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A retrospective study.To investigate the relationship between trunk muscle cross-sectional area (MCSA) and fusion rate after posterior lumbar interbody fusion using pedicle screw fixation (PLIF-PSF).Although trunk muscles of the lumbar spine contribute to spinal stability and alignment, effect of trunk muscles on spinal fusion rate and time to fusion is unclear.A total of 192 adult patients with degenerative lumbar disease who underwent PLIF-PSF at L3–L4 or L4–L5 were included. The MCSA of the flexor (psoas major, PS), extensor (erector spinae, ES; multifidus, MF) were measured using preoperative lumbar magnetic resonance imaging at 3 segments. Bone union was evaluated using lumbar dynamic plain radiography. Patients were divided into 2 groups according to the presence of bone fusion.Most PS MCSAs in the fusion group were significantly larger than in the nonfusion group, except for MCSA at the L2–L3 segment (all P<0.05). In cases of ES and MF MCSAs, 4 of 6 segments were significantly large. Multivariate analysis revealed that the PS MCSA at L4–L5 was an independent factor for decreased possibility of nonfusion status in both segments (OR=0.812, P=0.028). Pearson analysis demonstrated that the most trunk MCSAs were negatively correlated with time to fusion for both segments and PS MCSAs exhibited a significant correlation with time to fusion except for MCSA at the L2–L3 segment.Trunk MCSAs were significantly larger for a fusion group than a nonfusion group. As trunk MCSAs increased, fusion timing decreased.