Septic arthritis of the SI joint is a rare condition representing 1% to 2% of all forms of septic arthritis and bone infections. The source of infection is thought to be from hematogenous spread. The most common positive physical examination signs include maximal tenderness over the SI joint and increased pain with SI joint compression test. Patients will often have a positive straight leg raise because the sacral nerves running anterior to the SI join may be irritated. MRI is the gold standard test to diagnose an early SI joint infection. Laboratory findings typically include an elevated white blood cell count and elevated inflammatory markers (ESR and CRP), and blood cultures will often be positive for the causative bacterial agent. Identifying the responsible bacteria, if possible, helps guide the most appropriate antibiotic coverage. Obtaining an aspiration is extremely difficult due to the anatomy of the SI joint and is generally not attempted, particularly if blood cultures are positive. Most cases of SI joint infections can be treated successfully with a 4- to 8-week course of antibiotic therapy. Surgical debridement may be performed if pain does not improve despite bed rest and IV antibiotics, if the sepsis worsens, or if a large fluid collection or abscess is present.1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
Hodgson BF. Pyogenic sacroiliac joint infection. Clin Orthop Relat Res. 1989;246:146-149.
Vyskocil JJ, McIlroy MA, Brennan TA, Wilson FM. Pyogenic infection of the sacroiliac joint. Case reports and review of literature. Medicine (Baltimore). 1991;70(3):188-197.