Biomechanics Research On Nonfusion Technique Using Ni-Ti Alloy For Treatment Of Degenerative Lumbar Spondylolisthesis

Objective Degenerative lumbar spondylolisthesis (DLS) is a common spinal disease and also one of the major causes of low back pain. Long-term outcome of traditional surgical treatment for DLS is not satisfactory due to adjacent segments degeneration and sagittal imbalance. The purpose of this study is to evauluate the biomechanics property of posterior nofusion technique unsing Ni-Ti interfixation.Methods Digital model of DLS was established by Simleware and Ansys software. Biomechanics properties of DLS was evaluated by Ansys stimulation. Posterior nonfusion technique using Ni-Ti shape memory alloy was designed and assessed by digital stimulation and cadaver biomechanics experiment by strain gauge test. Biomechanics characteristics of lumbar spine after Ni-Ti fixation or Ti rod instrumentation was compared by Ansys stimulation and specimen experiment. Results Under the states of flexion and extension, the stress on L4-L5 when flexion is more than extension, but L5-S1 indicated no significance under the two states. Statistics analysis of detailed strength of stress confirmed that under the axial loading, stress between L4-5 disc was more than L5-S1, but stress between L4-5 facets was less than L5-S1. Stress on L5-S1 facts was much more than the same segment disc. analysis on Ni-Ti alloy compared with Ni alloy fixation and fusion were as follows: under the condition of axial loading, flexion-extension, side bending, Ni-Ti group and fusion group indicated no statistics difference (P>0.05) of stress on L5-S1 disc. But stress on L5-S1 facet joints of Ni-Ti group showed significant decrease than that of fusion group (P<0.05). Results of biomechanical experiment demonstrated that under the condition of axial loading and extension, most decrease of strain on L5-S1 facets was observed when the specimens were fixed by Ni-Ti rod. And similar distribution of strain was observed at the rest segments. Compared to the FE analysis of the first part and literature review, the mean stiffness has no statistics differences, which prove the efficiency of the FE stimulation. Conclusion When spondylolisthesis between L4-5 occurred, stress concentration can be observed on front annulus fibrosis at L4-5 disc and facet joints at L5-S1. This kind of barycenter translation may explain aggravation of low back pain caused by accelerating degeneration of posterior column due to spondylolisthesis. Digital stimulation indicated stress concentration on L5 bilateral pars can be seen most significantly whenever extension or flexion. And most stress increase occurred during flexion. The local intensity of stress makes it possible to transfer DLS to isthmic spondylolisthesis and aggravate the degree of slippage. Cadaver experiment verified that load bearing at L4-5 and L5-S1 facets showed significant decrease after Ni-Ti fixation. The stress loading acquired redistribution among the fixed lumbar spine.