SI joint pain is pain experienced at or in relation to the sacroiliac joint. This joint is found at the base of the spine where the sacral bone in the vertebral column meets the iliac bone in the hip. SI joint pain is felt just above the buttocks in the tailbone region, and it may radiate up the back or down the backs of the legs. It is typically caused by inflammation of the joint itself, which in turn can be caused by a host of factors, from degeneration of joint structures to trauma to the region.
The SI joint is a type of synovial or movable joint known as a planar joint, which means that the adjoining surfaces of the adjacent bones are fairly flat and can glide against one another, although these surfaces become increasingly irregular with age, and allow only a small degree of movement between them. There are two SI joints, one on either side of the sacrum where it meets each ilium, the large bone on either side of the upper pelvis that is shaped like a butterfly’s wing. Together they move as a unit as well as absorb a good deal of weight, two factors that are significant to the diagnosis of SI joint pain.
These articulations between the lateral or outside surfaces of the sacrum and the medial or inside surfaces of the ilia contain two types of articular cartilage, fibrocartilage and hyaline cartilage, which cushion the bones against one another during movement. They are also held together by several ligaments, including the anterior, posterior, and interosseous sacroiliac ligaments, which join the bones by their front, rear, and adjoining surfaces, respectively. The presence of these very strong ligaments, in combination with irregular articulating surfaces that lock the bones together, make dislocation of the sacrum and ilium very rare. Rather, SI joint pain tends to be a product of the degeneration of one or more of these joint structures.
As the two SI joints move as a unit, specifically to rotate the pelvis side to side while walking, damage to the joint structure on one or both sides can cause painful inflammation known as sacroiliitis that is aggravated by movement. Such damage, whether in the form of the degeneration of the cartilage or irritation of the ligaments, can be caused by muscle imbalances that lead to dysfunctional movement, such as abnormal walking or running patterns. In other words, muscles that become tight or weak by behaviors like sitting for long periods, such as shortened hip flexor muscles, can cause changes to posture, such as tilting the pelvis too far anteriorly, which in turn can affect the way a person moves.
Irregular movement means that undue stress may be placed on weight-absorbing structures and cause the breakdown of joint tissues themselves, leading to SI joint pain. This stress is typically experienced on one side more than the other, which disrupts the function of a joint that is intended to work as a unit. Similarly, carrying excess body weight, particularly in the abdominal region, can cause SI joint pain. This extra weight can pull forward on the pelvis in a way that negatively affects posture and gait and that places stress on the joint’s weight-bearing function.
Whether caused by muscle imbalances, obesity, or even injury, however, sacroiliitis presents with similar symptoms. The pain is usually dull and aching, is felt around the tailbone, though it may radiate down the buttocks and legs or up the low back, and is often experienced on one side only. SI joint pain may increase on certain movements, such as getting up from a sitting position, and it is often accompanied by stiffness after long periods in one position. In the absence of injury or the diagnosis of other spinal issues, this pain may typically be treated with exercise and stretching to return normal function to the joint.