| Part I Clinical and radiographic outcomes of cervical disc arthroplasty for the treatment of noncontinuous cervical spondylosis Objective:Cervical disc arthroplasty(CDA)has been widely performed in recent years,as it can maintain disc height(DH)and is beneficial for preserving cervical mobility.Several clinical trials have indicated the equivalence or superiority of CDA to treat one-level and contiguous two-level cervical spondylosis or cervical degenerative disc disease to the anterior cervical discectomy and fusion(ACDF).However,the outcomes of CDA on noncontiguous cervical spondylosis are still unclear,and whether CDA avoids or delays the degeneration of the intermediate segment is still controversial.Therefore,this study aimed to investigate the clinical and radiographic outcomes of CDA for the treatment of noncontiguous cervical spondylosis and to explore whether the intermediate segment degenerated faster after CDA than that of other non-operative adjacent segments.Materials and Methods:From January 2008 to December 2019,patients with noncontiguous cervical spondylosis who underwent Prestige-LP CDA in the Department of Orthopaedics,West China of Sichuan University were retrospectively analyzed.A total of 31 cases of two-level skip-level CDA,33 cases of skip-level hybrid surgery(HS),including 24 cases of two-level surgery and 9 cases of three-level surgery,were collected.A total of 97 Prestige-LP were implanted.Clinical outcomes were evaluated preoperatively and at routine postoperative intervals of 3 and 12 months and the last follow-up using Japanese orthopaedic association(JOA)score,neck disability index(NDI),and visual analogue scale(VAS).Radiographic outcomes were evaluated preoperatively and at routine postoperative intervals of 1 week,3,12 months,and at the last follow-up,including cervical lordosis,C2-C7 range of motion(ROM),segmental ROM,disc angle(DA)at the arthroplasty level,DH at the arthroplasty levels.Fusion status at the fused level,prosthesis subsidence,heterotopic ossification(HO),anterior bone loss(ABL)at the arthroplasty levels,adjacent segment degeneration(ASDeg)were also evaluated.Results:Sixty-four patients completed the at least 12 months follow-up.There were 31 patients in the CDA group(16 males and 15 females)with an average age of 46.16 ±6.48 years(range,31-59 years)and an average of 44.13 ±28.74 months follow-up(range,12-108 months).There were 33 patients in the HS group(18 males and 15females)with an average age of 48.97 ± 6.70 years(range,33-65 years)and an average follow-up of 42.55 ± 22.50 months(range,12-108 months).There was no significant difference in sex ratio,age,and follow-up time between the two groups(P >0.05).The C4/5 and C5/6 were the most involved intermediate segment,which accounted for 69% and 28%,respectively.The value of JOA,NDI,and VAS was significantly improved after surgery,but there was no significant difference between CDA and HS group at each follow-up period.There was no significant difference in DA and ROM at the arthroplasty before and after surgery,while the difference in cervical lordosis before and after surgery was statistically significant(P<0.05).The cervical lordosis in the CDA group was lower than that in the HS group at each followup period(P < 0.05).C2-C7 ROM decreased significantly after surgery(P<0.05).C2-C7 ROM in the CDA group was greater than that in the HS group at the last followup(P<0.05).The ROM of the intermediated segment in the HS group was significantly increased at the last follow-up(P<0.05),while the value in the CDA group was found similar to preoperative levels.At the last follow-up,one prosthesis was found subsidence(1.0%),44.3% arthroplasty levels developed HO,48.45%arthroplasty levels developed ABL and 20.6% superior adjacent segment,22.7%intermediated segment and 21.9% inferior adjacent segment developed ASDeg.No significant difference in the incidence of complications between the CDA group and HS group was found.Conclusion:Skip-level CDA or Skip-level CDA combined with ACDF to treat noncontiguous cervical spondylosis can achieve satisfactory medium-to-long-term clinical and radiographic outcomes and might be considered as safe and effective surgical strategies for the treatment of noncontiguous cervical spondylosis.The incidence of ASDeg is similar among non-surgical segments after CDA.The ROM of the intermediate segment is similar to that before surgery,indicating CDA theoretically does not increase the risk of degeneration of the intermediate segment.However,HS may increase 1°-2° ROM at the non-surgical segments,which may increase the risk of ASDeg during long-term follow-up.Part II Biomechanical evaluation of skip-level surgery for the treatment of noncontiguous cervical spondylosis: an in vitro human cadaveric studyObjective:Biomechanical studies confirmed that anterior cervical discectomy and fusion(ACDF)significantly increased range of motion(ROM)and intradiscal pressure at adjacent segments,while cervical disc arthroplasty(CDA)does not affect the ROM,intradiscal pressure,and facet joint force at adjacent segments.Hybrid surgery(HS)also less biomechanically affected adjacent segments when compared with two-level ACDF.However,limited biomechanical data concentrated on skip-level ACDF,CDA,or HS.The biomechanical results of one-level and contiguous two-level cervical spine surgery can not completely explain the special condition of noncontiguous cervical spine surgery.Therefore,the current study aimed to compare the kinematic and mechanical changes of non-surgical segments after different skip-level surgery for the treatment of noncognitions cervical spondylosis.Materials and Methods:Six fresh-frozen cadaveric cervical spines(C2-T1)were selected and biomechanically evaluated between levels C3 and C7.According to the experiment sequence,the specimens were divided into the Intact group,C3/4 CDA,and C5/6 CDA group(AA),C3/4 ACDF,and C5/6 CDA group(FA),C3/4 CDA and C5/6 ACDF group(AF),C3/4 ACDF and C5/6 ACDF group(FF).A 2.0 N ·m pure moment was used for flexion/extension,lateral bending,and axial rotation.The ROM values and left facet joints contact force at C3/4,C4/5,C5/6,and C6/7 segments were measured.Results:C3-C7 ROM significantly decreased in all directions in the FF group compared with the Intact group(P<0.05),while C3-C7 ROM of other groups was similar to that of the Intact group in all directions.The arthroplasty levels of AA,FA,and AF groups increased,but the difference was not statistically significant compared with the Intact group.The fused levels in FA,AF,and FF groups were significantly decreased after surgery(P<0.05).Except for lateral bending ROM of the inferior adjacent segment(C6/7),the intermediated segment(C4/5)and C6/7 in the AA group decreased in all directions.Still,the difference was not statistically significant when compared with the Intact group.The ROM of C4/5 and C6/7 in the FA group and AF group was similar to that of the intact group in all directions.The ROMs of C4/5 and C6/7 were decreased in all directions in the FF group except for C6/7 in flexion/extension direction compared with the Intact group.The increased ROM ratio of C4/5 in flexion/extension was significantly less than that in C6/7 in the FF group(P<0.05).Compared with the AA group or AF group,the facet joint pressure of C3/4 in the FA group decreased significantly(P<0.05).The facet joint force in the FA group was significantly lower than that in the AA group or AF group during extension movement(P<0.05).Compared with the AA group or AF group,the facet contact force at C3/4 in the FA group decreased significantly(P<0.05).Compared with the AA group or FA group,facet contact force at C5/6 significantly reduced in the AF group(P<0.05).Compared with the AA group,facet contact force at C3/4 and C5/6 in the FF group decreased significantly(P<0.05).However,the facet contact force at C4/5 and C6/7 in the FF group was greater than that in other groups,but the difference did not reach a significant.Conclusion:Skip-level CDA had the least effect on the ROM and facet contact force at the non-surgical segment,while skip-level ACDF had the most effect on the non-surgical segment.The intermediated segment may suffer additive biomechanical effects compared with the inferior adjacent segment in the skip-level ACDF construct.ROM at the non-surgical segment slightly increased after skip-level HS,but the intermediated segment did not experience additive biomechanical effects.Part III Finite element analysis of jumping surgery in the treatment of cervical spondylosis with discrete segmentsObjective:Finite element analysis(FEA)has incomparable advantages in the stress-strain analysis of the internal structure of the spine or implant.It is an essential supplement to in vitro cadaveric experiments.Therefore,the current study established a finite element model of noncontinuous cervical spondylosis to further explored changes of intradiscal pressure and facet contact force at non-surgical segments after skip-level anterior cervical discectomy and fusion(ACDF),cervical disc arthroplasty(CDA),and hybrid surgery(HS).Materials and Methods:A finite element model of a healthy cervical spine(C2-C7)was constructed.Based on the healthy cervical spine model,four surgical models were developed: FF group: C3/4 ACDF and C5/6 ACDF,AA group: C3/4 CDA and C5/6 CDA,FA group: C3/4 ACDF and C5/6 CDA,AF group: C3/4 CDA and C5/6 ACDF.A 75 N load with 1.0 N·m moment was applied to the top of the C2 vertebra in the intact model to simulate flexion,extension,lateral bending,and axial rotation.The range of motion(ROM),Intradiscal pressure(IDP),and facet contact force(FCF)of surgical models were measured under both load-control and displacement-control protocols.Results:Under the load-control mode,the C2-C7 ROM of FF,FA,and AF model decrease in all directions,while C2-C7 ROM of the AA model increases in all directions.In the FF model,the ROM of the non-surgical segment increased in all directions,and the magnitude of the increased ROM in the intermediated segment was greater.The increase of ROM at the non-surgical segment of the FA model and AF model was less than that of the FF group,while the increase of ROM in the non-surgical segment of the AA model was similar to that of the intact model.Similar to the trend of ROM,the IDP and FCF at the non-operative segment of the FF model increased,and the intermediated segment increased more than that of the superior and inferior adjacent segments.The IDP and FCF at the non-surgical segments of the FA and AF models were less than that of the FF model and increased more evenly.The IDP and FCF at the non-surgical segments of the AA model were similar to that of the intact model.The interface stress between the cervical disc prosthesis and the upper endplate of the vertebral body is the largest during rotation.Compared with the AA model,the pressure of the C6 upper endplate in the FA model and C4 upper endplate in the AF model increased in all directions.Under the displacement-control mode,the moment of the FF,FA,and AF model increased in all directions,and the moment of the FF model increases the most,while that of the AA model decreased.In the FF model,the motion contribution of nonsurgical segments increased unevenly.The magnitude of the increased motion in the intermediated segment was more extensive than those in the superior and inferior adjacent segments.The IDP and FCF also increased most,and IDP and FCF at the intermediated segment were more susceptible to impact when compared with superior and inferior adjacent.In FA and AF model,the motion contributions of the nonsurgical segment were evenly increased.The intermediated segment did not experience additive motion,IDP,and FCF.However,the changes of IDP and FCF were correlated to the relative location of fused and arthroplasty level.The motion contribution,IDP,and FCF of the non-surgical segments in the AA model were similar to that in the intact model.Conclusion:Skip-level ACDF significantly altered the kinematic and mechanical environment of non-surgical segments and showed additive biomechanical effects on the intermediated segments.Skip-level HS less altered the biomechanical environment of non-operative segments and showed no additive effects on the intermediated segment.However,the biomechanical effects of adjacent segments might be related to the relative positions of ACDF and CDA levels.Skip-level CDA hardly changed the biomechanical environment of non-surgical segments.Part IV Establishing an animal model of skip-level fusion for investigating intervertebral disc and facet joint degenerationObjective:In-vitro biomechanical study and finite element analysis can not completely simulate the changes of biomechanical environment after cervical spine surgery in vivo.Paravertebral muscle tissue and ligament play an essential role in maintaining cervical spine stability.Therefore,we aimed to establish an animal model of skiplevel fusion to achieve the different environmental stress at different vertebral segments in vivo and to investigate whether skip-level fusion will lead to the different degrees of intervertebral disc and facet joint degeneration at the different non-surgical segment.Materials and Methods:120 Spraque Dawley(SD)rats were randomly divided into three groups: skiplevel fusion group(SF),one-level fusion group(OF),and sham-operated group(Sham).Rats in the SF group underwent posterior fusion at L2/3 and L4/5,OF group only underwent L4/5 posterior fusion,and the Sham group did not perform fusion surgery.10 SD rats in each group were randomly selected for behavioral tests,which were carried out before surgery and at routine postoperative intervals of 8,12,16,20,24,28,and 32 weeks.The remaining SD rats in each group were evaluated intervertebral disc and facet joint degeneration by radiographic imaging at 8,20,and 32 weeks after surgery,respectively,and then sacrificed for histological and immunohistochemical evaluation.Results:After fusion surgery,the intervertebral disc and facet joints at the non-operative segments degenerated progressively.The disc height index(DHI)of L3/4 in the SF group was lower than that in the Sham and OF group.The MRI degeneration score and histological degeneration score of the intervertebral disc were higher than those in the Sham and OF group.Furthermore,the expression levels of Collagen II and Aggrecan in the intervertebral disc were lower than those in the Sham group and OF group,while the expression level of MMP-13 was higher than those in the Sham group and Of group(P<0.05).In the SF group,the MRI,histological and immunohistochemical evaluation showed more severe degeneration at L3/4 disc than those of superior and inferior adjacent segments(L1/2 and L5/6).However,the difference was not statistically significant.In the SF group,the degeneration of the L3/4 facet joint was severe at an early stage after fusion surgery.The radiographic and histological degeneration scores were significantly higher than those of the Sham and OF group(P<0.05).The expression levels of Collagen II and Aggrecan were lower than those of the Sham and OF group,while the expression level of MMP-13 was higher than those of the Sham and OF group(P < 0.05).In the SF group,the radiographic and histological degeneration scores of L3/4 facet joints were significantly higher than those of L1/2 and L5/6(P<0.05).The expression levels of Collagen II and Aggrecan were significantly lower than those of L1/2 and L5/6(P<0.05),while the expression level of MMP-13 was significantly higher than those of L1/2 and L5/6(P<0.05).Paw withdrawal threshold(PWT)and paw withdrawal latency to radiant heat in the sham group did not significantly change after surgery.PWT and paw withdrawal latency in SF and OF groups decreased significantly at 8 weeks after surgery(P<0.05),and then the level remained relatively stable.PWT and paw withdrawal latency in the Sham group were significantly lower than those in the SF and OF groups(P<0.05).However,no significant difference in PWT and paw retraction latency was found between SF and OF groups.Conclusion:The in-vivo biomechanical environment of the non-surgical segments after skiplevel fusion may be different.The intervertebral disc and facet joints at the intermediated segment degenerated more severely than those in superior and inferior adjacent segments.The Collagen II and Aggrecan expression were lowest,while the expression of MMP-13 was highest at the intermediated segment.There was no significant difference in the behavior changes between skip-level fusion and one-level fusion,indicating that clinical symptoms caused by adjacent segment degeneration were not related to the number of fused segments. |
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