| 1. BackgroudAlong with the development of cervical fusion techniques, cervical disease treatment effectiveness is obviously improved and the short-term effect is excellent. However, the adjacent segment degeneration or adjacent segment disease as one of major complication of cervical operation has threated the quality of life of the patients remarkable and also has been a challenge to modern treatment. The adjacent segment degeneration or adjacent segment disease has rich clinical manifestation such as disc herniation, osteophyte formation, facet hyperplasia and disc height reduction, could lead to cervical axial pain, loss of feeling, headache and movement disorder. In additional, a research had reported that the incidence rate was higher by using CT or MRI than X-ray, and as the imaging technology developing it tend to be more. Serious adjacent segment disease need operation overh aul that may add operation risk. What’s more, the quality of patients’ life was poor and it is also a heavy economic burden for them. The fusion techniques still should be improved, so the effects after fusion should be studied, too. And it could be an important evaluation index for a new designed fixation. The cause of adjacent segment degeneration or disease was still not clear, but the fact is that the biomechanical environment had changed which had proved by the clinical, biomechanical and finite element research. Based on the current technology level, the biomechanical methods is the only way to prevent adjacent segment degeneration or disease.Anterior transpedicular screw (ATPS) was first Invented by Koller, and some research has been done by him, too. ATPS is combined with the advantage of anterior and posterior the operation approach. ATPS has the stability as post-spedicular screw, but the anterior approach does not need to damage the paravertebral muscle, so the risk of operation is lower. It was reported that the cervical fracture, degeneration, tumour are found mainly in the anterior of spine. So the anterior approach is closer to the treatment area, it means more safety and more efficient in another way. The anterior approach has a large scope of operation which could be prolonged until T1, but the stabile of anterior fusion instruments is always criticized.with the poor stabile, the anterior approach some time need to combine with the posterior that mean an even worse damage for the patients. Comparing with the post-soedicular screw, APTS has a flexible insertion point which is also benefit for the design of instruments. In the second part of Koller’s study, the pull-out force was tested. The result shown the pull-out force of ATPS was 467.8N and the vertebral screw’s was 181.6N. ATPS’s pull-out force was 2.5-fold significant higher than vertebral screw. In security of insertion of ATPS, with the help of electronic conductivity device the percentage of non-critical screw positions was 100% in the ATPS group. The percentage of non-critical breaches in axial plane was 88.9% in post-spedicular screw group. In stability, the performance of ATPS is only poor in lateral bend than the post-spedicular screw, and better in other models. These biomechanical studies have made a solid foundation for the future application of ATPS.Nowadays, china is rapidly entering the aging society. And a survey shown that the incidence of osteoporosis can reach above 60% in the people who over 60 years old. The aging tend is a new challenge for traditional cervical operations. On one hand, the osteoporosis could cause the cervical disc degeneration. On the other hand, the traditional anterior plate fusion system could not keep enough pull-out force in osteoporosis. A research had proved that the pull-out force have no relationship with bone mineral density, that is to say in condition of osteoporosis the ATPS would be a credible guarantee for stability of the instruments. However, the solid fusion could lead to the loading of adjacent segment increase sharply, so that adds the risk of disc degeneration. In another word, AVB would be possible used in the older patients with osteoporosis it has complex effecting to adjacent segments, no matter good or not.it is still not clear, so it is remarkably important to evaluate the influence of ATPS in adjacent segments. This study will test the influence of adjacent segment though the strain, facet press and the rang of motion(ROM) in adjacent segments.The fusion technology of ATPS is not widely used yet in clinical. The lack of the data which reflex the influence of ATPS in adjacent segments is one of the limitation. Our group has measured the anatomy date of Asians to prove the practicability. And the pull-out force and fatigue test was also tested. The result was quite exciting. Based on the preliminary stage of the work, the Anterior Transpedicular Screw Artificial Vertebral Body System (AVB) was designed. And It also passed the stability test. This experiment is based on prior achievements.2. Materials and methods2.1 Specimen PreparationEight fresh-frozen cervical spines of adult males from C2 to C7 were used in this investigation. All the specimens had been scanned CT to exclude tumor, deformity, fracture, degeneration, severe osteoporosis and so on. The specimens were thawed at room temperature prior to preparation. All of them were prepared with standardized techniques. Paravertebral muscle, anterior longitudinal ligament, periost was removed to expose the surface of vertebra. Facet joint capsules, intervertebral discs, vertebrae were protected carefully. The C2 and C7 vertebrae were anchored with special mould in denture acrylic, and the other vertebrae could move freely, Facet joint capsules, intervertebral discs, vertebrae were protected seriously.2.2 Fixation PreparationAVB is composed of three parts, upper/lower L-plate and the cylindrical titanium cage in the middle. The vertical plate of upper/lower L-plate have some radian to fit the vertebral body, there are two ATPS holes in it, and one vertebral screw lower in the middle. Each of the horizontal plate has a cylindrical pin in the vertical direction, which links the titanium cage and could be locked by fastening screw in the on both ends of the titanium cage. The design model was created and produced by weigao company.AP is cervi-lock anterior cervical low profile plate screw locking system provided by weigao company.2.3 Group and the basic stepsBase on the CT scan,8 specimens were reconstructed by mimics, and grouped responsibly. And then the intact specimens were placed on load platform and tested the surface strain and three dimensional motions in six situations from C3 to C6, this group is the intact group. The specimens underwent corpectomy of C5, fixed with AP and AVB,followed the steps of above and collected dates, as AP group and AVB group responsibly. To exclude the effect of load order and the time of exposing in the air, AP and AVB would be exchanged after the forth test, all the order of specimens was random. After above test completed, the facet capsules of C3-C4 and C6-C7 would be opened, and pressure transducer films were inserted into every capsule to the facet press and the max pressure. Specimens were hydrated by frequently spraying them with saline solution throughout preparation and testing.2.4 The front surface strain of discAccording to the test area, the 660mm*480mm was chose. Following the steps of calibration to adjust the measuring system. The specimens should be sprayed the stochastic speckle by using the white and black matte ink, and then placed on loading platform with 1N/m to simulate six condition, which was flexion, extension, left/right lateral bending, left/right rotation. The non-contact strain measurement gauge recorded the whole loading process. And after the calculation of ARAMIS, the software could output the strain nephogram automatically. Then chosen stable loading strain nephogram and located the mean strain of C3-C4 and C6-C7 discs.2.5 ROMBecause the non-contact strain measurement gauge need to shooting the anterior surface, the Kirschner wires could only be placed in left/right transverse process and Spinous process, and a marker was been fixed in every end of the wire. The markers in left/right sides should be in the same distance to the center line. And the three markers in a lever also were in a same horizontal plane. Only in this way, the forth virtual marker in the center line could be set, and the precision could be ensured, too. There dimensional motion test used Motion analysis system, the ten cameras could catch infrared ray which reflexed from markers. And the system would record the whole loading process. The markers were synchronize with the motion of vertebral segments in the computer. And four markers in one level could define a vertebral level. So the range of motion (ROM) of segments would be calculated by software.2.6 Facet press and peak pressureTo test the press and peak pressure, T-scan was used. First, the facet joint capsules of C3-C4 and C6-C7 on both sides had to be opened to insert the pressure sensitive piece. The software could record the whole loading process and find the peak pressure area. The areas of interest were elected and the press, peak pressure, contact area could be out-putted automatically.3. Result3.1 The front surface strain of adjacent segments discs3.1.1 C3-C4 disc:The front surface strain of intact group of C3-C4 disc in flection was 8.834±3.164,3.457±1.892 in extension,5.092±1.427 in left lateral bending,5.092±2.021 in right lateral bending, 5.338±2.903 in left rotation,4.728±2.342 in right rotation. After AP and AVB was fixed, the front surface strain of C3-C4 disc was increased than the intact in the six models of motion,but no significant difference could be found among the three groups. The strain of AP group in flection was 9.96±3.565,3.744±1.741 in extension,5.518±1.443 in left lateral bending, 5.416±1.609 in right lateral bending,5.449±1.417 in left rotation,5.89±2.122 in right rotation. The strain of AVB group in flection was 9.94±2.404,3.533±1.367 in extension,5.878±1.759 in left lateral bending,6.213±1.264 in right lateral bending,6.716±1.667 in left rotation, 4.856±1.015 in right rotation. Comparing the six models of motion in the same group, the strain in flection was significant larger than other 5 models in all the three groups (p<0.01). Both in AP and AVB group, the strain was lower in extension than in other models, between some of them had statistical difference (p<0.05).3.1.2 C6-C7 disc:The front surface strain of intact group of C6-C7 disc in flection was 9.01±3.882,2.977±2.188 in extension,4.586±1.577 in left lateral bending,3.475±1.32 in right lateral bending,4.876±1.687 in left rotation,4.693±1.635 in right rotation. After AP and AVB was fixed, the front surface strain of C6-C7 disc was decreased than the intact in the six models of motion except the flexion and left/right lateral bending of AVB, but no significant difference could be found among the three groups. The strain of AP group in flection was 10.244±3.258,2.278±0.594 in extension, 4.008±1.271 in left lateral bending,3.714±1.256 in right lateral bending,4.389±1.405 in left rotation,4.204±1.303 in right rotation. The strain of AVB group in flection was 10.645±2.418, 2.004±1.066 in extension,4.622±2.101 in left lateral bending,4.11±0.723 in right lateral bending, 3.902±0.94 in left rotation,4.023 ±0.968 in right rotation. Comparing the six models of motion in the same group, the strain in flection was significant larger than other 5 models in all the three groups (p<0.01). Both in AP and AVB group, the strain was lower in extension than in other models, between some of them had statistical difference (p<0.05).3.1.3 The max strain of the instruments: The max strain of the instruments of AP group in flection was 14.562±4.559,12.793±3.349 in extension,12.901±3.692 in left lateral bending,12.857±3.445 in right lateral bending, 13.234±1.885 in left rotation,13.582±2.175 in right rotation. The max strain of the instruments of AVB group in flection was 9.699±2.679,9.515±2.148 in extension,9.988±3.111 in left lateral bending,9.43±4.118 in right lateral bending,9.742±2.99 in left rotation,9.638±1.953 in right rotation. In all six models of motion, the max strain of AP is larger than AVB,and there were significant difference in flection, extension and left/right rotation.3.2 ROM3.2.1 C3-C4 inter-vertebral:The ROM of AP group of C3-C4 inter-vertebral in flection was 7.389±1.592°,3.144±0.397° in extension,5.666±0.946°in left lateral bending,5.679±1.131° in right lateral bending, 5.003±1.136° in left rotation,4.981±0.796° in right rotation. The ROM of AP group in flection was 7.854±1.531°,3.548±0.88° in extension,5.325±0.868° in left lateral bending,5.516±1.242° in right lateral bending,5.288±1.183° in left rotation,5.251±1.047° in right rotation. The ROM of AVB group in flection was 7.564±1.658°,3.504±0.496° in extension,5.236±2.379° in left lateral bending,4.496±1.133° in right lateral bending,5.286±1.156° in left rotation,5.223±0.939° in right rotation. After AP and AVB was fixed, the ROM of C3-C4 disc was increased than the intact in the six models of motion, but no significant difference could be found among the three groups. 3.2.2 C6-C7 inter-vertebral: The ROM of AP group of C6-C7 inter-vertebral in flection was 7.12±1.402°,3.086±0.704° in extension,4.46±1.436° in left lateral bending,4.639±1.392° in right lateral bending,4.509±1.117° in left rotation,4.849±0.97°in right rotation. The strain of AP group in flection was 7.041±1.724±,2.823±1.184± in extension,5.019±0.272° in left lateral bending,4.768±1.129° in right lateral bending,4.351±1.035° in left rotation,4.486±1.115° in right rotation. The strain of AVB group in flection was 6.843±1.079°,2.858±0.511° in extension,5.03±1.509° in left lateral bending,4.826±1.883° in right lateral bending,4.068±0.906° in left rotation,4.386±0.819° in right rotation. After AP and AVB was fixed, the ROM of C6-C7 disc was increased than the intact in the six models of motion, but no significant difference could be found among the three groups.3.3 Facet pressIn Both sides of flection, the constrict side of lateral bending and the same side of rotation, the facets has no value, so they would not be discussed in this study.3.3.1 Facet press of C3-C4 inter-vertebralThe left facet of AP group of C3-C4 inter-vertebral in extension was 21.573±8.0561N, 34.312±6.678N in left lateral bending,36.131±6.725N in right rotation. The right facet of AP group in extension was 24.786±9.052N,35.887±10.307N in right lateral bending,35.708±3.961N in left rotation. The left facet of AVB group in extension was 22.489±9.793N,35.535±6.380N in left lateral bending,38.742±7.180N in right rotation. The right facet of AVB group in extension was 22.755±12.511N,35.747±8.038N in right lateral bending,36.252±5.964N in left rotation. In all the models of motion, no significant difference was observed in every facet.3.3.2 Facet press of C6-C7 inter-vertebralThe left facet of AP group of C6-C7 inter-vertebral in extension was 17.580±5.590N, 32.115±6.450N in left lateral bending,26.589±11.396N in right rotation. The right facet of AP group in extension was 21.133±11.306N,29.635±9.271N in right lateral bending,26.632±6.674N in left rotation. The left facet of AVB group in extension was 16.132±6.046N,29.960±11.377N in left lateral bending,28.534±11.337N in right rotation. The right facet of AVB group in extension was 17.066±12.227N,28.682±8.686N in right lateral bending,28.625±5.698N in left rotation. In all the models of motion, no significant difference was observed in every facet.4. Conclusion1. The adjacent segments were not significant affected by AVB.2. The performance of AVB was similar to AP, which could lead to adjacent segment degeneration, especially in the upper adjacent segment, though causing ROM, the surface strain and facet press unbalance contribution.3. The strain of instruments showed that the strain distribution of AVB was uniform that had great potential of resisting damages in structure itself. And compared with AP, the risk of fracture was significant lower. |
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