Motion Preservation Surgery inthe Spine

Ryan Murtagh, MD, MBAa,*, Antonio E. Castellvi, MDy,b

KEYWORDS

� Spine � Motion preservation � Total disc replacement

KEY POINTS

� The primary goal of motion preservation surgery is to maintain normal or near normal motion in anattempt to prevent adverse outcomes commonly seen with conventional spinal fusion, mostnotably the development of adjacent-level degenerative disc disease.

� There are several different approaches developed to preserve motion in the lumbar spine, includingtotal disc replacement, partial disc (nucleus) replacement, interspinous spacers, dynamic stabiliza-tion devices, and total facet replacement devices.

� The design of lumbar total disc replacement devices varies greatly. Commonly seen complicationsinclude subsidence, migration, fracture, heterotopic ossification, and even adjacent-segmentdegeneration.

� Cervical motion preservation surgery primarily consists of total disc replacement. The devices arecreated using a similar rationale but are unique in design relative to their lumbar counterparts.

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Discectomy and spinal fusion is considered thegold standard for the treatment of symptomaticdegenerative disc disease.1,2 Though often effec-tive in the short term, it is not without significantmorbidity. Perhaps one of the most commonlyrecognized adverse outcomes of spinal fusion isthe development of degenerative disc disease atthe level(s) adjacent to a spinal fusion procedure.

Motion preservation surgery in the spine, whilenot a novel concept, has seen a significant in-crease in use during the last decade, particularlywith respect to disc arthroplasty (total discreplacement) devices in the cervical and lumbarspine. The goal of motion preservation surgery,in general, is to closely replicate normal or nearnormal biomechanics in an effort to restore patientmobility andminimize the development of clinicallysignificant adjacent-segment disc disease.1

Although relatively new in the spine, motionpreservation surgery is a well-established surgicaltechnique in the appendicular skeleton, and thebenefits of hip and knee joint replacement, among

y Deceased.a University of South Florida, 2700 University Square DrivEducation, Florida Orthopaedic Institute, Tampa, FL 3363* Corresponding author.E-mail address: rmurtagh13@gmail.com

Neuroimag Clin N Am 24 (2014) 287–294http://dx.doi.org/10.1016/j.nic.2014.01.0081052-5149/14/$ – see front matter � 2014 Elsevier Inc. All

others, are clearly recognized. The spinal motionsegment, or functional spinal unit, is more complexthan the appendicular joints. The functional spinalunit is effectively composed of 3 joints: the discspace and 2 facet joints. As such, the relativelysimple principles of replacing a single joint in theappendicular skeleton are not applicable, and thecomplex interaction among these 3 articulatingcomponents must be taken into considerationbefore surgical intervention. As a result, there is avast array of devices available in spinal motionpreservation surgery, particularly in the lumbarspine. In the lumbar spine, devices can be looselygrouped into anterior and posterior motion-preserving devices. Cervical motion preservationsurgery presents with more limited options andmainly consists of total disc replacement (TDR).

In the lumbar spine, anterior motion preserva-tion devices, or devices intended to replicatethe normal motion of the disc space, includeTDR and partial disc (nucleus) replacement. TDRis a well-established surgical technique with a

e, Tampa, FL 33612, USA; b Orthopaedic Research and7, USA

rights reserved. neuroimaging.theclinics

Fig. 2. Example of interspinous spacer. Two-levelX-stop (Kyphon, Sunnyvale, CA) interspinous spacerimplanted at L3-L4 and L4-L5 levels.

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multitude of devices available. Although cervicaland lumbar TDR procedures are designed withthe same rationale (ie, replace the abnormal discwhile restoring near normal biomechanics), the de-vices are distinctly unique in design and, in manycases, the frequency and specific type of postop-erative complications. In partial disc replacement,the diseased nucleus pulposus is replaced oraugmented with an injectable or preformed device(Fig. 1). The complex, preformed partial discreplacement devices require relatively invasive sur-gery while some of the injectable materials can beadministered through a minimally invasive percuta-neous approach. The goal of partial disc replace-ment is to increase disc space height and maintainnear normal motion in an attempt to minimize thedevelopment of adjacent-segment disease.There are several types of posterior motion

preservation devices, including interspinousspacers, dynamic stabilization screws and rods,and total facet replacement. Interspinous spacers(Fig. 2) are devices placed between the spinousprocesses of adjacent levels. The primary goal ofthis procedure is to increase disc space height inan attempt to decrease canal stenosis and relieveneurogenic claudication. This procedure is rela-tively noninvasive, particularly in comparison with

Fig. 1. Magnetic resonance image of Nubac (PioneerSurgical Technology, Marquette, MI). The deviceconsists of 2 polyetherether ketone (PEEK) endplateswith a ball-and-socket articulation. (Courtesy of ChipBao, PhD.)

TDR, and typically does not preclude other typesof surgery in the event of failure. Dynamic stabiliza-tion devices are similar to traditional pediclescrew-and-rod constructs in many aspects. Thesedevices, however, are not rigid, allowing for a smallamount of movement through mobile portions ofthe screws or rods. The goal of these devices isto stabilize the spine while simultaneously allowingfor a small amount of motion. Facet replacement isa relatively invasive procedure whereby a laminec-tomy and bilateral facetectomy is performed and aprosthetic facet device implanted (Fig. 3). The goalof facet replacement is to relieve canal and foram-inal stenosis while maintaining some degree ofmotion at the affected level.Motion preservation surgery of the spine is a

complex, constantly evolving field. While thegeneral principles of spinal motion preservationsurgery remain relatively constant, many of thespecific devices in use as recently as 2 to 3 yearsago are no longer clinically available, having failedclinical trials, lost funding, or been simply deemedineffective. New devices enter clinical trials everyyear. Given the dynamic nature of this field, theremainder of this article emphasizes TDR. TDR isthe most commonly performed of the motion-preserving surgeries. There are many establishedcervical and lumbar TDR devices, several of whichhave been approved by the Food and Drug Admin-istration (FDA) for clinical use in the United States.As a result, there are many large randomized

Fig. 3. Frontal (A) and lateral (B) radiographs of the Acadia (Facet Solutions, Hopkinton, MA) total facetreplacement.

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controlled trials documenting the surgeries andcomparing TDR with conventional fusion.

LUMBAR TOTAL DISC REPLACEMENT

Lumbar TDR devices vary widely in shape andcomposition. Some, like the Charite device (DePuySpine, Raynham, MA), have a 3-componentdesign with metal endplates and a polyethyleneliner. Others, like the FlexiCore device (StrykerSpine, Summit, NJ) are a 2-part, metal-only designthat uses a ball-and-socket type articulation be-tween the endplates. Some of the endplates ofthese devices are relatively flat and must use small“teeth” along the endplates to maintain purchasewith the adjacent vertebral body; this is seen, forexample, in the Charite device (Fig. 4). Others,like the ProDisc-L device (Synthes Spine USAProducts LLC, West Chester, PA), use large, verti-cal endplate keels to maintain purchase.

Most of these devices require an invasive ante-rior approach. The exception is NuVasive’s XLTDR (Nuvasive Manufacturing LLC, Fairborn,OH), which is implanted indirectly through a far-lateral approach (Fig. 5). TDR surgeries in generalare relatively invasive, and revision is often difficultin the event of hardware failure.

From an imaging standpoint it is important forthe radiologist to be aware of the potential compli-cations of TDR. In the immediate postoperativesetting the radiologist should be able to identifysurgical complications related to the approach.Most of these are implanted from an anteriorapproach, and complications including inadver-tent peritoneal entry and vascular injury, thoughrare, can occur in the postoperative setting. Inthe case of the Nuvasive device a far-lateralapproach is used, and the radiologist shouldsearch for retroperitoneal or inadvertent peritonealcomplications related to the lateral approach.Other early complications include fracture of thevertebral body/endplates from aggressive implan-tation, and this should be investigated in the symp-tomatic postoperative patient (Table 1).

Chronic complications include subsidence,migration, and the development of heterotopicossification and adjacent-level disease. Subsi-dence refers to the slow settling of the deviceinto the endplates. Most of the surface area ofthe device will contact the softer bone locatedcentral to the outer ring apophysis. Subsidenceis particularly prevalent in cases where implant isundersized and the footplate of the device is

Fig. 4. Charite total disc replacement (TDR). Radiolu-cent polyethylene liner is denoted by metal band.Note small “teeth” along endplates to maintainpurchase.

Table 1Complications associated with lumbar totaldisc replacement

Early Hemorrhage, vascular injury, infection,peritoneal entry, vertebral fracture

Late Subsidence of implant, implantmigration, heterotopic ossification,adjacent-level degenerative discdisease

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significantly smaller than the surface area of theadjacent vertebral body. Migration is relatively un-common but can, like subsidence, occur in thesetting of undersized devices. In cases of migra-tion, the compressive forces of the vertebralbodies do not have sufficient strength to contain

Fig. 5. Nuvasive XL TDR. The device is implantedthrough an indirect far-lateral approach, similar tothat used with extreme lateral lumbar interbodyfusion technique. Note heterotopic ossification alongthe left lateral aspect of the device (arrow).

the device within the disc space. Migration is typi-cally along the operative path, and migration intothe paraspinal soft tissues or psoas muscle canpotentially occur if a far-lateral approach is used.Many devices use endplate keels (Fig. 6) or smallteeth to minimize migration. Heterotopic ossifica-tion refers to the gradual development of hetero-topic bone along the periphery of the interspace.Often this is asymptomatic, and is only significantfrom an imaging standpoint. With significant het-erotopic ossification there can, in some cases,be limitation of the range of motion, and in severecases there can even be bridging bone withautofusion.3

The clinical effectiveness of TDR has been asource of debate and extensive study. TDR is arelatively new concept and, while some deviceshave been in place for a decade or more, mostwell-designed randomized controlled studiesare still in the earlier stages. Recent trials havefailed to show superiority of TDR over fusion. Ameta-analysis of 6 randomized controlled trialsconsisting of 1603 patients reviewed the safetyand efficacy of lumbar TDR in comparison withlumbar fusion. Although there was significantsafety and efficacy of TDR versus fusion, TDRwas not be shown to be superior to fusion.4

Fig. 6. Scout computed tomography image of thelumbar spine with ProDisc at L4-L5 showing bridgingheterotopic ossification. Note the large vertical keelof the ProDisc device designed to prevent migration.

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Another meta-analysis of 837 patients enrolled in5 randomized controlled trials compared theeffectiveness and safety of lumbar TDR versusfusion.5,6 In this review the function, pain level,and patient satisfaction status between the 2groups was not significantly different at 5 years.The complication and reoperation rates werealso similar at 5 years. The investigatorsconcluded that lumbar TDR was not significantlysuperior to lumbar fusion for the treatment of lum-bar degenerative disc disease.5

The primary justification for lumbar TDR andmotion preservation surgery is to prevent or mini-mize the development of adjacent-level disease(ALD) and abnormality.1,3 In addition to debatingthe clinical efficacy of TDR, there has been signif-icant debate about the rate and even the signifi-cance of ALD and abnormality as regards fusionversus TDR. One theory for the development ofALD is that the altered mechanics of traditionallumbar fusion leads to abnormal stress on theadjacent motion segment and, in turn, promotesdegeneration. This situation would justify theuse of motion preservation surgery. Othersargue that the development of ALD is simplypart of the normal degenerative cascade. In2012, Wang and colleagues7 performed a litera-ture review in an effort to establish rates of ALDin lumbar TDR in comparison with fusion. The re-view, which included articles from 1990 through2012, found “moderate evidence” that clinicallysignificant ALD was almost 6 times more likelyto occur with fusion than with TDR. Specifically,the investigators found that the pooled risk ofclinically significant ALD requiring surgical in-tervention was 1.2% in TDR and 7.0% in fusion.Another study assessed 5-year results forradiographically demonstrated ALD in patientstreated with ProDisc-L versus circumferentialfusion. In this review, the patients treated withcircumferential fusion were greater than 3 timesmore likely to demonstrate findings of ALD(radiographically identified as loss of disc spaceheight, osteophytosis, endplate sclerosis, orspondylolisthesis).8

Fig. 7. Prestige (Medtronic, Memphis, TN) cervicalTDR. The device is composed of stainless steel andconsists of 2 articulating metal endplates held in placeby screws at the adjacent vertebral body levels.

CERVICAL TOTAL DISC REPLACEMENT

The principles of cervical TDR are similar to thosein lumbar TDR. In cervical TDR there is removal ofthe diseased disc and implantation of the arthro-plasty device. In addition to neural decompres-sion, the goal is to restore normal or near normalmotion so as to minimize ALD. ALD has beenshown to occur with increased frequency at thelevels adjacent to anterior cervical discectomyand fusion (ACDF) in several studies. In one study,

up to 25% of patients with ACDF experiencedsymptomatic ALD.9 Other post-ACDF problemspotentially alleviated by cervical TDR includestiffness, nonunion, hardware failure, anddysphagia.10 At present there are 3 devices withFDA approval in the United States but only 2 thatare available for commercial use. The 2 devicesapproved for commercial use are the ProDisc-C(Synthes Spine USA) and Prestige ST (Medtronic,Memphis, TN).

The devices, like lumbar TDR, vary greatly indesign. The Bryan Cervical Disc System device(Medtronic) consists of 2 titanium endplates witha polyurethane core. The Prestige device (Fig. 7)consists of 2 stainless-steel blade-shaped end-plates that are anchored to the vertebral bodiesby anterior screws. Other devices, such as theKineflex-C (Spinal Motion LLC, Mountain View,CA) are composed of cobalt-chromium-molybdenum alloy and use designs that moreclosely parallel those of their lumbar counterparts.Some devices, such as the ProDisc-C, use verticalkeels for stabilization, whereas others use teeth forendplate purchase. The Discover TDR device(Depuy Spine, Inc, Raynham, MA) consists of 2radiopaque titanium endplates and a radiolucentultra–high molecular weight polyethylene core(Fig. 8).

Complications of cervical TDR are similar tothose for the lumbar spine and include fracture,subsidence, migration, heterotopic ossification,and hardware failure or loosening. Subsidence,

Fig. 8. Discover (Depuy Spine, Inc, Raynham, MA) TDRdevice.

Fig. 9. Two-level cervical TDR using ProDisc-C (r)(Synthes Spine, West Chester, PA). There is subsidenceof both devices, to the point that they are nearlytouching. In addition, there is heterotopic ossificationseen at both levels, more so along the posterior aspectof the more caudal device (arrow).

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as in lumbar TDR, is a gradual process wherebythe device settles into the adjacent vertebralbody (Fig. 9). It is often seen with undersizeddevices that fail to contact the outer ring apoph-ysis. Migration is rare but typically occursanteriorly, along the operative approach (Fig. 10).Heterotopic ossification is seen with relative fre-quency in the cervical spine, such frequency vary-ing according to the device. In one study thefrequency of heterotopic ossification in 3 deviceswas 40%. More specifically, it was seen in 21%of Bryan devices, 52.5% of the Mobi-C devices,and 71.4% of ProDisc-C devices.11 There wasalso an increased prevalence of heterotopic ossifi-cation in men compared with women (47.6% vs29.2%). In another study that reviewed theProDisc-C device, 45% of devices demonstratedgrade III (extends into disc space and limitsmotion) heterotopic ossification and 18% demon-strated grade IV (bridges disc space and results infusion) heterotopic ossification.12 A third studyfound heterotopic ossification in 50% of Bryan im-plants, but only 4% of these were grade III andonly 2%were grade IV. The limitations of this latterstudy were a relatively small sample size (36 pa-tients) and a relatively short follow-up period(only 12 months).13

Like lumbar TDR, there are several trials in prog-ress. Recently, the 5-year results of cervical TDRwith ProDisc-C versus ACDF were published.Both groups showed clinically significant improve-ment at both 2 and 5 years versus baseline. At5 years the ProDisc-C group had statisticallysignificantly less severe and less frequent neckpain relative to the ACDF group. There were noreported device failures or implant migration. Inaddition, the ProDisc-C group had a statisticallysignificantly lower rate of reoperation relative toACDF (2.9% vs 11.3%).10 In similar fashion, a re-view of reoperation rates in a series of 6 FDAInvestigational Device Exemption studies usingcervical TDR in comparison with ACDF found asignificantly decreased reoperation rate in theTDR group when compared with the ACDF group(8.3% vs 21.2%). Specifically, the reoperationrate for ALD was 4.8% in the TDR group versus13.5% in the ACDF group. There was a signifi-cantly longer survival period before reoperationin the TDR group than in ACDF group.14 A meta-analysis of 13 reports from 10 randomizedcontrolled trials involving 2227 patients showedbetter function, less frequent reoperation, andlower major complication rates for TDR patients

Fig. 10. Migration immediately postoperatively (A) and 4 months after surgery (B) after 2-level cervical TDR usingProDisc-C (r) (Synthes Spine, West Chester, PA). At 4-month follow-up, the more cranial device was noted to havemigrated anteriorly (arrow).

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in comparison with patients undergoing fusionprocedures. This analysis, however, did not finda significant difference in the rate of reoperationattributable to adjacent-segment degeneration.15

Finally, a systematic review of the literature foundno significant difference in the frequency of radio-graphic or clinically significant adjacent-segmentabnormality in short-term to mid-term follow-upof patients treated with TDR rather than ACDF.16

DEDICATION

Dr Antonio Castellvi, or “Doc” as he is affection-ately known by peers, friends and family, passedaway on February 8th, 2014 at the age of 61.Dr Castellvi was a leader in the field of spinesurgery. He was head of the Spine Fellowship atthe University of South Florida and director ofthe biomechanics lab at the Foundation forOrthopedic Education and Research. Dr Castellvipresented and moderated at multiple meetings atthe national and international level but will perhapsmost fondly be remembered for his popular annualmeeting Current Solutions in Spine Surgery in DuckKey, FL. Dr Castellvi was widely respected by hispeers and patients and he will be dearly missed.

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