Design Of A New Lumbar Dynamic Pedicle Screw System And Its Effect On Adjacent Segments

Objective: Fusion technology has been widely used in surgical treatment of spinal trauma and degenerative diseases,but there are still complications such as fusion failure,internal stabilization fracture,pseudojoint formation and so on.Moreover,the traditional pedicle screw rod system and interbody fusion cage also have a long-term impact on the adjacent segments.It is reported that the incidence of adjacent segment degeneration(ASD)caused by biomechanical changes after fusion is more than 30%.It is generally believed that the development of ASD is related to compensatory hyperactivity of adjacent segments.The indicators of load sharing in adjacent segments include the pressure in intervertebral disc and facet pressure.The abnormal stress acting on adjacent segments leads to the accelerated degeneration of stable structures such as intervertebral disc and facet,which leads to more instability and leads to a vicious circle.The dynamic stabilization device fills the gap between simple decompression and fusion stabilization in function.The ideal dynamic stabilization must maintain the normal range of motion(ROM)of the fixed segment as much as possible,which can not be too rigid,but also maintain a certain stability.This can delay the process of intervertebral disc degeneration and effectively prevent iatrogenic instability after facet joint surgery.Dynamic stabilization devices are generally divided into two categories: inter spinous stabilization devices and pedicle screws based stabilization devices.Although their designs are different,most dynamic stabilization devices lack resistance to the anterior and posterior shear force of lumbar vertebrae,affecting the sagittal balance of lumbar vertebrae,and the repeated extension load during daily activities leads to a higher risk of loosening.Objective the dynamic remodeling of bone is usually considered in the design of internal stabilization device for interbody fusion.Traditionally,titanium alloy is mostly used in posterior internal stabilization device,which has complications such as metal fatigue,stress shielding,screw loosening and so on.Dynamic stabilization may make the load transfer behavior of the fusion segment closer to the normal segment,promote the bone integration at the bone screw interface,share the anterior and posterior load of the spine and reduce the stress value of the pedicle screw.Therefore,we designed a dynamic pedicle screw fixation system,which retains a certain range of relative sliding between the dynamic screw and the connecting rod,shares the axial load from the posterior column to the anterior and middle column,increases the stress stimulation of interbody bone grafting,is conducive to the dynamic remodeling of bone,and finally promotes interbody fusion,improves the fusion rate or shortens the fusion time;At the same time,reduce the abnormal stress of adjacent intervertebral disc and articular process,prevent degenerative diseases,and delay the occurrence time of ASD after interbody fusion,which is conducive to postoperative rehabilitation;Reduce the risk of internal fixation loosening and fracture and create a relatively stable environment for screw bone interface integration.Methods: 1.Design the lumbar posterior dynamic pedicle screw system,including dynamic pedicle screw(hereinafter referred to as dynamic screw),half moon pad,fixing ring,top wire and connecting rod.The upper end of the pedicle screw rod is provided with a U-shaped liner made of high cross-linked ultra-high molecular polyethylene.The half moon liner is located above the screw U-shaped liner,which is combined to form a circular space for the connecting rod to pass through,and a small amount of clearance is reserved between it and the connecting rod for sliding.The connecting rod is made of cobalt chromium molybdenum alloy.The jacking screw is located above the half moon pad.After the two are assembled and embedded,they are used for fixing the connecting rod.After locking,the universal ball head of the screw retains a certain degree of activity for the swing of the screw rod.2.After the preparation of 12 fresh porcine spine specimens,they were divided into dynamic group and rigid group.First,measure the ROM,take the specimen and fix it in the neutral position,and apply the axial compression force.Continuous torque is applied in six directions: flexion and extension,lateral flexion and rotation.The movement track of the specimen is continuously recorded by the photoelectric camera,the position of the spine from zero load to 7.5N·m load is recorded,and the angle information is converted.After the dynamic pedicle screw and ordinary pedicle screw were placed in groups,the ROM of each segment in each direction was recorded and compared.According to the Panjabi loading protocol,force it to the same maximum ROM value as previously recorded,and compare the changes of each segment’s participation in the overall ROM.Then measure the pressure of intervertebral disc and articular process,put the pressure sensor completely into the joint and fibrous ring,apply 7.5N·m in 6 directions after applying axial compression force,read the pressure peak data of each sensor and output the retention results.3.Import the CT scanned L1-L5 lumbar spine image into the MIMICS program,create the generated model,import it into ANSA for meshing,assign the corresponding elastic modulus,Poisson’s ratio and other material parameters to each component,create the ligament according to the actual situation,set the material characteristics,and import it into ABQUS for calculation.Apply a vertical downward load on L1,and then apply torque in the six free movement directions of flexion,extension,lateral flexion and rotation to generate stress nephogram and displacement nephogram.L4-L5 segments were selected as fixed segments,and four internal stabilization methods were loaded in groups: dynamic stabilization group(S group),fusion stabilization group(R group),dynamic + interbody fusion cage group(S+IF group),fusion + interbody fusion cage group(R +IF group).The stress distribution characteristics of intervertebral disc,pedicle screw,connecting rod and intervertebral fusion cage under four groups of six direction working conditions were analyzed by computer finite element simulation.Results: After the design and manufacturing of the lumbar dynamic pedicle internal stabilization system,the ROM measurement showed that the dynamic group retained about 1 / 3 of the range of motion in the directions of forward flexion,backward extension and lateral flexion,but only 15.9% remained in the rotation.The ROM of adjacent segments in each group increased,especially L3-L4 segments.The increase of L1-L2 segments in dynamic group decreased slightly.There was no significant difference between the two groups on the effect of rotating ROM.In the flexion extension and lateral flexion States,the ROM curve of the dynamic group is closer to that of the complete state.Under the forced loading mode,the flexion extension and lateral flexion directions of L4-L5 segments in the dynamic group retain about 9% mobility,and only4.4% mobility in the rotation direction;L3-L4 segment has the highest overall participation.Under the condition of anterior flexion,the disc pressure of adjacent segments in the dynamic group and the rigid group increased,and the pressure in the dynamic group was lower than that in the rigid group.In the extension state,the pressure of articular process increased to 117.8% in the L3-L4 segment dynamic group and 130.3% in the rigid group.In the finite element analysis,the stress of L1-L4 intervertebral disc in each internal stabilization group was significantly higher than that in the normal state.Under flexion condition,group s and group S+IF increased by about 20% and 40% respectively,while Group R increased further than Group s and group R+IF than Group S+IF.The overall displacement amplitude in the six directions decreased,which was more obvious in group R than in group S and group R+IF than in group S+IF.Under each working condition,the stress of group S screw and connecting rod is less than that of Group R under the same working condition,and the internal stress distribution of group S screw is more uniform than that of group R.Conclusion: Through in vitro biomechanical analysis and computer finite element analysis,compared with rigid stabilization,dynamic pedicle screw system can maintain a certain stability,especially rotational stability,retain part of the range of motion,and reduce the compensatory activity of adjacent segments and the pressure of intervertebral disc and articular process.The stress of the screw and connecting rod in the dynamic internal stabilization system is less than that in the rigid internal stabilization system,and the stress distribution is more uniform;The compression effect of interbody fusion cage is stronger than that of rigid pedicle screw.It can be inferred that dynamic stabilization will reduce the incidence of degeneration of adjacent intervertebral discs and facets,improve postoperative lumbar function,promote interbody fusion,improve fusion rate or shorten fusion time;Reduce the failure risk of broken and loose screws and connecting rods.The fatigue,wear and toxicological experiments of dynamic pedicle screw system will be further studied by our research group.