| BackgroundAnterior cervical discectomy and fusion(ACDF) with autograft is widely employed in the surgical treatment of degenerative cervical spine diseases. However, segments of two or more than two sections at the same time involvement, a bone grafting and spans a large, poor stability related to fixation and bone graft, bone graft failure, pseudarthrosis formed thereby affecting operation,especially for multilevel procedures. Clinical application of anterior cervical internal fixation system, combined with osteoporosis caused by factors such as the ageing of the population as a result of fatigue or cyclical movement led to loosening and breakage of the screw. Need multilevel fusion as well as in patients with osteoporosis, the holding strength of VBS was limited. To improve stability strength of ACDF, on the one hand from the anterior placed to extend the screw lengths to strengthen the screws holding force, the other hand strengthens the bone-screw interface strength. For example, Koller proposed the anterior cervical pedicle screw concept and think possible. Domestic scholars, such as Zhao began to ACPS in clinical; use of bone cement strengthened anterior screw, to increase the bone-screw interface area to strengthen the stability of results.Through the anterior cervical pedicle screw and bone cement perfusion, enhance the stability of screws in the vertebral bodies in osteoporosis, reducing the risk of screws loose, improve treatment efficacy. This study for bone cement strengthening screw tunnel, VBS and ACPS in a state of fatigue and instant pull-resistant characteristics. By the pullout testing to analysis to evaluation factors of stability between ACPS and VBS stabilizing effect.This study using VBS and ACPS internal fixation devices, biomechanical in vitro experiments, test the screw in the normal bone density, osteoporosis and bone cement strengthening screw tunnel of cervical vertebrae, VBS and ACPS in a state of fatigue and instant pullout force, to evaluate the stability of internal fixation.ObjectiveEvaluation of anterior cervical pedicle screw fixation and vertebral body screws in the strengthening and fatigue conditions of biomechanical characteristics.The correlation between bone mineral density and strength of screw fixation.Analysis of the bone cement in the biomechanics of cervical spine strengthening effect.Materials and methods 1. Specimen preparation For biomechanical testing, twenty cervical spine segment (C3-7) were harvested from formalin-preserved from cadavers obtained from the Southern Medical University Department of Anatomy. Specimens with previous spinal surgery, posttraumatic abnormalities and structural disorders were excluded. Next, specimens were subjected to assessment of Bone Mineral Density(BMD), measured in the antero-posterior and postero-anterior plane using Dual-energy X-ray absorptiometry (DXA; Lunar Radiation Corp, Madison, WI) with LUNAR PRODIGY software(enCORETM version10.50.086; Lunar Radiation Corp). A total of60vertebral bodies ranging from C3-7included in this study by bone mineral density values, and60VBs were dissected from the segments. The paravertebral musculature and fascia were removed and the individual cervical vertebrae separated by incising the ligaments and annulus. They were wrapped in double plastic bags and frozen at-20℃. Prior to testing, specimens were thawed over night at4℃and prepared at room temperature right before testing. The vertebral were embedded in Polymethylmethacrylate(PMMA) cement using a specially designed jig, with the vertebral body completely embedded but both anterior border of the cervical vertebral body free for inspection during testing.2. Experimental methods DXA measurements confirmed30osteoporotic human cadaveric cervical vertebrae (mean BMD<0.63g/cm2) and30healthy, nonosteoporotic cervical vertebrae (mean BMD>0.63g/cm2).Based on their BMD, the specimens were assigned randomly to three groups in pullout force, respectively, were tested until comparable BMDs were present in each group. The vertebrae were divided into instant group and fatigue group, A1,2,C1,2for normal BMD group, B1.2,D1.2for Osteoporotic group. The cadaver specimen were subjected to manual insertion of ACPS and VBS, which were performed at the university anatomical institute. The specimen was fixed to a dissection table. The screw entry point was selected in accordance with the parameters established by Koller et al. The embedded vertebrae, mounted in the testing jig, were placed in a variable axis frame attached to the ElectroForce(?)3510Test Instrument(Bose Corporation-ElectroForce Systems, USA). The test instrument was attached to the screw head by a universal joint and aligned so that pullout would be co-axial to the screw. Axial pull-out testing was done to evaluate the fixation strength between the bone and ACPS. In group, pilot holes were drilled parallel to the sagittal plane, they were then injected with0.3ml-0.5ml of PMMA before the vertebral screw was inserted. Axial pullout was performed at5mm/min displacement. Pullout loads and displacement were recorded until failure occurred. Data were acquired at a rate of50Hz. The maximum load during testing was defined as the maximum pullout force(Fmax).ResultsA1, B1, A2, B2group of BMD(mean±standard deviation)values for0.909±0.13g/cm2,0.600±0.07g/cm2,0.885±0.10g/cm2,0.532±0.06g/cm2and bone mineral content(BMC) of2.737±0.40g,1.283±0.33g,2.170±0.55g,1.535±0.36g respectively. A1, B1, A2, B2group of VBS pullout force for196.54±81.77N,185.55±89.40N,106.38±69.37N,82.70±90.65N, respectively. There was significant statistical difference between the group A1and group A2, B2(P=0.008,P=0.001). There was significant statistical difference between the group B1and group A2. B2(P=0.018,P=0.003). After the fatigue, the pullout force was reduced by45.87%, 55.43%, respectively, between normal and osteoporotic groups;C1, D1, C2, D2group of ACPS pullout force for646.71±234.82N,525.80±190.14N,556.22±207.58N,510.19±195.56N, respectively. There was no significant statistical difference among the groups (P=1.470,P=0.232). A1, B1, A2, B2group of augmented VBS pullout force for284.38±80.21N,231.50±36.97N,164.11±45.78N,143.18±78.08N, respectively. There was significant statistical difference between the group Ai and group A2. B2(P=0.000,P=0.000). There was significant statistical difference between the group B1and group A2, B2(P=0.023,P=0.004). After the fatigue, the augmented VBS pullout force was reduced by42.29%,38.15%, respectively, between normal and osteoporotic groups.Stiffness of the structure means its ability to resist deformation, is to measure material elastic deformation index of how easy or difficult, mainly refers to deformation caused by the need to stress, the greater the stiffness of a structure, the less easy to deformation. There was significant statistical difference of Sv between the group A1and group A2, B2(P=0.004,P=0.045), There was significant statistical difference of Sv between the group B1and group A2. B2(P=0.000,P=0.001); There was no significant statistical difference of Sp among the groups (F=1.352,P=0.267), There was significant statistical difference of Sg among the groups (F=4.381, P=0.010).There was significant statistical difference of Fmax between the groups(t=-3.416,P=0.008;t=-5.516,P=0.000;t=-6.036,P=0.000;t=-6.641,P=0.000). Before and after augmentation, the Fmax of VBS in normal and osteoporotic groups were increased by44.69%and24.76%. Before and after augmentation and fatigue, the Fmax of VBS in normal and osteoporotic groups were increased by54.27%and73.13%.There was significant statistical difference of Fmax between group A1and C1in (t=-8.283,.P=0.000), There was significant statistical difference of Fmax between C1and augmented VBS in groups (t=-8.283,.P=0.000);There was significant statistical difference of Fmax between group B1VBS and D1(t=-8.181,P=0.000), There was significant statistical difference of Fmax between group D1and augmented VBS in group A2(t=6.111,P=0.000), The ACPS are working under different bone density has a strong pull-resistant characteristics. There was significant statistical difference of Fmax between group A2and C2(t=-6.975,P=0.000), There was significant statistical difference of Fmax between group C2and augmented VBS (t=6.082,P=0.000); There was significant statistical difference of Fmax between group B2and D2(t=-8.440,P=0.000), There was significant statistical difference of Fmax between group D2and augmented VBS (t=8.033,P=0.000). ACPS with good fatigue resistance characteristic, maintaining fixed stability.Conclusions1. ACPS technique can provide stronger internal fixation of cervical spine stability. The results showed that the pullout strength of ACPS fixed group was stronger than those of VBS fixed and the PMMA augmented VBS fixed groups, was not related with the bone mineral density and fatigue loading. 2. ACPS with good fatigue resistance characteristic, maintaining fixed stability. There was no difference in pullout strength between the acute group and the fatigue group.3. PMMA augmentation can improve the axial pullout strength of cervical vertebral body screw in acute. The PMMA augmentation groups had a stronger pullout force when compared with VBS fixed group without augmentation under the same condition of bone mineral density.4. PMMA augmentation can improve the axial pullout strength of cervical vertebral body screw in fatigue. The PMMA augmentation groups had a lower pullout force when compared with VBS fixed group after fatigue loading. |
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