the most important organizational landmarks in the anatomy of the gluteal region.

Structures Superior to Piriformis

The superior gluteal nerve and superior gluteal vessels normally pass through the greater sciatic foramen superior to piriformis.

After entering the gluteal region, the superior gluteal nerve travels between gluteus medius and gluteus minimus and supplies these muscles as well as tensor fasciae latae.

The superior gluteal vessels accompany the nerve and contribute substantially to the vascular supply of the superior gluteal region.

Structures Inferior to Piriformis

Most major structures leaving the pelvis through the greater sciatic foramen pass inferior to piriformis.

• Sciatic nerve
• Inferior gluteal nerve
• Inferior gluteal vessels
• Posterior femoral cutaneous nerve
• Pudendal nerve
• Internal pudendal vessels
• Nerve to obturator internus
• Nerve to quadratus femoris

The relationship of these structures to piriformis is fundamental to understanding the neurovascular anatomy of the gluteal region.

Relations

Anterior Relations

Within the pelvis, the anterior surface of piriformis is related to structures of the sacral plexus and pelvic fascia. The sacral plexus forms on the anterior surface of the muscle.

In the gluteal region, the anterior or deep surface of piriformis is related to the posterior aspect of the hip joint and neighboring deep structures.

Posterior Relations

In the gluteal region, piriformis lies deep to gluteus maximus. Gluteus maximus therefore covers the muscle in the intact buttock.

Reflection of gluteus maximus exposes piriformis and allows it to be used as a landmark for identifying the major nerves and vessels emerging from the greater sciatic foramen.

Superior Relations

Gluteus medius lies superior and superficial to portions of piriformis. The superior gluteal nerve and vessels emerge above the muscle and travel into the plane between gluteus medius and gluteus minimus.

Inferior Relations

Immediately inferior to piriformis lies the superior gemellus, followed by the obturator internus tendon, inferior gemellus, and quadratus femoris.

The sciatic nerve and several other neurovascular structures descend across the superficial aspect of these deeper muscles after emerging inferior to piriformis.

Relationship to the Sacral Plexus

The sacral plexus lies on the anterior surface of piriformis within the pelvis. It is formed primarily by the lumbosacral trunk and anterior rami of sacral spinal nerves.

Branches of the plexus converge toward the greater sciatic foramen and leave the pelvis in close relationship to piriformis.

The muscle therefore provides an important muscular landmark for understanding the organization and exit pathways of the sacral plexus.

Relationship to the Sciatic Nerve

The sciatic nerve is the largest nerve in the body and has an especially important relationship with piriformis. In the usual arrangement, the nerve leaves the pelvis through the greater sciatic foramen inferior to piriformis.

It then descends deep to gluteus maximus and superficial to the short lateral rotators before entering the posterior thigh.

The sciatic nerve is formed by tibial and common fibular components. Variations in the relationship of these components to piriformis are common enough to be clinically important.

Sciatic Nerve Variations

The relationship between piriformis and the sciatic nerve varies among individuals. In the most common arrangement, the undivided sciatic nerve passes below the muscle.

In some individuals, the sciatic nerve divides into tibial and common fibular components at a higher level. One component may pass through the piriformis while the other passes inferior to it, or other less common arrangements may occur.

These variations are important during surgery, imaging, intramuscular injections, and assessment of possible sciatic nerve entrapment.

Relationship to the Superior Gluteal Nerve

The superior gluteal nerve is distinguished by its course superior to piriformis. It leaves the pelvis with the superior gluteal vessels through the suprapiriform portion of the greater sciatic foramen.

It supplies gluteus medius, gluteus minimus, and tensor fasciae latae. Piriformis therefore provides the key landmark for identifying its exit from the pelvis.

Relationship to the Inferior Gluteal Nerve

The inferior gluteal nerve passes inferior to piriformis and supplies gluteus maximus.

After emerging from the pelvis, the nerve divides into branches that enter the deep surface of gluteus maximus.

The contrasting courses of the superior and inferior gluteal nerves relative to piriformis reflect their names and provide a useful anatomical distinction.

Relationship to the Pudendal Nerve

The pudendal nerve leaves the pelvis through the greater sciatic foramen inferior to piriformis. Unlike the sciatic nerve, it does not continue down the posterior thigh.

Instead, it curves around the region of the ischial spine and sacrospinous ligament and enters the perineum through the lesser sciatic foramen.

Piriformis therefore marks the initial exit pathway of the pudendal nerve before the nerve changes direction toward the perineum.

Relationship to the Internal Pudendal Vessels

The internal pudendal artery and vein follow a pathway similar to the pudendal nerve. They exit the pelvis inferior to piriformis through the greater sciatic foramen and then enter the perineum through the lesser sciatic foramen.

This pathway emphasizes the importance of piriformis as an orientation landmark for structures passing between the pelvis, gluteal region, and perineum.

Relationship to the Posterior Femoral Cutaneous Nerve

The posterior femoral cutaneous nerve normally exits the pelvis inferior to piriformis and descends beneath gluteus maximus.

It continues into the posterior thigh and provides sensory innervation to extensive areas of the posterior thigh and adjacent regions.

Its close proximity to the sciatic nerve beneath piriformis is important when identifying structures in the deep gluteal region.

Relationship to the Hip Joint

Piriformis passes posterior to the hip joint before inserting on the greater trochanter. Its close relationship to the joint allows it to produce rotation while also contributing to dynamic stabilization.

The muscle can generate a compressive component that helps maintain the femoral head within the acetabulum during movement.

Its mechanical action changes as the hip moves between extension and flexion because its line of pull shifts relative to the axes of the joint.

Relationship to the Gemelli and Obturator Internus

The superior gemellus, obturator internus, and inferior gemellus lie immediately inferior to piriformis in the deep gluteal region.

These muscles share several functional characteristics with piriformis. They laterally rotate the extended thigh, can assist abduction when the hip is flexed, and contribute to stabilization of the hip.

Their layered arrangement is particularly useful during anatomical dissection and posterior surgical approaches to the hip.

Relationship to Gluteus Maximus

Piriformis is largely covered by gluteus maximus. The much larger gluteus maximus forms the superficial muscular mass of the buttock, while piriformis lies deep to it.

Both muscles can contribute to lateral rotation of the hip, although gluteus maximus is also a powerful hip extensor.

During posterior dissection, gluteus maximus must be reflected to expose piriformis and the major neurovascular structures of the deep gluteal region.

Innervation

Piriformis is supplied by the nerve to piriformis, which arises directly from the sacral plexus and typically contains fibers from S1 and S2.

The nerve enters the pelvic surface of the muscle. Unlike many of the larger nerves of the sacral plexus, it does not travel distally into the lower limb.

The nerve to piriformis should be distinguished from the superior and inferior gluteal nerves and from the nerves to obturator internus and quadratus femoris.

Blood Supply

Piriformis receives arterial blood from branches of vessels supplying the pelvic and gluteal regions. Important contributions arise from the superior gluteal artery and inferior gluteal artery, with additional contributions possible from neighboring sacral vessels.

The superior and inferior gluteal arteries arise from the internal iliac arterial system and leave the pelvis through the greater sciatic foramen on opposite sides of piriformis.

This relationship reinforces the role of piriformis as both a muscular structure and a landmark separating major vascular pathways.

Actions

Lateral Rotation of the Extended Thigh

When the hip is extended, piriformis acts as a lateral rotator of the thigh.

Its insertion on the greater trochanter and its line of pull posterior to the hip's longitudinal axis allow contraction to turn the anterior surface of the femur laterally.

The muscle acts with gluteus maximus and the other short lateral rotators during this movement.

Abduction of the Flexed Thigh

When the hip is flexed, the orientation of piriformis relative to the femur changes. In this position, the muscle can assist abduction of the thigh.

This action is particularly relevant when the flexed femur must be moved away from the midline or stabilized during complex hip movements.

Hip Stabilization

Piriformis contributes to stabilization of the femoral head within the acetabulum. Its relatively short course and close proximity to the hip joint make it suited to controlled stabilization.

This function occurs together with the other short rotators and the passive restraints formed by the joint capsule, acetabular labrum, and hip ligaments.

Effect of Hip Position

The action of piriformis is strongly influenced by the position of the hip. In extension, its line of pull favors lateral rotation. With increasing hip flexion, the orientation of the muscle relative to the joint changes.

As a result, piriformis can become an effective abductor of the flexed thigh. Its rotational contribution may also change at greater degrees of hip flexion.

This position-dependent behavior illustrates why the functions of the deep hip muscles are more complex than a simple classification as external rotators.

Role During Walking

During gait, piriformis contributes to control of femoral rotation and stabilization of the hip as body weight is transferred through the lower limb.

The muscle acts together with other deep rotators and larger gluteal muscles to regulate the orientation of the femur relative to the pelvis.

Its role varies throughout the gait cycle as the hip moves through flexion, extension, and rotational adjustments.

Comparison with Other Short Lateral Rotators

Muscle	Origin	Innervation	Major Actions
Piriformis	Anterior sacrum	Nerve to piriformis	Lateral rotation, abduction of flexed thigh, stabilization
Superior gemellus	Ischial spine	Nerve to obturator internus	Lateral rotation, abduction of flexed thigh, stabilization
Obturator internus	Internal obturator membrane and surrounding bone	Nerve to obturator internus	Lateral rotation, abduction of flexed thigh, stabilization
Inferior gemellus	Ischial tuberosity	Nerve to quadratus femoris	Lateral rotation, abduction of flexed thigh, stabilization
Quadratus femoris	Ischial tuberosity	Nerve to quadratus femoris	Lateral rotation, adduction, stabilization
Obturator externus	External obturator membrane and surrounding bone	Obturator nerve	Lateral rotation and stabilization

Anatomical Variations

Piriformis shows clinically important anatomical variation, particularly in its relationship to the sciatic nerve. The muscle itself may also vary in size, shape, and number of muscular or tendinous divisions.

Accessory slips may occur, and portions of the sciatic nerve may pass through, above, or around different parts of the muscle.

Recognition of these variations is important when interpreting imaging and planning procedures in the deep gluteal region.

Clinical Significance

Piriformis Syndrome

Piriformis syndrome is a clinical term used for symptoms attributed to irritation or entrapment of the sciatic nerve in relation to piriformis. Symptoms may include buttock pain and pain or sensory symptoms extending along the sciatic distribution.

The diagnosis can be challenging because similar symptoms may arise from lumbar radiculopathy, other forms of deep gluteal nerve entrapment, hip disorders, and several musculoskeletal conditions.

The anatomical basis for the proposed syndrome is the close relationship between piriformis and the sciatic nerve, together with the considerable variation in how the nerve passes relative to the muscle.

Deep Gluteal Syndrome

Deep gluteal syndrome is a broader term describing non-discogenic sciatic nerve entrapment within the deep gluteal space. Piriformis is one potential structure involved, but it is not the only possible site or cause of compression.

The sciatic nerve passes through a region containing piriformis, the obturator internus-gemelli complex, quadratus femoris, fibrous bands, vessels, and other structures.

This broader anatomical framework is useful when sciatic symptoms originate in the gluteal region rather than from the lumbar spine.

Sciatic Nerve Entrapment

The sciatic nerve's normal passage inferior to piriformis places it close to the inferior border of the muscle. Anatomical variations can create even more intimate relationships.

Muscular hypertrophy, trauma, scarring, fibrous bands, or other abnormalities in the deep gluteal region may potentially alter the available space around the nerve.

Because several structures can produce similar symptoms, anatomical localization is important when evaluating suspected entrapment.

Intramuscular Injections

The relationship between piriformis and the sciatic nerve contributes to the anatomical rationale for avoiding inappropriate injection sites in the buttock.

Traditional gluteal injection landmarks are designed to reduce the risk of injury to the sciatic nerve and major gluteal neurovascular structures.

Understanding the position of piriformis helps explain why the sciatic nerve occupies a relatively deep but vulnerable course through the gluteal region.

Posterior Hip Surgery

Piriformis is an important landmark during posterior approaches to the hip. After gluteus maximus is divided or reflected, piriformis and the other short lateral rotators can be identified.

The sciatic nerve lies nearby and requires protection. Identification of piriformis helps orient the surgeon to the expected location of the nerve and other structures emerging through the greater sciatic foramen.

Muscle Injury

Piriformis may be affected by strain, direct trauma, or other injuries involving the deep gluteal musculature. Symptoms can include deep buttock discomfort and pain associated with particular hip movements.

Because the muscle is not superficially located and symptoms overlap with many neighboring structures, clinical assessment often requires consideration of the entire hip, lumbar spine, and deep gluteal region.

Imaging

Piriformis can be readily identified on CT and MRI. On axial imaging, the muscle can be followed from the anterior sacrum through the greater sciatic foramen into the gluteal region.

MRI can demonstrate muscular asymmetry, edema, atrophy, masses, and relationships between piriformis and the sciatic nerve.

Cross-sectional imaging is also useful for identifying anatomical variants in which components of the sciatic nerve pass through or around the muscle.

Surface Anatomy

Piriformis lies deep within the buttock and cannot usually be isolated by direct surface palpation. Its approximate course can be projected from the anterior sacral region through the greater sciatic foramen toward the greater trochanter.

Externally, the muscle lies deep to gluteus maximus and superior to the obturator internus-gemelli complex.

Its greatest anatomical importance comes from its position within the greater sciatic foramen. By separating structures that pass above it from the much larger group passing below it, piriformis serves as one of the principal landmarks for understanding the organization of the deep gluteal region.