| I.BackgroundWith the development of instruments and treatment techniques,the adult spinal deformity(ASD)can be corrected efficiently.However,the postoperative complications cannot be ignored,such as proximal junctional kyphosis(PJK).The incidence of PJK is high,and its risk factors have yet been fully elucidated.Biomechanical studies have shown that disruption of the proximal ligament complex can promote PJK,while strengthening the proximal ligament can prevent PJK.However,the currently reported ligament enhancement measures are complicated to operate and there is a risk of injury to the dura mater and spinal cord.Some biomechanical studies differ greatly from clinical surgical procedures.The prophylactic ligament augumentation increases the flexural stiffness of the active segment in the proximal junctional area,and mitigates the stiffness in the junctional area.Accordingly,reducing the stiffness of the proximal internal fixation structure can also mitigate stiffness in the junctional area.Therefore,a semi-rigid structure at the proximal structure may be a strategy to prevent PJK.II.The purpose of the current study1.The study aimed to assess the incidence of PJK and PJF after surgery in ASD patients,and analyzed its influencing factors such as damage to the posterior ligament complex.We further assessed the influencing factors of PJK by systematic review and meta-analysis.2.By establishing a finite element model of ASD,combined with "fresh frozen" corpse models,the current study assessed the ligament strengthening methods designed by our teams to prevent PJK theoretically.3.We further studied the preventive effect of proximal semi-rigid structure on PJK and PJF.III.Research Methods(A)Analysis of risk factors for the PJK in ASD patients after surgery.Section 1: A retrospective clinical study on PJK and PJF in ASD patients after surgery.We retrospectively collected the ASD cases treated in our hospital.The following were inclusion criteria:(1)ASD patients;(2)Age?40 years;(3)Provision of X-rays before,within 2 weeks after surgery and at the last follow-up;(4)Follow-up time?6 months;(5)fused segments?5.PJK diagnostic criteria: The Cobb angle formed by the lower endplate of UIV and the upper endplate of UIV+2 is greater than 10°,and increased by more than 10°,postoperatively.PJF diagnostic criteria: cases of revision surgery due to PJK,the failure of proximal junctional internal fixation,and UIV,UIV+1,UIV+2 vertebral fractures or dislocations.We collected the age,gender and so on.The sagittal spinal radiographic parameters were measured.When the variables met the normal distribution,independent sample t test was used to assess whether there were differences in parameters between groups.Otherwise,the Mann-Whitney U test was used.Logistic regression analysis was used to find the main influencing factors of PJK and PJF.Section 2: The incidence of PJK and its risk factors in ASD patients after surgery----a systematic review and meta-analysis.The current manuscript retrieved Pub Med,Web of Science,CNKI.Net,and Wanfang Data on September 30,2019.The inclusion criteria were:(1)A case-control study,or a cohort study;(2)Age?18 years;(3)fusion egments?4;(4)Follow-up time?12 months;(5)Report a PJK-related risk factor at least;(6)Writing in Chinese or English.We extracted the basic information on the literature,general information,sagittal parameters before and after surgery,the range of postoperative parameter changes,and information about the operation,and quality of life(ODI and SRS-22 scores)before surgery,and at the last follow-up.Statistical analysis was performed using Review Manager version 5.3(The Cochrane Collaboration,Oxford,UK)and STATA 12.0(Stata Corporation,College Station,TX,USA)software.(B)Biomechanical analysis of posterior ligament augmentation anchoring to spinous process to prevent PJK1.Finite element experiments.Based on an established ASD finite element model(T1-L5),four surgical models with different measures to prevent PJK were further established.The maximum torque is 10 N.M.The load was located on T1 vertebra.A: all pedicle screw fixation model.B: binding the spinous process(UIV+1/UIV/UIV-1)by nylon rope to strengthen the ligaments.C: binding the spinous process(UIV+1/UIV/UIV-1)by nylon rope to strengthen the ligaments and anchored the nylon rope to the cross-link to strengthen the ligament.D: binding the spinous process(UIV+1/UIV/UIV-1)by nylon rope to strengthen the ligaments and anchored the nylon rope to the rod extending to the UIV+1 segment for ligament reinforcement.Then,the study assessed the biomechanical changes of the vertebrae,intervertebral disc and internal fixation in the proximal junctional segments.2.Cadaver loading analysis.Five “fresh frozen” corpses(T5-L2)were selected.The maximum torque is 5N.M.The load is located on T5 vertebra.Each specimen was tested strictly in accordance with the following steps from A to G.Step one(group A): Measuring the range of motion(ROM)of the complete spinal specimen.Step 2(group B): Resection of ISL/SSL in T12/L1 segment and observe the ROM.Step 3(group C): Resected the ISL/SSL of T11/T12 segment and observe the ROM.The further steps fixed T9/T10/T11/T12 segments and assessed the ROM of different ligament augmentation methods in the proximal junctional area.Step 4(group D): All pedicle screw fixation.Step 5(group E): All pedicle screw fixation,nylon cords were used to bind the spinous processes of UIV+1/UIV/UIV-1.Step 6(group F): All pedicle screw fixation,nylon cords were tied to the spinous process of UIV+1/UIV/UIV-1,and the nylon cords were anchored to the transverse link.Step 7(group G): All pedicle screw fixation,nylon cords tied to the spinous process of UIV+1/UIV/UIV-1,and anchored to the rods extended to UIV+1.When the variables met the normal distribution,the LSD-T test was performed to assess whether there were differences in ROM between groups,and the other was to use the Non-parametric test.The statistical analysis software used SPSS18.0(Chicago,IL,The United States)version.When P <0.05,there was a statistical difference.(C)Biomechanical analysis of semi-rigid fixation for prevention of PJK.1.Finite element experiments.Four finite element models were established as following.Model A: All pedicle screw fixation;Model B: The laminar hook was used in UIV;Model C: UIV-1/UIV fixed with spring rod;Model D: UIV-1/UIV was fixed with Dynasts.The ROMs of different models(UIV/UIV+1 and UIV+1/UIV+2)were compared,respectively;maximum anterior pressure of vertebras(UIV,UIV+1,and UIV+2);intervertebral disc stress(UIV /UIV+1)and UIV+1/ UIV +2);UIV screw stress;as well as maximum rod stress.The maximum torque was10 N.M,located on the T1 vertebra.2.Cadaver loading analysis.Using the previous cadaver spinal specimens,the experiment was conducted with the following steps from group A to E.The maximum torque was 5N.M,located on the T5 vertebra.Group A: All pedicle screw fixation;Group B: UIV using laminar hook;Group C: UIV/UIV-1 using Dynesys;Group D: UIV/UIV-1 using spring rods(large pitch);Group E: UIV/UIV-1 uses spring rods(small pitch).Furthermore,the difference between spring rod and ligament strengthening was compared.The statistical method is the same as above.IV.Results(A)Analysis of risk factors for the PJK in ASD patients after surgery.Section 1: A retrospective clinical study of PJK and PJF in ASD patients after surgery.A total of 94 ASD patients were included in the current study,the incidences of PJK and PJF were 31.91% and 7.45%,respectively.We observed a higher OVCFs rate in the PJK group(P<0.001),as well as UIV spinous process destruction(P<0.001).The following parameters in the PJK group were larger before surgery: C2S(P=0.038);c-SVA(P=0.009);TLK(P=0.004)and CTPA(P=0.008).After surgery,TLK(P=0.031)in the PJK group was greater,while L1-L4 lordosis was smaller in the Non-PJK group(P=0.042).At final follow-up,the following parameters were greater in the PJK group: c-SVA(P=0.012);T1-Slope(P=0.005);TLK(P=0.005);SVA(P=0.002);CPA(P=0.038);PJA(P<0.001).However,the L1-L4 lordosis was smaller than the Non-PJK group(P=0.007).Univariate regression analysis found the following risk factors: age>65 years(OR=3.26,P=0.012),OVCFs(OR=9.02,P<0.001),UIV spinous process destruction(OR=0.12,P<0.001),postoperative LL(OR=3.06,P=0.045),preoperative TLK(OR=3.06,P=0.002),postoperative TLK(OR=6.02,P=0.040),TLK changes(OR=6.74,P<0.001).Univariate regression analysis found that the risk factors for PJF were: OVCFs(OR=27.50,P=0.001),UIV spinous process destruction(OR=0.09,P=0.030),PT change(OR=18.40,P=0.026),postoperative TLK(OR=16.33,P=0.016).Further multivariate analysis found that:(1)the OVCFs(OR=10.58,P=0.001)and UIV spinous process destruction(OR=0.14,P=0.003)were the main risk factors for PJK.(2)The main risk factor for PJF was the history of OVCF(OR=36.80,P=0.003).Section 2: The incidence of PJK and its risk factors in ASD patients after surgery----a systematic review and meta-analysis.A total of 1130 articles were retrieved,and 59 articles were included.A total of 8626 ASD cases were included,among which the incidence of PJK/PJF was 34.23%.The following parameters in the PJK group were greater: age(WMD=2.72,P<0.001);female(OR=1.55,P<0.001);age>65y(OR=2.39,P=0.003);osteoporosis(OR=1.58,P=0.01).While the T-score score(WMD=-0.5,P<0.001)was smaller in PJK group.Preoperatively,the following parameters in the PJK group were larger than the control group: SVA(WMD=19.19,P<0.001);PI-LL(WMD=2.64,P=0.02);PT(WMD=2.71,P<0.001);LL(WMD=-1.95,P<0.001)and SS(WMD=-3.41,P=0.007)was smaller than the control group,however,there were no differences in PI(P=0.73),TK(P=0.60)and PJA(P=0.80)between the two groups.In the early postoperative period,only PJA(WMD=5.82,P<0.001)was greater in the PJK group.At the last follow-up,the following parameters in the PJK group were greater: TK(WMD=6.68,P<0.001);PJA(WMD=6.50,P<0.001);SVA(WMD=7.07,P<0.001),and PT(WMD=0.99,P=0.01).The TK(WMD=6.63,P<0.001)increased more in the PJK group,while the PI-LL(WMD=-6.99,P<0.001)and SVA(WMD=-9.24,P=0.01)decreased more.However,there was no statistical difference in the changes in LL(P=0.07),PT(P=0.12)and SS(P=0.14).That LIV extended to S1 or the pelvis significantly increased the risk of PJK(OR=2.08,P<0.001),as well as UIV terminated to the lower thoracic segment(OR=1.64,P=0.04).Ligament augmentation(OR=0.34,P<0.001)significantly reduced the risk of PJK,as well as laminar hooks(OR=0.46,P<0.001).However,vertebral cement augmentation(P=0.98)could not prevent PJK.There was no significant difference in ODI index(P=0.07)and SRS-22 score(P=0.056)between groups before surgery.The ODI(WMD=6.08,P=0.002)was high in the PJK group at the last follow-up,while the SRS score(WMD=-0.24,P<0.001)was lower than the control group.(B)Biomechanical analysis of posterior ligament augmentation anchoring to spinous process to prevent PJK.1.Results of finite element simulation experimentsFor the UIV/UIV+1 segment,the ROM in group A was the largest under different conditions(forward flexion,backward extension,left bending,right bending,left rotation,and right rotation: 3.35°,3.13°,3.48°,3.43°,2.21°,and 1.92°),larger than group B(forward flexion,backward extension,left bending,right bending,left rotation,and right rotation: 3.14°,2.95°,3.25°,3.18°,2.01°,and 1.74°),group C(forward flexion,backward extension,left bending,right bending,left rotation,and right rotation: 2.90°,2.74°,2.94°,2.87°,1.79° and 1.58°)and group D(forward flexion,backward extension,left bending,right bending,left rotation,and right rotation: 2.92°,2.76 °,2.96 °,2.89 °,1.81°,and 1.60°).For the UIV+2/UIV+1 segment,the ROM was relatively greater than UIV+1/UIV segment,while the change trend is consistent with UIV+1/UIV.Since the moment load was added at the T1,the pressure on the cephalic vertebrae was larger(T6>T7>T8).As for the T8 vertebra,comparison analyses between groups detected largest stress in group A(backward extension,forward flexion,left bending,right bending,left rotation,and right rotation: 29.94 MPa,32.23 MPa,29.66 MPa,30.18 MPa,20.83 MPa,and 20.57MPa).The maximum pressure was higher than the ligament-reinforced groups,and the ligament-reinforced group D demonstrated the smallest pressure(backward extension,forward flexion,left bending,right bending,left rotation,and right rotation: 29.94 MPa,32.23 MPa,29.66 MPa,30.18 MPa,20.83 MPa,and 20.57MPa),the reducing trend was more pronounced during forward flexion.As for the T7 vertebrae,comparison between different groups found that the pressure in group A was the largest(backward extension,forward flexion,left bending,right bending,left rotation,and right rotation: 30.45 MPa,30.82 MPa,27.24 MPa,28.64 MPa,16.70 MPa and 16.72MPa),the smallest in group D(backward extension,forward flexion,left bending,right bending,left rotation,and right rotation: 27.20 MPa,21.93 MPa,24.53 MPa,25.61 MPa,14.66 MPa,and 15.01MPa).For the more cephalic T6 vertebra,comparison analyses between groups also found a similar trend.For the T7/8 intervertebral disc,comparison analyses found the highest pressure in Group A under all conditions(backward extension,forward flexion,left bending,right bending,left rotation,and right rotation: 20.98 MPa,22.4MPa,20.32 MPa,21.51 MPa,13.87 MPa,and 12.95MPa).The ligament strengthening can significantly reduce the disc pressure,with the smallest pressure in group D(backward extension,forward flexion,left bending,right bending,left rotation and right rotation: 18.55 MPa,6.53 MPa,18.73 MPa,19.92 MPa,6.17 MPa,and 6.55MPa).The T6/7 intervertebral space pressure also showed similar results.The UIV screw stress in group A was the highest,and the group D was the smallest.Ligament strengthening increasedd the stress on the proximal screw when it was twisted.However,ligament strengthening reduced rod stress without affecting stress distribution.2.Cadaver loading analysisWhen flexed forward,the ROM of group A,group B and group C was 8.57°±0.82°,9.16°±0.92°,and 9.94°±0.55°at 5N.M,respectively.In terms of UIV+1/UIV segments,forward flexion loading text detected that only the nylon strapping the spinous processes between UIV+1/UIV/UIV-1 does not restrict its ROM(Group E vs.Group D: 3.23°±0.15° vs.4.01°±0.80°,P=0.083).The group G was the smallest ROM(1.68°±0.24 °),which was significantly smaller than group F(2.77°±0.47°)and group E(3.23°±0.15°).When bending left,the ligament strengthening methods reduced the ROM in UIV+1/UIV segment,but there was no significant difference between different ligament strengthening methods.During right flexion loading,there was no difference in ROM between groups D and E,but ROM in group D was larger than that in groups F and G.(3)Biomechanical analysis of semi-rigid fixation for prevention of PJK.1.Results of finite element simulation experimentsIn terms of UIV/UIV+1 segment,the ROM was the largest in the forward bending condition.The ROM of group A was the largest(forward flexion,backward extension,left bending,right bending,left rotation and right rotation: 3.35°,3.13°,3.48°,3.43°,2.21°,and 1.92°),which was greater than Group B(Forward,Backward,Left,Right,Left and Right: 3.14°,2.95°,3.25°,3.18°,2.0° and 1.74°),Group C(forward flexion,backward extension,left bending,right bending,left rotation and right rotation: 2.90°,2.74°,2.94°,2.87°,1.79°,and 1.58°)and group D(forward flexion,backward extension,left bending,right bending,left rotation and right rotation: 2.92°,2.76°,2.96°,2.89°,1.81°,and 1.60°).As for the UIV vertebrae,comparison between different groups found that: Group A had greater stress(forward flexion,backward extension,left bending,right bending,left rotation and right rotation: 29.94 MPa,32.23 MPa,29.66 MPa,30.18 MPa,20.83 MPa,and 20.57MPa),followed by group B(forward flexion,backward extension,left bending,right bending,left rotation and right rotation: 29.80 MPa,31.10 MPa,29.78 MPa,30.26 MPa,16.57 MPa,and 16.41MPa),group C(forward flexion,backward extension,left bending,right bending,left rotation,and right rotation: 29.10 MPa,30.64 MPa,28.63 MPa,26.83 MPa,15.08 MPa,and 14.78MPa)and group D(forward flexion,backward extension,left bending,right bending,left rotation and right rotation: 28.10 MPa,31.08 MPa,28.25 MPa,27.68 MPa,14.70 MPa,and 14.29MPa).For the UIV+1 vertebra,the trend of stress was similar to T8 vertebra,but no difference was detected for the UIV+2 vertebra between different groups.The T7/8 disc pressure was greater in group A(forward flexion,backward extension,left bending,right bending,left rotation and right rotation: 20.98 MPa,22.40 MPa,20.32 MPa,22.51 MPa,13.87 MPa,and 12.95MPa),followed by group C,and group D was slightly smaller.T6/7 disc pressure demonstrated similar trendency.Comparing the maximum pressure of UIV screws between different groups,the stress of group C(spring rod group)was lower during forward flexion and backward extension.However,the all pedicle screw group demonstrated greater stress when bending and rotating.The all pedicle screw internal fixation method(group A)had the largest stress in the rod,and the spring rod group was relatively small.Comparison of different loading methods revealed that the maximum rod stress was larger when rotating.2.Cadaver loading analysisFor the UIV/UIV+1 segment,comparison analyses between groups found that the ROM of group A was relatively larger(forward flexion,backward extension,left bending and right bending: 4.01±0.80°,2.59±0.41°,3.31±0.18°,and 3.56±0.30°);followed by Group B(forward flexion,backward extension,left bending and right bending: 2.97±0.27°,2.60±0.41°,2.59±0.12°,and 2.01±0.74°).The smallest ROM for flexion was in group E(forward flexion,backward extension,left bending and right bending: 1.50±0.37°,2.53±0.22°,2.61±0.20°,and 2.35±0.20 °).V.Conclusion1.Surgical correction could reconstruct spinal alignment,but it didnot reverse the process of trunk inclined forward with age.The curvature of the fixed segments was relatively constant,and then the flexible segments in the proximal junctional area might become more compensatory kyphotic.PJK and PJF might be a manifestation of the decompensation.The cervical spine,with a large ROM,and had a strong ability to compensate for PJK/PJF.Preserving UIV spinous processes and avoiding damage to the supraspinous ligament and interspinous ligament between UIV and UIV+1 might reduce the risk of PJK and PJF.2.PJK could badly affect patients' quality of life.When PJA?10 ° is used as the diagnostic threshold,its incidence rate is about 30?35%.If the elderly and osteoporotic ASD patients had larger SVA,lower lumbar lordosis,larger PT,LIV to the pelvis,and UIV terminated in the lower thoracic region,the risk of PJK would increase.The SVA and LL should not be overcorrected.Proximal laminar hooks and ligament reinforcement were optional methods to prevent PJK.3.The ISL/SSL played an important role in restricting ROM.Disruption of ISL/SSL could significantly reduce the flexion stiffness.Ligament augmentation could significantly increase the flexion stiffness in the proximal junctional area and mitigate stiffness from the fixed segments to the flexible segments.Fixation of UIV-1/UIV/UIV+1 spinal process and anchoring to extended internal fixation rods were more effective in restricting ROM at the proximal flexible segments and mitigate stress.4.If no correction force was taken into consideration,the semi-rigid transition construct(Dynesys and spring rod)between UIV/UIV-1 could mitigate the rigidity of the junctional segments.The better the flexibility of the proximal fixation,the better the effect of reducing stress concentration.Both Dynesys and spring rods may not be appropriate flexible constructs,but the concept of semi-rigid fixation at proximal region deserved to be paid enough attention by surgeons and instrument developers. |
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