radiculopathy and myelopathy from degenerative , inflammatory , and traumatic processes have been successfully treated with anterior cervical discectomy and fusion ( acdf ) . despite clinical success with fusion treatment , there are concerns regarding the long - term effects of fusion on the cervical spine . .   reported on the incidence of radiculopathy and myelopathy at adjacent segment to a cervical fusion and found that 25% of acdf patients will have symptomatic adjacent segment deterioration ( asd ) within 10 years of acdf at a rate of 2.9% each year . it is believed that although some changes in asd occur naturally , the effects of fusion disturb the biomechanics , most likely increasing the incidence of asd . in vitro studies demonstrated that intradiscal pressures , shear strains , and motion increased in upper and lower segments adjacent to fusion levels [ 47 ] . recently , artificial cervical disc replacement ( acdr ) has become the alternative to fusion , with the potential to preserve the motion of the instrumented level and to prevent overload of the adjacent levels and subsequent asd . acdr has been proven to be beneficial in terms of avoiding the deleterious effects of fusion . however , indications for acdr are more stringent and hypermobility of the operative levels may occur , which lead to the limitation of multilevel acdr . clinical studies regarding hybrid combinations of fusion and non - fusion have been reported , with improved total motion and earlier recovery and return to work [ 1215 ] . biomechanical studies of 2-level hybrid acdf and acdr have demonstrated advantages of the hybrid in reducing compensatory adjacent motion and reduced internal stresses in the construct [ 4,1621 ] . in a clinical study , kang el al . reported that the hybrid construct is a safe and effective alternative for 3-level cervical disk disease . the hypothesis was that the motion response of disc replacement adjacent to fusion was comparable to a stand - alone disc replacement , and the non - operated segments in a hybrid construct would experience significantly less motion than with a fusion . this study is a progression to 3-level from the 1- and 2-level biomechanical studies because 3 levels of fusion are common and hybrid combinations either performed initially or for revision of asd are more common today . the objective of the present study was to compare the 3-level acdf to combinations of acdr and acdf with displacement controlled kinematics at instrumented and adjacent levels . eighteen fresh adult human cadaveric cervical spines ( c2-t1 ; age range , 5273 years ) were used for biomechanical testing . all cervical spines were evaluated for bone mineral density ( bmd ) using dual - energy x - ray absorptiometry scanning and measured bmd values ranged from 0.53 to 0.72 g / cm . cervical spines with degenerative diseases or traumatic pathology were excluded by anteroposterior and lateral screening radiographs before biomechanical testing . once harvested , all cervical specimens were immediately conserved in plastic bags and frozen at 20c . in preparation for biomechanical testing , the required spines were thawed at 4c for 12 hours . at room temperature on the testing day , the proximal vertebra ( c2 ) and distal vertebra ( t1 ) were separately mounted in a cylindrical container using a low - fusion point ( 72c ) alloy . and then , the c2 container was attached to the upper fixture while the t1 container was mounted to the lower testing platform . markers made of 4 plexiglas motion detectors were fixed to the anterior aspects of each vertebra from c2 to t1 . the 3-dimensional motion range of each vertebra was obtained with an optoelectronics measurement system ( optotrak , northern digital inc . , waterloo , ontario , canada ) capable of capturing the motion curve of the markers . biomechanical tests were performed under displacement control by an mts machine ( cmt6104 ; mts systems ( china ) corp . , shenzhen , china ) which can replicate physiologic motion with displacement control   ( figure 1 ) . the flexion  extension axis of each tested spine was placed eccentric to the load axis of the actuator . the bending moments were limited to the upper bound of physiological human bending ( 4.5 nm ) . an angular displacement transducer was assembled to measure the global rotation of the cervical spine ( c2-t1 ) . a displacement transducer was used to measure the changes in moment arm length between the upper container and the load axis of the mts machine . the spine specimens were tested by application of a 50 n preload in flexion - extension and lateral bending circumstances . during the biomechanical tests , after analysis of the intact spine , each specimen was sequentially reconstructed at c3-c6 3-level ) motion segments . the conditions were as follows ( figure 2 ) : acdr , acdf , acdr , or 3-level disc plate disc ( 3dpd ) ; acdf , acdr , acdf or 3-level plate disc plate ( 3pdp ) ; three - level acdf or 3-level plate ( 3p ) . prior to biomechanical tests , measurements included vertebral motion , applied load , and moment . in the biomechanical tests , the acdr was a titanium - ceramic alloy cervical disc ( prestige lp cervical disc , medtronic sofamor danek usa , inc . ) . the arthrodesis was performed using an interbody cage ( telamon tm , medtronic sofamor danek usa , inc . ) combined with an anterior cervical plating ( acp ) system ( doc cervical plate , depuy spine , inc . , raynham , ma , usa ) ( figure 3 ) . this biomechanical protocol limited motion to 20 degrees in flexion and extension as well as 15 degrees in lateral bending and axial rotation . the ratio of segmental rom to the total c2-t1 rom was applied to evaluate the operation effect by normalization method . one - way anova ( p<0.05 ) was used to determine the statistical differences in both c3-c6 motion and adjacent level motion under the 3-level anterior plate fusion ( 3p ) , the 3pdp , and 3dpd conditions . eighteen fresh adult human cadaveric cervical spines ( c2-t1 ; age range , 5273 years ) were used for biomechanical testing . all cervical spines were evaluated for bone mineral density ( bmd ) using dual - energy x - ray absorptiometry scanning and measured bmd values ranged from 0.53 to 0.72 g / cm . cervical spines with degenerative diseases or traumatic pathology were excluded by anteroposterior and lateral screening radiographs before biomechanical testing . once harvested , all cervical specimens were immediately conserved in plastic bags and frozen at 20c . in preparation for biomechanical testing , the required spines were thawed at 4c for 12 hours . at room temperature on the testing day , the proximal vertebra ( c2 ) and distal vertebra ( t1 ) were separately mounted in a cylindrical container using a low - fusion point ( 72c ) alloy . and then , the c2 container was attached to the upper fixture while the t1 container was mounted to the lower testing platform . markers made of 4 plexiglas motion detectors were fixed to the anterior aspects of each vertebra from c2 to t1 . the 3-dimensional motion range of each vertebra was obtained with an optoelectronics measurement system ( optotrak , northern digital inc . , waterloo , ontario , canada ) capable of capturing the motion curve of the markers . biomechanical tests were performed under displacement control by an mts machine ( cmt6104 ; mts systems ( china ) corp . , shenzhen , china ) which can replicate physiologic motion with displacement control   ( figure 1 ) . the flexion  extension axis of each tested spine was placed eccentric to the load axis of the actuator . the bending moments were limited to the upper bound of physiological human bending ( 4.5 nm ) . an angular displacement transducer was assembled to measure the global rotation of the cervical spine ( c2-t1 ) . a displacement transducer was used to measure the changes in moment arm length between the upper container and the load axis of the mts machine . the spine specimens were tested by application of a 50 n preload in flexion - extension and lateral bending circumstances . during the biomechanical tests , after analysis of the intact spine , each specimen was sequentially reconstructed at c3-c6 3-level ) motion segments . the conditions were as follows ( figure 2 ) : acdr , acdf , acdr , or 3-level disc plate disc ( 3dpd ) ; acdf , acdr , acdf or 3-level plate disc plate ( 3pdp ) ; three - level acdf or 3-level plate ( 3p ) . prior to biomechanical tests , positioning of implants measurements included vertebral motion , applied load , and moment . in the biomechanical tests , the acdr was a titanium - ceramic alloy cervical disc ( prestige lp cervical disc , medtronic sofamor danek usa , inc . ) . the arthrodesis was performed using an interbody cage ( telamon tm , medtronic sofamor danek usa , inc . ) combined with an anterior cervical plating ( acp ) system ( doc cervical plate , depuy spine , inc . , raynham , ma , usa ) ( figure 3 ) . this biomechanical protocol limited motion to 20 degrees in flexion and extension as well as 15 degrees in lateral bending and axial rotation . the ratio of segmental rom to the total c2-t1 rom was applied to evaluate the operation effect by normalization method . one - way anova ( p<0.05 ) was used to determine the statistical differences in both c3-c6 motion and adjacent level motion under the 3-level anterior plate fusion ( 3p ) , the 3pdp , and 3dpd conditions . as expected , 3-level arthrodesis resulted in significant reduction of rom at the three instrumented levels in all 6 loading conditions ( flexion , extension , left bending , right bending , left rotation , and right rotation ) . compared to intact spines , almost 80% of motion was successfully restricted at c3-c6 fusion levels in flexion , extension , and lateral bending , as well as 65% in axial rotation . for hybrid constructs , 3dpd construct resulted in slight increase at the 3 instrumented levels in extension , lateral bending , and axial rotation compared to intact ( p>0.05 ; maximal variation of + 7% ) . however , the 3dpd condition resulted in a slight decrease at c3-c6 in flexion ( p>0.05 ; maximal variation of 9% ) . as another 3-level hybrid construct , 3pdp construct resulted in significant decrease of rom at the c3-c6 instrumented levels in all 6 loading conditions except for left rotation ( mean variation of 21% ; maximal variation of 38% ) . although 3pdp and 3p conditions produced significant motion decrease at the 3 instrumented levels , there was significant difference within the instrumented levels between 3pdp and 3p conditions in all 6 loading conditions ( p<0.05 ) . on the other hand , there were also significant differences within the c3c6 levels between 3dp d and 3pdp conditions in all 6 loading conditions except for flexion and left rotation ( p<0.05 ) ( figures 4 , 5 ) . for each instrumented level , 3dpd and 3pdp hybrid constructs caused significant reduction of rom in all 6 loading conditions at the arthrodesis level compared to intact ( p<0.05 ) and produced motion increase at the arthroplasty level . for the 3dpd hybrid construct , implantation of upper - level ( c3-c4 ) acdr resulted in significant increase of rom only in right rotation ( p<0.05 ; maximal variation of + 36% ) , while implantation of lower - level ( c5-c6 ) acdr resulted in significant increase of rom in all 6 loading conditions except for flexion ( p<0.05 ; maximal variation of + 41% ) . for 3pdp hybrid construct , implantation of middle - level ( c3-c4 ) acdr resulted in significant increase of rom in extension , left rotation , and right rotation compared to intact ( p<0.05 ; maximal variation of + 67% ) but produced motion increase in flexion , left bending , and right bending , without significant difference ( p>0.05 ) ( figure 6 , table 1 ) . as suspected , 3-level arthrodesis resulted in an increased contribution of upper and lower adjacent levels . significant changes were noted at the lower adjacent level in all 6 loading conditions as well as upper adjacent level in flexion and left bending ( p<0.05 ; maximal variation of + 197% ) . concerning 3dpd hybrid construct , significant changes of motion increase were only noted at lower adjacent level in left bending , right bending , and right rotation ( p<0.05 ; maximal variation of + 57% ) . importantly , there was a significant decrease toward normal in adjacent segment motion adjacent to an acdr . concerning 3pdp hybrid construct , significant changes of motion increase were noted at both upper and lower adjacent levels and the largest motion increase was noticed at lower adjacent level in extension ( p<0.05 ; maximal variation of + 79% ) ( figures 4 , 5 ) . as expected , 3-level arthrodesis resulted in significant reduction of rom at the three instrumented levels in all 6 loading conditions ( flexion , extension , left bending , right bending , left rotation , and right rotation ) . compared to intact spines , almost 80% of motion was successfully restricted at c3-c6 fusion levels in flexion , extension , and lateral bending , as well as 65% in axial rotation . for hybrid constructs , 3dpd construct resulted in slight increase at the 3 instrumented levels in extension , lateral bending , and axial rotation compared to intact ( p>0.05 ; maximal variation of + 7% ) . however , the 3dpd condition resulted in a slight decrease at c3-c6 in flexion ( p>0.05 ; maximal variation of 9% ) . as another 3-level hybrid construct , 3pdp construct resulted in significant decrease of rom at the c3-c6 instrumented levels in all 6 loading conditions except for left rotation ( mean variation of 21% ; maximal variation of 38% ) . although 3pdp and 3p conditions produced significant motion decrease at the 3 instrumented levels , there was significant difference within the instrumented levels between 3pdp and 3p conditions in all 6 loading conditions ( p<0.05 ) . on the other hand , there were also significant differences within the c3c6 levels between 3dp d and 3pdp conditions in all 6 loading conditions except for flexion and left rotation ( p<0.05 ) ( figures 4 , 5 ) . for each instrumented level , 3dpd and 3pdp hybrid constructs caused significant reduction of rom in all 6 loading conditions at the arthrodesis level compared to intact ( p<0.05 ) and produced motion increase at the arthroplasty level . for the 3dpd hybrid construct , implantation of upper - level ( c3-c4 ) acdr resulted in significant increase of rom only in right rotation ( p<0.05 ; maximal variation of + 36% ) , while implantation of lower - level ( c5-c6 ) acdr resulted in significant increase of rom in all 6 loading conditions except for flexion ( p<0.05 ; maximal variation of + 41% ) . for 3pdp hybrid construct , implantation of middle - level ( c3-c4 ) acdr resulted in significant increase of rom in extension , left rotation , and right rotation compared to intact ( p<0.05 ; maximal variation of + 67% ) but produced motion increase in flexion , left bending , and right bending , without significant difference ( p>0.05 ) ( figure 6 , table 1 ) . as suspected , 3-level arthrodesis resulted in an increased contribution of upper and lower adjacent levels . significant changes were noted at the lower adjacent level in all 6 loading conditions as well as upper adjacent level in flexion and left bending ( p<0.05 ; maximal variation of + 197% ) . concerning 3dpd hybrid construct , significant changes of motion increase were only noted at lower adjacent level in left bending , right bending , and right rotation ( p<0.05 ; maximal variation of + 57% ) . importantly , there was a significant decrease toward normal in adjacent segment motion adjacent to an acdr . concerning 3pdp hybrid construct , significant changes of motion increase were noted at both upper and lower adjacent levels and the largest motion increase was noticed at lower adjacent level in extension ( p<0.05 ; maximal variation of + 79% ) ( figures 4 , 5 ) . this study demonstrated that the motion response of an acdr adjacent to a fusion maintains normal motion at the acdr level and normalizes adjacent segment motion in 3-level hybrid constructs . in longer fusions , the intradiscal pressures and the compensatory hyper - mobility this study confirms the work of lee et al . and others [ 4,1321 ] that a longer fusion affects all adjacent levels with hyper - mobility . this study shows that an acdr normalized the adjacent level motion in a 3-level construct . acdr is the accepted alternative to anterior cervical fusion for single - level disc disease . favorable outcomes and the prospect of a lower incidence of adjacent - level disease have encouraged surgeons to expand current acdr indications to multilevel disc disease . however , the evidence in multilevel acdr is not as well established as its role in single - level disease , and some levels may be too degenerative for acdr . with that in mind , authors are reporting combinations of fusion and arthroplasty as an alternative to multi - level acdf or acdr . described 2-level , 3-level , and 4-level hybrid surgery results as safe and reliable without revision . shin el al .   compared hybrid construct to acdf in 2 levels , with improved ndi , pain , return of motion , and reduced asd . kang el al .   confirmed these findings in 3-level hybrid treatments , with improvement in recovery of total motion and maintenance of adjacent segment motion . jia et al . , in a systematic review of 8 biomechanical and 7 clinical papers , found a paucity of quality evidence to support hybrid surgeries and recommended prospective randomized trials . previous biomechanical studies have investigated the operative- and adjacent - level kinematic properties of 2-level acdf and combined acdr / acdf . acdf plus acdr had less severe biomechanical effects on adjacent levels when compared to 2-level acdf procedure . reported that the acdf at c6-c7 and acdr at c5-c6 produced increased segmental motion at the arthroplasty level , particularly in axial rotation and flexion - extension . barrey et al .   analyzed the biomechanics of acdr placed above acdf and found similar kinematics to single - level acdr and adjacent to a previously implanted acdr . compared the biomechanics of 2-level acdf and hybrid ; the hybrid construct had a better biomechanical performance than the fusion and the hybrid avoided excessive increase of adjacent level motion and loads . .   found that the location of the fusion ( cephalad or caudad ) did not affect the behavior of the disc replacement . biomechanical studies investigating the kinematic properties of 3-level hybrid arthroplasty - arthrodesis reconstruction are limited . however , multilevel cervical surgeries , such as 3-level anterior surgeries , are more common in clinical practice . hanai et al .   reported that there was excellent clinical success and a 100% union rate for 3-level and 4-level cervical corpectomy and autograft strut graft reconstruction . .   revealed that the likelihood of pseudarthrosis was 10% for 1-level surgery , 44% for 2-level surgery , and 54% for 3-level surgery . one study investigated acdr as a promising treatment for symptomatic adjacent level after prior 2-level cervical fusion . characterized acdr kinematics above 2-level fusion and found that acdr placed adjacent to a 2-level fusion was subjected to a more challenging biomechanical environment as compared to a stand - alone acdr . two groups reported both 2-level and 3-level hybrid surgeries are comparable to acdf and acdr in terms of safety and feasibility with a minimum follow - up of 2 years . . stated that hybrid surgeries may be an alternative to acdf for 3-level cervical disease due to the equivalent or improved early clinical outcomes , better overall c2-c7 range of motion , and less impact at adjacent levels . we found that rom in instrumented levels after 3p condition was systematically reduced in all planes . in contrast , the 3dpd condition appeared to preserve motion at adjacent intact levels to the acdr segments . compared to 3dpd condition , rom in instrumented levels after 3pdp condition was slightly reduced but the difference was not significant . in addition , we did not observe an abnormal increase of motion for 3dpd and 3pdp conditions . at the adjacent levels , 3p condition resulted in increased adjacent segment motion , especially in the caudal adjacent segment , which may potentially result in accelerated adjacent segment degeneration . on the contrary , and as expected , although 3dpd and 3pdp conditions did not fully restore motion within the instrumented levels , they did not demonstrate hyper - mobility , and in fact induced only minimal changes in rom at adjacent levels . this confirmed the finding that the hybrid construct could avoid large motion increase in adjacent levels and clinically may reduce the risk of adjacent segment degeneration . the 3dpd construct did produce a smaller motion change in adjacent segments compared with 3pdp construct . this study has the limitations of any cadaveric biomechanical study of cervical fusion in that it allows only observation of the immediate effects of the intervention . it did not remove all motion in the fusion segments and can not represent the long - term effect of increasing stiffness as the fusion progresses . these biomechanical findings suggest that while hybrid conditions failed to restore normal motion within the construct , they significantly normalized motion in adjacent segments compared with the 3-level acdf condition . the artificial disc in 3-level constructs has biomechanical advantages compared to fusion in normalizing motion .
backgroundthe ideal surgical approach for cervical disk disease remains controversial , especially for multilevel cervical disease . 
 the purpose of this study was to investigate the biomechanics of the cervical spine after 3-level hybrid surgery compared with 3-level anterior cervical discectomy and fusion ( acdf).material / methodseighteen human cadaveric spines ( c2-t1 ) were evaluated under displacement - input protocol . after intact testing , a simulated hybrid construct or fusion construct was created between c3 to c6 and tested in the following 3 conditions : 3-level disc plate disc ( 3dpd ) , 3-level plate disc plate ( 3pdp ) , and 3-level plate ( 3p).resultscompared to intact , almost 65~80% of motion was successfully restricted at c3-c6 fusion levels ( p<0.05 ) . 
 3dpd construct resulted in slight increase at the 3 instrumented levels ( p>0.05 ) . 
 3pdp construct resulted in significant decrease of rom at c3-c6 levels less than 3p ( p<0.05 ) . both 3dpd and 3pdp caused significant reduction of rom at the arthrodesis level and produced motion increase at the arthroplasty level . 
 for adjacent levels , 3p resulted in markedly increased contribution of both upper and lower adjacent levels ( p<0.05 ) . 
 significant motion increases lower than 3p were only noted at partly adjacent levels in some conditions for 3dpd and 3pdp ( p<0.05).conclusionsacdf eliminated motion within the construct and greatly increased adjacent motion . artificial cervical disc replacement normalized motion of its segment and adjacent segments . 
 while hybrid conditions failed to restore normal motion within the construct , they significantly normalized motion in adjacent segments compared with the 3-level acdf condition . 
 the artificial disc in 3-level constructs has biomechanical advantages compared to fusion in normalizing motion .