| 【Background】Adolescent idiopathic scoliosis is a common spinal deformity,involving lateral translation in the coronal plane,thoracic hypokyhosis of the sagittal plane and vertebral rotation of the transverse plane.With the continuous development of internal fixation technology,especially the application of pedicle screw instrumentation system,surgical outcomes of AIS has been significantly improved.However,with pedicle screws becoming standard instrumentation for surgical correction of AIS,the disadvantages such as high insertion risk and high implant expenses are becoming more and more concerned by researchers.In order to improve the above disadvantages,most researchers began to reduce the number of screws to optimize the instrumentation strategy,but these optimized strategies have not been validated by the biomechanical experiments.The influence of these optimized strategies on the stress distribution of pedicle screws instrumentation system and surgical outcomes is still not well studied.【Objectives】1.To establish a three-dimensional finite element model of scoliosis using pre-operative CT scan data,and using dynamic biomechanical parameters of the patient`s spine to adjust and verify the scoliosis model.2.To explore how different instrumentation strategies and implant densities affect the stress distribution in the instrumentation system and the surgical outcomes by simulating the surgical correction process using the scoliosis model above.3.To evaluate the effects of implant density on coronal and sagittal correction in the treatment of Lenke 1 AIS by reviewing database of surgically treated patients.【Methods】1.Establishment and validation of the biomechanically optimized three-dimensional finite element model of the scoliotic spine.A typical Lenke 1 AIS patient was recruited in the study.Using self-developed in vitro Cobb angle measuring instrument and scoliosis traction frame,the coronal and sagittal Cobb angle of thoracic curve were dynamically monitored under different traction force and the dynamic force-cobb angle reactive curve was recorded.Finite element model of the scoliotic spine was built according to the pre-operative CT scan data.In order to personalize the mechanical property of the model,the traction process above was simulated and the soft tissue parameters were adjusted to best reproduce the Cobb angle change of the spine observed experimentally.Then the same method was used to validated the finite element model.2.A finite element analysis of different strategies of pedicle screws placement for the treatment of Lenke 1 adolescent idiopathic scoliosisA 3-dimensional model of T4-T12 spine segments was extracted from the pre-existed model for finite element analysis.Five screw-placement strategies(A,B,C,D,E)were designed to simulate the correction of scoliosis.Surgical procedures of these five screw-placement strategies were simulated on the finite element model and the stress in the implants structure during curve correction and after correction were compared,including the Mises stresses of screws-vertebral interface,screw-rod interface and rods.3.Correction of Lenke 1 AIS using pedicle screws: the relationship between correction results and implant densityThirty-one consecutive patients with Lenke 1 AIS underwent single-stage posterior correction and instrumented spinal fusion with pedicle screw fixation between January 2008 and January 2010.The data were retrospectively analyzed.The radiographs were analyzed before surgery,immediately after surgery,and at the 2-year follow up.General information of patients was recorded.Pearson correlation analysis was used to analyze the correlation between implant density,coronal Cobb angle correction,and correction index.The relations between implant density and magnitude of coronal and sagittal curve correction were also investigated.According to the implant density,patients were divided into two groups: high-density group(>0.7)and low-density group(≤0.7).The differences were compared between the related parameters in the two groups.【Results】1.The finite element model of the scoliotic spine was successfully built involving 12 thoracic vertebrae,4 lumbar vertebrae,15 intervertebral discs.And the dynamic biomechanical property of the spine was included in the model,and the validation showed that the model can accurately simulate the anatomical and biomechanical properties of the patience’s spine.2.The number of screws used in five screw-placement strategies was 14,11,9,8 and 7,respectively,and the densities of pedicle screws were 100%,79%,64%,57% and 50%,respectively.With regard to the stress during curve correction with rod rotation at concave side,the maximum stresses of screws,rods and pedicle screw-rod interface in each strategy was 2673 MPa,2673MPa,2660 MPa,2918MPa,2918 MPa,and 1229 MPa,1229MPa,1377 MPa,1167MPa,1167 MPa,and 289 MPa,289MPa,305 MPa,278MPa,278 MPa,respectively.The maximum interaction force between screws and vertebrae was 113 N,113 N,289 N、172N,172 N,respectively.After the fixation of rod at convex side,the maximum stresses on screws in each strategy were 1743 MPa,1689MPa,1435 MPa,1683MPa,1414 MPa,respectively.The stress of rods was 561.8Mpa,522.4Mpa,994.4Mpa,548.8Mpa,446.3MPa,respectively.In addition,the correction rate of each strategy was 48%,48%,46%,47% and 46%,respectively.3.The mean patient age at the time of operation was 15.3 years.The mean preoperative main thoracic curve of(48.5±8.5)°was corrected to(16.0±6.9)°(68% correction)2 weeks postoperatively.There were no significant correlations between implant density and curve correction(r=-0.19,P=0.315).No correlations were detected between implant density and correction index(r=0.10,P=0.589),between implant density and correction loss(r=0.04,P=0.837),and there were also no correlations between implant density and magnitude of sagittal thoracic kyphosis change(r=0.09,P=0.619).The average implant density of the high and low groups were 0.85 and 0.61 respectively(P<0.01).No obvious differences were found in parameters of the two groups.【Conclusions】1.In this study,the authors innovatively used the pre-obtained dynamic biomechanical property of the spine to optimize and validate the finite element model.The result showed that the model can accurately simulate the anatomical and biomechanical properties of the patience’s spine.2.Screw-placement strategies and densities of pedicle screws have little influences on the curve correction rate for this case.The screw-placement strategy E has better biomechanics properties than other strategies,except for the more 50% of maximum interaction force between screws and vertebrae all segment screws instrumentation.3.Taking curve flexibility into consideration or not,implant density is not correlated with coronal Cobb curve correction of main thoracic curve.No significant correlations are found between screw density and correction index.There are no associations between implant density and magnitude of sagittal thoracic kyphosis change before and after surgery. |
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