MJB Stallmeyer and Gregg H Zoarski

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Sacroiliac (SI) joint dysfunction or arthopathy is thought by many to be a significant source of low back pain and referred lower extremity pain. Bernard and Kirkaldy-Willis1 reported that 22.5% of 1293 patients with low back pain were symptomatic as a result of SI joint disease. Schwarzer et al.,2 using fluoroscopically guided SI joint injections, estimated that the prevalence of SI joint pain in patients with low back pain was between 13 and 30%. From the results of provocation tests and SI joint blocks, Maigne et al.3 concluded that 18% of patients experienced pain attributable to the SI joint.

Sacroiliac joint pain is presumed to be caused by abnormal movement or malalignment of the SI joint. It may result from a variety of causes including spondyloarthropathy,4-6 crystal7 and pyogenic arthropathy,8 pelvic and sacral fractures,9 and diastasis resulting from trauma, pregnancy, or childbirth,10,11 but it also may be idiopathic.12-13

The patterns of pain referral from the SI joint are variable and are thus difficult to distinguish from other causes of low back pain.2,14-16 Presenting symptoms and signs include lower lumbar pain, buttock pain, groin pain, lower abdominal pain, pain radiating to the leg or foot, and focal pain and tenderness over the joint.1,2,14,15,17,18 The complex pain referral patterns are explained by the innervation of the joint. The SI joint and the sacroiliac ligaments contain myelinated and un-myelinated axons that are thought to conduct proprioception and pain sensation from mechanoreceptors and free nerve endings in the joint.19-21 The anterior aspect of the sacroiliac joint likely derives the majority of its innervation from the dorsal rami of the L1-S2 roots but may also be innervated by the obturator nerve, superior gluteal nerve, and lumbosacral trunk.13,22-24 The posterior aspect of the joint is innervated by the dorsal rami of L4-S4, with major contributions from S1 and S2.19,22-24 Additionally, the piriformis muscle, which originates from the ventrolateral aspect of the sacrum and inserts into the greater trochanter, may contribute to the production of pain; spasm of the pir-iformis may produce a compression syndrome of the sciatic nerve, which may pass through or just beneath this muscle.25 Patterns of innervation vary between individuals and may even vary slightly from side to side in an individual patient.

The SI joint has been classified as an amphiarthrosis (two hyaline cartilage surfaces connected by fibrocartilage). In an alternative classification scheme, the superior portion of the sacroiliac joint has been defined as a synarthrosis (articular surfaces connected by fibrous tissue), while the anterior portion and inferior third of the SI joint has been described as a true synovial joint13 (Figure 13.1A). In adults, the joint is S or C shaped. On cross-sectional imaging, the joint space, which is usually 0.5 to 4 mm, is oriented along a posteromedial-to-anterolateral plane (Figure 13.1B).

The SI joint is stabilized by a strong ligamentous support system composed of the interosseus sacroiliac ligament, the dorsal and ven-

Synovial Portion Joint

Figure 13.1. (A) Coronal reconstructed CT image of the sacroiliac joint. The upper portion of the joint is a synarthrosis, while the inferior third is a true synovial joint. (B) Axial CT image of the sacroiliac joint, demonstrating orientation of the joint along a posteromedial-to-anterolateral plane.

Figure 13.1. (A) Coronal reconstructed CT image of the sacroiliac joint. The upper portion of the joint is a synarthrosis, while the inferior third is a true synovial joint. (B) Axial CT image of the sacroiliac joint, demonstrating orientation of the joint along a posteromedial-to-anterolateral plane.

tral sacroiliac ligaments, and the sacrospinous and sacrotuberous ligaments.26-28 Although the SI joint is mobile, motion is limited to only a few millimeters of translation and 3° of rotation.29,30

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