Decoding the Multifidus Muscle: Fat Infiltration as a Predictor of Outcome After Lumbar-Lumbar Fusion Surgery

To identify independent risk factors for residual pain following posterior lumbar interbody fusion (PLIF) and assess if multifidus muscle fat infiltration (MFI) predicts persistent postoperative pain.

A retrospective study of 336 patients (mean age: 67.18 ± 8.73 years) undergoing PLIF for lumbar degenerative diseases between January 2021 and October 2023, with ≥12 months of follow-up. Visual analog scale (VAS) and Oswestry Disability Index scores were recorded preoperatively, and at 1 day, 1 month, and 12-18 months postoperatively. Patients were divided into nonpain (VAS < 3, n = 180) and pain (VAS ≥ 3, n = 156) groups. Radiological parameters, including Hounsfield unit (HU) values, MFI, multifidus muscle index, spondylolisthesis, nonunion, and screw loosening, were assessed.

Both groups showed significant improvements in VAS and Oswestry Disability Index (P < 0.05), yet 46.43% of patients reported VAS ≥ 3 at final follow-up. The pain group had higher rates of spondylolisthesis, multisegmental surgery, nonunion, and screw loosening (P < 0.05). MFI was the strongest correlate of pain, followed by surgical extent and nonunion. Multivariate regression identified higher MFI (odds ratio [OR = 1.094, P = 0.001), more fused segments (OR = 1.960, P = 0.015), lumbar spondylolisthesis (OR = 2.231, P = 0.018), and lower HU values (OR = 0.991, P = 0.015) as predictors of pain.

MFI, lower HU values, greater surgical extent, and spondylolisthesis are independent predictors of postoperative pain after PLIF, emphasizing the need for targeted preoperative and rehabilitation strategies.