Chronic inflammatory involvement of the sacral multifidus insertion, predominantly observed after lumbar surgery

Persistent sacral pain following lumbar spine surgery is a common yet poorly understood condition. When conventional imaging fails to identify a structural cause, symptoms are often attributed to nonspecific postoperative pain. Although postoperative alterations of the multifidus muscle are well documented, focal inflammatory involvement of its sacral insertion has not been specifically described.

We conducted a prospective single-center observational case series including 48 consecutive patients (13 men, 35 women; mean age 61.8 years) presenting with persistent sacral pain and MRI evidence of inflammatory involvement of the multifidus muscle. All patients underwent pelvic MRI including fat-suppressed T2-weighted and STIR sequences. Histopathological material was available in three patients undergoing subsequent surgical procedures. Follow-up data were available for 25 patients.

Patients presented with focal sacral pain exacerbated by prolonged sitting, reproducible tenderness over the posterior sacral region, and no objective neurological deficits. MRI consistently demonstrated inflammatory hyperintensity involving the multifidus muscle. Involvement was bilateral in 40 patients (83.3%) and unilateral in 8 (16.7%), with a strong correspondence between pain laterality and MRI findings. Histopathological material available in three patients demonstrated chronic inflammatory changes with focal fascial involvement. Among patients with follow-up data, mean pain intensity decreased from 6.0 ± 1.2 to 2.4 ± 2.6 (p = 0.025), while MRI abnormalities markedly improved in most reassessed patients.

Inflammatory abnormalities involving the sacral insertion of the multifidus muscle may represent an underrecognized imaging correlate of persistent sacral pain, particularly following lumbar spine surgery. Recognition of this characteristic MRI pattern may facilitate earlier diagnosis and improve patient management.