The pelvis is a complex ring structure composed of multiple articulations that connect the spine to the lower limbs. This bony basin acts as a mechanical bridge, transferring the weight of the upper body downward. It also accommodates subtle movements necessary for walking and posture. The different joints within the pelvis are classified into distinct types, each contributing uniquely to the structure’s overall function.
The Bones That Form the Pelvic Girdle
The foundation of the pelvis, or pelvic girdle, is built from four primary bones. The two large, symmetrical hip bones (coxal or innominate bones) form the sides and front of the structure, each being a fusion of the ilium, ischium, and pubis. These paired hip bones articulate with the sacrum, a spade-shaped bone at the base of the spine formed by the fusion of five vertebrae. The sacrum is part of the axial skeleton, while the hip bones are part of the appendicular skeleton. The entire ring structure, including the two hip bones, the sacrum, and the small coccyx (tailbone), provides a stable attachment point for the lower limbs.
The Sacroiliac Joints: A Hybrid Classification
The sacroiliac joints (SIJs) connect the sacrum to the ilium of each hip bone, forming the posterior connection of the pelvic ring. These joints are classified as a complex hybrid articulation, possessing features of both a synovial joint (capsule and fluid) and a syndesmosis (fibrous joint). The syndesmosis classification is due to the dense network of ligaments binding the bones. The interosseous sacroiliac ligaments are among the strongest in the body and severely restrict movement. This structure permits only minimal gliding and rotational movements, such as nutation (sacral flexion) and counternutation (sacral extension), which are typically limited to a few millimeters and degrees of rotation.
The Pubic Symphysis: A Cartilaginous Connection
The pubic symphysis forms the anterior connection of the pelvic ring, joining the two pubic bones at the midline. This articulation is classified as a secondary cartilaginous joint, or a symphysis type. The joint is characterized by a thick pad of fibrocartilage, known as the interpubic disc, sandwiched between the hyaline cartilage-covered ends of the pubic bones. The joint’s stability is reinforced by surrounding ligaments, including the superior and inferior pubic ligaments. The pubic symphysis typically allows for roughly 2 millimeters of vertical motion, which increases significantly in women during pregnancy due to the hormone relaxin.
Integrated Function: Load Bearing and Stability
The combined structure of the sacroiliac joints and the pubic symphysis functions as a kinetic link for the entire body. The primary purpose of this integrated system is to efficiently transfer gravitational forces from the upper body and spine to the lower limbs. This transfer is achieved through “form closure,” which is stability derived from the shape and tight fit of the bones themselves. The interlocking surfaces of the SIJs and the wedge shape of the sacrum create a self-locking system requiring minimal muscular effort. This passive stability is supplemented by “force closure,” the active compression generated by muscles, fascia, and ligaments, ensuring the pelvic ring maintains high stability necessary for fundamental movements.