3D Morphometric Analysis of Normal Sacroiliac Joints: A New Classification of Surface Shape Variation and the Potential Implications in Pain Syndromes.

BACKGROUND Normal sacroiliac (SI) joints vary widely in shape, and it is unclear whether SI joint morphology plays a role in the development of pain. To answer this question, we used 3D-CT surface rendering to image healthy study participants and used the resulting images to develop a classification system for SI joint morphology. In a case-control study comparing health research participants to patients with SI pain, we assessed whether the classification of SI joint morphology was associated with pain. OBJECTIVES We aimed to define normal variation in area and shape of the synovialized SI joint. We also aimed to determine interactions between joint area and shape and SI pain in men and women. STUDY DESIGN A retrospective cohort study. SETTING University hospital tertiary care center. METHODS We conducted a retrospective case-control study. We acquired 3D-CT surface rendered images of the SI joint in 223 normal controls and 34 patients with SI pain syndrome. We measured the sacral and iliac surface areas and performed morphologic 3D assessment of both articular surfaces. We classified SI joints into 3 types based on morphology (types one, 2, and 3). We used descriptive statistics to provide a reference standard for normal SI joints. We used multivariate models to assess whether articular surface area differed between study participants with and without SI pain. We also assessed the association between morphology type and the presence of pain. RESULTS Sacral and iliac surface areas differ by participant gender. Sacral and iliac surface area was associated with SI joint pain in both men (P = 0.0007) and in women (P = 0.02). In women (P = 0.04), but not in men (P = 0.11), joint shape was associated with pain. LIMITATIONS A retrospective study may create potential for misclassification bias if SI joint symptoms/histories were present but not well-documented in the electronic medical record. A lack of clinical standardization in the pre-procedural assessment of SI joint pain via provocative maneuvers (FABER, etc.) is also a limitation. CONCLUSIONS Our study provides insight into the association between shape and joint surface area and SI joint pain. Further, prospective studies will allow us to determine the role of joint shape and surface area in the patho-etiology of SI joint pain, and thus provide information for patients and physicians about prevention or treatment. KEY WORDS Sacroiliac joint, spine pain, 3D reformation, SI joint area, SI joint shape, SI joint pain syndrome.