In vitro biomechanics of cervical disc arthroplasty with ...

Approximately 187,000 anterior cervical spine proce-dures are performed annually in the US.29 Since its de-scription in the 1950s by Cloward9 and Robinson andSmith,40 ACDF has accounted for the majority of theseprocedures. More recently, the long-term outcome afterACDF has been described in sobering terms. Adjacent-segment degeneration with new, symptomatic radiculop-athy occurs after ACDF in 2 to 3% of patients per year ona cumulative basis.25,26 An estimated 7 to15% of patientsultimately require a secondary procedure at an adjacentlevel.5,7 It has been suggested that the increased stressplaced on the adjacent segments, particularly the inferiordisc, after successful ACDF may increase the rate of fu-ture symptomatic disc disease at those segments.8,10,31,38,42,44

Spine surgeons are now becoming interested in alter-natives to fusion, such as total disc arthroplasty. The goalin using these devices is to replace the diseased discwhile preserving and/or restoring motion at the treatedlevel. The hope is that such devices will help protect pa-tients from experiencing problems in adjacent segments.Although early clinical experience is growing, the biome-chanics of cervical disc arthroplasty have not been fullydelineated in the literature.

Our objective in the current study was to determine thebiomechanics of the ProDisc-C cervical disc prosthesis(Synthes Spine Solutions, West Chester, PA) in an estab-lished cadaveric model. Because ACDF remains the stan-dard of care, simulation of a single-level fusion at theimplanted site was compared with the harvested and disc-implant conditions.

MATERIALS AND METHODS

Specimen Preparation and Spine Conditions

Six fresh human cadaveric subaxial cervical (C2–T1)spines were procured from the Medical Education Re-search Institute (Memphis, TN). Three spines were ob-tained in female and three in male cadavers; the meanage of the specimens was 74.8 � 3.25 years. Each spinewas harvested and immediately double-wrapped in plas-tic bags and stored at �20˚C until the spinal constructswere prepared. Before preparation, the spines werethawed in a refrigeration system for 12 hours. All spineswere screened with anteroposterior and lateral radio-graphs to exclude any specimens with gross osteopeniaor anatomical abnormality. Bone density measurementswere not done, but any specimen that did not provide ade-quate screw purchase, as determined by the spine surgeon(K.T.F.), was not used.

The specimens were evaluated sequentially in three dif-

Neurosurg Focus 17 (3):E7, 2004, Click here to return to Table of Contents

In vitro biomechanics of cervical disc arthroplasty with the ProDisc-C total disc implant

DENIS J. DIANGELO, PH.D., KEVIN T. FOLEY, M.D., BRIAN R. MORROW, B.SC., JOHN S. SCHWAB, M.SC., JUNG SONG, PH.D., JOHN W. GERMAN, M.D., AND EVE BLAIR, B.SC.

Department of Biomedical Engineering, University of Tennessee Health Science Center;and Image Guided Surgical Research Center, Memphis, Tennessee

An in vitro biomechanical study was conducted to compare the effects of disc arthroplasty and anterior cervicalfusion on cervical spine biomechanics in a multilevel human cadaveric model. Three spine conditions were studied:harvested, single-level cervical disc arthroplasty, and single-level fusion. A programmable testing apparatus was usedthat replicated physiological flexion/extension, lateral bending, and axial rotation. Measurements included vertebralmotion, applied load, and bending moments. Relative rotations at the superior, treated, and inferior motion segmentunits (MSUs) were normalized with respect to the overall rotation of those three MSUs and compared using a one-wayanalysis of variance with Student–Newman–Keuls test (p � 0.05). Simulated fusion decreased motion across the treat-ed site relative to the harvested and disc arthroplasty conditions. The reduced motion at the treated site was compen-sated at the adjacent segments by an increase in motion. For all modes of testing, use of an artificial disc prosthesis didnot alter the motion patterns at either the instrumented level or adjacent segments compared with the harvested condi-tion, except in extension.

KEY WORDS • biomechanical testing • cervical disc prosthesis • cervical fusion •biomechanics

Neurosurg. Focus / Volume 17 / September, 2004 44

Abbreviations used in this paper: ACDF = anterior cervical disc-ectomy and fusion; MSU = motion segment unit; VB = vertebralbody.

Unauthenticated | Downloaded 06/09/22 02:57 AM UTC

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ferent conditions, as follows: 1) harvested; 2) single-levelcervical disc prosthesis (ProDisc-C); and 3) single-level(C5–6) fusion. The ProDisc-C cervical implant (SynthesSpine Solutions, West Chester, PA) was used for the discprosthesis condition. This implant (Fig. 1) consisted oftwo forged CoCr alloy endplates and an ultra–high mole-cular weight polyethylene inlay element. The final con-dition studied was that of the fused spine. Fusion was sim-ulated across the treated level by using custom-designedfixtures (Fig. 2). The fixtures were similar to an externalfixation system used by orthopedic surgeons. The remain-ing vertebrae were left free to move unimpaired. The threedifferent spine conditions are shown in Fig. 3.

Before preparation and testing, the bone surfaces of theC-2 and T-1 VBs were cleaned and mounted in cylindricalpots by using an alignment frame to position the cervicalspine in a neutral (upright) orientation. The flexion/exten-sion axis was estimated at the anterior aspect of the fa-cet joint of each vertebra. Positioning screws that passedthrough the sides of the pots initially held the end bodiesin place; a low-melting-point bismuth alloy (Small Parts,Miami Lakes, FL) provided final fixation of the end bod-ies in the pots.

The surrounding paravertebral soft tissues were dissect-ed, with care taken to preserve the spinal ligaments, discs,and bone. Threaded rods were placed into the lateralaspects of the VBs that secured light-emitting diode tar-gets used with the motion tracking system.13 The target at-tachment site did not interfere with the installation of thetotal disc prosthesis or with the attachment of the fusionhardware.

Biomechanical Testing Apparatus

In this study we used a portable programmable testingapparatus. The rigid two-column frame housed a servo-motor load actuator (International Device Corp., Novato,CA) connected to a robotic controller (Adept, Inc., SanJose, CA).6,11,12 A single-axis load cell (Transducer Tech-

nologies, Temecula, CA) was in line with the shaft of theload actuator. The other end of the single-axis load cellwas coupled with fixtures containing a pinned connectionand a linear bearing for attaching the cervical spine. Theflexion/extension testing arrangement is shown in Fig. 4.

Mounting fixtures added to the biomechanical testingsystem converted the single controlled input from the loadactuator to a coupled motion input (unconstrained transla-tions and rotation in a plane) and a combined loading state(axial compressive force and flexion/extension or lateralbending moment). With the flexion/extension axis of thespine placed eccentric to the load axis of the actuator, acompressive load and flexion/extension bending momentwere applied to the upper pot. The specimens were mount-ed in an inverted neutral orientation with the T-1 pot at-tached to the upper fixture and the C-2 pot mountedto the lower base fixture, thereby inducing a greater mo-ment at T-1 than at C-2, simulating in vivo conditions.22,43

For lateral bending tests, the spine was rotated 90˚ in themounting fixtures and the base was unconstrained in axialrotation. A rotational displacement transducer (Data In-struments, Acton, MA) was attached to the upper pinnedassembly and measured the global rotation of the spine.The displacement transducer recorded changes in the mo-ment arm length between the upper pot and load axis ofthe actuator during flexion/extension or lateral bendingtests. A separate loading system was used for axial rota-tion and is shown in Fig. 5.

Nondestructive Testing Protocol and Sequence

Various in vitro testing methods have been used tostudy the stability of cervical fusion devices. Typicalprotocols have involved bovine tissue or single-level hu-man cervical motion segment units, and these are usuallytested under pure-moment loading conditions. Althoughpure-moment methods permit ranking between the differ-ent fixation systems, they do not replicate physiologicalconditions. We have developed a testing protocol that dis-

D. J. DiAngelo, et al.

45 Neurosurg. Focus / Volume 17 / September, 2004

Fig. 1. Digital drawing of a ProDisc-C cervical disc prosthesis.

Fig. 2. Digital drawings showing adjustable devices custom de-signed to simulate fusion.


http://www.aans.org/education/journal/neurosurgical/sept04/17-3-7f1.html


tributes a bending moment across the spinal construct thatincreases in the caudal direction.2 This protocol has beenused to study other cervical fusion15,20 and motion restora-tion16 devices. The in vitro flexion/extension motion re-sponse associated with the modified testing protocol isshown in Fig. 6 and was similar to that in published invivo data. The nondestructive protocol involves applica-tion of a bending moment of 3 Nm with limit checksof 35˚ total spine rotation, 5-Nm flexion/extension mo-ment at T-1, or an applied load of 75 N. These values werebased on our preliminary test findings11 and conformedwith the limits used by other researchers.35,36

For each condition listed, the spines were nonde-

structively tested with flexion/extension, lateral bending,and axial rotation loading. All tests were performed un-der displacement control. For flexion/extension and later-al bending, the spine offset was 200 mm from the loadaxis. The testing apparatus was programmed to output atriangular displacement–time waveform of 6.4 mm/sec-ond, which corresponded to approximately 2˚/secondoverall spine motion.

Before the formal testing sequence began, each spinewas preconditioned with five cycles at low displace-ment levels. Each test trial included three loading cycles.Throughout the entire testing sequence the spines weremoistened at regular intervals with a normal saline mist.

Neurosurg. Focus / Volume 17 / September, 2004

Biomechanics of artificial cervical disc arthroplasty

46

Fig. 3. Photographs showing the spine conditions tested. Left: Harvested. Center: Cervical disc prosthesis (C5–6).Right: Simulated fusion (C5–6).

Fig. 4. Photographs showing the extension testing set-up. Overview (A) and closeup (B) photographs of mountedspine. For flexion testing, the spine was rotated 180˚ in the mounting fixtures.




The spines were first tested in the harvested condition.After completion of these tests, a conventional discecto-my was performed by the spine surgeon to prepare fordisc implantation. After the procedure was completed, thespine was tested in the implanted state. All procedureswere performed at the Medical Education Research In-stitute.

Data Management and Analysis

Signals from the transducers were collected with a ded-icated analog-to-digital data acquisition system (National

Instruments, Inc., Austin, TX) and sampled at 10 Hz. Thedata were processed using custom-designed softwareroutines (Labview; National Instruments) and collectedin a spreadsheet file for later computational processingand statistical analysis (Sigma Stat; Jandel Scientific, SanRafael, CA). A three-dimensional, noncontact, real-timemeasurement system was used to track segmental cervicalmotion for each testing condition.13

The vertebral displacement data were simultaneouslycollected and displayed to the user in real time. The mo-ment applied to the spine at T-1 (designated Ma) wasdetermined by calculating the vertical force reported bythe inline load cell (designated Fa), the total rotation ofthe upper pot reported by the rotational transducer(qrdt), and the displacement offset (da � dtdt) between theupper pot and load axis according to the following formu-la: Ma = Fa(da � dtdt)/cos(qrdt), where da is the initialoffset distance between the load axis and the center of theupper pot.

Measurements of the global rotational motion and ap-plied load data were combined to calculate the overallspine flexibility. Flexibility data were compared at thelargest reported end limit of motion common to all spines.Variations in the motion patterns were also analyzed at anend limit of global (C2–T1) moment common to all spineconditions within each specimen by comparing the per-cent contribution of the rotation at the superior (S), treat-ed (T), or inferior (I) MSUs relative to overall rotation ofthose three MSUs (S�T�I). The instrumented spine con-ditions were normalized to the harvested condition toaccount for intrinsic differences in tissue specimens. Tostudy the effects of the disc-implant or fusion treatmentson the adjacent segment biomechanics, the contribution ofmotion at the remaining segments relative to the overalltotal motion was normalized to its contribution in the har-vested state and then compared.

A one-way analysis of variance with a Student–New-man–Keuls test was used to compare differences in theflexibility and normalized motion data at the treated andadjacent segments. The alpha value was set at 0.05 for alltests. The distribution of the relative rotations of eachMSU was also compared across the entire cervical spine.

RESULTS

Global Stiffness and Normalized Flexibility

Typical global stiffness curves for flexion and extensionloading are shown in Fig. 7. The degree of hysteresis be-tween the loaded and unloaded cycles remained simi-lar among the three different spine conditions, indicatingthat minimal tissue/ligamentous relaxation had occurredthroughout the testing sequence. A similar response oc-curred in lateral bending and axial rotation.

The global flexibility values, or the inverse of stiffness,for the instrumented spine conditions were normalized tothe harvested condition and compared at the largest re-ported applied moment common to all spine conditions;results are given in Tables 1 and 2. Significant differencesoccurred in extension between the ProDisc-C and harvest-ed (151% of harvested) and ProDisc-C and fusion (151%compared with 109%) conditions, and in right axial rota-tion there were significant differences between the Pro-



Fig. 5. Photograph showing the axial rotational testing arrangement.

Fig. 6. Graph showing in vitro flexion/extension rotations of theharvested spine compared with in vivo data for C2–7 bodies.




Disc-C and harvested (148% of harvested) and ProDisc-Cand fusion conditions (148% compared with 112%).

Normalized Motion

The mean rotational values at the treated MSU level ofthe instrumented conditions were normalized to the har-vested condition and compared at common limits of glob-al moment. For the normalized motion data, the rotation atthe treated level was expressed relative to the sum of therotations at the treated, superior adjacent, and inferior ad-

jacent segments. All initial and normalized motion dataare given in Tables 3 and 4. Graphs of the normalized mo-tion for combined flexion and extension, right plus left lat-eral bending, and right plus left axial rotation are shown inFig. 8. A normalized value identical to the harvested con-dition equals 1 and can be expressed as equivalent to100% of the harvested condition. Significant differencesbetween the spine groups and loading conditions are indi-cated on the charts (Fig. 8) and included in Table 4. Theonly significant difference between the ProDisc-C and ha-rvested spine conditions occurred in extension (57% of



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Fig. 7. Graphs showing typical flexion/extension stiffness curves for different spine conditions of specimen EV. Deg = degrees.

TABLE 1 Flexibility data in cadaveric spines tested in various conditions

Harvested ProDisc Fusion

Test Moment Global Rotation Moment Global Rotation Moment Global Rotation

flexion 2.88 � 0.55 31.30 � 7.36 2.81 � 0.54 32.67 � 4.51 2.83 � 0.52 32.33 � 4.03extension 1.86 � 1.07 27.68 � 6.40 1.82 � 1.04 39.29 � 1.94 1.84 � 1.04 28.94 � 4.60lt lat 3.51 � 0.18 26.67 � 6.74 3.46 � 0.23 27.24 � 7.37 3.45 � 0.21 26.79 � 4.22rt lat 3.25 � 0.78 26.75 � 6.17 3.25 � 0.84 28.59 � 5.74 3.22 � 0.84 27.68 � 5.08lt axial 4.00 � 0.24 16.29 � 2.99 3.96 � 0.22 21.52 � 6.86 4.01 � 0.26 16.72 � 3.24rt axial 3.95 � 0.16 17.07 � 5.01 3.97 � 0.13 24.66 � 5.91 3.99 � 0.11 19.34 � 6.60



harvested), but was not seen in combined flexion plus ex-tension or any other individual or combined loadingmode. Nevertheless, significant differences occurred inflexion plus extension between the fusion and harvested(32% of harvested) and between the fusion and ProDisc-C groups (32% compared with 95%); in combined lateralbending between fusion and harvested (32% of harvested)and between fusion and ProDisc-C groups (32% com-pared with 102%); and in combined axial rotation, be-tween the fusion and harvested (12% of harvested) and be-tween fusion and ProDisc-C (12% compared with 107%).

Mean Relative Rotations

The distribution of the relative MSU rotations for theharvested, ProDisc-C, and fused spines during flexion/extension, left and right lateral bending, and left and rightaxial rotation are shown in Fig. 9. The flexion and exten-sion motion profile of the harvested cervical spine testedin vitro was similar to that in published in vivo data.2,17,28,43

The greatest range of flexion and extension occurred atthe C5–6 MSU, as was observed by Lysell.30 Use of adisc prosthesis joint maintained biomechanical integrityof the spine that was comparable to the harvested condi-tion for all modes of loading. Nevertheless, simulation ofa single-level fusion shifted motion from the treated re-gion to the adjacent segments. The reduced motion at thetreated MSU level was compensated for by an increase inmotion in the remaining segments. To study further theeffects of fusion on the adjacent segments, the changes inthe contribution of each MSU level relative to the overallglobal rotation (C2–T1) of the instrumented spine condi-tions were normalized to its contribution in the harvestedcondition and then compared.

Changes in MSU Contribution

Treated MSU Level. The rotation of the surgicallytreated MSU level (C5–6), expressed as a percentage ofthe overall global rotation (C2–T1) for the ProDisc-C andfused spines, was normalized to its corresponding contri-bution in the harvested condition. This ratio, calculatedas follows: ([C5–6 rotation of surgically treated spine]/[C2–T1 rotation of treated spine]) divided by ([C5–6rotation of harvested spine]/[C2–T1 rotation of harvest-ed spine]) is shown in Figs. 10 through 12 for the com-bined loading conditions. There were no significant dif-ferences between the ProDisc-C and harvested conditionsat the treated MSU level. Nevertheless, significant differ-ences occurred between the fusion model and both theProDisc-C and harvested conditions for all loading modescombined.

Adjacent MSU Levels. The changes in the MSU contri-bution at the remaining segments are also shown in Figs.10 through 12. Significant differences between the har-vested and ProDisc-C conditions only occurred in com-bined left plus right lateral bending at C2–3 and C6–7.Nevertheless, significant differences occurred between theharvested and fusion conditions in combined flexion plusextension at C3–4 and C6–7, in combined left plus rightlateral bending at C6–7, and in combined left plus rightaxial rotation at C6–7. Significant differences between theProDisc-C and fusion conditions occurred in flexion plusextension at C3–4 and C4–5, in combined left plus rightlateral bending at C2–3, and in left plus right axial rotationat C6–7.

DISCUSSION

Biomechanical Testing Protocols

The two most common methods for studying cervicalspine mechanics in vitro are load control and displace-ment control.23 For load control, a pure or constant mo-ment is incrementally applied to the spine and the spine isloaded in one motion plane at a time. For displacementcontrol, the translations and rotations of the end VBs arecontrolled. In this study we controlled the displacement ofthe spine by using custom fixtures that induced a “mo-ment distribution” throughout the spine similar to the invivo situation.

The question arises as to which testing method betterreplicates the in vivo motion behavior of the cervical spine



TABLE 2 Flexibility of disc implant and fused spines

normalized to harvested spine

Test Normalized ProDisc/Harvested Normalized Fusion/Harvested

flexion 1.11 � 0.27 1.09 � 0.23extension 1.51 � 0.37*† 1.09 � 0.25†lt lat 1.10 � 0.41 1.05 � 0.19rt lat 1.10 � 0.15 1.07 � 0.13lt axial 1.35 � 0.41 1.04 � 0.20rt axial 1.48 � 0.36*† 1.12 � 0.15†

* Denotes significant difference from the harvested condition.† Denotes significant difference between the ProDisc and fusion conditions.

TABLE 3Rotational motion data obtained in cadaveric spines*

Harvested ProDisc Fusion

Test S�T�I Treated S�T�I Treated S�T�I Treated

flexion 17.79 � 7.81 6.84 � 4.20 21.99 � 3.23 9.46 � 1.99 17.85 � 4.10 0.34 � 0.30extension 14.56 � 4.26 4.95 � 1.30 18.92 � 3.05 3.76 � 1.90 11.50 � 3.50 0.15 � 0.13lt lat 13.55 � 3.79 5.01 � 2.30 14.16 � 5.64 5.37 � 3.20 10.77 � 3.15 0.46 � 0.42rt lat 9.90 � 2.20 3.76 � 1.48 13.99 � 1.82 4.72 � 2.91 9.16 � 2.27 0.26 � 0.27lt axial 7.10 � 1.93 3.88 � 2.24 12.19 � 6.16 5.78 � 4.75 7.43 � 2.70 0.95 � 0.44rt axial 9.22 � 4.24 3.42 � 1.06 14.02 � 5.26 7.14 � 4.48 8.95 � 3.34 0.68 � 0.37

* S�T�I = overall data for superior, treated, and inferior segments combined.


and would thus be better suited to delineate differencesbetween disc arthroplasty and fusion instrumentation.Miura, et al.,33 recently described a method for simulatingin vivo cervical spine kinematics by using a preload andpure-moment protocol. Application of a follower load wasused in conjunction with a pure moment. The followerload concept directs a compressive load through the cen-ter of rotation of each MSU. In their study, the rotationalaxis was placed near the lateral masses and remained fixedfor the flexion and extension tests. Nevertheless, the in-stant axis of rotation position was based on three citedstudies,3,18,41 none of which included an analysis of thepropagation of error associated with the theoretical instantaxis of rotation calculation itself, nor were the instant cen-ters determined over small ranges of cervical motion (thatis, 2 to 3˚ increments). We have previously shown that theinstant axis of rotation error can be large (as high as � 10mm) for small angular changes (2 to 3˚) and that theinstant axis of rotation position is significantly different inflexion and extension.14 In the end, use of the followerload restricts the spine from following its natural path.

The flexion/extension motion response obtained usingMiura and colleagues’32 pure-moment protocol with a fol-lower load is shown in Fig. 13, along with a mean in vivodata set17,19,27,29,34,37 and the MSU rotational patterns fromour testing protocol. No significant difference was report-ed between the pure-moment response and the in vivodata. Nevertheless, the combined mean flexion/extensionrotational values did not always follow the in vivo patternand in some instances went in the opposite direction orremained constant across multiple MSU levels (Fig. 13) atthe region where the predominant amount of motion oc-curs in the cervical spine (that is, C4–5 and C5–6). Fur-thermore, the trend in Miura and colleagues’ data in-dicates that if the sample size were increased, significantdifferences would exist between the pure-moment re-sponse and the in vivo response.

Using displacement control methods to apply a momentdistribution across the cervical spine produced an in vitromotion response (Fig. 13) that matched the in vivo pattern.Therefore, this method should be used when studying theeffects of nonfusion instrumentation on spine biomechan-ics. When using this protocol to evaluate disc arthroplas-ty, the motion of the spine at the superior, implanted, orinferior MSUs was comparable to the harvested condition.

Few biomechanical studies have been conducted toanalyze the effects of disc arthroplasty on cervical jointbiomechanics in a multiple-body cadaveric model. We

have previously analyzed the biomechanical properties ofthe Prestige cervical disc under flexion/extension and lat-eral bending conditions by using the modified testing pro-tocol.16 McAfee, et al.,32 investigated the role of the poste-rior longitudinal ligament following discectomy, anteriordisc replacement, and anterior disc arthrodesis. The por-ous coated motion prosthesis was used as the disc replace-ment device. In their study the different spine conditions



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TABLE 4Rotational motion of disc and fused spines

normalized to harvested spine

Test Normalized ProDisc/Harvested Normalized Fusion/Harvested

flexion 1.35 � 0.51* 0.05 � 0.04*†extension 0.57 � 0.22*† 0.04 � 0.05*†lt lat 1.05 � 0.37* 0.13 � 0.12*†rt lat 0.92 � 0.47* 0.07 � 0.05*†lt axial 1.07 � 0.88* 0.29 � 0.15*†rt axial 1.26 � 0.41* 0.24 � 0.20*†

* Denotes significant difference between the ProDisc and fusion conditions.† Denotes significant difference from the harvested condition.

Fig. 8. Bar graphs showing MSU rotations of the instrumentedspines normalized to the harvested spine condition. * = significantdifference with the harvested condition; # = significant differencebetween the ProDisc-C and fusion conditions.



were tested under the pure-moments method by using ashortened (C3–7) spine model. Although pure-momentmethods permit relative comparisons among the differentspine conditions, they do not replicate physiological con-ditions and are less suitable for biomechanical testing ofarthroplasty or nonfusion devices. In addition, use of few-er spinal segments further limited the analysis of the adja-cent-segment biomechanics.

Clinical Experience

The issue of adjacent-segment disease in the cervicalspine has recently been reviewed by Azmi and Schlenck.4

It should be emphasized that the problem of adjacentsegment degeneration may be best managed by avoidanceof ACDF. Patients with unilateral radicular symptomswho require surgery may very well be best served by pos-

terior cervical foraminotomy and not ACDF. Whether it isperformed in a minimally invasive manner or as an openprocedure, posterior cervical foraminotomy has a highsuccess rate with respect to alleviation of radicular symp-toms, and it preserves spinal motion. For patients with bi-lateral radicular symptoms, significant ventral cord com-pression, or significant axial neck pain, ACDF remainsthe standard of care. These patients constitute a groupof individuals who may potentially benefit from cervicaldisc arthroplasty to minimize symptomatic adjacent seg-ment degeneration and the need for future surgical inter-vention. It has been argued that adjacent-segment diseaserepresents progression of the underlying disease of cervi-cal spondylosis and may not necessarily be related to thebiomechanical effects of successful fusion. Despite thiscontention, radiological follow-up studies, biomechanicalstudies, and clinical experience indicate that the presenceof a successful ACDF places increased strain on the adja-cent levels and may accelerate the degenerative process.The ultimate test of this proposition will be the long-termclinical outcomes of cervical disc arthroplasty.



Fig. 9. Bar graphs showing the mean relative MSU rotations.

Fig. 10. Bar graphs showing the percent contribution of flexion/extension MSU rotations of each MSU level of the implanted andfused spines, normalized to their respective rotational contributionin the harvested state. * = significant difference with the harvestedcondition; # = significant difference between the ProDisc-C andfusion conditions.




The concept of cervical disc replacement is not new: theinitial clinical efforts are attributed to Fernstrom,20 Reitzand Joubert,39 and Alemo-Hammad.1 Over the last decade,clinical experience with the Cummins artificial cervicaljoint10 and the Bryan Cervical Disc prosthesis (as de-scribed by Goffin, et al.24) have been reported.

The Cummins artificial cervical joint is a stainless steelball-and-socket joint that allows some translation. It re-quires internal fixation with AO screws similar to thoseused for anterior cervical plating. The device is insertedafter a standard anterior cervical discectomy and resectionof posterior osteophytes as needed. The initial clinical ex-perience includes implantation of 22 single-sized devicesin 20 patients for myelopathy, radiculopathy, and axialpain. Two patients underwent implantation of two de-vices, one as a primary procedure, and the second patientreceived the device in a staged procedure. Nineteen of the20 patients had either a congenital fusion or had under-gone a previous surgical fusion. Sixteen of the patients re-ported pain relief, whereas three had continued pain. Onepatient sustained a transient hemiparesis related to a drill

injury. Screw pullout, screw breakage, and joint subluxa-tion were all reported. One device was ultimately explant-ed. Joint motion was shown to be present up to 5 yearspostsurgery. No patient required further surgery at an adja-cent motion segment. Wear debris and bone incorporationinto the device were not observed. The report by Cum-mins, et al.,10 demonstrated that a stainless steel joint canbe implanted in the cervical spine. Suggested future re-finements of the device include development of multiplesizes, a decreased profile, and a total of four screws forbetter fixation.

The Bryan Cervical Disc prosthesis consists of two tita-nium shells enclosing a polyurethane nucleus. Implanta-tion of the device requires milling of the endplates afterestablishing the center of the disc space by using a gravi-tational reference system. The milling affords a precisionfit such that the milled surfaces match the titanium shells’outer surface. This allows the shells to be protected by arim of bone and provides immediate stability. Goffin, etal.,24 reported the initial clinical experience with the BryanCervical Disc prosthesis in 103 patients who underwent



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Fig. 11. Bar graphs showing the percent contribution of lateralMSU rotations of each MSU level of the implanted and fusedspines, normalized to their respective rotational contribution in theharvested state. * = significant difference with the harvested con-dition; # = significant difference between the ProDisc-C and fusionconditions.

Fig. 12. Bar graphs showing the percent contribution of axialMSU rotations of each MSU level of the implanted and fusedspines, normalized to their respective rotational contribution in theharvested state. * = significant difference with the harvested con-dition; # = significant difference between the ProDisc-C and fusionconditions.




single-level implantation and 43 who underwent bilevelimplantation.

Patients were enrolled if they needed treatment of ra-diculopathy or myelopathy and were excluded only if theyneeded treatment of axial neck pain. In the single-levelstudy, 90% of patients were classified as having attainedexcellent, good, or fair outcomes at 2 years. In the bilevelstudy, 96% of patients were classified as having attainedexcellent, good, or fair outcomes at 1 year. Reported rea-sons for repeated operation included the following: evac-uation of prevertebral hematomas (two patients), evacua-tion of an epidural hematoma, posterior foraminotomy forresidual symptoms, posterior decompression for residualmyelopathy, repair of a pharyngeal tear caused by intuba-tion, and anterior decompression. No device failures havebeen reported. The device provided more than 2˚ of mo-tion at 2 years in 93% of the patients who received single-level implants and in 86% of the ones who receivedbilevel implants at 1 year. It was believed by the investi-gators that the results equaled or surpassed those achievedwith ACDF, based on a metaanalysis of the literature.Nevertheless, the clinical experience does not constitute arandomized clinical trial of cervical disc arthroplasty andACDF, which would be ideal.

Both the Cummins and the Bryan devices have distinctadvantages and disadvantages. The Cummins device has arelatively high rate of instrumentation-related complica-tions, including device subluxation, whereas the Bryandevice appears to be well tolerated at 1 to 2 years. TheCummins device is easily implanted, whereas the Bryandevice requires a precise milling process. The ultimateevaluation of the success of both devices will depend onlong-term follow up with documentation of adjacent-seg-ment degeneration when compared directly with results inpatients undergoing ACDF. It is likely that a minimum of5 years of follow up will be required before any conclu-sions regarding adjacent-segment disease can be made. Aswith any new surgical procedure, time will be the finalarbitrator.

CONCLUSIONS

Although pure-moment protocols are more commonlyused to evaluate spinal fixation hardware, they are lesssuitable for evaluating spinal arthroplasty or nonfusion

devices. An improved testing protocol that replicatedthe in vivo motion behavior of the cervical spine was usedto study the biomechanics of a disc prosthesis in vitro.Under this protocol the ProDisc-C implant maintainedthe biomechanical integrity of the cervical spine. Never-theless, simulation of fusion significantly reduced motionat the surgical site, which was compensated for by in-creased motion at the adjacent segments. This increasedmotion at the adjacent segments may accelerate degener-ation of adjacent disc segments.

In this biomechanical study we have emphasized themaintenance of normal motion at all segments of the spinewith placement of the cervical prosthesis, compared withdecreased motion of the adjacent segments after surgicalfusion. Use of a prosthetic total disc replacement devicesuch as the ProDisc-C to treat symptomatic degenerativecervical disc disease may minimize or alleviate the ad-jacent-segment disease associated with fusion surgery.Prosthetic disc replacement may become a viable alterna-tive to cervical fusion for the surgical treatment of me-chanical neck pain.

Acknowledgments

We thank the Medical Educational Research Institute inMemphis, Tennessee for the use of their surgical facilities and tis-sue. We are grateful for assistance with manuscript preparation byHenry Bonin.

References

1. Alemo-Hammad S: Use of acrylic in anterior cervical discecto-my: technical note. Neurosurgery 17:94–96, 1985

2. Amevo B, Macintosh JE, Worth D, et al: Instantaneous axes ofrotation of the typical cervical motion segments: I. an empiricalstudy of technical errors. Clin Biomech 6:31–37, 1991

3. Amevo B, Worth D, Bogduk N: Instantaneous axes of rotationof the typical cervical motion segments: A study in normal vol-unteers. Clin Biomech 6:111–117, 1991

4. Azmi H, Schlenk RP: Surgery for postarthrodesis adjacent–cer-vical segment degeneration. Neurosurg Focus 15 (3):Article 6,2003

5. Baba H, Furusawa N, Imura S, et al: Late radiographic findingsafter anterior cervical fusion for spondylotic myeloradiculopa-thy. Spine 18:2167–2173, 1993

6. Chen J: Development of a Flexible Biomechanical TestingApparatus. MS Thesis. Memphis: University of Tennessee–Memphis, 1996

7. Cherubino P, Benazzo F, Borromeo U, et al: Degenerativearthritis of the adjacent spinal joints following anterior cervicalfusion: clinicoradiologic and statistical correlations. Ital J Or-thop Traumatol 16:533–543, 1990

8. Clements DH, O’Leary PF: Anterior cervical discectomy andfusion. Spine 15:1023–1025, 1990

9. Cloward RB: The anterior approach for removal of rupturedcervical disks. J Neurosurg 15:602–617, 1958

10. Cummins BH, Robertson JT, Gill SS: Surgical experience withan implanted artificial cervical joint. J Neurosurg 88:943–948,1998

11. DiAngelo DJ, Faber HB, Dull ST, et al: Development of an invitro experimental protocol to study the extensional mechanicsof the cervical spine, in Simon B (ed): 1997 Advances in Bio-engineering. New York: ASME, 1997, Vol 36, pp 211–212

12. DiAngelo DJ, Foley KT, Vossel KA, et al: Anterior cervicalplating reverses load transfer through multilevel strut-grafts.Spine 25:783–795, 2000

13. DiAngelo DJ, Jansen TH, Eckstein EC, et al: Measurements for



Fig. 13. Graphs showing in vitro flexion/extension rotations ofthe harvested spine compared with pure-moment and in vivo datafor C2–7 bodies.



the in vitro failure of multi-level instrumented cervical spine, inSimon B (ed): 1997 Advances in Bioengineering. New York:ASME, 1997, Vol 36, pp 223–224

14. DiAngelo DJ, Robertson JT, Metcalf NH, et al: Biomechanicaltesting of an artificial cervical joint and an anterior cervicalplate. J Spinal Disord Tech 16:314–323, 2003

15. DiAngelo DJ, Vossel KA, Foley KT: The instant axis of rota-tion of the cervical spine in flexion and extension, in Proceed-ings of the 2000 Cervical Spine Research Society. Rosemont,IL: CSRS, 2000, pp 203–204 (Abstract)

16. DiAngelo DJ, Vossel KA, Jansen TH: A multi-body opticalmeasurement system for the study of human joint motion, inYoganathan AP (ed): 1998 Advances in Bioengineering. NewYork: ASME, 1998, Vol 39, pp 195–196

17. Dvorak J, Froehlich D, Penning L, et al: Functional radiograph-ic diagnosis of the cervical spine: flexion/extension. Spine 13:748–755, 1988

18. Dvorak J, Panjabi MM, Grob D, et al: Clinical validation offunctional flexion/extension radiographs of the cervical spine.Spine 18:120–127, 1993

19. Dvorak J, Panjabi MM, Novotny JE, et al: In vivo flexion/ex-tension of the normal cervical spine. J Orthop Res 9: 828–834,1991

20. Fernstrom U: Arthroplasty with intercorporal endoprosthesis inherniated disc and in painful disc. Acta Chir Scand Suppl 357:154–159, 1966

21. Foley KT, DiAngelo DJ, Rampersaud YR, et al: The in vitro ef-fects of instrumentation on multilevel cervical strut-graft me-chanics. Spine 24:2366–2376, 1999

22. Goel VK, Clark CR, McGowan D, et al: An in-vitro study of thekinematics of the normal, injured and stabilized cervical spine.J Biomech 17:363–376, 1984

23. Goel VK, Wilder DG, Pope MH, et al: Biomechanical testing ofthe spine. Load-controlled versus displacement-controlled anal-ysis. Spine 20:2354–2357, 1995

24. Goffin J, Van Calenbergh F, van Loon J, et al: Intermediate fol-low-up after treatment of degenerative disc disease with theBryan Cervical Disc Prosthesis: single-level and bi-level. Spine28:2673–2678, 2003

25. Grundy P, Nelson RJ: The long-term outcome of anterior cervi-cal discectomy and fusion (ACDF). J Bone Joint Surg BR(Suppl II):102, 2002 (Abstract)

26. Hilibrand AS, Carlson GD, Palumbo MA, et al: Radiculopathyand myelopathy at segments adjacent to the site of a previousanterior cervical arthrodesis. J Bone Joint Surg Am 81:519–529, 1999

27. Holmes A, Wang C, Han ZH, et al: The range and nature offlexion-extension motion in the cervical spine. Spine 19:2505–2510, 1994

28. JP Morgan, US Equity Research: Medical Supplies & Devices.Spine Market Update: Thoughts and Observations Post-NASS. New York: JP Morgan, 2003

29. Lind B, Sihlbom H, Nordwall A, et al: Normal range of motionof the cervical spine. Arch Phsy Med Rehabil 70:692–695,1989

30. Lysell E: Motion in the cervical spine. An experimental study

on autopsy specimens. Acta Orthop Scand (Suppl 123):1–61,1969

31. Matsunaga S, Kabayama S, Yamamoto T, et al: Strain on inter-vertebral discs after anterior cervical decompression and fusion.Spine 24:670–675, 1999

32. McAfee PC, Cunningham B, Dmitriev A, et al: Cervical discreplacement-porous coated motion prostheis: a comparative bi-omechanical analysis showing the ket role of the posterior long-itudinal ligament. Spine 28 (Suppl 20):S176–S185, 2003

33. Miura T, Panjabi MM, Cripton PA: A method to simulate invivo cervical spine kinematics using in vitro compressive pre-load. Spine 27:43–48, 2002

34. Ordway NR, Seymour RJ, Donelson RG, et al: Cervical flexion,extension, protrusion, and retraction. A radiographic segmentalanalysis. Spine 24:240–247, 1999

35. Parsons JR, Zimmerman MC, Lee CK, et al: Examination of thefailure mode of several cervical spine fixation devices, in Lan-grana NA, Friedman MH, Grood ES (eds): 1992 Bioengineer-ing Conference. New York: ASME, 1993, Vol 24, pp 175–178

36. Pelker RR, Duranceau JS, Panjabi MM: Cervical spine stabi-lization. A three-dimensional, biomechanical evaluation of ro-tational stability, strength, and failure mechanisms. Spine 16:117–122, 1991

37. Penning L: Normal movements of the cervical spine. AJR 130:317–326, 1978

38. Pospiech J, Stolke D, Wilke HJ, et al: Intradiscal pressure re-cordings in the cervical spine. Neurosurgery 44:379–384,1999

39. Reitz H, Joubert MJ: Intractable headache and cervico-brachial-gia treated by complete replacement of cervical intervertebraldiscs with metal prosthesis. S Afr Med J 38:881–884, 1964

40. Robinson RA, Smith GW: Anterolateral cervical disc removaland interbody fusion for cervical disc syndrome. Bull JohnsHopkins Hosp 96:223–224, 1955

41. van Mameren H, Sanches H, Beursgens J, et al: Cervical spinemotion in the sagittal plane. II. Position of segmental averagedinstantaneous centers of rotation—a cineradiographic study.Spine 17:467–474, 1992

42. Weinhoffer SL, Guyer RD, Herbert M, et al: Intradiscal pres-sure measurements above an instrumented fusion. A cadavericstudy. Spine 20:526–531, 1995

43. White AA III, Panjabi MM: Clinical Biomechanics of theSpine. Philadelphia: Lippincott Williams & Wilkins, 1990

44. Wigfield C, Gill S, Nelson R, et al: Influence of an artificial cer-vical joint compared with fusion on adjacent-level motion in thetreatment of degenerative cervical disc disease. J NeurosurgSpine 96:17–21, 2002

Manuscript received July 7, 2004.Accepted in final form August 16, 2004.This work was funded in part by Synthes Spine Solutions (West

Chester, PA).Address reprint requests to: Denis J. DiAngelo, Ph.D., Depart-

ment of Biomedical Engineering, University of Tennessee HealthScience Center, 920 Madison Avenue, Suite 1005, Memphis, Ten-nessee 38103. email: [email protected].



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