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Objective:We present the investigation of the biomechanical factors of the degeneration process after lumbar fusion. Methods:Thirty-five patients underwent L4-5 fusion operation. We measured radius(cm), top angle(TA)(degree), and flexion-extension angle(FEA) at L3-4 joint in pedicle screw fixation(PSF)(n=27) and posterior lumbar interbody fusion(PLIF)(n=8)groups. They were divided into early(3 to 6 months) and late(over 1 year) groups according to post-operative period. Results:The pre-operative radius, TA and FEA in PSF were 6.2±0.4, 9.0±0.8, and 8.2±0.8. The early and late post-operative radius, TA and FEA in PSF were 6.4±0.5, 6.9±0.8, 6.8±0.8, and 5.5±0.7, 12.1±0.9, 11.0±0.9, respectively. The pre-operative radius, TA and FEA in PLIF were 5.4±0.5, 8.9±0.8, and 9.5±1.2. The early and late post-operative Radius, TA and FEA in PLIF were 5.0±0.4, 5.8±0.9, 5.2±0.7, and 5.0±0.5, 8.4±0.9, 7.6±0.8, respectively. In late PSF, TA and FEA were significantly increased(p<0.05)but radius was significantly decreased(p<0.05)compared with pre-operative ones. But in late PLIF, all the factors were decreased compared with pre-operative ones. Conclusion:Biomechanical factors of chronic complications were different between PSF and PLIF. Increased FEA seems to be an important factor for post-operative degeneration in PSF, whereas increased stiffness and stress seems more important in PLIF. Key words:Degenerative complications;Pedicle screw fixation;Posterior lumbar interbody fusion;Biomechanical factors.