The buttocks, also known as the gluteal region, are a prominent area on the posterior surface of the lower trunk. This region is primarily composed of muscles and soft tissues, but its underlying structure is formed by several significant bones.
The Pelvic Bones
The central bony structure underlying the buttocks is the pelvic girdle, a ring-like assembly that connects the axial skeleton to the lower limbs. The pelvic girdle is formed by two large hip bones, which join anteriorly at the pubic symphysis and posteriorly with the sacrum. Each hip bone is not a single entity, but rather a fusion of three distinct bones: the ilium, ischium, and pubis. These three bones begin as separate structures in childhood, connected by cartilage, and gradually fuse during adolescence, typically between the ages of 12 and 16 years.
The ilium forms the broad, flaring upper part of the hip bone, contributing to the lateral and superior aspects of the pelvis. It is the largest of the three fused bones and plays a significant role in providing attachment points for muscles. The ischium constitutes the lower and posterior portion, featuring a prominent, roughened area called the ischial tuberosity, which bears the body’s weight when sitting. The pubis is the anterior part of the hip bone. All three of these bones converge to form a deep, cup-shaped socket called the acetabulum, which articulates with the head of the femur to form the hip joint.
The Sacrum and Coccyx
Beyond the main pelvic bones, two additional bones contribute to the posterior bony framework of the buttocks: the sacrum and the coccyx. The sacrum is a large, triangular bone located at the base of the spine, positioned between the two hip bones. It is formed by the fusion of five sacral vertebrae, a process that typically begins in the mid-teens and is complete by the early to mid-twenties. The sacrum connects the lumbar spine to the pelvic bones, articulating with the ilium on each side at the sacroiliac joints.
Inferior to the sacrum lies the coccyx, commonly known as the tailbone. This small, triangular bone represents the end of the vertebral column. The coccyx is typically formed by the fusion of three to five small, rudimentary vertebrae. While often considered a single bone in adults, the individual coccygeal vertebrae are separate during early development and typically fuse by the mid-20s or 30s.
Support and Movement Functions
The combined structure of the pelvic bones, sacrum, and coccyx forms a stable ring, providing a foundation for the upper body. This bony arrangement transfers body weight from the vertebral column to the lower limbs, especially during standing and walking.
These bones also serve as extensive attachment points for a multitude of muscles, including the large gluteal muscles of the buttocks, as well as muscles of the hips and thighs. These muscle attachments allow for a wide range of movements, such as walking, running, sitting, and standing. Furthermore, the bony pelvis forms a protective basin that safeguards internal pelvic organs, including parts of the urinary and reproductive systems, and the rectum.