Gluteal Region

Embryological Development and Segmental Innervation of the Limbs

During early development, the human trunk is organized into segments known as metameres. These metameres correspond exactly to and receive innervation from specific spinal cord segments. This segmental pattern remains visible in the mature body as dermatomes, though it is modified by limb growth and rotation.

Upper limb buds appear as elevations on the anterolateral body wall during the 4th4^{th} week of development, corresponding to the C5−T1C5-T1 segments. Following a cranial-to-caudal progression, the lower limb buds appear approximately one week later, during the 5th5^{th} week, growing laterally from broader bases formed by the L2−S2L2-S2 segments.

Initially, the limb buds flatten at their distal ends into paddle-like hand and foot plates. Early in their development, the thumb and the great toe are located on the cranial sides of the hand and foot, directed superiorly, while the palms and soles are directed anteriorly. As the limbs grow, flexures occur where gaps develop between long bone precursors, causing the limbs to bend anteriorly. This orientation directs the elbows and knees laterally and causes the palms and soles to face medially toward the trunk.

By the end of the 7th7^{th} week, the proximal portions of the limbs undergo a mandatory 90∘90^{\circ} torsion around their long axes. Importantly, this rotation occurs in opposite directions. The upper limb rotates laterally, directing the elbow caudally. Conversely, the lower limb undergoes medial rotation and permanent pronation (twisting), directing the knee cranially and positioning the great toe on the medial side of the foot. These dramatic movements cause mature dermatomes to spiral down the lower limb while producing a more limited modification in the upper limb.

Anatomy of the Gluteal Region: Surface Landmarks and Osteology

The gluteal region is defined by several surface landmarks: the superior boundary is formed by the level of the iliac crests, the medial boundary by the intergluteal cleft (also known as the butt crack), and the inferior boundary by the gluteal fold. This area connects the trunk to the lower extremity, specifically the thigh.

The underlying bony framework involves the os coxae, or innominate bone, which is composed of the ilium, ischium, and pubis. On the lateral surface of the ilium, three gluteal lines provide attachment points for muscles: the posterior gluteal line, the anterior gluteal line, and the inferior gluteal line. Key bony processes include the Posterior Superior Iliac Spine (PSIS), Posterior Inferior Iliac Spine (PIIS), Anterior Superior Iliac Spine (ASIS), and Anterior Inferior Iliac Spine (AIIS). The iliac tubercle is a specifically noted bony bump along the iliac crest that serves as an attachment site for the Tensor Fasciae Latae (TFL).

The posterior aspect of the pelvis features the greater sciatic notch and the lesser sciatic notch, separated by the ischial spine. The ischial tuberosity is the heavily weighted bone surface used for sitting. The acetabulum is the large socket that receives the head of the femur, containing the acetabular fossa and the lunate (articular) surface. Inferiorly, the obturator foramen is formed by the rami of the pubis and ischium.

Ligaments and the Sciatic Foramina

Two critical ligaments transform the sciatic notches of the innominate bone into functional foramina through which neurovascular structures and muscles pass. The sacrospinous ligament runs from the sacrum to the ischial spine. The sacrotuberous ligament runs from the sacrum to the ischial tuberosity.

The Greater Sciatic Foramen is divided into two spaces by the piriformis muscle. The space above the piriformis transmits the superior gluteal nerve, artery, and vein. The space below the piriformis transmits the sciatic nerve, inferior gluteal nerve, artery, and vein, the pudendal nerve, the internal pudendal artery and vein, the posterior femoral cutaneous nerve, the nerve to the obturator internus and gemellus superior, and the nerve to the quadratus femoris and gemellus inferior.

The Lesser Sciatic Foramen serves as a conduit for the tendon of the obturator internus muscle and its associated nerve. Additionally, the pudendal nerve and internal pudendal vessels pass through this foramen to enter the perineum after having exited the pelvic cavity via the greater sciatic foramen.

Muscles of the Gluteal Region

The gluteal region contains nine primary muscles, categorized into a superficial gluteal group and a deep layer of short external rotators/stabilizers.

The Superficial Group includes:

  • Gluteus Maximus: Originates from the ilium posterior to the posterior gluteal line, the dorsal sacrum, coccyx, and sacrotuberous ligament. It inserts into the iliotibial (IT) tract and the gluteal tuberosity of the femur. It is innervated by the inferior gluteal nerve and supplied by the superior and inferior gluteal arteries. Its actions include extending the flexed thigh, assisting in lateral rotation, and abducting the thigh.

  • Gluteus Medius: Originates between the anterior and posterior gluteal lines of the ilium and inserts on the lateral surface of the greater trochanter. It is innervated by the superior gluteal nerve. It abducts and medially rotates the thigh, though posterior fibers can assist in lateral rotation.

  • Gluteus Minimus: Originates between the anterior and inferior gluteal lines and inserts on the anterior surface of the greater trochanter. It is innervated by the superior gluteal nerve and abducts and medially rotates the thigh.

  • Tensor Fasciae Latae (TFL): Originates from the ASIS and the anterior iliac crest, inserting into the IT tract which continues to Gerdy's tubercle (lateral condyle of the tibia). It is innervated by the superior gluteal nerve and performs hip flexion, abduction, and internal rotation while maintaining knee extension.

The Deep Group (Short External Rotators) includes:

  • Piriformis: Known as the "key" to the region, it originates from the anterior surface of the S2−S4S2-S4 sacrum segments and passes through the greater sciatic foramen. It is innervated by the nerve to the piriformis (ventral rami of L5,S1,S2L5, S1, S2).

  • Triceps Coxae: This group consists of the Superior Gemellus (innervated by the nerve to the obturator internus), the Obturator Internus (innervated by its own nerve), and the Inferior Gemellus (innervated by the nerve to the quadratus femoris). Collectively, they laterally rotate the extended thigh and abduct the flexed thigh.

  • Quadratus Femoris: A quadrate-shaped muscle that laterally rotates the thigh. It features a quadrate tubercle for attachment.

  • Obturator Externus: While technically a short external rotator, it is typically grouped with the medial thigh muscles due to its innervation by the obturator nerve.

Neurovascular Supply and Clinical Applications

The nerves of the gluteal region include the superior gluteal (L4,L5,S1L4, L5, S1), inferior gluteal (L5,S1,S2L5, S1, S2), sciatic (L4,L5,S1,S2,S3L4, L5, S1, S2, S3), pudendal (S2,S3,S4S2, S3, S4), and various cutaneous nerves such as the posterior femoral cutaneous (S1,S2,S3S1, S2, S3). The sciatic nerve is the largest in the body and typically passes inferior to the piriformis; however, anatomical variations exist. Approximately 13.7%13.7\% of the population shows the sciatic nerve bifurcating through the piriformis, and 1.3%1.3\% shows the nerve passing superior to the muscle.

Blood supply to the lower extremity originates from the abdominal aorta, branching into the common iliac and then the internal and external iliac arteries. The internal iliac artery stays within the pelvis and gives rise to the superior gluteal, inferior gluteal, internal pudendal, and obturator arteries. The external iliac artery passes through the inguinal ligament to become the femoral artery.

Clinical diagnostic signs include the Trendelenburg gait. If the gluteus medius is weak, the pelvis will drop on the side opposite the weakness when the patient stands on the affected leg. For safe intramuscular gluteal injections, the upper lateral quadrant of the buttock is preferred to avoid damaging the sciatic nerve or major gluteal arteries.