Biomechanical Evaluation Of The U Shape Lumbar Biological Dynamic Fixation System

Background:Low back pain is a frequently-occurring orthopedic disease,which is caused by inflammation(tuberculosis,infection),cancer,trauma and degeneration (spondylolisthesis,instability,multi-segmental intervertebral disk hernia,spinal stenosis and iatrogenic factors).As the trend of aging process going on and the pressure for life and work being increasingly intensified,the incidence rate of Low back pain has been increased gradually,which has seriously affected patients daily life and led to great economic losses.Currently,the efficacy of the common non-surgical treatment is poor.Implementing a strong fixation and fusion on basis of the decompression is of key importance on the treatment of low back pain caused by lumbar spinal instability.Although a lot of patients had been effectively integrated, the clinical effect is still poor.It is possible to support most of the load by the strong fixator which can greatly cut down the load shared by the fixed sect,while resulting in stress-shielding effect which can change the load of adjacent segment of intervertebral disk and the small joints,expand their activity and increase pressure of intervertebral disk.All of these result in a series of questions such as:a decline in the quality of bone fusion,the osteoporosis of the fixed segment,bone atrophy,the degeneration of adjacent segment of the intervertebral disk and facet joint,the stress caused by overly focusing on the broken nails,broken rods and so on.These urge the spinal surgeons to think about the real causes of chronic low back pain and the possibilities of applying the non-fusion technology to treat the lumbar degenerative disease.On the premise of unsacrificing the posterior dynamic stabilization,the ideal method increases load sharing and reducing stress shielding and stress concentration. Thus,the concept of posterior dynamic stabilization(semi-firm fixation,flexible fixation,elastic fixation,PDS:posterior dynamic stabilization) was proposed and gradually admitted in the world.Mulholland and the others think that the direct cause of low back pain is the abnormal and non-anomalous activities of intradiscal stress distribution patterns caused by abnormal activities.They also put forward the concept of dynamic stabilization.Dynamic stabilization,also known as flexible fixation or elastic fixation,that is,by using of internal fixation devices,it can not only stabilize spinal segments but also allow them to move in a limited area to increase the load sharing and reduce stress shielding and stress concentration.This concept is based on the hypothesis "shoes with the sand" of the causes of low back pain,that is,the normal intervertebral disk has the natural characteristics of isotropic structure. Normal nucleus pulposus is composed by proteoglycans and the glias in uniform nature.However,the glias for degeneration of intervertebral disk become inhomogeneous fragment and being reduced to be gels,with a mixture of water-bearing gas inside the tissue.Sometimes,isolated annulus fibrosus or endplate debris can enter into the internal disk.Those abnormal and loose pieces like the stones in shoes,resulting in the high centralization of stress and the pain caused by the asymmetry distribution of the load passing the endplate,the physiological condition of the transmitting method for load and the change of the sharing mode.In addition,through abnormal load distribution of intervertebral disk,the pain can also be caused by the abnormal direction and degree for spinal segmental displacement.Therefore,it is considered that simply preventing abnormal displacement is not the factor to ease back pain.It is important to create a normal pattern of load bearing for clinical success,thus a stationary but non-integration new concept comes up.Panjabi MM and the others think that the range of Motion for spine can be divided into the neutral zone and flexibility zone.Neutral zone is a non impedance excitable area,from the joint neutral position to the elastic displacement of the starting point,while flexibility zone is the impedance excitable area from the point of origin to point of maximum displacement for resistance exercise area,from the origin point of elastic displacement to the point of maximum displacement.The impedance in the flexibility zone come from ligament stretch and in the neutral zone the ligament will maintain relaxation.The purpose of dynamics internal fixation is to fix spinal motion segment so that it is possible to maintain in the neutral zone activities.By controlling the lumbar segments of the abnormal movements,it can provide the physiological states which are close to load conduction mode and load-sharing mode and displacement for pain relieving to prevent approaching segmental degenerative diseases.Objective:To evaluate biomechanical characteristics of a new U shape lumbar biological dynamic fixation system(USLBDFS) through biomechanical tests.To be compared with traditional rigid fixation to provide a theoretical basis for further clinical application of this system.Methods:Select 6 pieces of fresh specimens of calf lumbar(L2-L5),excluding diseases such as lumbar trauma,tumors and hyperosteogeny.Use multilayer preservative film to wrap the specimens,and save them in -20℃environment,in which there are no significant changes to the bone and ligament biomechanical properties for preserving fresh specimens of the calf.Before the trial,put the specimens under room temperature and humidity to thaw out for 12-18h,carefully remove the muscles,fat and connective tissue around the vertebral body and retain the integrity of lacertus medius and ligamenta supraspinale,then used UHMWPE materials polymethyl methacrylate to embedded L2 & L5 vertebral body separately.Based on the test sequence,it is divided into the experimental model of normal group,injury instability group,rigid fixation fixed group and U-shaped elastic biological fixation group.Separately put the above four kinds of specimens to successively connect to the load plate and the test-bed of the spine three-dimensional motion experimental aircraft,imposed 8 Nm of flexion,extension,left bending,right bending,left rotation and right rotation of pure torque on the spine specimens,made the spine move in the same way.From the images recorded by the laser camera on load zero and the largest load(8Nm) when the state of spinal exercise,by computer image processing system to calculate the angle between segmental spinal change, analysis with load the same direction as that of spinal range of motion(range of motion,ROM),calculated the ROM between L3 and L4.According to the experimental sequence,flexion-extension compression test has been tested on 6 specimens by using of MTS 858 testing machine in turn.These 6 six specimens are in four states,which are integrity,injury and the after using rigid, flexible fixed on.Calculate Stiffness and then analyze statistically.Results:1.Three-dimensional Spinal Exercise Test(Test packet) Group A:normal Group B:spine instability model Group C:rigid internal fixation system fixed Group D:a flexible fixation system fixed group Flexion and extension exercise:significant differences exist between group C and group D and group A,so as group C and D,with statistical significance.Group D increased by an average of 2.220 than group C in flexion and extension ROM,the difference was significant(P＜0.05).About scoliosis exercise:group C and group D and group A,exist significant differences,significantly differences existed between group C and D under the right side of bending(P＜0.05),but there was no significant difference under left side bending state(P＞0.05).2.Compression Stiffness DeterminationThe results show that there is a significant difference between group C and the other groups under the flexion condition(P＜0.05),there is a significant difference between group D and group C,with statistical significance(P＜0.01).When under extension condition between group D,group C and group A,B,there is a significant difference(P＜0.05),between group D and group C,there is a significant difference(P＜0.01) with statistical significance.Conclusion:The biomechanical test shows that flexible fixation and rigid fixation can significantly enhance segmental stability in flexion and extension on the direction of flexion and extension,left and right lateral bending movements.At the same time, elastic fixation stiffness is lower than rigid fixation,and the difference is significant (P＜0.01),thus flexion and extension ROM is significantly wider than that of the rigid fixation.Therefore,when the application of flexibility in the spine is fixed,the load sharing will be increased and the stress-shielding effects will become smaller correspondingly.Thereby,in order to create an ideal environment for the mechanics of a fixed segment and adjacent segments,it is necessary to cut down postoperative spinal stress shielding and stress concentration,to reduce the fixed segment of osteoporosis,bone atrophy,adjacent segment of the disc,facet joint degeneration,etc.