Objective:To investigate the changes of thickness and area of the ligamentum flavum after lateral lumbar interbody fusion (LLIF) for lumbar degenerative diseases.Methods:From 2019 to 2021, a total of 54 patients with lumbar degenerative diseases who underwent LLIF combined with percutaneous pedicle screw internal fixation were retrospectively analyzed. There were 9 males and 45 females, aged 59.46±6.91 years (range, 45-76 years), followed up for 14.69±6.87 months (range, 12-33 months). The disc height (DH), midsagittal canal diameter (CD), dural sac axial cross-sectional area (DCSA), ligamentum flavum area (LFA) and ligamentum flavum thickness (LFT) before and after surgery and at the last follow-up were evaluated and compared. Pearson correlation analysis was used to assess the relationship between the amount of change in the DCSA and LFA in the immediate postoperative period and at the last follow-up, as well as the correlation between the two and the amount of change in the DH. The data of patients at the last follow-up of 12 months after operation were extracted. Pearson correlation was used to evaluate the changes in DCSA and LFA at the last follow-up and the visual analogue scale (VAS) of low back pain and leg pain and Oswestry disability index (ODI) at 1 year after surgery.Results:All patients were followed up for 14.69±6.87 months (range, 12-33 months). The differences in DH ( F=354.93, P<0.001), sagittal CD ( F=44.78, P<0.001) and DCSA ( F=130.97, P<0.001) before, immediately after surgery and at the last follow-up were statistically significant. The DH, sagittal CD, and DCSA immediate after surgery and last follow-up were higher than those before surgery ( P<0.05). The differences in LFA ( F=51.59, P<0.001) and bilateral LFT ( F=53.49, P<0.001; F=50.53, P<0.001) before and after surgery and at the last follow-up were statistically significant, and both LFA and bilateral LFT at immediate after surgery and last follow-up were smaller than those before surgery ( P<0.05). Pearson correlation analysis showed that the change of DH immediately after surgery was moderately correlated with the change of DCSA ( r=0.57, P<0.001), and was strongly correlated with the change of LFA ( r=0.65, P<0.001). The change of DH at the last follow-up was moderately correlated with the change of DCSA ( r=0.43, P<0.001), and was weakly correlated with the change of LFA ( r=0.25, P=0.042). The differences in VAS-leg ( F=199.51, P<0.001), VAS-low back ( F=233.90, P<0.001), and ODI ( F=199.17, P<0.001) were statistically significant in patients before operation, 3 months after operation and 12 months after operation. There was no correlation between the changes of DCSA and LFA at the last follow-up and the changes of VAS and ODI at 1 year after operation ( P>0.05). Conclusion:LFA and LFT decrease and DCSA increase in patients with lumbar degenerative diseases after LLIF. LFA and LFT gradually decrease with time, and VAS and ODI are significantly improved compared with those before surgery. The DH loss caused by a certain degree of cage subsidence after surgery does not affect the clinical efficacy. There is no correlation between the improvement of DCSA and LFA and the improvement of clinical symptoms.