Biomechanical Predictors of Sacroiliac Joint Uptake on SPECT/CT

Introduction: The sacroiliac (SI) joint connects the spine to the pelvis. It plays a major role in force resistance of physiologic loads. Single Photon Emission Computed Tomography/ Computed Tomography (SPECT/CT) is an emerging imaging modality that identifies sites of heightened bone metabolism in response to increased stresses. The relationship between SI radiotracer uptake and anatomic biomechanical parameters is poorly understood.

Methods: We performed a retrospective review of all SPECT/CT scans done at our institution between 2021-2023. Only adult patients with SPECT/CT of the thoracolumbar spine with a corresponding standing scoliosis film met inclusion criteria. Patient charts were reviewed for demographic factors, including age, gender, and prior thoracolumbar fusion history. Radiology reports were reviewed for evidence of adjacent segment degeneration, disc disease, facet arthropathy, or pseudoarthrosis. Biomechanical parameters including pelvic incidence, sacral slope, pelvic tilt, lumbar lordosis, sagittal and coronal balance, pelvic harmony, and coronal scoliosis were measured from standing scoliosis films. Two authors reviewed SPECT/CT scans at the SI joint. SI uptake greater than iliac crest uptake was designated hot. Patients were stratified by presence or absence of SI uptake. Cohen’s Kappa was measured to assess interrater reliability. Statistically significant differences were determined via t-tests and chi-squared analysis for continuous and categorical variables, respectively.

Results: 115 patients met inclusion criteria. Patients were slightly more male (57%) with average age 55 ± 15 years. 50 patients (43%) had evidence of increased SI activity. Interrater reliability showed substantial agreement (kappa = 0.62). The hot cohort had greater incidence of prior thoracolumbar fusion (p = 0.02). Of those with prior fusions, the hot SI cohort trended towards longer constructs (p = 0.06). Patients were otherwise similar between cohorts (p >0.05).

Conclusion : Thoracolumbar fusions may cause increased stresses at the SI joint. SPECT/CT may be a valuable tool to assess SI degeneration.