Biomechanical And Clinical Study Of Monoplanar Pedicle Screws Fixation Surgery In The Patients With Thoracolumbar Vertebral Fractures

Objective:Correction loss is a common phenomenon in the patients with Thoracolumbar Vertebral Fracture(TLVF)who undergo the percutaneous posterior pedicle screw fixation.The incidence varies with the kinds of fixation instrumentation used.There is higher incidence in Polyaxial Pedicle Screw(PAPS)group than in Fixed-Axis Pedicle Screw(FAPS)group.However,because of the difficulty of rod insertion,FAPS has been limited,especially in minimally invasive techniques.A novel screw design,named Monoplanar Pedicle Screw(MPPS),which combines the two advantages,solves the difficulty of rod insertion and reduces the correction loss.We expect to evaluate the biomechanical property and clinical advantages and outcomes of the novel pedicle screw.Materials and Methods:Biomechanical study: 30 porcine spinal units(L2-L4)were used for the static and dynamic tests,which were randomized into 6 groups(A1,A2,A3,B1,B2 and B3),with 5spinal units each group.Static test was performed in A1,A2 and A3.In this test,FAPS,MPPS and PAPS were performed in A1,A2 and A3 respectively.The ultimate load(UL)was noted after tested.Additionally,dynamic test was performed in B1,B2 and B3,used FAPS,MPPS and PAPS respectively.Correction loss(Tail-Shank Angle shift(TSAs)and Anterior Vertebral Body Height shift(AVBHs))was obtained and analyzed in each mode.Clinical study: between January 2010 and June 2015,75 patients with TLVF,which had finished the posterior pedicle screw fixation surgery,were classified into 3 groups according to implant types,group A(FAPS),group B(PAPS)and group C(MPPS).General Information,Operation Time(BT),Blood Loss(BL),Visual Analogue Scale(VAS,back pain),Anterior Body Compression Index(ABCI),Vertebral Body Angle(VBA)and Cobb Angle(CA)were collected and analysed.Results:Biomechanical study: In static test,FAPS and MPPS had significantly higher UL than PAPS(P< 0.05)and FAPS had a little higher UL than MPPS(P< 0.05).In dynamic test,TSAs and AVBHs were minimal in FAPS,medium in MPPS and maximal in PAPS.However,the differences were statistically significant in all comparisons but no in the comparison of FAPS and MPPS(P> 0.05).Clinical study:(1)In terms of age,follow-up period,gender and distribution of fractured vertebral,there was no significant difference among the 3 groups.(2)BL and OT in PAPS and MPPS groups were less than that in FAPS Group(P<0.05),but no significantly different between PAPS and MPPS groups(P>0.05).With respect to VAS,it was no significantly different among the 3 groups before operation(P>0.05).In 1 day post-operation,VAS in PAPS and MPPS were obviously lower than one in FAPS(P<0.05),but the difference couldn't be found in the comparison between PAPS and MPPS.There was no significantly different VAS among the 3 groups in 3 months post-operation and the final visit(P>0.05).Comparing with pre-operation,VSA was decreased obviously in post-operation(P<0.05).To compare with 1 day post-operation,VAS in 3 months post-operation and final visit were lower significantly in all groups(P<0.05),but difference was no significant between 3 months post-operation and the final visit(P>0.05).(3)To compare with pre-operation,CA and VBA were lower but ABCI was higher in post-operation(P<0.05).Comparing 1 day post-operation with the final visit,all image parameters in all groups had no significant difference,except for the CA in group PAPS.In terms of image parameters,it was no significantly different among the 3 groups before operation(P>0.05).In 1 day post-operation,CA and ABCI were similar among 3 groups(P>0.05).VBA in FAPS and MPPS were lower than one in PAPS(P<0.05),but the difference between FAPS and MPPS was no significant(P>0.05).In the final visit,VBA and CA in FAPS and MPPS were lower than one in PAPS(P<0.05),but the difference between FAPS and MPPS was no significant(P>0.05).About ABCI,there was no significant difference among 3 groups(P>0.05).Conclusions:The findings from the current study suggest that MPPS can significantly increase the stiffness in axial direction and it is similar to FAPS and significantly stronger than PAPS.At the same time,the clinical outcomes are good in minimal invasive surgery.Compared to PAPS,because of reducing the risks of correction loss,MPPS is a better optional instrumentation for a part of patients with TLVF.