Assessment Of The Posterior Ligament Complex As Well As Its Effects To The Stability Of Thoracolumbar Spine-a Biomechanical Study

Objective:Thoracolumbar spine injuries or deterioration can bring aboutspinal weaknesses. physicians nowadays attach more and more importance tothe effects of the posterior ligament complex. The accurate evaluation andanalysis of its status will inevitably influence on determining therapeuticprinciple and corresponding strategies for traumatic thorac-lumbar fractures.The term instability is used to describe a wide variety of spinal conditions,including clinical, radiological and biomechanical abnormalities. The mostwidely used general definition is that of White and states that: The loss of theability of the spine under physiological loads to maintain its pattern ofdisplacement so that there is no initial or additional neurological deficit, nomajor deformity, and no incapacitating pain, Instability missed during theinitial management phase may produce or exacerbate a spinal cord injury orpermit displacement of fractures or dislocations. Thereby precipitating theneed for more invasive interventions. In the long term it can lead to chronicinstability, pain on movement, and a higher risk of degenerative changes,particularly if overall sagittal alignment is lost. By using embalmed vertebralbody specimens, this study was designed to investigate the posterior ligamentcomplex as well as its effects to the stability of thoracolumbar spine throughbiomechanical comparison.Methods:Eight embalmed specimens were harvested for this study. Allof them are of the same region, race, and sex. Their weights ranges from50-75Kg， and their heights ranges from163-176cm. Specimens were cleanedof all soft tissue and sectioned from T10to L4with care to preserve spinalligaments and capsules. Based on plain radiographs, dual energy x-rayabsorptiometry scans, direct inspection, and review of patient history, no specimens had fractures, tumor and hereditary malformation, or obviousosteoporosis. They were divided into group A and group B according to thetackling methods. Each group has4specimens. Then different forces wereloaded in seven directions: axial compression, flexion, extension, right and leftlateral bending, and right and left axial rotation. Group A：four specimens withintegrated PLC were fixed in a biomechanical testing frame. Following intacttesting, burst fractures were simulated by completely resecting the loweraspect of the L1vertebral body and removing the L1–L2intervertebral disc.Then pedicle screws were inserted in vertebral pedicle for fixation with fivedifferent methods: First, one-above-one-below (short construct) with anteriorfixation and bone grafting. Second, one-above-one-below with another one atthe fracture level. Third, one-above-two-below with anterior fixation and bonegrafting Fourth, two-above-two-below (long construct).Fifth two-above-one-below with another one at the fracture level. Specimens were tested randomlyin order to minimize the error caused by a single process. The two groupswere tested in the same experimental conditions. Group B：Four Specimenswith rupturing PLC made by Panjabi[34]method were fixed in a biomechanicaltesting frame. Following intact testing, burst fractures were simulated bycompletely resecting the lower aspect of the L1vertebral body and removingthe and L1-2intervertebral disc. Then pedicle screws were inserted invertebral pedicle for fixation with five different methods: First,one-above-one-below (short construct) with anterior fixation and bonegrafting. Second, one-above-one-below with another one at the fracture level.Third, one-above-two-below with anterior fixation and bone grafting. Fourth,two-above-two-below (long construct).Fifth two-above-one-below withanother one at the fracture level. Specimens were tested randomly in order tominimize the error caused by a single process. The two groups were tested inthe same experimental conditions. datum were measured during axialcompression, flexion, extension, right and left lateral bending, and right andleft axial rotation. In this study, our goal was to compare the properties ofloading-displacement, axial stiffness and torsional strength. A11of the datum were analysed by software—SPSS13.0,using paired sample T test to analyzethe difference,p＜0.05is statistical significance.Results: Under different stress, there is statistical significance betweenthe two groups (p＜0.05）.1、The displacement of thoracolumbar spine. Forexample, At the load of500N,the displacement of one-above-one-below (shortconstruct) with anterior fixation and bone grafting in axial compression,rupturing PLC increased by20.7%compared to integrated PLC（P﹤0.05）.Inlateral bending, rupturing PLC increased by20.6%compared to integratedPLC at the load of300N（P﹤0.05）.2、Axial stiffness in thoracolumbar spine:Axial stiffness means the resistance capacity to deformation undercompression. the formula is “EF=pl/△I”,“p” means axial load.“I” means theheight of the vertebral body,“△I”means the relative displacement of thevertebral body. Take the one-above-two-below with anterior fixation and bonegrafting in the status of500N for example. In contrast with integrated PLC,the rupturing PLC is lower in axial stiffness, and the ratio is1:0.83(P<0.01),which means that the axial stiffness is considerably lower inrupturing PLC3、The experiment of torsional strength: Take the torque of5Nm for example, rupturing PLC are11.4%more than integrated PLC at themeasurement of degree(P<0.01). Torsional strength of thoracolumbar spine isthe resistance capacity to deformation under torsion. The formula is“GJp=Mnl/ф”,in which,“GJp” means Torsional strength,“Mn” meanstorsion,“1”means the length of the specimen,“ф”means torsional angle. As canbe seen in the formula, because of the inverse ratio between“GJp”and“ф”,with the increasing of“ф”,Torsional strength of thethoracolumbar spine is decreasing.Conclusion: Posterior ligment complex is vital to the stability ofthoracolumbar spine. And the injury of posterior ligment complex will comeup with significantly unstable thoracolumbar spine.