The Anatomic And Biomechanical Study Of Pedicle Rib Unit And The Finite Element Analysis Of The Strategy In Scoliosis

Scoliosis is a three-dimensional(3D) abnormal deformation of the spine, which is regarded as one of the most difficult work for spinal surgery.Because of its prominent effect and few complications,the posterior pedicle screw fixation system has been widely used in scoliotic spine correction.Concerns have been raised regarding the safety of pedicle screw instrumentation in patients with idiopathic scoliosis who have more complex and varied thoracic pedicles,whose pedicles are too narrow or complex sometimes to accommodate transpedicular fixation,a safe alternative incorporating screw fixation in the thoracic spine allowing for application in this excluded population is sought.A new concept of pedicle rib unit fixation has been introduced recently.This new extrapedicular method was widely accepted because of the benefits of three column fixation and the lower risk of injury to surrounding structures.However, recent researches focus on the width of pedicle rib unit without considering its height,which lead to increase the risk of lesion.This investigation includes three parts:(1)the anatomic,radiographic morphometry of pedicle rib unit.(2)pullout strength of pedicle rib unit screws versus pedicle screws in the thoracic spine.(3)three-dimensional finite analysis of surgical strategies for scoliosis correction.Part one:the anatomic,radiographic morphometry of pedicle rib unitObjective:To investigate the spatial structure of pedicle rib units in the thoracic spine in normal thoracic human spines and to compare the dimensions of the pedicle rib unit with corresponding dimensions.Methods:Four fresh adult cadaveric thoracic spine specimens were harvested.Computerized tomographic (CT) images(including two-dimensional,three-dimensional reconstruction) of the thoracic spine were obtained.Measurement parameter includes:the width,the height,the chord length and the sagittal angles of the pedicle rib unit compared with pedicle,especially for the pedicle-rib overlap height.Results:The pedicle rib unit is not a simple two-dimensional structure but a three-dimensional structure. The height of pedicle rib unit and the height of pedicle were significantly larger than that of the pedicle-rib overlap,while there was no significantly difference between the pedicle rib unit and the pedicle.Conclusions:The pedicle rib unit is a spatial structure.The height of pedicle-rib overlap is the real height of the unit, and only when the screw was in the pedicle-rib overlap space could the safety and biomechanic performance of the screw be obtained.Partâ…¡:Pullout strength of pedicle rib unit screws versus pediele screws in the thoracic spineObjective:In vitro biomechanical pullout testing was performed to evaluate the axial pull-out strength of pedicle rib unit screws by comparison with pedicle screws in the thoracic spine.Methods:Five fresh human cadaveric thoracic spine specimens(T₆-T₁₀) were harvested and disarticulated into 25 vertebra rib units at the following lever:intervertebral disk,facet articulation and inferior costovertebral articulations.According to the principle of paired grouping,one side of each vertebral pedicle was prepared for pedicle screw instrumentation randomly,and the other side was regarded as pedicle rib unit(the proximal ribs of both sides have been preserved until grouping finished).The diameter of pared screw was same in each group,but the screws length of pedicle rib unit side was 10 mm longer than that of pedicle side.Twenty five pared groups were obtained and the axial pull-out strength of 50 screws was measured respectively and the velocity of the axial force is 5mm/min.Matched pair t-tests were used.Results: The data of 21 groups were gained successfully and the tests of the other four groups were failed.We found that the axial pull-out strength of pedicle rib unit screw was weaker than that of pedicle screw significantly(P=0.002).Conclusions: The pull-out strength of pedicle rib unit screw was in inferior position compared with the pedicle screw,so the former may be just regarded as a supplementary of the latter in some instance such as the transversal width of thoracic pedicle was too little to screw.Part 3:finite element analysis of correction strategies for scoliosis correctionObjective:An effective numerical method for simulating the correction process of scoliosis was presented in order to investigate the biomechanics issues in correction to determine the impact of alternative correction strategies,and to provide guidance for surgical planning.Methods:A detailed three-dimensional finite element model of L₁-L₅ and screw were constructed using the commercial finite element software ANSYS.The model geometry was extracted from a computed tomography reconstruction.A three-dimensional finite element model of a 40°scoliotic lumbar and pedicle screw instrument were then constructed.The correction process including rod rotation and spring-back was simulated with rigid body dynamic software ADAMS.The correction strategies were designed into 5 types(1-5) according to the lever of no screw.Results:The correct angle was 30°,23°,25°,22°,29°respectively from strategy 1 to 5.The maximum of torque of strategy 2(3.3 Nm) and strategy 4(3.4 Nm) were just 70%of the maximum of torque of strategy 3(4.6 Nm) or less.Either resultant or pull out force strategy 2 and 4 were less than other three strategies.Conclusions:Scoliosis could be well treated by posterior pedicle screw instrument and the safety of vertebrae could be ensured.Different surgical strategies could be considered to reduce surgical risks and save implant,which would make contribution to surgical planning.In the present study,we suggest to keep enough fusion length and to reduce one screw in the fusion range,which would also yield good coronal and lateral correction.