Piriformis Anatomy

Origin

• Anterior Sacrum: From the front of the sacrum, involving segments S2, S3, and S4 (between the anterior sacral foramina).
• Gluteal Surface of Ilium: Near the PIIS (Posterior Inferior Iliac Spine).
• Sacrotuberous Ligament: A minor origin contribution.

Course

• Leaves the pelvis through the Greater Sciatic Foramen.
• Fills the foramen almost completely, leaving small gaps above and below.

Insertion

• Greater Trochanter: Specifically the superior border and medial aspect of the apex.
• The tendon often blends with the tendons of the Obturator Internus and Gemelli.

Nerve to Piriformis

• Roots: S1, S2 (sometimes L5).
• Branching: Arises directly from the anterior divisions of the sacral plexus within the pelvis.
• Entry: Enters the anterior (deep) surface of the muscle immediately.

This direct innervation means it is rarely affected by extra-pelvic pathology, unlike the Sciatic nerve.

Key Relations

• Anterior (Pelvic): Sacral plexus, portions of Rectum (medially).
• Posterior (Gluteal): Gluteus Maximus covers it completely.
• Superior: Superior Gluteal Nerve and Artery.
• Inferior: Sciatic Nerve, Inferior Gluteal Neurovascular bundle, Posterior Cutaneous Nerve of Thigh.

The Sciatic Nerve typically emerges at the inferior border, deep to the muscle belly.

Piriformis Line

• Draw a line from the PSIS (Posterior Superior Iliac Spine) to the Greater Trochanter.
• The superior border of the Piriformis roughly corresponds to this line.

Palpation

• Palpation: Deep palpation in the mid-buttock, midway between the PSIS and Greater Trochanter, may elicit tenderness in Piriformis syndrome.
• Difficulty: Direct palpation is difficult due to the overlying Gluteus Maximus.
• Seated Palpation: Palpating while the patient is seated (flexed hip) may make the muscle more accessible as it emerges from under the gluteus maximus.

Tenderness at the greater sciatic notch is a sensitive sign for deep gluteal pathology.

Beaton & Anson: Sciatic Nerve Relationship

Describes the relationship between the Piriformis and the Sciatic Nerve (SN).

• Type A (Unsplit): Normal (85-90%). SN passes undivided below Piriformis.
• Type B (Split): Common Peroneal Nerve (CPN) pierces Piriformis; Tibial Nerve (TN) passes below.
• Type C (Split): CPN passes ABOVE (superior); TN passes below.
• Type D (Unsplit): Entire SN pierces muscle (Rare).
• Type E (Split): CPN passes ABOVE; TN pierces muscle (Very Rare).
• Type F (Unsplit): Entire SN passes ABOVE muscle (Extremely Rare).

Significance: Type B is the most common variant and predisposes to entrapment (Piriformis Syndrome).

Etiology

• Primary: Anatomic anomaly (Beaton Type B). (less than 15%)
• Secondary: Macro-trauma (fall on buttock), Micro-trauma (wallet neuritis, overuse), or Hypertrophy.
• Mechanism: Compression of the Sciatic nerve (Usually the peroneal division) by the muscle belly or tendon.

The peroneal division is more lateral and susceptible to compression against the ischium.

Clinical Features

• Buttock Pain: Deep, aching.
• Radiculopathy: Pain radiating down the posterior thigh/leg (Sciatica) but usually sparing the foot (unlike true diskogenic sciatica).
• Sitting Intolerance: Pain worsens after sitting for 20 minutes.
• Exacerbation: Walking up stairs or inclines.

Typically, straight leg raise (SLR) is negative or only positive in extreme ranges, unlike disk herniation.

MRI Neurography

• Gold Standard.
• Can show Piriformis hypertrophy, asymmetry, or anomalous course of the nerve.
• T2 Sequence: Increased signal in the sciatic nerve indicates neuritis.

Diagnostic Injection

• CT or Ultrasound-guided local anesthetic into the muscle belly.
• Positive Test: Immediate relief of sciatic symptoms confirms the diagnosis.

This is often considered the most definitive confirmation of the diagnosis.

Electrophysiology

• Usually normal in early changes.
• H-Reflex: May be delayed on the affected side when the hip is placed in the FAIR position (dynamic testing).

EMG helps rule out L5/S1 radiculopathy (paraspinal muscles should be normal in Piriformis Syndrome).

Treatment by Stage

• Botox: Paralyses the muscle, inducing atrophy and relieving compression. Effects last 3-6 months.

Surgical release should only be considered after a failure of at least 6 months of conservative therapy.

Piriformis Release

• Indication: Failure of conservative management and positive diagnostic block.
• Approach: Posterior (Kocher-Langenbeck) or Endoscopic.
• Technique:
  1. Identify Sciatic Nerve distally.
  2. Trace proximally to inferior border of Piriformis.
  3. Identify any bands or split nerve (Type B).
  4. Tenotomize the Piriformis at the reflected tendon (insertion).
  5. Observe decompression of the nerve.
• Caution: Protect the Superior Gluteal vessels superiorly!

Endoscopic release is becoming increasingly common due to lower morbidity.

Posterior Hip Approach

• Role: The Piriformis is a key landmark.
• Tagging: The tendon is tagged and released to mobilize the femur.
• Repair: Often repaired at the end of surgery to restore stability (reduce dislocation risk).
• Identification: Usually the highest tendon identifiable (Gemelli are smaller).

"Find the nerve, then find the muscle" is a safe adage.