Our five-year-old son was recently diagnosed with an infection of his sacroiliac joint. The doctor told us this is a fairly uncommon condition. What causes it?

Infections of the sacroiliac (SI) joint are most often caused by strep or staph bacterial infection. Trauma or some other type of infection in the body are the most common causes. The child may have had an earlier ear infection, strep throat, or skin infection. Even sinus infection or urinary tract infection can be the culprit.

Bacteria such as streptococcus or staphylococcus can travel through the bloodstream to the joint and cause a joint infection. This may be what happened in your son’s case.

Usually the immune system kills these invading bacteria. But in the young child whose immune system is still developing, the bacteria may survive. It then travels across the protective layers of the joint through the blood.

Antibiotics almost always take care of this problem. Only rarely (usually if the infection comes back) will surgery be needed. A quick visit to the pediatrician is needed any time a child develops fever, joint pain, and starts to limp. Early diagnosis of most infections can prevent further problems from developing.