Background Adjacent segment degeneration could seriously affect the long-term prognosis of lumbar fusion.Dynamicfixation such as the interspinous fixation,which is characterized by retaining the motion function of the spinal segment,has obtained satisfactory short-term effects in the clinical setting.But there are few reports about the biomechanicalexperiments on whether dynamic fixation could prevent adjacent segment degeneration.Methods The surgical segments of all 23 patients were L4/5.Thirteen patients with disc hiation of L4/5 underwentWallis implantation surgery,and 10 patients with spinal stenosis of L4/5 underwent posterior lumbar interbody fusion(PLIF).L3-S1 segmental stiffness and displacement were measured by a spine stiffness gauge (SSG) device duringsurgery when the vertebral plate was exposed or during spinal decompression or intal fixation.Five fresh,frozencadavers were used in the self control experiment,which was carried out in four steps:exposure of the vertebral plate,decompression of the spinal canal,implantation of a Wallis fixing device,and PLIF of L4/5 after removing the Wallis fixingdevice.Then,L3-S1 segment stiffness was measured by an SSG device.Results The experiments showed that the average stiffness of the L4/5 segment was (37.1±8.9) N/mm after exposure of the vertebral plate,while after spinal decompression,the average stiffness fell to (26.2±7.1) N/mm,decreasing by 25.8％ (P ＜0.05).For the adjacent segments L3/4 and L5/S1,their stiffness showed no significant difference between the L4/5 segment decompression and the exposure of the vertebral plate (P ＞0.05).After Wallis implantation of L4/5,the stiffness of the cephalic adjacent segment L3/4 was (45.8±10.7) N/mm,which was 20.5％ more than that after the exposure of the vertebral plate (P ＜0.05); after L4/5 PLIF surgery,the stiffness of L3/4 was (35.3±10.7) N/mm and was decreased by 12.4％ more than that after the exposure of the vertebral plate (P ＜0.05).The stiffness of the cephalic adjacent segment L3/4 after fixation in the Wallis group was significantly higher than that of the PLIF group (P ＜0.05).Cadaver experiments showed that the stiffness of the cephalic adjacent segment in the Wallis group was significantly higher than that of the PLIF group after L4/5 segment fixation (P ＜0.05); the stiffness of the L5/S1 segment showed no significant difference between PLIF surgery and Wallis implantation (P ＞0.05).Conclusions After interspinous (Wallis) fixation,the stiffness of the cephalic adjacent segment increased.After PLIF with pedicle screw fixation,the stiffness of the cephalic adjacent segment decreased.An interspinous fixation system (Wallis) has a protective effect for cephalic adjacent segments for the immediate post-operative state.