The pelvic girdle is a sturdy, ring-like bony structure located at the base of the trunk. It functions as the anatomical link between the upper body (axial skeleton) and the lower limbs (appendicular skeleton). Designed for stability and weight bearing, the complete bony pelvis forms a basin that protects internal organs and serves as a major attachment point for muscles involved in locomotion and posture.
The Primary Components: The Hip Bones
The pelvic girdle consists primarily of two large, symmetrical bones called the hip bones (Os Coxae or innominate bones). These bones join anteriorly at the pubic symphysis and articulate posteriorly with the sacrum. Each hip bone is formed by the fusion of three distinct bones: the ilium, the ischium, and the pubis.
The ilium is the largest and most superior of the three, forming the wide, fan-shaped flare often felt just below the waist. The ischium forms the posteroinferior portion of the hip bone and includes the ischial tuberosity, which is the roughened area that bears the body’s weight when a person is sitting. The pubis is the most anterior part of the hip bone and contributes to the anterior ring structure.
In childhood, these three components are separate, joined by cartilage within the hip joint socket (the acetabulum). Fusion begins early, typically between the ages of four and eight years for the ischium and pubis. The complete fusion process, including the ilium, concludes between the mid-teens and early twenties, resulting in a single, robust hip bone in adults.
The Central Connection: Sacrum and Coccyx
While the two hip bones form the girdle, the bony ring is completed posteriorly by the sacrum and the coccyx. The sacrum is a large, triangular bone situated below the last lumbar vertebra, formed by the fusion of typically five sacral segments. It articulates with the ilium of the hip bones at the sacroiliac joints, forming a key connection point for stability.
The coccyx, or tailbone, is located inferior to the sacrum and is composed of three to five small, partially fused segments. While it provides minor support for the pelvic floor, its main role is completing the posterior wall of the pelvis. The combined structure of the two hip bones, the sacrum, and the coccyx is collectively referred to as the bony pelvis, which functions as a single, largely immobile unit.
Structural Role and Function
The completed bony pelvis serves a mechanical role as a strong, stable foundation for the body. Its structure efficiently transfers the weight of the upper body and head from the vertebral column, through the sacrum and sacroiliac joints, down to the hip joints. This weight is then transmitted to the femurs of the lower limbs, supporting the body during standing, walking, and running.
The bony basin created by the pelvis provides protection for internal organs housed within the lower abdomen. These viscera include the urinary bladder, portions of the lower digestive tract, and the internal reproductive organs. The stability required for weight transfer is maintained primarily by the sacroiliac joints and surrounding ligament tension.
The extensive surface area of the hip bones provides numerous attachment sites for powerful muscles. These muscles generate the force needed for locomotion, control posture, and maintain the stability of the trunk and lower limbs. The structural integrity of the pelvic ring is fundamental to both mobility and the protection of internal anatomy.