| Spinal fusion is the most common treatment of degenerative disc diseases. Along with the development and spread of this technique worldwide, its complications have become to be known and studied by people. Adjacent segment diseases are among the mostly researched complications of spinal fusion in recent years.Researches on adjacent segment disease and questioning on the traditional firm internal fixation represented by spinal fusion eventually lead to birth of the concept "non-fusion". But the problems haven't been solved even after the posterior spinal dynamic stabilization system appeard. Debate about spinal fusion and the mechanism of adjacent segment disease continued. Arguments focus on the lacking of direct and enough biological mechanical evidences that spinal fusion leads to the degeneration of adjacent segments:On one hand, biomechanical changes of adjacent segments caused by spinal fusion didn't have the support of convincing biomechanical data, on the other hand, the role-of non-fusion spinal fixation in prevention of ASD is not quite clear yet.Intervertebral disc degeneration is an important aspect of spinal degeneration. Yet researches on how fusion causes its degeneration still vary or even contradicted with each other. The reason lies on that the test methods used in these studies have inevitable technical restriction. Therefore, how much of an impact spinal fusion has on the biomechanical behavior of adjacent segments still has no clear conclusion, and further researches are neededCartilage end-plate is an important component of an intervertebral disc, it can buffer forces and pass stresses applied on intervertebral disc, which is very important in spinal biomechanical behavior. It is also the main approach interverbral disc gets nutrients. Degeneration of cartilage endplate is closely related with the intervertebral disc degeneration, it played a very important role in intervertebral disc degeneration, cartilage endplate degeneration occurred before intervertebral disc degeneration. Although cartilage endplate degeneration is a natural aging process, it can be accelerated by excessive inflammation, abnormal stress and cell apoptosis. When degeneration takes place, calcification of cartilage endplate appears first. Along with the development of the disease, disordered structure and micro-cracks appear on cartilage endplate. When the nutrition transportation is blocked because of cartilage end-plate calcification and/or rupture, intervertebral disc degeneration occurs due to insufficient supply of nutrients.Measurement of three dimensional movements of the spine, is still the major means of researches on spinal Biomechanics characteristics in vitro, which is also an important indicator for effects of internal fixations. Panjabi, as early as 1977, pioneered a proven method for measuring three dimensional movement of the spine. With the development of measurement technology, a continuous improvement was made by posterities, precision of this technique continues to increase. But now most of in vitro measurement of the spine is statically, no in vivo environmental condition of the motion are considered. Furthermore, the results are only morphological data, lacking data of pressure inside the intervertebral disc, thus cannot fully reflect the Biomechanics of the spine.Most of current studies on Biomechanics of the spinal columns use static or quasi static loading strategies. But in daily activities human bodies are always on the move in one way or another. Therefore spine is subject to alternating forces of different frequency and amplitude. The cartilage end-plates also response these load. So, it is necessary to impose on the spinal column with alternating dynamic load, testing the behavior of the spine under alternating forces, so that a deeper understanding of the Biomechanics of the spine can be achieved.This research intend measure the distribution of pressure in cartilage-bone interface of segments near lumbar vertebral segment fixed with firm or dynamic posterior spinal fixation using Tekscan pressure distribution measurement system. Imposing a certain vertical alternating compression on the specimen, observe the behavior of spinal column under alternating load, with a view to obtain more detailed and full-scale biomechanical information about effects of traditional spinal fusion and dynamic fixation on adjacent segments. On the same time measure the pressure distribution on the adjacent segments when testing ROM of the specimen, with a view to explicitly understand the relation between pressure distribution and kinematics of adjacent segments under different ROM. Therefore provide information support for comprehension of mechanism how spinal fusion inducing ASD and how dynamic fixation preventing ASD. The study is divided into two parts:1 Effects of different posterior internal fixation on vertical compression stiffness of lumbar vertebra and pressure distribution of intervertebral disc cartilage end-plate.First, pre-experiments are done with calf specimens, and skills in experiment are exercised. Fresh human body specimens of lumbar vertebra are chosen for the experiment, muscle and soft tissue are removed as routine. After Thawed the specimens are excavated on either side of the chosen centrums, following a protocol designed by ourselves. Complete cartilage endplates are exposed. Placing Tekscan pressure sensors, attach them to computer system via a special pressure-reception handlebar. Compression stiffness and pressure distribution of normal specimens are measured. And then models of unstable spine made by resecting part of the posterior annex, posterior 1/2 of annulus fibrosus of L3/4 segment, the same measurements done. And third, short-segment pedicle screw Rod fixation system and posterior spinal dynamic upass5.5 system are applied on the specimens in turn and the same measurements continue. The fourth step, the specimens are placed under alternating loads of different frequency, the measurements before repeated, informations under alternating load obtained. Fifth step, effects of two loading strategy are compared to see if there is any difference between them.Results:With the increase in vertical compression, compression stiffness of lumbar specimens also increases. For unstable model of spine and specimens with posterior fixation the overall compression stiffness of whole specimens has no significant change, illuminating mechanical properties of lumbar column are not affected by structural changes in one segment. For alternating loads, stiffness increases with the increase in loading frequency. About pressure distribution on intervertebral disc cartilage endplate:pressure and load are highly correlated, with a linear relationship between the two. Pressure on cartilage endplate of L2/3 is higher than that of L4/5; on the spatial distribution, under vertical compression loading, pressure on the normal intervertebral disc cartilage endplate is evenly distributed Specimens of instability and posterior spinal internal fixation have a higher pressure with the centers move to rear. When an alternating load applied, pressure on the intervertebral disc cartilage endplate is not only related with the amplitude of the load, but also varys with the frequency of load. Plate loading strategy and blade loading strategy have no difference in results of pressure distribution on cartilage endplate, but as to stiffness and dynamic stiffness, plate loading strategy has a relatively higher outcome.2 Three dimensional movements of spine and the pressure distribution on adjacent segment cartilage endplates under different posterior spinal fixations.Specimens pre-processed are fit on spinal three dimensional movement test machine, caudal sides buried in denture acrylic and fixed on special clamp firmly, cephalic sides of specimens fixed into the loading disc. Loads are applied via steel wire,4kgs on each side. According to experimental design, movements of extension and flexion, left turn and right turn, left rotation and right rotation are performed. Images are taken by laser camera. Use Geomagic studio8.0 system for data reading and processing. Ranges of motions of each segment (ROMs) are obtained.Results:ROMs of flexion and extension in specimens of instability are 1.8 times larger than those of normal group. But firm fixation group and dynamic fixation group have smaller ROMs than normal group, which are 23.09% and 39.56%, respectively. ROMs of dynamic group are slightly larger than those of firm fixation group. Normal group and dynamic fixation group, group of instability have bigger ROMs in flexion than extension, but in firm fixation group, ROMs of flexion and extension are almost the same and much smaller than in other groups. In lateral movements, instability group's ROMs are much larger than those of normal group, 1.53 times those of the normal group. Firm fixation group and dynamic fixation group bend much less on either side than normal group,24.85% and 45.66%, respectively. Bigger ROMs are found in dynamic fixation group's lateral bending compared with firm fixation group. In measurements of rotation ranges:instability group are much greater than normal group, are 2.61 times greater than the normal group, while Firm fixation group and dynamic fixation group are less than normal. Observation of the neutral zone under different conditions shows that:in normal group, the neutral zones of extension and flexion, bending, rotation become smaller in turn. All these neutral zones greatly increased in instability group compared with those in the normal group. Firm fixation group's neutral zones substantially decreased compared with normal group, with their flexion extension neutral zone, bending neutral zone, rotation neutral zone 14.88%,48.53%,75.85% that of normal group, respectively. Dynamic fixation has slightly reduced neutral zone compared to normal group.Dynamic fixation and firm fixation can greatly reduced the ranges of motion of the spine in all directions and the neutral zone, increasing the stability of spine. Compared with firm fixation, dynamic fixation has a larger ROM in all directions, thus it allows the spine has a range of activities, so as to reduce the possibility of adjacent segment degeneration. But from the results, we can see dynamic fixation group's ROMs are very close to firm fixation group, and far less than normal.After spinal posterior fixation, the ROMs of adjacent segments are slightly increased, no significant difference found between two internal fixation groups.When three dimensional movements of the specimens are tested, the pressure distribution on upper and lower intervertebral discs cartilage endplates are measured. In flexion of the spine, pressure on upper and lower cartilage endplates of spinal instability specimens and posterior fixation specimen is higher than that of the normal group, accompanied by change of pressure center. In extension, bending and rotation position, no effects are found of posterior internal fixation on endplate pressure.Results of pressure distribution on cartilage endplate obtained by two different Loading strategies are almost the same. As to pressure distribution on cartilage endplate of L2/3, results obtained by blade loading strategy are slightly higher, but no significant differences are found. when come to compression stiffness measurements, results obtained by plate loading strategy are slightly higher, with the increasing of loading frequency, the stiffness of plate loading group increases quickly while the result is more gentle when applied with blade.Conclusions:①Under the vertical compressive loading, pressure on upper cartilage endplate is higher than that on the lower one.②When load is less than 800N, pressure on intervertebral disc cartilage endplate is closly linear related to load.③In an unstable or posteriorly fixed specimen, higher pressure on upper and lower intervertebral disc cartilage endplate can be found, with pressure center tending to move backward.④When alternating compression load applied, pressure on intervertebral disc cartilage endplate is not only correlated with amplitude, but also varies with the changes of frequency.⑤When flexion of the specimens are tested, the pressure on upper and lower intervertebral disc cartilage endplate in instability group and posterior fixation group is higher than that on normal group, with transfer of pressure center.⑥Dynamic fixation and firm fixation can greatly reduce the ranges of motions of the spine in all directions and the neutral zones, increasing the stability of spine. dynamic fixation has larger ROMs in all directions, thus it allows the spine has a range of activities, so as to reduce the possibilities of adjacent segment degeneration.⑦Results of pressure distribution on cartilage endplate obtained by two different Loading strategies have no significant difference, but plate loading strategy has a higher stiffness or dynamic stiffness outcome compared with blade loading strategy.
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