| BackgroundIn the United States,the annual incidence of low back pain(LBP) is 60%-80%. According to preliminary statistics from Peking University Third Hospital,the incidence of LBP is up to 60%in China each year.At present,surgery is manegment,which include the rigid internal fixation,the dynamic of internal fixation and the replacement of artificial lumbar intervertebral disc.Lumbar fusion and rigid internal fixation are often applied in the treatment of a wide variety of spinal disorders.However,there are numerous clinical studies showed accelerated disc degeneration adjacent to fused and/or rigidly instrumented segments.Degeneration developed at mobile segments above or below a fused spinal segment is known as adjacent segment disease(ASD),which is a term with broad meaning,since it can describe nearly any abnormal process that develops in the mobile segment next to a spinal fusion.ASD has been found to occur more often than originally expectation and is now considered as a potential long-term complication of spinal arthrodesis. With dramatic increase in spinal fusions sugery performed in recent years,ASD is becoming much more widespread.The development of ASD is problematic because it can necessitate further surgical intervention and adversely affect functional outcomes.In order to maintain the mobility of a motion segment and to prevent negative effects on adjacent segments,dynamic non-fusion systems have become more and more popular.The systems range from completely replacing the intervertebral disc with a prosthesis,replacing the nucleus with an elastic spacer while preserving the annulus,to a device that maintains a reduced and controlled motion on the segment.However,clinical outcome and radiological results after dynamic fixation are inhomogeneous and not always satisfactory.The efficacy of dynamic non-fusion systems is still a matter of discussion.ObjectiveAim of the current study was to elucidate the intervertebral disc and strain of adjacent segments with a device controlling the ROM of L3/4 and to evaluate the effect of the stabilization on the adjacent segments.Our data would help to determine the best ROM of single segment and promote the further improvement of ideal dynamic internal fixation devices.Material and methodsInstrument and apparatusMTS 858 bionix machine,Three-dimensional motion tester,Three-dimensional laser scanner,YE2538A programmed static strain gauge,Micro-pressure sensors with 5mm in diameter,Resistance strain gauge(Model:BE120-03AA(11),base size: 2.7×2.7mm),Digital Inclinometer with measurement accuracy at 0.01°,Lumbar single segment motion control devices with ROM control at 0～10°,α-cyanoacrylate ester,Silicon glue,Alcohol,Acetone and surgical instruments. Specimen and treatmentSix normal adult lumbar spine specimens(L1-S1),were carefully removed the surrounding muscles and kept ligaments and facet joint integrity,After embedded in PMMA(polymethylmethacrylate) cement,the specimens were sealed in double plastic bags and stored at -20℃.Before the experiment,the samples were placed at room temperature for 9 hours to thaw completely.Experimental methodsThe bone tunnel for disc pressure measurement at coronal was prepared on L2 vertebral body,and then implanted the rods on L3 and L4 vertebral body in accordance with the requirements of a single lumbar segment motion control devices. To paste the resistance strain gauges,the end of L2 and L5 pedicle must be polished, degreased the surface of bone with acetone and cleaned acetone with alcohol.In accordance with the requirements of the strain gauge adhesion,long-axis of strain gauge was parallel to the direction of the pedicle.According to the specific local circumstances of the bone,the appropriate substrate size of strain gauge was chosen and lines of strain gauge were appropriately connected.Loading experiments were performed under the control of MTS in a confrontational way.Data were collected in a fully automated way controlled by computer,which improved the quality and accuracy of data.The intervertebral disc pressure sensor connected to MTS,which was related to the software of the automated data collection.Y2385A static strain gauge program connected with the computer via the serial interface and then collected data through the matched computer software. Statistical analysisData were compiled with the software package SPSS 13.0.All data were recorded in a mean±SD way.The linear relationship between strain and loading was analyzed using curve fitting analysis.ANOVA for two repeated measurement data was applied to test the significance of differences of the L2/3 intervertebral disc pressure in the intact and injured spine for flexion,extension and lateral bending.ANOVA for three repeated measurement data was employed to determine whether the differences of the L2 strain in the intact and injured spine for flexion,extension and lateral bending had statistical significance.The level of significance was defined as P＜0.05.ResultsThe changes of the L2/3 intervertebral disc pressure for the different ROM of L3/4 and loading in different planeIn flexion,the changes of the L2/3 intervertebral disc pressure occurring during the ROM of L4/5 from 0°to＞6°were significantly different.However,for extension and lateral bending,the differences of pressure change were not statistically significant.When the ROM of L4/5 was 0-6°and＞6°,the L2/3 intervertebral disc pressure remarkably decreased compared to the intact spine.In all tested motion planes,the changes of the L2/3 intervertebral disc pressure occurring during the course of loading from 2NM to 8NM were significantly different. Furthermore,our data revealed that the L2/3 intervertebral disc pressure showed a linear increase as the loading increased in flexion and lateral bending. The linear relationship between pedicle strain and loadingTo determine the effect of resistance strain gauges when they were pasted in the pedicle,we measured the strain of L2 pedicle during the course of gradual loading from 50N to 400N.Our data showed that there was a linear relationship between pedicle strain and loading of spine.The changes of L2 pedicle strain for the different ROM of L3/4 and loading in different planeIn all tested motion planes,the changes of the L2 pedicle strain occurring during the ROM of L4/5 from 0°to＞6°showed no statistically difference.However,in flexion,extension and lateral bending,the changes of the L2 pedicle strain occurring during the course of loading from 2NM to 8NM were significantly different. Moreover,the L2 pedicle strain showed a linear increase along with the increase of spine loading.In addition,no statistically differences were found between the left and right pedicle strain not only in flexion/extension but only in lateral bending.Conclusions1.The ROM of a motion segment should be maintained to＜6°,because this ROM has the smallest effects on adjacent segment.2.The intervertebral disc pressure of adjacent segment has no linear correlation with the loading in extension,suggesting that extension has smaller influence on the intervertebral disc pressure than flexion and lateral bending. 3.The main spine loading is borne by facet joint,so keeping a lasting extension status will lead to facet joint degeneration.4.The biomechanical influence of rigid internal fixation on adjacent segment is positively correlated with the spine loading.5.The left and right pedicle strain has no significant difference in flexion,extension and lateral bending.6.The resistance strain gauges pasted on the pedicle have satisfactory results for measurement of the spine posterior column loading in all tested planes,suggesting that it is a good method used to measure the strain of spine posterior column. |
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