Biomechanical Predictors of Sacroiliac Joint Uptake on Single-Photon Emission Computed Tomography/Computed Tomography

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 sacroiliac joint (SI) radiotracer uptake and anatomic biomechanical parameters is poorly understood. Adult patients with SPECT/CT scans performed at our institution between 2021-2023 for the workup of low back pain were included. Patient charts were reviewed for demographic factors, including age, gender, and prior thoracolumbar fusion history. Biomechanical spinopelvic parameters were measured from standing scoliosis x-rays. SPECT/CT scans were reviewed for uptake at the SI joint. Patients were stratified into two cohorts; patients with SI uptake greater than iliac crest uptake were designated "hot," whereas those with less or equal uptake were labeled "cold." 160 patients met inclusion criteria. Patients were slightly more male (55%) with average age 55 ± 14.9 years. 68 patients (43%) had evidence of increased SI activity. Interrater reliability showed substantial agreement (kappa = 0.62). The hot cohort demonstrated greater pelvic incidence (54.8 ± 14.0° vs. 51.0 ± 11.0°, p = 0.031) and pelvic tilt (20.8 ± 9.5° vs. 18.4 ± 8.6°, p =0.047) compared to the cold cohort. Patients were otherwise similar between cohorts (p >0.05). Increased pelvic incidence and pelvic tilt angles are associated with SPECT/CT uptake at the SIJ, which may reflect altered biomechanics at the spinopelvic junction. SPECT/CT may be a valuable tool to assess SI degeneration. Future studies are warranted to better characterize the clinical applications of these findings.