ORIGINAL ARTICLE

Cervical arthroplasty with Discocerv‰ ‘‘Cervidisc Evolution’’surgical procedure and clinical experience 9 years afterthe first implantation of the first generation

A. Ramadan, A. Mitulescu, S. Champain

22, chemin Beau-Soleil, CH-1206 Geneva, Switzerland

Abstract: Long time considered as gold standard in thesurgical treatment of the cervical disc disease, anteriordiscectomy and fusion is, however, associated with animportant rate of complications and low quality of life levels,especially in young and active populations. As literatureemphasized a correlation between fusion (and the resultingsuppression of motion) and adjacent segment degeneration,motion-preserving techniques have been imagined; most ofthem involve cervical disc prostheses that are underclinical and biomechanical evaluation. Various materialsand designs are proposed, with the common point thatmotion axis is located downwards (with regard to theprosthesis), thus allowing not only a physiological motion ina sagittal plane but also some possible mechanical conflictsbetween unci and the upper vertebra. Uncus resection maysolve the conflict, however, with a great risk of postoperativeossification of bone spurs. For a prosthesis whose motionaxis is located upwards, motion in frontal plane isphysiological, avoiding the aforementioned mechanicalconflict and the need for uncus resection. However, themovements in a sagittal plane may be altered, so we studiedsuch a concept of prosthesis from a clinical and bio-mechanical point of view. The population consisted in36 patients who underwent cervical arthroplasty withDiscocerv‰ ‘‘Cervidisc Evolution’’ for cervical degenerativediseases without instability. Among this group, 21 (8 men,13 women, mean age: 49 ± 9 years) patients reached the12-month follow-up. The following data were analyzed – (A)clinical: surgery data, complications, return to work,pain (VAS), function (NDI) and overall outcome (Odom’scriteria); (B) radiological: flexion-extension motion, meancenters of rotation, local and C1-C7 lordosis, signs ofdegeneration or implant migration. Surgery duration wasof 65 ± 15 min and the average hospital stay of 4 days.There were one case of per-operative minor (importantbleeding) and three cases of postoperative complications(unrelated to the prosthesis). Twelve months after surgery, animportant cervical and radicular pain relief was underlined bypostoperative values of respectively 22.6 and 20.3 versus 69.1

and 65.3 observed before surgery. Functional disabilitydiminished from 55.4/100 before surgery to 18.7/100 at 1-yearfollow-up and 83% of patients declared high levels ofsatisfaction. These findings are in agreement with Odom’scriteria showing 76% of excellent and 24% of good results,12months after surgery, andwith the good return towork rate(83% of patients resumed their previous activities within6months).Radiological analysis showedpreservedmotion forthe treated level (average: 9.2 ± 4�) and adjacent ones at12 months, with normal centers of rotation in 64% of cases(in all postoperative examinations) and altered but stable in36% of cases, where motion axis was located more on thesuperior plate of the prosthesis. A statistically significantdifference between pre- and postoperative values of locallordosis showed a restoration of sagittal alignment. Moreoverthe absence of a significant postoperative evolution as well asthe statistically significant differences between long-termpost- andpreoperative values of C1-C7 lordosis confirmed thisprogressive realignment. The findings of this ongoingprospective study highlighted encouraging preliminaryresults suggesting the efficacy of the device in symptomsrelief and motion preservation. Nevertheless, they need to bevalidated by the long-term results, which will clearly show ifthe device is able to maintain motion while preventingadjacent segment disease.

Keywords: Cervical arthroplasty – Total disc replacement –Cervical spine motion – Outcomes

Introduction

Over 40 years ago, Smith and Robinson [1] described the useof anterior cervical discectomy and fusion in the treatment ofdegenerative disorders of the cervical spine. Since that time,this procedure has gained wide acceptance, with excellentresults. Historical reviews of this procedure have reportedover 80-90% fusion rates using autogenous iliac crest bonegraft [2,3].

Correspondence: aymenram@yahoo.com

Interact Surg (2008) 3: 187–200© Springer 2008DOI 10.1007/s11610-007-0040-8

Although anterior discectomy with fusion (ADF) is anaccepted procedure for symptomatic cervical disc diseaseand cervical spondylosis, an important percentage ofpatients show disappointing long-term results followingfusion, especially in young and active populations. Long-term follow-up studies of ADF describe re-operation for newdisease at an adjacent level in 7-16% of cases [4-8]. Patientshave undergone re-operation at an adjacent level for newradiculopathy due to progressive spondylosis or herniateddisc [9-11].

As more patients undergo primary anterior cervicalfusions (ACF), and as patients live longer after theseprocedures, it is likely that adjacent segment disease of thecervical spine will develop in increasing numbers of patients.In a study of the natural history of adjacent segment diseasein the cervical spine, Hilibrand et al. [12,13] found that 19% ofall patients still being followed 10 years after surgery had newdisease at an adjacent level. Adjusting for the loss of follow-up over time, aKaplan-Meier survivorship analysis predictedthat over 25% of all patients undergoing an anterior cervicaldecompression and fusion procedure contract adjacentsegment disease during the first 10 years after the indexprocedure [13].

Little information is available concerning positive MRfindings at an adjacent level to the fused segments. Ross [14]found an increase in degenerative changes at the leveladjacent to the fused area in 40 of 73 patients followingcervical surgery. Wu et al. [15] reviewed pre- and post-operative fast spin-echo (FSE) MR images of disc herniation(DH) and spondylosis in patients after spinal cervicalsurgery. They found that ADF at one or two intervertebraldisc spaces in the cervical spine produces a significantaccelerative formation of osteophytes at the fused levels. DHand osteophytes above, below and at the operated leveloccurred at a higher rate in the fusion-operated patients thanin the controls. In particular, the incidence of bony stenosissecondary to hypertrophy of the fusion graft and theuncovertebral structure at the fused level was much higherin the operation group than in the control group. Thesefindings of post-ADF increased degenerative changes aboveand below the fused space may be due to an inability ofthe fused space to absorb its share of the stress applied to thecervical spine. This stress must then be absorbed by theadjacent disc spaces thus increasing their exposure totrauma. Furthermore, the traumatic effects on the bonyfusion mass and vertebral body could accelerate theformation of osteophytes.

Gore and Sepic [16-18] found that 78%of the patientswithrecurrent symptoms after ADF had symptoms related to anintervertebral disc level other than the fused ones. Inaddition, Lunsford et al. [19,20] stated that patients withrecurrent cervical pain were more likely to have progressionof spondylosis above and below the fused segments thanthose without recurrent pain. On plain films, DePalma et al.[21] found degenerative changes at adjacent cervical inter-vertebral disc level in 81% of operated patients.

Cherubino et al. [11] hypothesized that the functionaloverloading of the discs both above and below the level offusion can accelerate the progression of degenerativearthritis and create new areas of compression. To test thishypothesis, a series of patients who had undergone anteriorcervical decompression using Cloward’s technique werereviewed after 7-16 years. Close attention was paid to theradiographic changes in the disc spaces bordering the fusion.

The conclusion is that fusion using Cloward’s techniqueprovokes a functional overload of the discs bordering thefusion that is directly correlated to the number of fusedspaces. The overloading and subsequent degeneration areless evident than expected. Radiographic observations showa greater mechanical stress on the discs, especially inarthrodeses involving more than one space. Nevertheless,in two groups with moderate and long follow-ups (8 and14 years, respectively), no size difference in the disc wasfound. This could mean that the changes in the disc due tooverloading take place soon after spinal fusion, and that thecervical spine subsequently finds a new biomechanicalequilibrium.

Finite element models were used to quantify thebiomechanical effects of C4-C5 and C5-C6 level fusionswith different graft materials on the adjacent unalteredcomponents [22-24]. Intact and surgically altered modelswere subjected to physiological loads and motion. Resultsshowed that fusion produced an increase in the cervical spinerigidity as well as an increase in adjacent vertebral bodystresses. In vitro analysis on human spine cadavers confirmsthese results and reveals an increase in intradiscal pressure[25]. It is now well known that the changes in disc pressureresult in metabolic disturbances. Biochemistry studiesundertaken on canine discs show that alteration of discmatrix composition causes the intervertebral disc tissue todegenerate [26].

Long-term clinical studies and experimental outcomedata suggest then a correlation between fusion, i.e. suppres-sion ofmotion, and degeneration of the levels adjacent to thefused level [27]. Reconstruction of a failed intervertebral discwith functional disc prosthesis should offer the same benefitslike fusion while simultaneously providing motion andthereby protecting the adjacent level discs from the abnormalstresses associated with fusion.

Although several cervical disc prostheses are availabletoday, all of them are still under biomechanical and/orclinical evaluation [28-46]. Regarding the Discocerv‰‘‘Cervidisc Evolution’’ prosthesis, mechanical tests, i.e. staticand dynamic compression shear tests, expulsion tests, teststo measure the load-induced subsidence of intervertebralbody fusion device under static axial compression and weartests, concluded to the resistance of the implants inphysiological conditions.

Furthermore, a biomechanical study (intact specimensversus instrumented segment with Discocerv‰ ‘‘CervidiscEvolution’’ prosthesis in C5-C6) has been performed on12 segments C4-C6 of 12 spines (5 women and 7 men, mean

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age at death 68.5 years old [54-74]) documented mobilitypreservation within physiological ranges after the implanta-tion of Discocerv‰ ‘‘Cervidisc Evolution’’ prosthesis.

Clinical experience with the previous version of thedevice Cervidisc®, implanted in 52 patients, showedencouraging clinical outcomes [37,38] in terms of pain reliefor functional ability recovery. Besides, in spite of asubsidence rate reaching 43%, motion was preserved in96% of cases at 7 years postoperatively. Discocerv‰‘‘Cervidisc Evolution’’ – the new version of the device –conserved the main concept in terms of materials andbiomechanical principles, but its design was adjusted inorder to better fit the human anatomy and to facilitateinsertion. Therefore, a clinical follow-up study on patientshaving received the last version of the device is necessary toconfirm the ability of this new device to relieve symptomsand to restoremotion while preventing adjacent disc disease.

In order to have an as realistic as possible overview on allpossible incidents that may occur with the use of Discocerv‰‘‘Cervidisc Evolution’’, the current study was conducted bysurgeons previously trained on the surgical technique buthaving no prior experience with the device, meaning that allpatients operated by the investigators were included in thestudy, including those operated during the learning curve.The authors believe that by doing so, the study couldhighlight possible pitfalls that might be associated with theuse of the device, especially during the learning curve, andthus provide the manufacturer with valuable clinicalinformation related to the use of the device.

Material

Medical device

Discocerv‰ ‘‘Cervidisc Evolution’’ is an implant intended forthe surgical treatment of the cervical spine. The Cervical DiscProsthesis Discocerv‰ is intended for patients sufferingfrom degenerative disorders of the cervical spine, withoutinstability.

The Cervical Disc Prosthesis Discocerv‰ has beendesigned in order to preserve cervical mobility. Thepreserved mobility is within physiologic ranges, with 18� ofrange of motion in flexion-extension and lateral bending.

The Cervical Disc Prosthesis Discocerv‰ takes the formof a ceramic joint, consisting of a cup and a spherical ball.The two parts that form the joint are embedded in titaniumplates that come into contact with the vertebral bodies. Theupper surface of the disc replacement inserted into the discchamber is convex in the sagittal plane, and has a convexprofile on its lower surface, in order to match the anatomicalgeometry of the vertebral plate as closely as possible. Severaldifferent thicknesses are supplied in order to fit differentheights of the intervertebral spaces (Fig. 1).

The design of the prosthesis prevents dislocation due tothe lip on the lower ceramic component and to the grooveinto the upper ceramic component (Fig. 2).

The Cervical Disc Prosthesis Discocerv‰ ismanufacturedfrom ELI titanium (according to ISO5832-3 or ASTM F136),and from two different types of ceramics: zirconia andalumina (ISO 13356 and ISO 6474, respectively). The titaniumendplates have been plasma sprayed (with Titaniumaccording to ISO5823-2 or ASTM F67).

Given its titanium and ceramic composition, Discocerv‰is MRI compatible making it possible for patients to undergopostoperative MRI examination if needed [41-43].

Biomechanical concept

The biomechanics of the cervical spine has been largelydescribed in the literature [47-51]. In terms of mobility, thecervical spine is the most mobile spinal segment, accountingfor approximately 120� of mobility in flexion-extension, 80�

in lateral bending and 135� in axial rotation, 60% of the latterbeing concentrated at the C1-C2 level. In vivo studies alsodemonstrated that the range of motion decreases with age[Watier, 1998].

Most of the disc prostheses on the market today allow forthe preservation of mobility within the physiologic range.However, the range ofmotion, although essential, is only oneof the biomechanical components of the cervical mobilityfunction. The second one is the nature of movement. Themanner in which the movement is accomplished at the levelof each spinal motion unit has been described in theliterature by several authors. The most popular ones arePearcy et al. [52] for the lumbar spine and Dvorak et al.[50,51] for the cervical spine.

These authors described the concept of mean centerrotation (MCR) that characterizes the nature ofmovement at

Fig. 1. Discocerv‰ ‘‘Cervidisc Evolution’’: inferior and superio plates:titanium TA6V ELI (ASTM F136 or ISO 5832-3); superior ceramic part(convex): alumina (AL203 ISO 6474 type A); inferior part (concave):zirconia (ZrO2HfO2Y2O3 ISO 13356); the shape of the device, convexupward in sagittal plane and downward in frontal plane, is designed tofit exactly the disc space

Fig. 2. Design of the ceramic components to prevent dislocation

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each spinal level. To understand theMCR concept, onemustimagine a pure rotational movement that assimilates thecombined movement of one vertebra with respect to thesubjacent one (Fig. 3) from point A to point A0.

Studies on asymptomatic volunteers demonstrated thatthe physiologic MCR of a cervical vertebra is situated in theupper posterior area of the subjacent vertebra (Fig. 4).

Given these considerations, most of the currentlyused cervical prostheses on the market have the centerof rotation located downwards. Indeed, it seems ratherlogical that any alteration of the MCR localization mightlead to improper functioning of the spinal motion unitand thereby result in pathological behaviour with possibleconsequences on the integrity of the spinal structures.However, the risk of an artificial disk is to give too muchmobility in the frontal plane and to allow the uncus to

touch the upper vertebra at the end of the lateral bendingon each side (Fig. 5, left). This is not the case when theMCR is located upwards (Fig. 5, right).

Therefore, a prosthesis with a center of rotation locateddownwards in the frontal plane might somehow alter thenatural mobility of the cervical spine, just as much as aprosthesis with a center of rotation oriented upwards mightalter the movement in the sagittal plane.

If we look deeper into the consequences of such inversedmechanical behaviour in sagittal plane (Fig. 6), one willnotice that when the center of rotation of the prosthesis islocated downwards, the sliding of the facet joints is purelyphysiologic, whereas a risk of excessive facet distraction isnoted when the center of rotation is located upwards.Therefore, a small distraction will be an advantage and willkeep the physiological sliding motion of the facet joint.

Fig. 3. Description of the MCR in flexion-extension

Fig. 4. Localization of the MCRs in the cervical spine in the sagittal plane(50,51)

Fig. 5. Biomechanical effect of the center of rotation of the prosthesisDiscocerv‰ (right side) in the frontal plane

Fig. 6. Biomechanical effect of the center of rotation of the Discocerv‰(right side) prosthesis in the sagittal plane

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However, literature shows that heterotopic ossification(HO) does occur in a significant number of patients whoreceived prostheses with a center of rotation locateddownwards [53], whereas it never occurred in the series ofpatients operated with the Cervidisc® prosthesis, 7 yearspostoperatively. The explanation of this phenomenon mightbe that a little distraction on the facets enhancesmobility andprevents ossification and ultimately fusion. However,overdistraction must certainly be avoided to prevent theoccurrence of postoperative facet joint syndrome [36].

Conversely, as alreadymentioned above,when the frontalplane is considered, the prosthesis with a center of rotationlocated upwards provides a physiologic movement andprevents mechanical conflicts between the unci and theupper vertebra, unlike traditional designs. Therefore, nouncus resection is needed with such prostheses, whichsignificantly reduces both peroperative complication factorsand the risk of postoperative ossification of bone spurs [54].

Indications/contraindications

The Cervical Disc Prosthesis Discocerv‰ is a surgical implantintended to preserve the physiological cervical mobility of asegment of the spine, the disc height and lordosis. The use ofprostheses prevents fusion-related comorbidity such aspseudarthrosis and risks usually associated with bone graftsand immobilization. However, as aforementioned, theultimate goal of cervical arthroplasty is to preserve theadjacent levels from postoperative degeneration thanks tothe recovery of the biomechanical function.

Therefore, the disc replacement is only indicated forlevels from C3 to C7. The main indication is DD (discdegeneration) that does not display any instability. Theindications include the following:

– herniated disc;– degenerative disc disease (DDD);– myelopathy associated with a spondylotic stenosis

of the foramen or spinal canal.Clinically, there must be a root disease associated with a

neurological deficiency that does not respond to medicaltreatment. Disc arthroplasty is contraindicated in cases ofactive infection, osteoporosis, tumoral disease, trauma,deformity, instability, rheumatoid arthritis or metabolicbone diseases.

Surgical technique

Surgical protocol-patient positioning

The surgery is performed under general anaesthesia. Thepatient is placed in dorsal decubitus on an ordinary table.The head should be placed in the neutral position (no flexionor hyperextension); it may be held in place with an adhesiveband. It is vital to keep a strict profile to avoid losing themedian line. The success of surgery depends on this. It is

possible to position the head on a transparent headrest ora transparent extension of the table. The approach is astandard right anterior pre-sternocleidomastoid approach(Cloward or Smith-Robinson).

Under scopic control, determine the correct interver-tebral level (image intensifier). In our institute, the C-arm(image intensifier) is always kept in place during the wholeprocedure. The surgeon stands on the right side of the patientand the assistant at the head. After a right side skin incisionand incision of the anterior longitudinal ligament, performthe disc resection using appropriate rongeurs and/or curettes.

Discectomy and preparation of the surgical site

When the disc has been excised, proceed with a bilateralpostero-lateral foraminal decompression. If the disc space istoo tight, an intersomatic retractor may be used. Decom-pression may require a high-speed drill or a kerisson to freethe bone (osteophytosis). However, the resection of theosteophytes must be carefully performed in order topreserve the natural contours and the surface of theendplates. We always open the posterior longitudinalligament (PLL) in order to avoid leaving any disc fragmentbehind it and to enhance the interbody distraction. Thissurgical gesture should be performed under microscope.

Preparation of the endplates

The vertebral endplates should be prepared and cleanedcarefully with the curette, removing the cartilaginousendplate without removing the vertebral bone. Never usethe high-speed drill on the vertebral endplate! A damagedbony cartilage may cause impaction of the prosthesis in thevertebral body. This procedure is essential to ensureoptimum contact between the implant and adjacent vertebralplates, inducing osteo-integration and avoiding secondarydisplacement.

Insertion of the prosthesis trials

Control of the intervertebral space: width

Once the site preparation is finished, a flat probe ‘‘footprint’’(available in two sizes) is inserted and indicates the width ofthe intervertebral space created. It is important to insert thetwo flat probes one after the other to determine the size of themost suitable prosthesis. In order to support a better loadsdistribution on the vertebral endplates, the prosthesiscovering the largest part of the endplate should be chosen.

Control of the intervertebral space: height

The set of prosthesis trials is used to determine the idealheight of the prosthesis to be inserted. The design of the trialsperfectly matches the dimension of the prosthesis (notconsidering the anchorage elements). The trials are designed

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to determine the best-suited implant to the patient’s anatomyand requirements.

The trials can be introduced and removed easily sincetheir surface has no retaining edges.

The choice of the right size of the prosthesis is themost critical part. If the prosthesis is too small, it maymigrate and if it is too big, it may interfere with mobility.Continuous sagittal flexion-extension motion can bechecked by C-arm with the trial in place, showing themovement of the facet joints and comparing the height ofadjacent levels. It is also recommended to control and toclean again both vertebral endplates with the curettes afterthe removal of the trials.

Positioning of the prosthesis on the holder

The holder tips fit on the lateral sides of the prosthesis toensure a firm grasp of the implant.

The inscription ‘‘TOP’’ on the holder allows the surgeonto insert the prosthesis in the proper position.

First, the upper plate of the prosthesis is positionedmanually on the two upper arms of the holder. The lowerplate of the prosthesis is then inserted in the two lower armsof the holder using the flexibility of the instrument.

It is important to obtain a perfect coupling between thetwo ceramic parts.

Insertion of the prosthesis

The interbody space is opened by pulling up the chin ofpatient by the operative-assistant.

As soon as the device has entered the space, the chin isimmediately released.

The holder is used for the impaction of the prosthesis inthe intervertebral space, with a very gentle hammer tapingand under slight compression. The holder takes up littlespace because it is in line with the prosthesis, leaving theoperative zone totally visible. The effort during the insertionof the prosthesis should be along the ancillary axis and isdone under scopic control (C-arm).

No lever or torsion movement should be applied to theholder during impaction of the prosthesis.

The posterior edge of the prosthesis must be positionedas close as possible to the posterior wall of the operatedvertebrae. That is why it is necessary to correctly preparethe intervertebral space and to remove all osteophytes.After scopic control, if necessary, specifically designedsuperior and inferior keys may be used to reposition theprosthesis.

Closure of the approach wound

The approach wound is closed after rinsing. Haemostasis isverified and drainage may be required. The platysmamuscleis carefully reconstituted and the skin closed by absorbableintradermic sutures.

Removing the implant

If it becomes necessary to remove the prosthesis, the cervicalapproach should be performed again until the exposure ofthe instrumented zone. The two parts of the device should beseparated from the bone and the two retrieval keys placedon the prosthesis to facilitate withdrawal of the implant.

Clinical experience

To date, over 2000 devices have been implanted worldwide.Since June 2006, a multicenter prospective observational

non-comparative study has been conducted in seven centersin France and Switzerland. Over 100 patients have beenenrolled in the study so far.We will report hereafter our ownexperience with patients, operated at Clinique Beaulieu,Geneva, Switzerland, who are part of the study populationenrolled into the multicenter study.

Patient population

Since April 2006, 36 patients underwent cervical arthroplastywith Discocerv‰ ‘‘Cervidisc Evolution’’ in our center forcervical degenerative diseaseswithout instability. Twenty-onepatients with single level arthroplasty reached the 12-monthfollow-up. The current study focuses on this subgroup.

There were 8 male and 13 female patients in the studypopulation, mean age: 49 ± 9 years (range: 35-67 years).Eighty-five percent of the patients were professionally activeprior to surgery. Sixty-one percent of the active patients inthis series were on sick leave at the time of consultation.

All patients complained of neck and upper limb pain(cervico-brachialgia). Paresthesiae was observed in 20patients. Eighteen patients had sensory deficit and 20 hadmotor deficit. Seven patients had surgical history related tothe cervical spine (one with discectomy alone, four withdiscectomy and one-level ACDF with cage, and two withprevious one-level arthroplasty with Cervidisc®).

Preoperative investigation revealed DH in 14 patients(66%), foramen stenosis with degenerative disc disease(DDD) in seven patients, with affected levels from C3-4 toC6-7 (i.e. 10% C3-4; 14% C4-5; 66% C5-6; 10% C6-C7).

Methods

All patients underwent single level cervical arthroplasty withthe Discocerv‰ ‘‘Cervidisc Evolution’’ prosthesis accordingto the aforementioned procedure.

Instrumented levels

One-level Discocerv‰ ‘‘Cervidisc Evolution’’ arthroplastywas performed in all patients and the distribution of theinstrumented levels is presented in Fig. 7. One patient in thisgroup was fused with a cage (ACDF) at the suprajacent levelduring the same operation.

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Evaluation criteria

Clinical evaluation

Surgery

Surgery duration, hospital-stay and peroperative complica-tions were recorded to assess the primary safety of use of thedevice.

Clinical outcomeThe clinical outcome evaluation is based on clinicalsymptoms and pain relief, functional recovery, complicationrate, return to work rate and patient satisfaction.

Neurological condition was assessed before and after thesurgery, at each follow-up visit. Symptoms such as pares-thesiae, cervical myelopathy, pyramidal syndrome, para-plegia, and tetraplegia, were checked.

Pain was assessed by means of a Visual Analogic Scale(VAS). This scale consists of a horizontal line of 100 mmwithout graduation. The left side (0) corresponds to ‘‘nopain’’ and the right side (100) corresponds to ‘‘unbearablepain’’. The patient places a vertical mark in accordance withhis personal assessment of the current pain. Cervical andradicular pain was evaluated.

Additionally, the analgesics intake was also assessed pre-and postoperatively according to the WHO classification.

FunctionThe Neck Disability Index (NDI) was used to evaluate thefunctional abilities of a patient with cervical pain to performdaily life activities. The higher the score, themore the patientpretends having incapacitating pain.

Patient satisfactionA sub-division of the North American Spine Society (NASS)Outcome Questionnaire called Patient Satisfaction Index(PSI) was used to measure patient satisfaction 1 year aftersurgery. The patient had to chose among four propositions(Table 1).

Overall outcomeAn assessment of the overall surgical outcome was made bythe surgeon using Odom’s criteria in addition to the otheroutcome measures detailed previously.

It consists of four levels detailed in Table 2.The NDI and VAS are measured preoperatively and

postoperatively up to the last follow-up.

The return to work rate, the PSI and ODOM score aremeasured postoperatively up to the last follow-up.

Radiographic criteria

Qualitative criteriaIn order to evaluate the evolution of the adjacent levels, allchanges in the disc, such as disc narrowing, presence ofosteophytes or vertebral endplate sclerosis were recorded.Also, the migration of the device was checked directly ontothe sagittal X-ray images taken in neutral position.

The following quantitative parameters were measuredpre- and postoperatively up to the last follow-up.

Static X-raysRegional lordosis: angle between the most tilted superiorendplate and the most tilted inferior endplate of the cervicalcurve, most often C1-C7.

Local lordosis: angle between the extreme endplates of theinstrumented segment.

Dynamic X-rays (Fig. 8)Flexion-extension mobility: difference between the inter-vertebral angle calculated on the flexion X-ray and theintervertebral angle calculated on the extension X-ray.

Mean center of rotation (MCR): the calculation of theMCR of a vertebra in dynamic analysis is an indicator of thenature of the intervertebral movement. While the above-mentioned mobility quantifies the rotation involved in thismovement, the MCR gives the position of the equivalent axisof rotation of a vertebra in relation to the one below.

All radiological parameters previously described weremeasured by means of a specific application softwarededicated to X-rays measurements of spinal, pelvic andvertebral parameters (SpineView‰ 2.4, Surgiview, Paris,

Fig. 7. Instrumented levels

Table 1. Patient satisfaction index

Patient responses PSI

Surgery met my expectations 1

Surgery improved my condition enough so thatI would go through it again for the same outcome

2

Surgery helped me but I would not go throughit again for the same outcome

3

I am the same or worse compared to before surgery 4

Table 2. ODOM criteria

Grade Definition

Excellent All preoperative symptoms relieved; abnormalfindings improved

Good Minimal persistence of preoperative symptoms

Fair Definite relief of some preoperative symptoms;other symptoms unchanged or slightly improved

Poor Symptoms and signs unchanged or exacerbated

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France). The accuracy of this software has already beendocumented elsewhere [55]. All X-rays were digitized with anX-rays digitizer (Vidar Systems Corporation, Herndon, VA,USA).

Flexion-extension mobility was measured both for theindex level and the adjacent ones.

An independent observer, who was not part of thesurgical team, performed the measurements.

Statistical analysis

The postoperative evolution of all quantitative parameterswas assessed and the mean, maximum, minimum andstandard deviation values were calculated and comparedwith the values obtained preoperatively.

Regarding the different scores, any change fromone classto another was considered as a discriminating factor.

Appropriate statistical tests (Student’s t-test-Wilcoxon)were employed to assess statistically significant differencesbetween preoperative and postoperative values of allquantitative parameters.

Results

Follow-up

Clinical and radiological data are available for analysis todate with a mean follow-up of 13 ± 2 months [range: 11-18].

Population

The general features of both groups are presented in Table 3.

Surgery

For the entire population the mean duration of the surgerywas 64.7 ± 15.4 minutes (range: 35-90 minutes).

Per-operative complications

No per-operative complications were reported, except forexcessive bleeding in one patient, without any furtherconsequences. The hospital stay was 4 ± 1.6 days (range:3-8 days).

Clinical outcome

All patients recovered completely from their preoperativeparesis and sensory loss and no neurological deficit was everfound in any patient at all postoperative examinations.

Postoperative complications

Postoperative complications are reported in Table 4 (Fig. 9).

Pain evolutionBoth cervical and radicular, as assessed by means of the VASare presented hereafter for the entire population at differentfollow-up periods (Fig. 10).

Pre- and postoperative analgesics intake according to theWHO classification is reported in Fig. 11.

Function evolutionAs described by the NDI is presented hereafter for the entirepopulation at different follow-up periods (Fig. 12).

Patient satisfactionAmong patients who filled in the PSI (N = 10), 15 (83%)declared themselves globally satisfied with their treatment(i.e. surgery met their expectations), while three patients(17%) declared that surgery helped somehow but they wouldnot go through it again for the same outcome.

Fig. 8. Angular mobility (flexion-extension, lateral bending) and MCR

Table 3. General features of the study population

Parameters Values

Mean follow-up (min-max) 13 (11-18) monthsMean age (min-max) 49 ± 9 (35-67)Women (percentage) 13 (62)Men (percentage) 8 (38)Active (prior to surgery) 18 (85%)Sick leave (prior to surgery) 11 (61%)

Table 4. Observed postoperative complications

Type of complication Patient population

Implant related: subsidence,expulsion, migration, etc.

One patient presented a slightforward migration of thedevice 12 months after surgery(migration < 2 mm) (Fig. 9)

Absence of motion(mobility 3� and less)

Four cases presented loss ofmotion at the index level aftersurgery

Dysphagia One case at 1-month,improved at 3 months

Revision surgery atadjacent level

One patient underwentsurgery for subjacent levelinstrumentation with cage(not related to arthroplasty),which further on demonstra-ted pseudarthrosis ; revisionsurgery is to be expected

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Overall outcomeThe surgeon’s postoperative evaluation of the symptomsrelief as per Odom’s criteria is presented in Table 5.

Return to work rateOut of the active population (N = 18), all patients returned totheir previous work within the first year after surgery (83%

returned within the first 6 months). Four patients convertedto a lighter work condition (same job at part time).

Radiological outcome

Radiological analysis was performed to date for all patientsbut one. This latter did not have appropriate X-ray images toallow a full radiographic analysis.

As presented in Table 4, radiographic analysis documen-ted migration in one patient. No subsidence was observed inthis series.

Fig. 9. Patient with device migration 12 months after surgery

Fig. 10. Neck and arm pain evolution in time

Fig. 11. Pain medication (number of patients/class at each follow-upinterval)

Fig. 12. Function postoperative improvement, as reflected by theevolution of the NDI (%). A statistically significant difference wasobtained for postoperative VAS and NDI values versus preoperativevalues (P > 0.05)

Table 5. Global outcomes issued from Odom’s criteria

Odom criteria Group 1

Excellent 16 (76%)Good 5 (24%)Fair 0Poor 0

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Regional lordosis (C1-C7)

C1-C7 lordosis was stable postoperatively (non-significantevolution from early postoperative to last follow-up).However, a statistically significant difference was observedat long-term with regard to preoperative values (Fig. 13).

Local lordosis

A statistically significant difference was found between earlypostoperative values and preoperative ones (1.8 ± 5�

[6�, 15.5�) preoperatively and 8.4 ± 5� [0.9�, 16�] post-operatively). The further long-term evolution was non-significant (constantly 8 ± 5�[1-16]) for the entire population,except for one patient who had a significant change oflordosis (patient presented in Fig. 9). In this case, local C5-C6lordosis evolved from 0� before surgery to 8� in earlypostoperative and 11� at 6 months; however, the value of thisparameter at the 12-month examination was 2.5� showingsignificant changes of local geometry.

Flexion-extension mobility

Mean ROM for the index levels evolved from 10.3 ± 5�

preoperatively to 8.2 ± 4� at 3-6-month follow-up examina-tion and at 9.2 ± 4� at the last follow-up visit. The variationsobserved until the 3-6-month follow-up examinations arestatistically significant and not significant afterwards.

The gain of mobility (i.e. calculated as the difference indegrees between the preoperative value and the post-operative one) equals 3.3� at the last follow-up examinationin 17 patients. A loss of mobility at the index level wasobserved in four patients (Fig. 14).

Regarding the suprajacent levels, postoperative valueswere normal in all cases, passing from 11.6 ± 5� before surgeryto 12.6 ± 6� at 3-6 months and 12.8 ± 6� at 12-24 months.

As for subjacent levels, preoperative mean values were8.2 ± 7� and evolved to 8.2 ± 6� at 3-6 months and to7.2 ± 5� at last follow-up.

MCR

The MCR in flexion-extension was calculated pre- andpostoperatively in all patients. The localization of the MCRwas normal in 64%of patients at all examinations and alteredwith regard to the physiologic localization in 36%, in whichmost MCRs were located at the level of the upper endplate ofthe prosthesis (Fig. 15).

Regarding the suprajacent levels, preoperatively 64% ofpatients whose X-rays allowed the calculation of this level’sMCR (N = 14) were normal. At last, follow-up of the MCRswere normal in 68% of patients. Twenty-six of the patientshad an MCR located lower than normal, at the level of thelower endplate of the subjacent vertebra.

As for subjacent levels, preoperativelyMCRswere normalin 62%of the patient population and in 15% they could not beprocessed. However, only 13 patients had X-rays allowing the

calculation of the subjacent level’s MCR. Postoperatively,subjacent level’sMCRswere normal in 50%of patients and in28% of the patients MCRs could not be processed because ofa flexion-extension ROM less than 3� (in patients with cage atthe subjacent level).

Discussion and conclusion

Cervical disc replacement has now been validated to be a safeand efficient tool in the treatment of various degenerativecervical pathologies.Many studies are bringing evidence thatcervical discs can restore disc height while maintainingsegment’s motion, and that this new technology is free ofserious complications at both short- and mid-term.

Regarding the Bryan Cervical Disc (Medtronic SofamorDanek), in a prospective multicenter study, Goffin et al.

Fig. 13. Evolution of the regional lordosis

Fig. 14. Flexion-extension mobility of the treated levels (main treatedlevel is C5–6)

Fig. 15. Examples of patients with normal localization of the MCR of theindex level in flexion-extension. (Left) Prosthesis only; (right)prosthesis and ATCD with cage (CMR = MCR)

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[56,57] reported good results in 86% of patients at 6 monthsand 90% at 1 year postoperatively. Moreover, at 1 year,motion was preserved in 88% of patients and only onemigration was noted. These results were confirmed at longerfollow-up and several authors reported similar clinicalimprovement rates and motion preservation (mean: 7.8� at2 years) [41-43,46,57-59].

A multicenter, prospective, randomized and compara-tive study of the Prestige (Medtronic SofamorDanek) versusACF for single level DDD showed that the device maintainedmotion (mean: 5.9�) at 12-month follow-up. A better clinicalimprovement was found in patients with TDR versus ACF,according to VAS, NDI and SF-12 criteria [35].

In a study on 82 PCM (Porous CoatedMotion: Cervitech)discs in 53 patients with DDD associated with radiculopathyand myelopathy, Pimenta et al. [34] reported significantimprovement assessed by means of VAS, NDI, andTreatment Intensity Gradient Test (TIGT) at 1-year follow-up. More specifically, 80% of patients had good or excellentresults at 1 week, improving to 90% of patients being judgedto have good or excellent results by 1 month (Odom’scriteria). Only one migration was observed at 3 months.

Our results are comparable with aforementioned data.Indeed, a good rate of recovery from main symptoms wasfound in all our patients after surgery, reflected by ODOMscores, VAS self-reported pain, NDI self-reported functionalcondition as well as by the decrease in pain medication afterthe surgery. Moreover, flexion-extension mobility wasrestored in most of the patients both at the index level andat the adjacent ones. The PSI reflects these results with 83%ofpatients declaring that the surgery met their expectations.

Last but not least, an excellent return to work rate wasobserved in this series with all active patients havingreturned to their previous job within the first year after thesurgery.

The per-operative complication rate is very low and noimplant-related complication was noted. There was onepatient with excessive bleeding, but this per-operativeinconvenient had no consequences on the surgery outcomeand another case presenting an early postoperative dyspha-gia recovered completely within the first 3 months aftersurgery.

However, some postoperative complications deservediscussion. As mentioned in the section 6, four patients inthis series lost their preoperativemobility after surgery at theindex level. Two of these patients had a surgical history withACDF with cage at the subjacent level, which might explainthis abnormal biomechanical behaviour. As for the twoothers, both had an important gain of mobility (6�) at thesuprajacent level, meaning that the overall mobility isrestored with a patient specific distribution between theindex and adjacent levels. Indeed, the analysis of C1-C7lordosis shows a statistically significant difference betweenpreoperative values and long-term values (i.e. last follow-up)demonstrating a ‘‘re-equilibrium’’ of the sagittal balance andintrinsically of each vertebra with regard to the adjacent

ones. This ‘‘rebalance’’ within a mobile structure mightexplain, at least partially, a transfer of mobility from onelevel to another.

Nonetheless, in this series one patient demonstrated aworsening of the local lordosis, with a kyphotic profile atthe index level. This patient is presented in Fig. 9 fordevice migration. The X-rays suggest that the implant wasprobably oversized for this patient, since the sagittalneutral position shows the prosthesis in extension. Theslight frontward migration of the prosthesis is a normalconsequence of the re-adaptation of the patient to theoverload generated into the index level by an excessivelyconstrained implant.

However, except for this particular case, no migrationwas observed in the rest of the group, which demonstratesthe stability of the device when properly dimensioned andinserted.

This case highlights the importance of the implant under-or over-sizing. Indeed, the dimensions of the implant areextremely important in the context of arthroplasty becausethey will contribute significantly to the biomechanicalbehaviour of the index and adjacent levels by generatingexcessive constraints that would ultimately results in eitherspontaneous fusion or hypermobility with excessive liga-ment destruction that could eventually lead to a facet jointsyndrome.

As mentioned in Table 4, one patient underwent surgeryfor subjacent level ACDF fusion with cage and furtherdemonstrated pseudarthrosis to this fused level. A revisionsurgery related to pseudarthrosis is to be expected. Thepatient was a 47-year-old woman with polyarthritis operatedfive times at both hands and elbow with a 2-year history ofcervico-brachial neuralgia with constant pain to theshoulders and deltoid weakness on one side. MRIs showedC4-C5 DD responsible for the symptoms. PreoperativeX-rays showed severe osteophytosis from C4 to C6 andsevere discarthrosis at C5-C6, although this level wasasymptomatic. Therefore, C4-C5 arthroplasty seemed agood option to mobilize the spine in this patient whilerelieving pain and neurological symptoms. Although thisfirst procedure completely relieved all symptoms, 6 monthsafter the surgery C5-C6 became symptomatic and requiredsurgical treatment as well. Given the severe osteophytosis atC5-C6, ACDFwith cage filled with bone graft was performed.Control at 1 month after the second surgery showed asignificant relief of all symptoms, but 4months later patient’scondition worsened and kinetic X-rays images showedmobility and thus pseudarthrosis at the C5-C6 fused level,responsible for the new symptoms, which would probablyrequire a revision surgery.

Although this complication is not related to the cervicalprosthesis and does not concern the levelwith arthroplasty, itis important to note that arthroplasty might have been anexcessive indication in this patient.

This case demonstrates once again that cervical arthro-plasty, although well developed lately and very popular,

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remains a nice indication, with very strict indications. Therespect of these indications is of paramount importance forthe overall patient outcome.

The aim of this study was not to compare differentmobile cervical prostheses but to evaluate the evolution ofthe second generation of the Cervidisc® prosthesis, whichdemonstrated preservation of mobility at 7-year follow-up[38]. Therefore, the main purpose of this intermediateanalysiswas to validate the primary safety and stability of theDiscocerv‰ ‘‘Cervidisc Evolution’’ and to estimate itsefficiency in restoring instrumented level mobility.

The most important point at this stage was to confirmthat the device is safe, allowing neither any secondarydisplacement nor instability. A statistically significantdifference between pre- and postoperative values of locallordosis showed a restoration of sagittal alignment, and thestatistically significant differences between long-term post-and preoperative values of C1-C7 lordosis confirmed thisprogressive realignment.

Except for the four cases discussed, the mobility ispreserved at index and adjacent levels. Indeed, one mustalways keep in mind that cervical arthroplasty is mainlyjustified for the adjacent level preservation in young andactive patients. Therefore, the preservation ofmobility in sub-and suprajacent levels is as important as at the index level.

The orientation of the spherical joint is importantbecause it defines the location of the MCR of the uppervertebra versus the lower one. Beyond restoring the range ofmotion, respecting the nature of the physiological motion iscritical. Indeed, surrounding tissues must be loaded in anatural way in order to reproduce adequate motion pattern.The posterior edge of the device must be aligned with theposterior wall of the cervical spine in order to get the centerof rotation at the right place.

A non-physiologicalmotion can lead to an overloading ofthe surrounding structures, and mechanical conflicts withthe implant.

TheMCR at each level of the cervical spine is known to belocated downward and corresponds with the upper part ofthe inferior vertebra. The Discocerv‰ ‘‘Cervidisc Evolution’’prosthesis is a ‘‘ball and socket’’ prosthesis with a center ofrotation located upward! This concept brings new andimportant considerations related to the biomechanics of thespine: the device has a more natural motion in the frontalplane, thus allowing optimal preservation of the uncusduring lateral bending unlike traditional designs. A secondadvantage is the facet release/distraction during flexion inthe sagittal plane. Of course, maximum attention must bepaid not to overdistract the facet joints with the risk ofimpairing or reducing mobility and thus creating pain. Theradiographic analysis of the flexion-extension MCR in thisgroup of patients demonstrates that in almost 70% of thecases the MRCs at the index and adjacent levels are properlylocated (i.e. ‘‘normal"), demonstrating that the specificdesign of this prosthesis does not necessarily alter thebiomechanical behaviour of the cervical spine. Particular

attention will be paid to those patients who showed anatypical motion pattern with abnormal localization of theMCRs. In some of the cases, adjacent instrumentation(ancient Cervidisc® prosthesis at one of the adjacent levels orcage) might explain the abnormal pattern. However,the analysis of the MCRs requires a closer look into theradiographic data and a more thorough analysis of thepossible correlations between various factors.

Therefore, a more thorough analysis of the biomechani-cal behaviour of Discocerv‰ ‘‘Cervidisc Evolution’’ will bepresented and discussed in another paper. However, thesefinal considerations may prove useful to understand theperformance of the presented device, based on a newbiomechanical concept.

A final and important point is the absence of subsidenceof the inferior part of the device in this series of Discocerv‰.The Cervidisc® series showed 43% subsidence despite 96%mobility at 7 years and we now believe that the inversion ofthe MCR has certainly played a role in keeping the mobilitymuch longer than expected in the subsided Cervidisc®

devices. As already mentioned, another study is underwayandwill develop further the issue of the invertedMCR and itsrelation to the biomechanics of the human cervical spine.

In conclusion, the preliminary results of this ongoingprospective study are encouraging. Nevertheless, only thelong-term results to come can fully validate the usefulness ofsuch a device and its ability to maintain motion whilepreventing adjacent segment disease.

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