As with most other joints in the body, the SI joints have a cartilage layer covering the bone. The cartilage allows for some movement and acts as a shock absorber between the bones. When this cartilage is damaged or worn away, the bones begin to rub on each other, and degenerative arthritis (osteoarthritis) occurs. This is the most common cause of SI joint dysfunction. Degenerative arthritis occurs commonly in the SI joints, just like other weight-bearing joints of the body.
Another common cause of SI joint dysfunction is pregnancy. During pregnancy, hormones are released in the woman’s body that allow ligaments to relax. This prepares the body for childbirth. Relaxation of the ligaments holding the SI joints together allows for increased motion in the joints and can lead to increased stresses and abnormal wear. The additional weight and walking pattern (altered gait) associated with pregnancy also places additional stress on the SI joints.