| BackgroundSpinal kyphotic deformity can be caused by various diseases,including trauma,ankylosing spondylitis,Pott’s kyphosis,Schuermmann’s disease and degenerative scoliosis.Severe low back pain,spinal cord injury and sagittal imbalance due to spinal kyphotic deformtiy could influence quality of life.In such cases,spinal osteotomy surgery is often necessary to correct the deformity.Due to the stability of spine is totally destroyed by the procedure of osteotomy,safe and reliable long segment instrumentation is critical to restore spinal alignment and maintain correction effect.Proximal junctional kyphosis(PJK)is one of major complications after long segment instrumentation.There are two definitions of PJK both generally accepted.One of them is the kyphosis between upper instrumented vertebra(UIV)and 2 levels above the UIV greater than 20 degrees.And another definition is the proximal junction angle greater than 10 degrees and 10 degrees greater than the preoperative value.The incidence of PJK has been reported with a range of rates from 9% to 46% in adolescent idiopathic scoliosis whereas from 26% to 41% in adult spinal deformity.Until now,almost all studies on PJK have focused on the occurance in movable spinal segments,and it is believed that PJK does not occur in fused segments.However,a recent studie has observed the occurrence of PJK in rigid pinal segments.It is not clear the change of postoperative proximal junctional segments and the occurance of PJK after correction for ankylosing spondylitis.It is accepted that PJK is caused by the stress change in proximal junctional segments,which leads to fracture and fixation failure and revision surgery.Biomechanical study is able to reveal the mechanism of PJK.Traditional biomechanical experiment study needs cadaver specimens of human or animal which are always hard to obtian.Finite element(FE)analysis is a new biomechanical research method.In this method,the target structure is divided into a finite number of units,and the mechanical properties of each unit are set in accordance with actual situation.The mechanical responses of each unit and node can be obtained by calculation.An accurate finite element model can accurately simulate the structure of the spine and internal fixation,analyze the mechanical response of each part by applying different loads,and assist in guiding the surgical design according to the results.For complex spinal structures such as spinal deformity,biomechanical experiments are difficult to carry out.The mechanical responses of spine and internal fixation under different loads can be obtained qualitatively and quantitatively by means of finite element numerical simulation,so that the mechanism of mechanical complications can be analyzed,and preventive measures can be put forward accordingly.ObjectivesTo explore the evolution of proximal junctional segment and the occurrence of PJK after osteotomy in ankylosing spondylitis,and to analyze the related risk factors.To develop an accurate FE model of spine with kyphotic deformity following vertebral column decancellation and to provide a reliable model for further biomechanics analysis.To analyze the mechanical response of proximal junctional segments and to analyze the effect of osteoporosis on the construct by applying different moments on the developed FE model.To evaluate the biomechanical influence of cross-link(CL)on the internal fixation system construct,analyze the protective effect of the CL on the structure of the proximal junctional segments.Methods and ResultsPart IWe have reviewed 83 AS patients with thoracolumbar kyphosis surgically treated at our institution from the year of 2007 to 2013.Patients were divided into two groups based on presence or absence of PJK.The radiographic measurements including proximal junctional angle,sagittal parameters and pelvic parameters of these two groups were compared in different times i.e.before surgery,2 weeks and 12 months after operation,and the 2 years postoperatively.Oswestry Disability Index was also evaluated.There were 14.5% patients who developed into PJK.Most of PJK was identified as vertebra compression in proximal jucntioanl segments.Before operation,the mean PJA in two groups were 13.6 degrees and 8.5 degrees respectively(p = 0.008).There was no difference at age,gender and BMI between groups.Patients who suffered PJK had larger thoracolumbar kyphosis(50.8±12.6)and sagittal vertical axis(21.7±4.3)preoperatively than another group.The proportion of patients with PJK who fused to sacrum was 41.2(7/17),which was significantly higher than other choices of the lowest instrumented vertebrae.Oswestry disability index scores did not increase in PJK group compared with non-PJK group.Part IITo explore the mechanicsm of PJK,a FE model of spine with kyphotic deformity following vertebral column decancellation was developed in this part.A male patient with spinal kyphotic deformity was involved in this study.Vertebral column decancellation was performed at L1,and the segments from T10 to L4 were fixed with the pedicle screw and rod system.The patient’s full-spinal CT scanning images were obtained two weeks after surgery and imported into Mimics 17.0 software.The three-dimensional algorithm was used to generate three-dimensional point cloud data,and a thoracic and lumbar spine graphic model was established.The 3D point cloud data was imported into the software of 3-Matic and then smoothed to generate a three-dimensional point data model.The screw-rod fixation system was introduced into the model.All components were assembled to establish an FE model of spine following osteotomy and long segment instrumentation.The material parameters were set according to the previous literature.The model was verified based on the experimental results of the literature.The FE model of spine following osteotomy and long segment instrumentation was well developed,including vertebrae,intervertebral discs,spinal ligaments,pedicle screws and rods.The model consisted of 509,580 nodes and 445,722 hexahedrons.The model was verified by a comparison with previous studies.This model can be served as a reliable and accurate digital platform for biomechanical analysis and surgical planning.Part IIIUsing the established spine FE model,the biomechanical responses of the construct under different motion conditions were analyzed.All degrees of freedom of sacrum were constrained,and the loads were applied to the upper endplate of T1 vertebral body to simulate different working conditions.A load of 300 N acted perpendicularly on the upper surface of the T1 to simulate axial compression conditions.The moments of 10 N·m along the Z axis,the X axis,and the-X axis were applied respectively to simulate the motion of flexion,extension,and lateral bending.The overall stress distributions of the model under different working conditions were observed,and the stress(T6 to T11)and displacement(T9 to T11)of the vertebral body in the proximal junction area were measured.According to the literature,the mechanical parameters of different bone mass were set,and the difference of mechanical response between normal bone model and osteoporosis model under different working conditions was compared.Under axial compression,the peak stress of the entire model was 118.3 MPa on rods.Upon flexion,the proximal pedicle screws were the point of stress concentration with 221.1 MPa.Upon extension and lateral bending,the peak stress was found on L1,the osteotomy site,with 47.11 MPa and 60.2 MPa,respectively.Compared with normal bone mass,the pressure on osteotomized vertebra was decrease by 8.32%,1.92%,36.79% and 79.80% in osteoporosis model during axial compression,flexion,extension and lateral bending.Under the axial compression,the rods of both osteoporosis model and normal bone model bear the highest stress level.Compared with normal model,the stress on vertebra in osteoporosis model is lower.Under flexion loading,the stress on vertebra and screw in osteoporosis model decreased by 2.03% and 0.95%,respectively,while the stress levels on titanium rod increased from 131.7 MPa to 135.0 MPa.With the decrease of bone mass,the stress on vertebra and titanium rod decreased,while the stress of screw was higher than that on normal bone model.Under lateral bending,the stress on osteotomy site and screws in osteoporosis model was decreased,and the stress on titanium rod was increased.Part IVThe previous parts revealed that proximal screws and rods suffers greater stess which may become the reason for PJK occurance.In this part,the biomechanical influence of cross-link on the internal fixation system construct,analyze the protective effect of the cross-link on the structure of the proximal junctional segments were evaluated.Five fixation models were established to simulate different number and location of CLs: without CL(nCL),one CL at osteotomy site(CL-1),one CL at osteotomy site t and another at proximal segment(CL-2P),one CL at osteotomy site and another at distal segment(CL-2D),one CL at osteotomy site and other two at both proximal and distal segments(CL-3).Four loading conditions(flexion,extension,lateral bending and axial rotation)were applied on the models.An axial compression load of 280 N acted perpendicularly on the upper surface of the T1 to simulate the weight of the upper body.The moment of 10 N·m was applied to simulate the movement of flexion,extension,sidebend,and axial rotation of the spine.The influence of the number and position of the cross-links on the mechanical response of the fixation system in the spine were analyzed.The range of motion of the five FE models and the stress distribution of the pedicle screws,rods and vertebrae were measured under flexion,extension,lateral bending and axial rotation.With number of CL increasing,the ROM of instrumented segments were reduced by 2.37%,1.89% and 2.49% in flexion,extension lateral bending in average,respectively.When loading axial rotation condition,the ROM was reduced by 21.98%.With the number of CL increasing,the ROM tended to be limited.The ROM in CL-2P was less than CL-2D.The stress concentration was dispersed to other components with the use of CLs.In axial rotation,the stress on rods was reduced by CL.With the CLs number increasing,the stresses were reduced to 96.10 MPa,105.30 MPa and 113.20 MPa in CL-1,CL-2P and CL-3,respectively,which were still smaller than nCL.The stress on L1 vertebra was reduced,whereas the stress on screws was increased.In flexion,the stress concentration on screws was decreased by 17.73% in CL-1,19.86% in CL-2P and 17.00% in CL-3 compared to nCL.The stress on rods was slightly increased in CL-2P and CL-3.In extension,the stresses on screws,rods and L1 vertebrae were decreased by CLs.Especially on L1 vertebra,the stress concentration part,CL-2P had the greatest impact on stress reducing.In lateral bending,CL-2P reduced stress concentration on L1 vertebrae when CL-1 and CL-3 made the stress increasing.The stress on rods was decreased when CL number adding.The use of cross-links reduces the stress on screw and rod in the proximal junctional segments,but has no significant effect on the stress of vertebral body.Conclusions1.It was confirmed that PJK can occur at rigid spine segments.In AS postoperative patients,the incidence of PJK was 14.5%.The main manifestation was local kyphosis caused by vertebral compression at the proximal junctional segments.There are four main factors affecting the occurrence of PJK: severe sagittal imbalance before surgery,severe kyphosis at proximal junctional area,insufficient correction and progress of AS disease.2.By establishing an effective and reliable three-dimensional FE model for biomechanical analysis,the occurrence of PJK is mainly due to the stress concentration on proximal junctional segments and the difference of stress and displacement between fixed and non-fixed segments.Reducing the difference is helpful for the prevention of PJK.3.Under different bone mass conditions,the distribution patterns of stress in vertebrae,screw and rod were relatively similar.Collectively,the stress level on vertebrae was decreased and the stress levels of screw/rod system were increased in osteoporosis model compared to normal bone model.Hence,these may increase the risk of fracture and internal fixation failure.4.The application of CLs is able to enhance the rigidity of the construct and disperse the stress concentration on the construct.The use of CLs reduces the stress on the fixed structure in the proximal junctional area and also reduces the risk of internal fixation fracture after operation.These two effects may play a preventive role in PJK occurrence.Comparing various CLs configurations in different motion conditions,we recommend that the optimal method is to place two CLs at both osteotomy site and proximal segment. |
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