| Background:Traditional transpedicular fixation is a major spinal rigid fixation, and has been widely used in clinic as it offers biomechanic stability, and is an easier and safer practice. The treatment with rigid fixation and spinal fusion has been regarded as a gold standard for spine surgery. Even though the application of rigid fixation increased the fusion rate and improved the clinical outcome, the clinical satisfaction rate was still lower than the fusion rate, which did not related to the improvement of clinical outcome significantly. There are complications, such as accelerated degeneration of adjacent segments, loosening and rupture of internal fixators, associated with the rigid spinal fixation.To address the problems caused by the treatment with rigid fixation and spinal fusion, a dynamic fixation technique has been emerged, which may provide the stability required for the fixed segments and reserve the segment motion to a certain extent while enabling the anterior vertebral bodies to share more loads and reducing stress of the instrument. A transpedicular dynamic fixation system may restrict the excessive spinal motion while allowing part of motion for the fixed segments. It shares load with the spine column and may reduce degeneration of the adjacent segments and failure of implants. At present, the posterior transpedicular dynamic fixation instruments are mainly classified into the following categories:(1) Pedicle screw-ligament system, such as Dynesys system;(2) Semi-rigid fixation rods, such as Isobar TTL;(3) Dynamic pedicle screw, such as Cosmic system;(4) felxible materials, such as PEEK rod;(5) Smaller rods with diameter of3.0mm or4.0mm. However, the current dynamic implementation mechanisms for dynamic fixation are mainly a combination of multiple factors. Still, it has been hard to explore the exact mechanism for dynamic implementation of fixation, and the biomechanical study on various fixation hasn’t been investigated enough. To address these issues, the present study aims to anaylze each possible mechanism implementing dynamic fixation and conduct an independent analysis for each, and discussed the biomechanical effect produced by each dynamic fixation, so as to find a optimal fixation. After reclassification of the modes described above, we called the fixation modes using the ligament system, plastics and thinner Ti-rods "flexible fixation", and called other fixation modes "dynamic fixation". Moreover, we developed various representative dynamic and flexible fixation instruments and evaluated their biomechanical performance in the process of in vitro biomechanical testing.Objectives:Develop a spine testing machine applying a pure moment to the spine specimen in flexion,extension,lateral bending and axial rotation.Develop four types of dynamic fixation instruments with sliding-rotating rod, linear sliding rod, ball-socket screw, and rotating screw.Compare the difference in three-point bending between the dynamic and flexible fixation.Evaluate the effects of the dynamic and flexible fixation on the spinal stability and load sharing.Discuss the effects of sliding rod and rotating screw in stability/load distribution of different sliding modes and different rotation angles respectively.Evaluate the performance of anti-compression of different dynamic and flexible fixation.Methods:1. Development of spinal mechanical testing machineTo meet the requirements of our study, this paper presents a spine testing machine that, as a type of "loading device based on the joint-arm", may provide a pure moment in different direction for spine specimen.2.Design of dynamic and flexible fixationThe dynamic and flexible fixation instruments in test were in comparison with "the5.5mm-in-diameter rod with the locking fixation of the screw and rod".(1) Flexible fixation instrument:In our study, it specifically referred to the3.0mm-in-diameter titanium rod, which was made of the same material as the5.5mm-in-diameter rod.(2) Dynamic fixation instruments:a. Sliding-rotating rod:The vertical connecting rod for posterior fixation was designed dynamically. A joint was set up in the middle of the rod. The joint had one end fixed with a5.5mm-in-diameter titanium rod; meanwhile, another5.5mm-in-diameter titanium rod was inserted into the joint. Thus, the original rod could slide freely and rotate axially in this joint.b. Linear sliding rod:Its design was similar with the sliding-rotating rod. The main difference between them was that the linear sliding rod could only slide axially without rotating axially.c. Ball-socket screw:It was similar with the polyaxial pedicle screw clinically used at present. Our design enabled it to rotate polyaxially in the range of10°after the locking of screw-rod joint.d. Rotating screw:Its design was similar with the ball-socket screw described above. Based on this, its design was improved. The rotation angle was restricted to the range of6°, and the rotation joint underwent a special strengthening treatment.3.Evaluate the performance of resistance to three-point bending of different dynamic and flexible fixation.The polyvinyl plastic blocks were used to simulate the vertebral bodies. Pedicle screw was placed into the vertebral bodies,and then fixed with the testing instruments. The midpoint of the connecting rod served as the loading point for the three-point bending test. Instron E1000was used for loading. For different instruments, their specific bending displacements were tested.The testing instruments included:a. the5.5mm-in-diamater rod,as a rigid fixation; b. the3.0mm-in-diamater rod; c. the sliding-rotation rod; d. the linear sliding rod. Pull out the inserted rod by0,1/4,2/4and3/4respectively for loading of the two sliding rods, in order to test the variation of their three-point bending strength during the elongation to shorten process of the sliding rod.4. Study on stability and load sharing of spines implanted with dynamic and flexible fixatorsIn this part of test, we carried out the in-vitro biomechanical testing on the pig specimens(n=7,L2-L5). The testing status included:a. the intact status(Intact); b. the injured status,:the staus of wide posterior decompression (including the bilateral small joints) for L3/4(WPD, state A); c. the additional status based on WPD, the state of discectomy and intervertebral cage fusion for L3/4(Cage, state B). After the pedicle screw was inserted, each specimen in the two injured status underwent the fixation treatment in the two injured modes respectively. The testing instruments included:the5.5mm-in-diameter titanium rod,3.0mm-in-diameter titanium rod, sliding-rotating rod, linear sliding rod, ball-socket screw.The spinal testing machine was used to apply a pure moment of5Nm in directions of flexion/extension, left/right lateral bending and left/right axial rotation respectively on each specimen.The repeated measures design was used in test. Each specimen was firstly tested in intact status, and then underwent the treatment of5.5mm-in-diamater rod fixation, dynamic and flexible fixation respectively in the two injured status. In this process, it was unnecessary to replace the pedicle screw. In order to reduce the influence of testing order, the fixation instruments were used for testing in a random order.The test focused on the following two features:①Evaluation of stability:With the infrared marking points placed on the appropriate vertebral bodies which connected via K-wire. Optotrak Certus3D motion measurement system was used to collect the motion data of those marking points, so as to calculate the range of motion (ROM) and the the neutral zone (NZ) of the segments.②Evaluation of load sharing:The method of attaching a resistance strain gauge to the anterior column of the spine, which make an indirect testing to reflect load sharing. Specifically, a strain gauge was attached to the anterior column of the spine to evaluate the load on the anterior column.5.Biomechanic study on different sliding way of the sliding-rotating rod and different number of rotating screw fixation. In this part of test, we carried out the in-vitro biomechanical testing on the cadaver specimens(n=7,T12-S1). To study the stability and load sharing in different sliding way of the sliding-rotating rod and different number of rotating screw fixation.The testing status included:a. the intact status(Intact); b. injured status,the staus of medial facetectomy(MF), c. Medial facetectomy plus cage inserted (cage).On L4～L5segments, the sliding-rotating rod fixation were studied, the sliding-rotating rod fixation included two forms:(1) put the telescopic rod in the sleeve middle position, the sliding rod can be shortening and elongate freely, referred to as tube+/-.(2) In specimen of neutral state, telescopic rod top to the bottom of the sleeve, the telescopic rods can only elongate displacement, limited its shortening displacement, referred to as tube+. In L2-L3segments,the rotating screw fixation were studied, the rotating screw fixation included two forms:(1) the superior vertebral body with two rotating screws fixation, the lower vertebral body adopts two common screw fixation, referred to as rotating*2.(2) two vertebral body all fixed with rotating screw, referred to as rotating*4.The spinal testing machine was used to apply a pure moment of5Nm in the directions of flexion/extension, left/right lateral bending and left/right axial rotation respectively on each specimen.The test focused on the following two features:①Evaluation of stability:include the range of motion (ROM) and the the neutral zone (NZ) of the segments.②Evaluation of load sharing.6. Study on compression strength of dynamic and flexible fixationThis paper compares the variation of intervertebral disc height among the specimens of porcine lumbar spines implanted with three types of flexible and dynamic fixators under axial compression load at eight different positions, to evaluate their difference in compression strength.In this part of test, the testing instruments included:the5.5mm-in-diamater titanium rod with rigid fixation, the3.0mm-in-diamater titanium rod, the sliding-rotating rod, and the ball-socket screw fixation.Resusts:1.Evaluation of three-point bending strength of dynamic and flexible fixation Results show that,3.0mm rods and the linear sliding rod,compared with5.5mm rod, all show the ability of resistance to three-point bending significantly lower. The sliding-rotating rod, compared with5.5mm rod, there was no statistically significant difference in the three-point test.Results confirm that,the design of sliding-rotating rod is feasible, which has good three-point bending resistance. At the same time found the defects in the design of linear sliding rod.2.Study on stability and load sharing of spines implanted with dynamic and flexible fixation (pig specimens)(1)The stability and load sharing of spines with3.0mm rod fixationWhen compared with intact specimens, two rods(3.0mm rod and5.5mm rod) fixation similarly restricted motion in each mode of bending, except axial rotation (p<0.05). When compared with two diameter rods, two fixation similarly restricted motion (p>0.05) in all bending modes in status WPD; In lateral bending,there were significant differences in the range of motion between the2rod in status with Cage (p<0.05),Two fixation couldn’t constrain the motion in axial rotation.In flexion and lateral bending,the3mm rod measured29%～38%less strain than the5.5mm rod.(2) The stability and load sharing of spines with the sliding-rotating rod fixationCompared to the intact specimens, two fixations(rigid fixation and sliding-rotation rod fixation) reduced segmental motion significantly in flexion, extension and lateral bending, but couldn’t constrain the motion in axial rotation. Compared to the rigid fixation, the sliding-rod fixation limited the motion lesser and resulted in larger compressive strain on vertebral bodies in flexion and lateral bending. There was interaction whereby cage insertion resulted in better stabilization for the sliding-rod fixation and worse stabilization for the rigid fixation in flexion and extension. Length of the sliding-rotating rod changed1.9mm in flexion-extension,1.1mm in lateral bending and0.1mm in axial rotation at WPD status.(3) The stability of linear sliding rod fixationIn status of WPD, Compared to the intact specimens, the linear sliding rod fixation reduced segmental motion in flexion, extension and lateral bending.Except in lateral bending,there was no significant difference of ROM in other direction of motion. In status with Cage, the linear sliding rod fixation only reduce the ROM in flexion and extension, but there were no significant differences in each direction.In flexion/extension and lateral bending, the stability of the linear sliding rod fixation show lower stability than the sliding-rotating rod fixation. Combining three-point bending test, confirmed that the design of linear sliding rod existing flaw,and not to study in the follow-up experiment.(4) The stability and load sharing of spines with ball-socket screw fixationComparison of ROM:When compared with intact specimens, each fixation(rigid fixation and ball-socket screw fixation) similarly restricted motion in each mode of bending, except axial rotation (p<0.05), there were no significant difference between the two fixations (P>0.05).Comparison of NZ:When compared with intact specimens, two fixations similarly restricted motion in all bending modes except axial rotation in status WPD, the NZ decrease significantly in status with fixation.In flexion-extension the NZ with rigid fixation decrease significantly when compare with intact status,And there were no significant difference between the ball-socket screw fixation and rigid fixation/intact status.Comparison of strain:Compared to the rigid fixation, the joint screw fixation limited the motion lesser and resulted in larger compressive strain on vertebral bodies in every direction,in flexion94%-299%, in lateral bending160%-172%,and in axial rotation61%-99%.3. Biomechanic study on different sliding way of the sliding rod and different number of rotating screw fixation.(cadaver specimens)(1) Biomechanic study of single/double sliding fixationWith fixators, segmental ROM decreased than the intact status.Except in axial rotation, the range of motion reduced mostly in rigid fixation,and then tube+(single sliding) fixation, tube+/-(double sliding) fixation reduced at least. With three different fixations, NZ of fixed segment reduced mostly in rigid fixation, and then tube+fixation, the tube+/-fixation reduced at least.With three different fixations, Vertebral strain all reduced.Except in axial rotation.The strain redeced mostly in rigid fixation, and then the tube+fixation, tube+/-fixation redeced at leastThe strain of vertebral increased in tube+/-and tube+fixations,when compared to rigid fixation,the tube+/-fixation increased mostly.(2) Biomechanic study on different number of rotating screw fixation. In flexion-extension and lateral bending direction,when compared with the intact status,the ROM of fixed segment reduced significantly in4rotating screw fixation and2rotating screw fixation.When compared with rigid fixation,ROM increased in rotating screw fixation. In addition to the lateral bending direction,there were no significant differece between4rotating screw fixation and2rotating screw fixation in ROM.With4rotating screw fixation, segmental ROM reduced significantly when compared with rigid fixation. In axial rotation, the stability of fixed segment reduced in4rotating screw fixation,when compared with2rotating screw fixation.Compared to the intact status, All fixations increase the strain of vertebral body. And the rigid fixation increase strain least, the rotating screw resulted in larger compressive strain on vertebral bodies than the rigid fixation.4.Study on compression strength of dynamic and flexible fixationUnder the flexion-compression, lateral-bending-compression and lateral-flexion compression, there were no significant differences in the disc height change between the intact and each fixation condition (P>0.05). In extension-compression, the disc height change with the3.0mm rod was significantly different from the intact or the fixations with5.5mm rod (P<0.05), while no significant difference among other groups; In lateral-extension-compression, there was significant difference in the disc height change between each fixation and intact status (P<0.05), and between fixations with3.0mm rod and the ball-socket joint screw fixation (P<0.05).Conclusions:This paper presents a spine testing machine that, as a type of "loading device based on the joint-arm", can provide a pure moment in different direction for spine specimen.We have developed four types of dynamic fixation instruments with sliding-rotating rod, linear sliding rod, ball-socket screw, and rotating screw.And then evaluate the effects of the dynamic and flexible fixation on the spinal stability and load sharing.3.0mm-in-diameter titanium rod:In directions of flexion/extension and lateral bending, it can provide the spinal stability such as the5.5mm-in-diamater rod fixation. With the3.0mm-in-diamater rod fixation, it can increase the load sharing on the anterior column of spine.The three-point bending strength and compression strength of the3.0mm-in-diamater rod are insufficient.Sliding-rotating rod:It can provide a better stability in the directions of flexion/extension and lateral bending. Compared to the rigid fixation, it increased the range of motion for the fixed segments. The sliding-rotating rod fixation has a better adaptability for the motion of fixed segments and reduces the stress on instrument.The sliding-rotating rod shows good three-point bending strength and has good compression strength.Linear sliding rod:Range of motion for the fixed segments, compared with the intact state, decreasesed in the directions of flexion/extension and lateral bending. Compared to the sliding-rotating rod, it has a lower stability and shows significantly less strength for three-point bending. This proves that the linear sliding rod has some flaw in instrument design.Ball-socket screw:In directions of flexion/extension and lateral bending, the range of motion with fixation is significantly less than the intact status, and the stability against rotation is insufficient.The ball-socket screw fixation shows less strength under the compression of lateral anterior flexion and anterior flexion while performing better under the compression of extension and lateral posterior extension.Rotating screw:Compared to the rigid fixation, the rotating screw fixation increase the range of motion for the fixed segments significantly. The fixation using two rotating screws may provide a good stability in all directions, while the fixation with four rotating screws has a significantly lower stability against rotation. In the direction of lateral bending, the difference in stability is the most significant between different fixation modes. |
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