Background and indications: The sacroiliac (SI) joint is another common source of pain affecting patients presenting to chronic outpatient pain clinics. These patients commonly complain of buttock pain, possibly posterior thigh pain, and have a pinpoint tenderness overlying the superior portion of the SI joint.

Anatomy: The SI joint is, as its name implies, composed of the junction of the articular surfaces of the sacrum and ileum. The diarthrodial joint is enclosed in a fibrous capsule (Fig. 56.1). It normally has minimal mobility and is typically a stable joint secondary to extensive support by surrounding muscles and fascia. Innervation to the posterior aspect of the SI joint is primarily via the lateral branches of the L4-S2 nerve roots, in addition to some contributions from the superior gluteal nerve and S3 nerve roots. The anterior SI joint is innervated by L2-S2.

Patient position: Prone with towel rolls placed under the abdomen for alleviation of lumbar lordosis.

Probe: Broadband low-frequency curved array transducer.