Are sacral stress fractures in young women truly rare? a retrospective MRI analysis of 1970 patients

Background Sacral stress/insufficiency fractures (SSFs) are underrecognized causes of postpartum low-back/pelvic pain and are frequently misattributed to lumbar disc disease. To determine the MRI-based detection rate and clinical/ imaging characteristics of SSFs in women aged 18–50 years undergoing lumbosacral MRI, with subgroup analyses for postpartum versus non-postpartum status. Methods Retrospective review of 2400 lumbosacral MRI examinations (January 2020–December 2024) corresponding to 1970 unique women. SSFs were identified by a fellowship-trained musculoskeletal radiologist. Prespecified exclusions (chronic corticosteroid use, major pelvic trauma, incomplete follow-up) yielded the analytic cohort. Group comparisons reported effect sizes (Hedges’ g for continuous; risk difference [RD]/odds ratio [OR] for binary) with 95% CIs. Results Crude MRI-based detection rate: 1.52% (30/1,970; 95% CI, 1.07–2.17); analytic MRI-based detection rate: 1.17% (23/1,970; 95% CI, 0.78–1.75). Median age 34 years (range 22–49); 10/23 (43.5%) postpartum. Five cases (21.7%) were initially misdiagnosed. Fractures most often involved the sacral ala (right 12, left 10, bilateral 1). Compared with non-postpartum patients, postpartum patients had lower hemoglobin (**11.5 vs 12.6 g/dL; p = 0.007; Hedges’ g = 1.21, 95% CI 0.34–2.09) and hematocrit (**33.9% vs 37.3%; p = 0.007; g = 1.21, 95% CI 0.34–2.09); vitamin D was lower but nonsignificant (**15.4 vs 20.3 ng/mL; p = 0.204; g = 0.53, 95% CI − 0.28–1.34). Overall vitamin D status (n = 21): deficient < 20 ng/mL 11/21 (52.4%), insufficient 8/21 (38.1%), sufficient ≥ 30 ng/mL 2/21 (9.5%). The reported frequency reflects an MRI-based detection rate within a selected cohort of symptomatic women undergoing lumbosacral MRI and should not be interpreted as the true population prevalence of sacral stress fractures. Conclusion Sacral stress fractures in young women, particularly in the postpartum period, may be underrecognized in symptomatic patients presenting with persistent lo