The pelvis is a basin-shaped bony structure located at the base of the spine, situated between the abdomen and the thighs. It connects the trunk of the body to the lower limbs. It supports the weight of the upper body, whether sitting or standing, and helps transfer this weight to the legs during movement. The pelvis also provides protection for various internal organs, including parts of the urinary and reproductive systems, as well as the rectum.
The Hip Bones
The human pelvis is largely formed by two substantial, irregularly shaped bones known as the hip bones, also called coxal or innominate bones. Each hip bone develops from the fusion of three distinct bones: the ilium, the ischium, and the pubis. This fusion process typically completes by around 20 to 25 years of age.
The ilium is the largest and uppermost part of each hip bone, providing a broad surface for the attachment of muscles involved in hip movement and stability. Beneath the ilium, the ischium forms the lower and posterior portion of the hip bone, bearing the body’s weight when sitting. The pubis constitutes the anterior and lower part of the hip bone, positioned at the front of the pelvis, and provides attachment points for various muscles and ligaments. All three bones converge at the acetabulum, a cup-shaped socket that articulates with the thigh bone.
The Sacrum and Coccyx
Completing the pelvic structure are the sacrum and the coccyx, positioned posteriorly. The sacrum is a large, triangular bone at the base of the vertebral column. It is formed by the fusion of five individual sacral vertebrae, a process that gradually completes between 18 and 30 years of age. This fused bone creates a strong foundation, connecting the spine to the hip bones and aiding in the transfer of body weight.
Inferior to the sacrum, at the end of the vertebral column, lies the coccyx, commonly known as the tailbone. This small, triangular bone forms from the fusion of rudimentary vertebrae. The coccyx articulates with the sacrum, contributing to the overall integrity of the pelvic floor.
Connecting the Pelvic Structure
The individual bones of the pelvis articulate to form a stable, ring-like structure known as the pelvic girdle. This framework is essential for transferring forces from the upper body to the lower limbs. A primary connection occurs at the sacroiliac joints, where the sacrum articulates with the ilium of each hip bone. These joints are strong and allow for limited movement, which is important for stability and weight transmission.
Anteriorly, the two pubic bones meet at the pubic symphysis. This cartilaginous joint connects the left and right sides of the pelvis. While allowing for minimal movement, this joint provides flexibility that helps the pelvis absorb shock during activities like walking and running. Together, the articulations at the sacroiliac joints and the pubic symphysis create a strong pelvic ring, supporting body weight, protecting internal organs, and providing attachment points for muscles that move the trunk and lower extremities.