Objective:To investigate the mid-term clinical efficacy and imaging changes of Waveflex semi-rigid internal fixation system combined with posterior lumbar interbody fusion (PLIF) in the treatment of double segmental lumbar degenerative diseases.Methods:The data of 51 patients with lumbar degenerative diseases who underwent surgery from September 2014 to September 2015 were retrospectively analyzed, including 29 males and 22 females, aged 65.5±5.6 years (range 58-73 years). Preoperative intervertebral space degeneration grade by University of California at Los Angeles (UCLA) and Pfirrmann intervertebral disc degeneration grade were recorded. 23 cases of primary responsible segments were treated with decompression, fixation and fusion, and adjacent secondary responsible or degenerative segments were treated with Waveflex semi-rigid internal fixation (combined group); 28 cases of double segments were treated with decompression, fixation and fusion (fusion group). Disc height index (DHI) and intervertebral foramina height (IFH) of the semi-rigid fixation segments, DHI and IFH of the upper adjacent intervertebral space, and horizontal displacement of the upper adjacent vertebral body (HD) were measured on lateral X-ray films of lumbar spine; In the fusion group, DHI and IFH adjacent to the upper vertebral space and HD adjacent to the upper vertebral body were measured. The efficacy was evaluated by short-form McGill Pain Questionnaire (SF-MPQ) and Oswestry disability index (ODI).Results:51 cases were followed up for 5.4±0.3 years (range 5.2-6.3 years). The low back and leg pain and function in the combined group and fusion group were significantly improved compared with those before operation. SF-MPQ and ODI at 3 months, 1 year, 5 years after operation were significantly different from those before operation ( P<0.05). In the combined group, the DHI of semi-rigid internal fixation segments before operation and 3 months, 1 year, 5 years after operation were 37.8%±7.6%, 37.9%±7.4%, 36.5%±6.9% and 36.0%±7.1% respectively ( P>0.05); The IFH of semi-rigid internal fixation segments before operation and 3 months, 1 year, 5 years after operation were 21.5±2.8, 21.4±2.8, 20.4±2.7, 19.4±2.4 mm respectively ( P<0.05); The DHI of the upper segment adjacent to semi-rigid internal fixation before operation and 3 months, 1 year, 5 years after operation were 37.1%±9.3%, 36.8%±9.1%, 35.2%±9.1%, 33.9%±8.8% respectively ( P>0.05); The IFH of the upper segment adjacent to semi-rigid internal fixation before operation and 3 months, 1 year, 5 years after operation were 21.9±3.0, 21.4±3.0, 20.4±2.9, 19.5±2.7 mm, respectively ( P<0.05). The HD of upper vertebral body adjacent to semi-rigid internal fixation before operation and 3 months, 1 year, 5 years after operation were 2.2±0.7, 2.3±0.5, 2.5±0.5, 2.8±0.5 mm respectively ( P<0.05). At the last follow-up, one case of semi-rigid titanium rod fracture, one case of screw loosening at semi-rigid internal fixation segment, three cases with unsatisfied numbness relief, and 2 cases of facet joint spontaneous fusion at semi-rigid fixation segment occurred in the combined group. Conclusion:Waveflex semi-rigid internal fixation can protect the degenerative lumbar intervertebral disc, and delay the degeneration of semi-rigid internal fixation segment and adjacent upper segment after interbody fusion, but long-term follow-up and study are needed.