The hip bone, or os coxae, consists of three separate bones: the ilium, ischium, and pubis. These bones meet at the acetabulum, which is a cup-shaped socket that articulates with the head of the femur. The sacrum, located at the base of the spine, also connects to the hip bone. Specifically, the sacrum and ilium join at the sacroiliac (SI) joint.

The SI joint is a strong, yet highly flexible joint. Its articulation of the ilium and sacrum allows for the back and hips to move fluidly. It also acts as a shock absorber during physical activity, helping to protect the spinal cord and other internal organs from strain.

The SI joint is comprised of four parts:

• Ilium – the curved, superior part of the hip bone
• Sacral Raphe – the posterior midline ligamentous connection between the ilium and sacrum
• Sacrum – the triangular bone at the base of the spine
• Sacroiliac Joint – the articulation of the ilium and sacrum

It is important to maintain the health of the SI joint, as it is integral to mobility and balance. Muscle imbalances may cause the joint to become out of alignment, resulting in pain and discomfort. Osteopathic manipulation and physical therapy may help to restore balance and mobility to the joint.

The SI joint provides mobility and shock absorption for the hips and spine. It is comprised of the ilium, sacral raphe, sacrum, and the articulating joint between them. Proper care should be taken to maintain its health and mobility.