The Biomechanical Study Of The Upper Adjacent Level Lumbar Vertebrae In Spondylolysis

Objective:To study the upper adjacent level lumbar vertebral stability in spondylolysis of lumbar vertebra by biomechanical methods. Methods: 1,Nine fresh-frozen human lumbar spinal specimens were used for this study. The bony defects were created on the L5 pars articularis bilaterally. The range of motion(ROM) were measured by machines with 10N.m-moment, including extension -flexion, and axial rotation, bilateral compression, were applied on the specimens in two different conditions as follows: 1) intact spine; 2) bilateral pars defects on the L5. 2.The pressure of the upper adjacent level intervertebral discs of lumbar vertebrae were measured by a material testing machine(MTS 858 Bionix test system ), including extension -flexion, and axial rotation, bilateral compression, were applied on the specimens in two different conditions as follows: 1) intact spine; 2) bilateral pars defects on the L5. Result: (1).After creating the pars interarticularis defects at L5, mobility at the L4-L5 which were the upper lumbar vertebral in spondylolysis, were increased in all loading conditions. Mobility at the L4-L5 in bilateral pars defects were 12.71°,4.79°,3.71°,and in intact spine were 10.41°,3.70°,2.78°in which conditions of extension–flexion and axial rotation, the differences were significant (P < 0.01). The range of motion were compared with the intact specimens showed that the pars defects increased the mobility at the upper adjacent level (L4-L5) to 122.1% and 129.4% in extension -flexion and to 133.8% in axial rotation; However, the range of motion in bilateral compression was 7.74°of bilateral pars defects,it was 7.67°of intact spine, it were not different significantly (P>0.05); Mobility at the L5-S1 in bilateral pars defects were12.69°,6.29°,4.10°,and in intact spine were10.12°,5.19°,3. 01°in which conditions of extension–flexion and axial rotation, the differences were significant (P < 0.01). The range of motion were compared with the intact specimens showed that the pars defects increased the mobility at the level (L5-S1) to 125.4% and 144.1% in extension -flexion and to 136.4% in axial rotation; However, the range of motion in bilateral compression was 6.12°of bilateral pars defects, it was 6.06°of intact spine, it were not different significantly (P>0.05). (2).After creating the pars interarticularis defects at L5, the pressure of the upper adjacent level intervertebral discs of lumbar vertebrae, were increased in all loading conditions. The pressure were compared with the intact specimens showed that the pars defects increased to 101.3% and 101.5% and 101.7% in axial compression with 600N,800N,1000N, it were not different significantly (P>0.05), to120.97%,124.45%,129.79% in 15°of extension with 300N,500N,700N, the differences were significant (P < 0.01), to 114.15%,117.86%,124.92% in 15°of flexion with 300N,500N,700N, the differences were significant (P < 0.01), to103.54%,102.12%,101.14% in 15°of latero-compression with 300N,500N,700N, it were not different significantly (P>0.05). Conclusion: This biomechanical study exhibited that bilateral pars interarticularis defects made an effect on the intervertebral mechanics seriously, not only at the involved level but also at the upper adjacent level, and it can lead to instability of lumbar spine.