Dynamic reconstruction of the degenerative functional spinal unit (FSU) is a rapidly growing field iη spinal surgery. Procedures such as : nucleus replacement, posterior dynamic stabilization, interspinal distraction biological methods to regenerate the disk are being tested in experimental or clinical studies

1. Biomechanics of CervicalBiomechanics of Cervical
Disk ReplacementDisk Replacement
GEORGE SAPKASGEORGE SAPKAS
Associate ProfessorAssociate Professor
11stst
Orthopaedic DepartmentOrthopaedic Department
Medical School Athens UniversityMedical School Athens University
Athens GreeceAthens Greece

2. Synovial jointsSynovial joints
Movement by sliding articulationMovement by sliding articulation

13. Cervical Disk ReplacementCervical Disk Replacement
Dynamic reconstructionDynamic reconstruction
of the degenerativeof the degenerative
functional spfunctional spiinal unitnal unit
(FSU)(FSU)
is a rapidly growis a rapidly growiing fieldng field
iiηη spinal surgeryspinal surgery

14.  ProceduresProcedures
such as :such as :
– nucleus replacement,nucleus replacement,
– posterior dynamicposterior dynamic
stabilization,stabilization,
– interspinal distractioninterspinal distraction
– biological methodsbiological methods
to regenerate the diskto regenerate the disk
are being tested inare being tested in
experimental orexperimental or
clinical studiesclinical studies
PDN
3K - Fradis
ISOLOCK

15.  Among spineAmong spine
arthroplastyarthroplasty
techniques, total disktechniques, total disk
replacementreplacement
in the lumbar spinein the lumbar spine
is the most advanced,is the most advanced,
with promising earlywith promising early
results seen not onlyresults seen not only
ίίn empirical studies,n empirical studies,
but meanwhilebut meanwhile ίίnn
prospectiveprospective
randomized studies asrandomized studies as
well.well.
Maverick

16.  It seems logical thatIt seems logical that
solutions for dynamicsolutions for dynamic
stabilization as anstabilization as an
alternative to spinalalternative to spinal
fusion have now alsofusion have now also
been found for thebeen found for the
cervical spine.cervical spine.
 Here as well, totalHere as well, total
disk replacementdisk replacement
seems to be the firstseems to be the first
choice and probablychoice and probably
the most easilythe most easily
realizable technique.realizable technique.
 New implants for totalNew implants for total
cervical diskcervical disk
replacement havereplacement have
been developedbeen developed iiηη thethe
past few years.past few years.
3K - Fradis Prestige

17. Cervical Disk ReplacementCervical Disk Replacement

18. Cervical Disk ReplacementCervical Disk Replacement

22. Research has identified a total of eightResearch has identified a total of eight
patentspatents
 Two implants are currentlyTwo implants are currently
undergoundergoiing controlled clinicalng controlled clinical
evaluationevaluation iiηη multicentermulticenter
studies:studies:
– the Bryan Discthe Bryan Disc
– the Prodisc-Cthe Prodisc-C
– whereas others are nearing thewhereas others are nearing the
stage of clinical application.stage of clinical application.

23. Some dataSome data::
Το date, we cannotΤο date, we cannot
precisepreciselly describe they describe the
mechanics of themechanics of the humanhuman
cervical spcervical spiine underne under iiηη
ννiiνο conditions, ί.e.,νο conditions, ί.e., iiη theη the
activities of daily lactivities of daily lifife.e.
InIn ffact, the mechanics ofact, the mechanics of
the humanthe human cervicacervicall spinespine
inin ννiiνο most probabνο most probablly arey are
a resua resullt oft of bendingbending
around different axls,around different axls,
shear, and axiashear, and axiall
compression forcescompression forces..

24.  CompressionCompression
The weight of the head is passedThe weight of the head is passed
throughthrough the occipitathe occipitall condylescondyles
οοnn both sides to the atboth sides to the atllanto-axialanto-axial jointjoint
into the vertebrainto the vertebrall body of C-2.body of C-2.
 Load is then passedLoad is then passed
throuthrouggh the subaxiah the subaxiall spinespine
via the vertebravia the vertebrall bodiesbodies
andand both facets.both facets.
 GoeGoell and CΙausenand CΙausen (1998)(1998)
have beenhave been llookingooking
atat the amountthe amount of compressionof compression lloadoad
that is passedthat is passed
throughthrough ththe vertebrae vertebrall bodiesbodies
and the disk:and the disk:
They found that 88 %They found that 88 %
ofof a compressa compressiionon loadload
isis passed through the vertebrapassed through the vertebrall bodbodiieses
ofof the cervthe cerviicacall spspiine,ne,
with the amount estimatedwith the amount estimated
to range trom 110 to 1200 Νto range trom 110 to 1200 Ν

25.  BendinBendingg momentsmoments..
Το investigate the compΤο investigate the compllexex
scenarioscenario
ofof lloading and movingoading and moving the spine,the spine,
defined loadingdefined loading
hashas been proposed:been proposed:
– 1.8-2.5 Nm are wide1.8-2.5 Nm are widelly recommendedy recommended oror
used toused to lload the human cervical spineoad the human cervical spine
underunder inin vitro conditions.vitro conditions.
– This usually produces segmentaThis usually produces segmentall
range of motion which can berange of motion which can be
observed underobserved under inin νίνο conditionsνίνο conditions;; itit
is approximate!y 10is approximate!y 1000
for flexion-for flexion-
extension, left right axial rotation, andextension, left right axial rotation, and
left right lateral bending. This range ofleft right lateral bending. This range of
motion increases if a diskectomy ismotion increases if a diskectomy is
performedperformed

26.  Shear forcesShear forces
of 39 Ν have been applof 39 Ν have been appl iieded
toto the cervical spine,the cervical spine,
resuresulltingting inin 1.6-1.9 mm of trans1.6-1.9 mm of trans llationation
((Panjabi et alPanjabi et al, 1986 -, 1986 - Moroney et alMoroney et al, 1988), 1988)
 these motions induced bythese motions induced by
shear forcesshear forces
coucoulld resud resulltt inin earearlly ory or llate faiate faillureure
of a cervicaof a cervicall spine disk prosthesis.spine disk prosthesis.

27. ApplAppliication of these datacation of these data
to the cervical spineto the cervical spine disk prosthesdisk prosthes iiss
 StabilStabiliize a segment following diskectomyze a segment following diskectomy
 Preserve "physioPreserve "physiollogicaogicall" range of motion of" range of motion of
approximateapproximatelly 10y 1000
inin every motion pevery motion pllaneane
 Resist bending moments of atResist bending moments of at lleast 2.5 Nmeast 2.5 Nm
appappllied to the segmentied to the segment
 RReesist shear forces of atsist shear forces of at lleast 40 Ν appleast 40 Ν appliied toed to
thethe segmentsegment
 Take compression forces of at least 1200 ΝTake compression forces of at least 1200 Ν

28. Bryan Cervical DiskBryan Cervical Disk
ProsthesesProstheses

29. DESIGN OBJECTIVEDESIGN OBJECTIVE
SUMMARYSUMMARY
 Provide range ofProvide range of
motion (ROM) tomotion (ROM) to
permit normalpermit normal
functionfunction
 Long-term stabilityLong-term stability
 Durability:Durability:
withstand loads ofwithstand loads of
ADL forADL for >>10 years10 years

30. DESIGN FEATURESDESIGN FEATURES
Shell with Rigid Wings
Sheath (shown cut away)
Retaining Wires (shown cut away)
Nucleus
Porous Coating on Shell Dome

31. DESIGN FEATURESDESIGN FEATURESShell
Wings: anterior stop
Post: “soft” stop in maximum ROM
Internal polished concave spherical surface
External convex surface with porous coating
Low friction, wear resistant, elastic material.
2 convex spherical surfaces
Nucleus

32. OBJECTIVE: RANGE OFOBJECTIVE: RANGE OF
MOTIONMOTION
 Articulates via axially symmetricArticulates via axially symmetric
spherical bearing surfacesspherical bearing surfaces
 1111° of F/E and lateral bending° of F/E and lateral bending
 2 mm translation2 mm translation
 Rotationally unconstrainedRotationally unconstrained
 Motions also determined by softMotions also determined by soft
tissue interactionstissue interactions
– Allows coupled motion of normal spineAllows coupled motion of normal spine
– Maintains normal biomechanics ofMaintains normal biomechanics of
adjacent FSU’sadjacent FSU’s

33. OBJECTIVE:OBJECTIVE:
CONSTRAINTCONSTRAINT
 Unconstrained overUnconstrained over
normal ROMnormal ROM
 Semi-constrained inSemi-constrained in
maximum ROM:maximum ROM:
Internal geometryInternal geometry
and mechanicsand mechanics
provides “soft”provides “soft”
stopsstops
 Mechanically stableMechanically stable
against dislocationagainst dislocation
or subluxationor subluxation

34. OBJECTIVE:OBJECTIVE:
ELASTICITYELASTICITY
 Polymer nucleus has elasticityPolymer nucleus has elasticity
more like natural disc (vs.more like natural disc (vs.
UHMWPE)UHMWPE)
 May help protect adjacent levelsMay help protect adjacent levels
against excessive loadsagainst excessive loads

35. OBJECTIVE: ACUTEOBJECTIVE: ACUTE
STABILITYSTABILITY
 Machined endplatesMachined endplates
provide interference fitprovide interference fit
 Porous coating: highPorous coating: high
friction betweenfriction between
bone/shellbone/shell
 Polished shell: lowPolished shell: low
friction betweenfriction between
shell/nucleusshell/nucleus
– minimizes stressminimizes stress
transfer to implant/bonetransfer to implant/bone
interfaceinterface

36. OBJECTIVE: LONG-OBJECTIVE: LONG-
TERM STABILITYTERM STABILITY
 Ingrowth surface hasIngrowth surface has
appropriate porosityappropriate porosity
for bony fixationfor bony fixation
 Five sizes allowFive sizes allow
precision fit andprecision fit and
maximum contact areamaximum contact area
to prevent subsidenceto prevent subsidence
or migrationor migration
 Shell flanges provideShell flanges provide
resistance to posteriorresistance to posterior
migrationmigration

37. OBJECTIVE:OBJECTIVE:
DURABILITYDURABILITY
 Proprietary composite nucleusProprietary composite nucleus
construction resists abrasive wearconstruction resists abrasive wear
 Material properties: low friction andMaterial properties: low friction and
wearwear
 Sheath creates “diarthrodial” jointSheath creates “diarthrodial” joint
allowing:allowing:
– Maintenance of internal lubricated regionMaintenance of internal lubricated region
– Contains any particulate debrisContains any particulate debris
– Segregates articulating elements fromSegregates articulating elements from
surrounding tissue/fluidsurrounding tissue/fluid
 Testing has demonstratedTesting has demonstrated
functionalityfunctionality

38. OBJECTIVE: ACCURATEOBJECTIVE: ACCURATE
PLACEMENTPLACEMENT
 Precision instrumentationPrecision instrumentation
controls prosthesiscontrols prosthesis
implantation positionimplantation position
 Helps ensure safety of criticalHelps ensure safety of critical
anatomical structuresanatomical structures
 Sagittal centeringSagittal centering

39. OBJECTIVE: ACCURATEOBJECTIVE: ACCURATE
PLACEMENTPLACEMENT
 Precision powered cuttingPrecision powered cutting
instrumentsinstruments
 Helps ensure
accurate bone
preparation
 Minimizes bone
removal

40. OBJECTIVE: BIO-OBJECTIVE: BIO-
COMPATIBILITYCOMPATIBILITY
 All metallic materials have aAll metallic materials have a
history of use in orthopedichistory of use in orthopedic
devicesdevices
 All polymer materials have aAll polymer materials have a
history of use in cardiovascularhistory of use in cardiovascular
devicesdevices
 All materials have establishedAll materials have established
stability in a biologicalstability in a biological
environmentenvironment

41. DESIGN VERIFICATIONDESIGN VERIFICATION
 Conduct Risk Analysis processConduct Risk Analysis process
 Identify potential failure modesIdentify potential failure modes
 Conduct testing to establishConduct testing to establish
– Mechanical performance (static andMechanical performance (static and
fatigue)fatigue)
– Safety testing (biocompatibility andSafety testing (biocompatibility and
sterility)sterility)
– In vivoIn vivo performance (cadaver andperformance (cadaver and
animal)animal)
– Clinical performanceClinical performance

42. Shell with Rigid Wings
Sheath (shown cut away)
Retaining Wires (shown cut away)
Nucleus
Porous Coating on Shell Dome
PROSTHESIS

43. MECHANICALMECHANICAL
PERFORMANCE: SHELLPERFORMANCE: SHELL
 Compression fatigueCompression fatigue
– Determine shell fatigue strength underDetermine shell fatigue strength under
simulatedsimulated in vivoin vivo axial compressiveaxial compressive
loadingloading
– Purpose: ensure shell will not fracturePurpose: ensure shell will not fracture
during activities of daily living (ADL)during activities of daily living (ADL)
with “worst case” bony supportwith “worst case” bony support
– Safety factorSafety factor >> 3.5 at 10 MM cycles3.5 at 10 MM cycles
 Shear fatigueShear fatigue
– Determine post fatigue strength under cyclicDetermine post fatigue strength under cyclic
shear loadingshear loading
– Purpose: ensure shell post will not fracturePurpose: ensure shell post will not fracture
during ADL with maximum translation ofduring ADL with maximum translation of
shellsshells
– Safety factorSafety factor >> 2 at 10 MM cycles2 at 10 MM cycles

44. MECHANICALMECHANICAL
PERFORMANCE: NUCLEUSPERFORMANCE: NUCLEUS
 Static testingStatic testing
 Creep testingCreep testing
 Compression fatigue testingCompression fatigue testing

45. MECHANICALMECHANICAL
PERFORMANCE:PERFORMANCE:
NUCLEUSNUCLEUS
 Static testingStatic testing
– Determine maximum compressive load prosthesis canDetermine maximum compressive load prosthesis can
support without shell to shell contactsupport without shell to shell contact
– Safety factor > 9 for a single load cycleSafety factor > 9 for a single load cycle
 Creep testingCreep testing
– Establish long term load application will not result inEstablish long term load application will not result in
unacceptable loss of prosthesis heightunacceptable loss of prosthesis height
– Maximum ADL loadingMaximum ADL loading
– Safety factor > 3 for 700 hoursSafety factor > 3 for 700 hours
 Compression fatigue testingCompression fatigue testing
– Establish ADL cyclic loading will not result in degradationEstablish ADL cyclic loading will not result in degradation
of the nucleus that could lead to shell contactof the nucleus that could lead to shell contact
– Safety factor > 12 at 10 MM cyclesSafety factor > 12 at 10 MM cycles

46. MECHANICALMECHANICAL
PERFORMANCE: SHEATHPERFORMANCE: SHEATH
 Sheath testingSheath testing
– Establish sheath canEstablish sheath can
withstand worst casewithstand worst case
loading conditionsloading conditions
(maximum tension and(maximum tension and
torsion) without leakagetorsion) without leakage
– Safety factorsSafety factors >> 7.57.5

47. MECHANICALMECHANICAL
PERFORMANCE: WEARPERFORMANCE: WEAR
 Wear (Durability)Wear (Durability)
– Establish prosthesis can maintain functionalityEstablish prosthesis can maintain functionality
under ADL loads and motions for minimum ofunder ADL loads and motions for minimum of
10 years10 years
– Tested in custom cervicalTested in custom cervical
spine simulatorsspine simulators
– No failure of componentsNo failure of components
– Less than 2% weight lossLess than 2% weight loss
– Minimal wear observedMinimal wear observed
at 10 MM cyclesat 10 MM cycles

48. MECHANICALMECHANICAL
PERFORMANCE:PERFORMANCE:
STABILITYSTABILITY
 Prosthesis Expulsion:Prosthesis Expulsion:
– Establish forceEstablish force
required to dislodgerequired to dislodge
the prosthesis.the prosthesis.
– Bone analog modelBone analog model
under ADL loadunder ADL load
– Safety factors ofSafety factors of >> 22

49. MECHANICALMECHANICAL
PERFORMANCE:PERFORMANCE:
STABILITYSTABILITY
 Establish shear load required toEstablish shear load required to
cause subluxation of thecause subluxation of the
prosthesisprosthesis
– Tested in human cadaver modelTested in human cadaver model
– ADL axial loadADL axial load
– Passed with safety factor > 7Passed with safety factor > 7

50. SAFETY / PACKAGINGSAFETY / PACKAGING
TESTINGTESTING
 Bio-compatibilityBio-compatibility
– Meets requirements of EU/ISO/FDA standardsMeets requirements of EU/ISO/FDA standards
 Device SterilityDevice Sterility
– Sterilized in accordance with EU/ISO/FDASterilized in accordance with EU/ISO/FDA
standardsstandards
 Instrument Cleaning/SterilizationInstrument Cleaning/Sterilization
– can be cleaned and sterilized in accordance withcan be cleaned and sterilized in accordance with
EU/ISO/FDA standardsEU/ISO/FDA standards
 PackagingPackaging
– Meets shipping requirements of EU/ISO/FDAMeets shipping requirements of EU/ISO/FDA
standardsstandards

51. ANIMAL TESTINGANIMAL TESTING
• Surgical feasibility & safetySurgical feasibility & safety inin
vivovivo
• DurabilityDurability in vivoin vivo
• Salvage fusionSalvage fusion
• Optimal human analogOptimal human analog
 AnatomyAnatomy
 PhysiologyPhysiology
 KinematicsKinematics
 BehaviorBehavior
SheepSheep HumanHuman GoatGoatChimpanzeeChimpanzee

52. ANIMAL TESTINGANIMAL TESTING
 6 animals for 6 months6 animals for 6 months
– Safety establishedSafety established
– Bone ingrowth data obtainedBone ingrowth data obtained
– Design modifications determinedDesign modifications determined
 4 animals for 3 months4 animals for 3 months
– Design modification verifiedDesign modification verified
– Bony ingrowth verified with fluorochromeBony ingrowth verified with fluorochrome
labelinglabeling
– No prosthesis migration seenNo prosthesis migration seen
 All animals successfully fused usingAll animals successfully fused using
allograft bone after prosthesis removalallograft bone after prosthesis removal

53. New bone shown in green
Fluorochrome Labeling

54. Bryan disc prosthesesBryan disc prostheses
SUMMARYSUMMARY
 Prosthesis performance has beenProsthesis performance has been
challenged in static, dynamic,challenged in static, dynamic,
fatigue, durability and in vivo “worstfatigue, durability and in vivo “worst
case” modelscase” models
 All results have exceeded designAll results have exceeded design
requirements with adequate factorrequirements with adequate factor
of safetyof safety
 Based on these results, clinicalBased on these results, clinical
evaluation was initiatedevaluation was initiated

56. The Prodisc-C:The Prodisc-C:
Concept forConcept for
Cervical DiskCervical Disk
ArthroplastyArthroplasty

57. Biomechanical choicesBiomechanical choices
 The mechanical design of a prosthesis must fulfillThe mechanical design of a prosthesis must fulfill
several criterseveral criteriiaa
 TThe mechanical construct must be adapted to thehe mechanical construct must be adapted to the
cervcerviical biomechanics when the disk and thecal biomechanics when the disk and the
anterior longituanterior longituddinalinal lliigament, and often thegament, and often the
posterιor one, have been resectedposterιor one, have been resected
 The reconstruction must combine the capacity forThe reconstruction must combine the capacity for
stability with that for motion, with neutralization ofstability with that for motion, with neutralization of
the shear forcesthe shear forces
 This motion must also be compatίble with the onlyThis motion must also be compatίble with the only
parts that remain from the mobile unitparts that remain from the mobile unit i.e.i.e. TheThe
posterposterioior structures, facets, capsulae, andr structures, facets, capsulae, and
lίgamentslίgaments
 The choice to date has been a ball-and-socket joint,The choice to date has been a ball-and-socket joint,
with a radius of motion and a center of rotationwith a radius of motion and a center of rotation
compatible with those remaining posteriorcompatible with those remaining posterior
structures and a tolerance of settstructures and a tolerance of settiing whichng which
generallv adapts to local situatgenerallv adapts to local situatiionsons
 ThisThis semiconstrained concept is the only onesemiconstrained concept is the only one
acceptable after the anterior release that removesacceptable after the anterior release that removes
ΑLL, ΡLL, and diskΑLL, ΡLL, and disk..

58.  TheThe primary anchorage is provided byprimary anchorage is provided by
a keel that stabilizes thea keel that stabilizes the iimplant;mplant;
secondary anchorage will be provίdedsecondary anchorage will be provίded
by osteointegration. All of thoseby osteointegration. All of those
solutions have been tested and usedsolutions have been tested and used
in thousands of cases in thein thousands of cases in the fieldfield ofof
diskdisk arthroplasty with the lumbararthroplasty with the lumbar
PPrοdίsc-L experience,rοdίsc-L experience, whichwhich startestartedd
15 years ago15 years ago
 TheThe range of motion covers 20°range of motion covers 20° inin
flexion-extension (physiologicallyflexion-extension (physiologically
around 17°),20°around 17°),20° inin lateral inclinationslateral inclinations
(1(111°), and unl°), and unliimited rotation (12°). Themited rotation (12°). The
posterior elements retain as muchposterior elements retain as much
physiological control over the rangephysiological control over the range
of the mobility as possible.of the mobility as possible.

59. SizesSizes
 Different footprints are avaίlable:Different footprints are avaίlable:
– medium (15 mm width) with two depths (12 andmedium (15 mm width) with two depths (12 and
14 mm),14 mm),
– large (17 mm width, 14 and 16 mm depth),large (17 mm width, 14 and 16 mm depth),
– extra large (19 mm width, 16 and 18 mm depth).extra large (19 mm width, 16 and 18 mm depth).
 The different cores allow a global height ofThe different cores allow a global height of
5, 6, and 7 mm (the physiological maximum5, 6, and 7 mm (the physiological maximum
measured is 7.5 mm).measured is 7.5 mm).

60. TestingTesting
 The tests were performedThe tests were performed inin a laboratory toa laboratory to
evaluateevaluate ::
– compression shear (10 million cydes)compression shear (10 million cydes)
– ccompression fatigue (10 million cycles),ompression fatigue (10 million cycles),
– the amount ofthe amount of wear debris (2 mg for one milΙwear debris (2 mg for one milΙiiοηοη
cycyclcles, multes, multi-i-directional motion),directional motion),
– thethe ssnap-locking of thenap-locking of the inlinlay,ay,
– the creep (slow change ofthe creep (slow change of dimdimensions underensions under
stress).stress).

61. Chord compression at C4-C5 left side
Lateral view after
Prodisc-C implantation
ΑΡ view after
Prodisc-C
implantation
MRI pre-surgery.
DDD multilevels -
Chord
compression at
C4-C5

62. Chord compression
at C5-C6 pre-surgery
Flexion and Extension
after Prodisc-C
implantation at level
C5-C6
Prodisc – C in neutral
position and in lateral
bending

63. Mobi–C Cervical Disk ProsthesesMobi–C Cervical Disk Prostheses

70. Pearsall Elastomeric Textile ProsthesisPearsall Elastomeric Textile Prosthesis

82. Biointegration testingBiointegration testing

85. ConclusionsConclusions

86.  The amount of clinical dataThe amount of clinical data
available does not allow aavailable does not allow a
detailed evaluation of thedetailed evaluation of the
benefits and risk of thesebenefits and risk of these
implants, but the earlyimplants, but the early
success rates, thesuccess rates, the
perioperative morbidity,perioperative morbidity,
and the complications andand the complications and
adverse side effects, asadverse side effects, as
well as the patients'well as the patients'
satisfaction, seem tosatisfaction, seem to
support taking ansupport taking an
optimistic view of this newoptimistic view of this new
technology .technology .

87.  Is the implantationIs the implantation
procedure less invasive thanprocedure less invasive than
interbody fusion with ainterbody fusion with a
cage?cage?
 Can segmental mobility beCan segmental mobility be
achieved and/or maintained?achieved and/or maintained?
 Can the physiologicalCan the physiological
curvature be restored andcurvature be restored and
retained?retained?
 What will be the rate ofWhat will be the rate of
spontaneous fusions?spontaneous fusions?
 How does the implantHow does the implant
behavebehave iiηη the long term?the long term?