About 80% of adults will have low back pain at some point in their lives, according to the National Institutes of Health. Some episodes are caused by specific events, like a sprain, traumatic injury or herniated disc. But others may have more subtle origins—like strained ligaments or the gradual wearing down of cartilage around the joints.
Problems with the sacroiliac joint—the junction at the base of the spine connecting the backbone to the hip bones—often fall under that second category, says Dr. David Borenstein, rheumatologist and clinical professor of medicine at George Washington University. Also called the SI joint, it’s a common cause of low back pain, but not always an obvious one.
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The body has two sacroiliac joints, one on each side of the spine. Each joint’s role is to act as a shock absorber and support the weight of the upper body, while remaining relatively stable. “It’s not like the knee or elbow that bends back and forth,” says Dr. Alice Chen, a physiatrist at the Hospital for Special Surgery in New York City. “It’s a joint that doesn’t have much motion—or at least it’s not supposed to.”
Women tend to have more SI joint-related pain than men, since their joints are naturally a bit looser. The SI joint is often associated with premenstrual syndrome–related back pain, as well as back pain during and after pregnancy, says Chen. (Ligaments around the pelvis stretch to prepare for birth, putting extra stress on the joints and the nerves around them.)