Three-dimensional Finite Element Study Of Posterior Surgical Correction Of Infant Congenital Scoliosis Caused By Hemivertebrae

Objective:Congenital scoliosis is a seriours disease that impact onchildren physical and mental health. There is a high incidence betweenchildre. Especially the younger age group is very serious. They have thoracicwith poor development, poor cardiopulmonary dysfunction. So there aremore serious surgical complication and higher anesthesia risk. That is adifficulty in the field of scoliosis treatment. This deformity requires earlysurgical correction, but the operation must minimize the impact of spinalgrowth. Choice of orthopedic way and vertebral fusion segment is a keyfactor to affect treatment and prognosis. Now the main treatment is posteriorhemivertebra resection and pedicle screw fixaion.However, due tocomplicated deformitis and big individual change, the choice of the fixedsegment is according to the docetors'clinical experience and lack ofbiomechanical research evidence. We often see poor orthopedic orpostoperative complications such as decompensation of spine. Based on this,three-dimensional finite element model was established to simulatedcorrection posterior surgery by a infant congenital scoliosis patient. We wantto compare the biomechanical characteristics with differernt fixed methodsand provide a theoretical basis for doctors.Methods:1Obtaining images: We select one congential scoliosis child caused byhemivertebrae, who is male,2years9months, weight14kg, height90cm.The hemivertebrae is12thoracic. Siemens64-slice CT transverse scanningin supine position was done from C₂to sacrum. Layer thickness was1.25mm and space between two scan was0mm. The scanning range included allosteal structure and discs. We obtain470CT dicom images and148pictures were applied for modeling.2Establishing a three-dimensional model: The images that we get wereimported into Mimics14.0. Using original image, we mask, split, smooth togenerate three-dimensional model of the thoracic, lumbar and intervertebraldisc. And then we use this model to compare with Lateral spine X-ray of thispatient, measure the hemivertebra size, volume and Cobb angle.3Simulating correction surgery: The model after simple handling wasfurther delivered to3-Matic5.1to simulate posterior hemivertebra resection.During the simulation of surgery, we fixed the pedicle screws, resected thehemivertebrae, install orthopedic rods, and gradually closed vertebral gapbetween the T₁₁and L₁. Compared with the postoperative X-ray, orthopediceffect is satisfactory.4Establishing a three-dimensional finite element model: In3-matic5.1,we assemble of pedicle screw, orthopedic rods, vertebrae and discs. Themodel after assembly and element mesh was delivered to Mimics14.0onceagain to set the parameters that conclude elastic modulus and Poisson ratio.A three-dimensional scoliosis finite element model was build.5Finite element analysis: The younger congenital scoliosis FE modelwas imported to ANSYS13.0and then constrained, loaded respectively andsolved. We applied to investigate different correction effectiveness withdifferent fixed way and find out the biomechanical characterisics withdifferent instrumentation stages.Results:1A three-dimensional finite element models of infant congenitalscoliosis were built caused by hemivertebrae, including thoraco lumbarspine, intervertebral discs and joint. There are total105252elements and185849nodes in the model, including2thoracic vertebra models, whichconsisted of31576elements,56105nodes;2lumbar vertebra models, whichconsisted of37890elements,67281nodes; and5intervertebral discs modelswhich include nucleus pulposus and fiber ringconsisted of11577elements,21163nodes; 2The finite element model of the congenital scoliosis show realisticcharacteristics of this patient's spine. The infant vertebra is oval, have morenutrient foramen; vertebral height is significantly higher than the discthickness; the region of cancellous bone and compact bone is difficult todefine. It is very similar to the X-ray films of patient's spine. The volume ofhemivertebra is3793mm³, the maximum diameter is31.27mm, themaximum height is15.13mm, the Cobb angle is40.2°.3From the simulation of the surgery,we plan four personalizedprograms according to different fixed way, one of them is fixing10,11thoracic vertebra and1lambar vertebra two sides that called T₁₀^L₁group.The others conclude T₁₀^L2group, T₁₁^L₁group, T₁₁^L₂group.4In Ansys13.0we obtain equivalent stress, total deformation andsafety factor of the FE model of the four groups. We found that with loadingforce (below104N) gradually increased, the shape of cloud of totaldeformation and equivalent stress basically unchanged, the value of cloudchanged. There was a line correlation between them. The maximumequivalent stress of four groups FE model are mainly concentrated in thepedicle screws, between the pedicle screws and rods, pedicle screws andpedicle. The max total deformations of four groups lie on top of vertebra.The total deformation of four groups reduce trend in turn.5When loading300N vertical instead on the concave side and convexside, the equivalent stress of four groups are52.552Mpa,59.422Mpa,55.215Mpa,59.624Mpa, the equivalent stress in convex side more thanconcave side about3-6times. The equivalent stresses of top and bottomvertebra are more than middle vertebra. The equivalent stresses of pediclescrews in T₁₁^L₁group more than other groups. If we add one fixed segment,the stress of pedicle screws decress obviously, suce as T₁₁^L₂group, T₁₀^L₁group. If we add one fixed segment once again, the stress of pedicle screwshave no effect, like T₁₀^L2group. The cloud area of min safety factor in thebar of T₁₁^L₁group is4-8times than other groups.The bigger cloud area ofmin safety factor in the bar, the easilier it could fatigue failure.So the T₁₀^L₁, T₁₁^L₂groups are better choice.Conclusion:1In this sudy, a three-dimensional individual FE model of infantcongenital scoliosis caused by hemivertebrae in details was firstly built,according to CT images, using advanced computer-aided engineering software.The simulation of surgery of hemivertebrae resection is very sucessful.2The three-dimensional FE model could do mechanical analysis andsurgical simulation in every conditions, provides a good biomechanicstheory and clinical research for infant congenital scoliosis caused byhemiverbrae,which can help us obtain a personalized surgical strategies andprovide a biomechanics research.