Buttock

The buttock refers to the rounded bulge in the lower part of the gluteal region. The inferior extent of the buttock is marked by a gluteal fold of skin below. Medially, an intergluteal cleft separates the two buttocks from each other, while laterally they are bounded by the hip regions.

The rounded shape of the buttock is due to the gluteus maximus muscle. It overlies two additional gluteal muscles, namely the gluteus medius and the gluteus minimus. Together they form the superficial group of gluteal muscles. The skin and fascia overlying the buttock region are thick and contain abundant fat.

The gluteus medius and minimus muscles originate from the external surface of the iliac bone, with medius lying over the minimus. The gluteus maximus has a wide area of origin, involving not only the external surface of the ilium but also arising from the sacrum and the sacrotuberous ligament.

Distally, the gluteus maximus inserts into the gluteal tuberosity of the femur and the iliotibial tract of the fascia lata, causing lateral rotation of thigh at the hip. It also extends the trunk on the lower limb while climbing stairs or standing up from a sitting position. Unlike the maximus, the gluteus medius and minimus muscles insert into the greater trochanter of the femur, causing medial rotation of the thigh at the hip. They also act as abductors of the hip and prevent the pelvis from dropping on the side of the unsupported/swinging limb. Hence, if paralyzed, the pelvis sags, also known as the Trendenlenburg’s sign, i.e. walking with a ‘waddling gait’.

Deep within the buttocks (i.e. underneath the gluteal muscles), a group of short muscles can be found originating medially from the hip bone and the sacrum and extending laterally–crossing behind the hip joint–and inserting into the greater trochanter of the femur in the hip region. These include the piriformis, superior and inferior gamelli, obturator internus, and quadratus femoris muscles. All of them cause lateral rotation at the hip joint.

Muscles of the buttocks or the gluteal region are innervated by branches of the sacral plexus, including the superior and inferior gluteal nerves, and several motor branches to the deep short lateral rotator muscles. Since the plexus is situated within the pelvic wall, the sacral branches have to traverse through the greater sciatic foramen to reach the gluteal region.

The sciatic nerve–the largest branch of the sacral plexus–can also be found underneath the gluteus maximus muscle in the buttocks. It traverses through the greater sciatic foramen, descends through the gluteal region, and reaches further down into the hamstrings.

The pudendal nerve–a branch of sacral plexus–innervates the perineum. It also arises from the sacral plexus but takes a long route, first traversing through the greater sciatic foramen to enter the gluteal region. Next–deep within the buttocks region–it passes through the lesser sciatic foramen to reach the perineum.