This article is divided into several pages for easier reading:

3.0     Anatomy of the Gluteal Region

The gluteal region overlies the side and back of the pelvis, extending from the iliac crest above to the gluteal fold below. The lower part of the gluteal region which presents a rounded bulge due to excessive amount of subcutaneous fat is known as the buttock or natis.

The anterosuperior part of the region, seen in a side view, is called the hip. The muscles, nerves and vessels emerging from the pelvis are covered by gluteus maximus and buttock.

Morphologically, the erect posture of man has led to extension at the hip and appearance of gluteal fold, which is a transverse skin crease of the hip joint. This puts greater responsibility on gluteus maximus which makes the body erect and maintains it in erect posture at the hip; this involves raising and supporting the trunk against gravity.

The gluteus maximus, covering the hip joint is, therefore, one of the most powerful and bulkiest muscles in man.

The Piriformis is one of nine muscles of the gluteal region and is located both within the pelvis and posterior to the hip joint. The nine muscles of the gluteal region include the (Chaurasia, 2005, p.71; Hamil, Knutzen & Derrick, 2015, p.185-186):

  1. Gluteus maximus (superficial muscle).
  2. Gluteus medius (superficial muscle).
  3. Gluteus minimus (superficial muscle).
  4. Piriformis (deep muscle).
  5. Superior gemellus (deep muscle).
  6. Inferior gemellus (deep muscle).
  7. Obturator internus (deep muscle).
  8. Obturator externus (deep muscle).
  9. Quadratus femoris (deep muscle).

3.1     Piriformis Muscle

“Immediately before leaving the pelvis, the sciatic nerve has an intimate relationship with the ventral surface of the piriformis muscle.” (Agnollitto et al., 2017).

Simplistically, the piriformis muscle is a small (pyramidal or pear-shaped) muscle (Figure 2) located deep in the buttock (behind the gluteus maximus) which:

  • Starts at the lower spine and connects to the upper surface of each femur (thighbone).
  • Functions to assist in rotating the hip and turning the leg and foot outward.
  • Runs diagonally, with the sciatic nerve running vertically directly beneath it (although in some people the nerve can run through the muscle).

The piriformis muscle originates largely on the anterior surface of the sacrum, passes through the greater sciatic notch, and follows the inferior border of the posterior gluteus medius to insert above the other lateral rotators into the medial aspect of the greater trochanter.

The piriformis and gluteus maximus are the only two muscles that cross the sacroiliac joint.

The sciatic nerve, the largest nerve in the body, enters the gluteal region just inferior to the piriformis muscle as both structures pass through the greater sciatic notch.

The piriformis muscle has a number of functions including (TePoorten, 1969; Williams & Warwick, 1980; Papadoupolos & Khan, 2004; Hamil, Knutzen & Derrick, 2015, p.176 & 187):

  • Providing postural stability during ambulation and standing.
  • Contributes to external rotation of the hip when the thigh is extended.
  • Contributes to hip abduction when the hip is flexed.
  • Creating the movement on lifting the leg into abduction with the toes pointing upward in external rotation.
  • Producing sacral torsion “…in a side-bending exercise of the trunk.”

3.2     Sciatic Nerve

“The different anatomical variations of the sciatic nerve have been studied in association with gluteal surgeries and have been suggested as a possible trigger for piriformis syndrome.” (Lewis, 2016, p.15).

Alongside blood vessels, the large sciatic nerve passes from the pelvis posteriorly into the thigh via the greater sciatic notch.

In the majority of cases the sciatic nerve runs distal to the piriformis muscle. However, variations have been observed involving a split or single sciatic nerve passing proximal, through or distal to a split or single piriformis muscle (Lewis, 2016).

“When it is related to an abnormality in the piriformis muscle, neuropathy of the proximal segment of the sciatic nerve can be called piriformis syndrome, although there is controversy in the literature about the existence of this causal relationship.” (Agnolletti et al., 2017, p.193).

Research suggests that piriformis syndrome has been identified to account for up to 6-8% of sciatica (Jankovic, Peng & van Zundert, 2013).

It is common practice that injections in the buttock should always be given well away from this area to avoid possible nerve damage.

Return to Part 02 Continue on to Part 04
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