Relationship between the postoperative variations of paraspinal muscles and adjacent-segment degeneration in patients with degenerative lumbar spinal stenosis after posterior instrumented lumbar fusion

OBJECTIVE This study aimed to quantify pre- and postoperative paraspinal muscular variation following posterior lumbar interbody fusion (PLIF) in patients with degenerative lumbar spinal stenosis (DLSS) and measure the association of this variation with adjacent-segment degeneration (ASD). METHODS Data from 149 patients who underwent L4–S1 PLIF for DLSS were collected. Patients were divided into radiologically confirmed ASD and control groups according to follow-up radiological findings. MRI was performed before surgery and at the last follow-up. Muscular parameters including the relative cross-sectional area (rCSA), relative functional cross-sectional area (rFCSA), relative total cross-sectional area (rTCSA), and fatty infiltration (FI) of the multifidus (MF), erector spinae (ES), and psoas major (PM) muscles were measured on preoperative and follow-up L2–S1 MR images. Logistic regression was used to investigate risk factors for ASD. RESULTS The rate of radiological ASD was 42.3% at the final follow-up (mean 25.71 ± 8.35 months). At surgical levels, the rFCSA and rTCSA of the MF and ES muscles decreased. The FI of the MF from L2–3 to L5–S1 and ES muscles at L5–S1 significantly increased after surgery, while the rFCSA and rTCSA of the PM muscle increased and its FI decreased. At adjacent levels, the rFCSA and rTCSA of the MF muscle and rTCSA of the ES muscle decreased and the FI of the MF muscle increased postoperatively (p < 0.05), but the rFCSA and rTCSA of the PM muscle increased and its FI decreased (p < 0.05). The FIs of the MF, ES, and PM muscles at adjacent levels significantly differed between the ASD and control groups. Logistic regression analysis indicated that higher BMI (p = 0.002) and FI of the PM muscle at adjacent levels (p = 0.025) were significant risk factors for ASD. CONCLUSIONS The functional area decreased in the MF and ES muscles and increased in the PM muscle after L4–S1 PLIF. A compensatory postoperative decrease in FI of the PM muscle at the adjacent level was a protective factor for ASD in DLSS patients after PLIF.