Biomechanics of Biomechanics of Cervical Disk Cervical Disk ReplacementReplacement

GEORGE SAPKASGEORGE SAPKASAssociate ProfessorAssociate Professor

11stst Orthopaedic Department Orthopaedic DepartmentMedical School Athens UniversityMedical School Athens UniversityAthens GreeceAthens Greece

Synovial joints Synovial joints Movement by sliding Movement by sliding

articulationarticulation

Cervical Disk Cervical Disk ReplacementReplacement

Dynamic reconstruction Dynamic reconstruction of the degenerative of the degenerative functional spfunctional spiinal unit nal unit (FSU) (FSU) is a rapidly growis a rapidly growiing field ng field iiηη spinal surgery spinal surgery

Procedures Procedures

such as :such as :– nucleus replacement, nucleus replacement, – posterior dynamic posterior dynamic

stabilization, stabilization, – interspinal interspinal

distraction distraction – biological methods biological methods

to regenerate the to regenerate the disk disk are being tested in are being tested in experimental or experimental or clinical studiesclinical studies

PDN

3K - Fradis

ISOLOCK

Among spine Among spine

arthroplasty arthroplasty techniques, total disk techniques, total disk replacement replacement in the lumbar spine in the lumbar spine is the most advanced, is the most advanced, with promising early with promising early results seen not only results seen not only ίίn empirical studies, n empirical studies, but meanwhile but meanwhile ίίn n prospective prospective randomized studies as randomized studies as well. well.

Maverick

It seems logical that It seems logical that

solutions for dynamic solutions for dynamic stabilization as an stabilization as an alternative to spinal alternative to spinal fusion have now also fusion have now also been found for the been found for the cervical spine. cervical spine.

Here as well, total disk Here as well, total disk replacement seems to be replacement seems to be the first choice and the first choice and probably the most easily probably the most easily realizable technique. realizable technique.

New implants for total New implants for total cervical disk replacement cervical disk replacement have been developed have been developed iiηη the past few years.the past few years.

3K - Fradis Prestige

Cervical Disk ReplacementCervical Disk Replacement

Cervical Disk ReplacementCervical Disk Replacement

Research has identified a total of eight Research has identified a total of eight patentspatents

Two implants are currently Two implants are currently undergoundergoiing controlled ng controlled clinical evaluation clinical evaluation iiηη multicenter studies: multicenter studies: – the Bryan Discthe Bryan Disc– the Prodisc-C the Prodisc-C – whereas others are nearing whereas others are nearing

the stage of clinical the stage of clinical application.application.

Some dataSome data::Το date, we cannot Το date, we cannot precisepreciselly describe the y describe the mechanics of themechanics of the human human cervical spcervical spiine under ne under iiη η ννiiνο conditions, ί.e., νο conditions, ί.e., iiη η thethe activities of daily lactivities of daily lifife. e. InIn ffact, the mechanics of act, the mechanics of the humanthe human cervicacervicall spine spine inin ν νiiνο most probabνο most probablly are y are a resua resullt oft of bendingbending around different axls, around different axls, shear, and axiashear, and axiall compression forcescompression forces..

CompressionCompression

The weight of the head is passed The weight of the head is passed throughthrough the occipitathe occipitall condyles condyles οοnn both sides to the at both sides to the atllanto-axialanto-axial joint joint 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 spine spine via the vertebravia the vertebrall bodies bodies andand both facets. both facets.

GoeGoell and CΙausen and CΙausen (1998) (1998) have been have been llooking ooking atat the amountthe amount of compression of compression lload oad that is passed that is passed throughthrough th the vertebrae vertebrall bodies bodies and the disk: and the disk: They found that 88 %They found that 88 % ofof a compress a compressiion on load load isis passed through the vertebra passed through the vertebral l bodbodiies es ofof the cerv the cerviicacall sp spiine, ne, with the amount estimatedwith the amount estimated to range trom 110 to 1200 Νto range trom 110 to 1200 Ν

BendinBendingg moments moments. .

Το investigate the compΤο investigate the compllex ex scenarioscenario of of lloading and movingoading and moving the spine, the spine, defined loading defined loading hashas been proposed:been proposed:– 1.8-2.5 Nm are wide1.8-2.5 Nm are widelly recommendedy recommended or or

used to used to lload the human cervical spineoad the human cervical spine under under inin vitro conditions. vitro conditions.

– This usually produces segmentaThis usually produces segmentall range of motion which can be range of motion which can be observed underobserved under in in νίνο conditions νίνο conditions;; it it is approximate!y 10is approximate!y 1000 for flexion- for flexion-extension, left right axial rotation, and extension, left right axial rotation, and left right lateral bending. This range of left right lateral bending. This range of motion increases if a diskectomy is motion increases if a diskectomy is performedperformed

Shear forcesShear forces

of 39 Ν have been applof 39 Ν have been appliied ed toto the cervical spine, the cervical spine, resuresullting ting inin 1.6-1.9 mm of 1.6-1.9 mm of transtransllationation ((Panjabi et alPanjabi et al, 1986 - , 1986 - Moroney et alMoroney et al, 1988), 1988)

these motions induced bythese motions induced by shear forces shear forces coucoulld resud resullt t inin ear earlly or y or llate faiate faillure ure of a cervicaof a cervicall spine disk prosthesis. spine disk prosthesis.

ApplAppliication of these data cation of these data to the cervical spineto the cervical spine disk prosthesdisk prosthesiiss

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 p every motion pllaneane Resist bending moments of at Resist bending moments of at lleast 2.5 Nmeast 2.5 Nm

appappllied to the segmentied to the segment RReesist shear forces of at sist shear forces of at lleast 40 Ν appleast 40 Ν appliied to ed to

thethe segmentsegment Take compression forces of at least 1200 ΝTake compression forces of at least 1200 Ν

Bryan Cervical Disk Bryan Cervical Disk ProsthesesProstheses

DESIGN OBJECTIVE DESIGN OBJECTIVE SUMMARYSUMMARY

Provide range of Provide range of motion (ROM) to motion (ROM) to permit normal permit normal functionfunction

Long-term stabilityLong-term stability Durability: Durability:

withstand loads of withstand loads of ADL for ADL for >>10 years10 years

DESIGN FEATURESDESIGN FEATURES

Shell with Rigid Wings

Sheath (shown cut away)

Retaining Wires (shown cut away)

Nucleus

Porous Coating on Shell Dome

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

OBJECTIVE: RANGE OF OBJECTIVE: RANGE OF MOTIONMOTION

Articulates via axially symmetric Articulates 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 soft Motions also determined by soft

tissue interactionstissue interactions– Allows coupled motion of normal Allows coupled motion of normal

spinespine– Maintains normal biomechanics of Maintains normal biomechanics of

adjacent FSU’sadjacent FSU’s

OBJECTIVE: OBJECTIVE: CONSTRAINTCONSTRAINT

Unconstrained over Unconstrained over normal ROMnormal ROM

Semi-constrained in Semi-constrained in maximum ROM: maximum ROM: Internal geometry Internal geometry and mechanics and mechanics provides “soft” provides “soft” stopsstops

Mechanically stable Mechanically stable against dislocation against dislocation or subluxationor subluxation

OBJECTIVE: ELASTICITYOBJECTIVE: ELASTICITY

Polymer nucleus has elasticity Polymer nucleus has elasticity more like natural disc (vs. more like natural disc (vs. UHMWPE)UHMWPE)

May help protect adjacent May help protect adjacent levels against excessive loadslevels against excessive loads

OBJECTIVE: ACUTE OBJECTIVE: ACUTE STABILITYSTABILITY

Machined endplates Machined endplates provide interference provide interference fitfit

Porous coating: high Porous coating: high friction between friction between bone/shell bone/shell

Polished shell: low Polished shell: low friction between friction between shell/nucleusshell/nucleus– minimizes stress minimizes stress

transfer to transfer to implant/bone interfaceimplant/bone interface

OBJECTIVE: LONG-TERM OBJECTIVE: LONG-TERM STABILITYSTABILITY

Ingrowth surface has Ingrowth surface has appropriate porosity appropriate porosity for bony fixationfor bony fixation

Five sizes allow Five sizes allow precision fit and precision fit and maximum contact area maximum contact area to prevent subsidence to prevent subsidence or migrationor migration

Shell flanges provide Shell flanges provide resistance to posterior resistance to posterior migrationmigration

OBJECTIVE: OBJECTIVE: DURABILITYDURABILITY

Proprietary composite nucleus Proprietary composite nucleus construction resists abrasive wearconstruction resists abrasive wear

Material properties: low friction Material properties: low friction and wearand wear

Sheath creates “diarthrodial” Sheath creates “diarthrodial” joint allowing:joint allowing:– Maintenance of internal lubricated regionMaintenance of internal lubricated region– Contains any particulate debrisContains any particulate debris– Segregates articulating elements from Segregates articulating elements from

surrounding tissue/fluidsurrounding tissue/fluid Testing has demonstrated Testing has demonstrated

functionalityfunctionality

OBJECTIVE: ACCURATE OBJECTIVE: ACCURATE PLACEMENTPLACEMENT

Precision instrumentation Precision instrumentation controls prosthesis controls prosthesis implantation positionimplantation position

Helps ensure safety of Helps ensure safety of critical anatomical critical anatomical structuresstructures

Sagittal centeringSagittal centering

OBJECTIVE: ACCURATE OBJECTIVE: ACCURATE PLACEMENTPLACEMENT

Precision powered cutting instrumentsPrecision powered cutting instruments

Helps ensure accurate bone preparation

Minimizes bone removal

OBJECTIVE: BIO-OBJECTIVE: BIO-COMPATIBILITYCOMPATIBILITY

All metallic materials have a All metallic materials have a history of use in orthopedic history of use in orthopedic devicesdevices

All polymer materials have a All polymer materials have a history of use in cardiovascular history of use in cardiovascular devicesdevices

All materials have established All materials have established stability in a biological stability in a biological environmentenvironment

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 and Mechanical performance (static and fatigue)fatigue)

– Safety testing (biocompatibility and Safety testing (biocompatibility and sterility)sterility)

– In vivoIn vivo performance (cadaver and animal) performance (cadaver and animal)– Clinical performanceClinical performance

Shell with Rigid Wings

Sheath (shown cut away)

Retaining Wires (shown cut away)

Nucleus

Porous Coating on Shell Dome

PROSTHESIS

MECHANICAL MECHANICAL PERFORMANCE: SHELLPERFORMANCE: SHELL

Compression fatigueCompression fatigue– Determine shell fatigue strength under Determine shell fatigue strength under

simulated simulated in vivoin vivo axial compressive axial compressive loadingloading

– Purpose: ensure shell will not fracture Purpose: 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 factor Safety factor >> 3.5 at 10 MM cycles 3.5 at 10 MM cycles

Shear fatigueShear fatigue– Determine post fatigue strength under Determine post fatigue strength under

cyclic shear loadingcyclic shear loading– Purpose: ensure shell post will not Purpose: ensure shell post will not

fracture during ADL with maximum fracture during ADL with maximum translation of shells translation of shells

– Safety factor Safety factor >> 2 at 10 MM cycles 2 at 10 MM cycles

MECHANICAL MECHANICAL PERFORMANCE: NUCLEUSPERFORMANCE: NUCLEUS

Static testing Static testing Creep testingCreep testing Compression fatigue Compression fatigue

testingtesting

MECHANICAL MECHANICAL PERFORMANCE: NUCLEUSPERFORMANCE: NUCLEUS

Static testingStatic testing– Determine maximum compressive load prosthesis Determine maximum compressive load prosthesis

can support without shell to shell contactcan support 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 in Establish 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 Establish ADL cyclic loading will not result in

degradation of the nucleus that could lead to shell degradation of the nucleus that could lead to shell contact contact

– Safety factor > 12 at 10 MM cyclesSafety factor > 12 at 10 MM cycles

MECHANICAL MECHANICAL PERFORMANCE: SHEATHPERFORMANCE: SHEATH

Sheath testingSheath testing– Establish sheath can Establish sheath can

withstand worst case withstand worst case loading conditions loading conditions (maximum tension and (maximum tension and torsion) without torsion) without leakageleakage

– Safety factors Safety factors >> 7.5 7.5

MECHANICAL MECHANICAL PERFORMANCE: WEARPERFORMANCE: WEAR

Wear (Durability)Wear (Durability)– Establish prosthesis can maintain functionality under Establish prosthesis can maintain functionality under

ADL loads and motions for minimum of 10 years ADL loads and motions for minimum of 10 years – Tested in custom cervical Tested in custom cervical

spine simulatorsspine simulators

– No failure of componentsNo failure of components– Less than 2% weight lossLess than 2% weight loss– Minimal wear observed Minimal wear observed

at 10 MM cyclesat 10 MM cycles

MECHANICAL MECHANICAL PERFORMANCE: PERFORMANCE:

STABILITYSTABILITY Prosthesis Expulsion:Prosthesis Expulsion:

– Establish force Establish force required to dislodge required to dislodge the prosthesis.the prosthesis.

– Bone analog model Bone analog model under ADL loadunder ADL load

– Safety factors of Safety factors of >> 2 2

MECHANICAL MECHANICAL PERFORMANCE: PERFORMANCE:

STABILITYSTABILITY Establish shear load required Establish shear load required

to cause subluxation of the to cause 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

SAFETY / PACKAGING SAFETY / 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/FDA Sterilized in accordance with EU/ISO/FDA

standardsstandards Instrument Cleaning/SterilizationInstrument Cleaning/Sterilization

– can be cleaned and sterilized in accordance can be cleaned and sterilized in accordance with EU/ISO/FDA standardswith EU/ISO/FDA standards

PackagingPackaging– Meets shipping requirements of EU/ISO/FDA Meets shipping requirements of EU/ISO/FDA

standardsstandards

ANIMAL TESTINGANIMAL TESTING

• Surgical feasibility & safety Surgical feasibility & safety in in vivovivo

• Durability Durability in vivoin vivo• Salvage fusionSalvage fusion• Optimal human analogOptimal human analog

AnatomyAnatomy PhysiologyPhysiology KinematicsKinematics BehaviorBehavior

SheepSheep HumanHuman GoatGoatChimpanzeeChimpanzee

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 fluorochrome Bony ingrowth verified with fluorochrome

labelinglabeling– No prosthesis migration seenNo prosthesis migration seen

All animals successfully fused using All animals successfully fused using allograft bone after prosthesis removalallograft bone after prosthesis removal

New bone shown in green

Fluorochrome Labeling

Bryan disc prostheses Bryan disc prostheses SUMMARYSUMMARY

Prosthesis performance has Prosthesis performance has been challenged in static, been challenged in static, dynamic, fatigue, durability and dynamic, fatigue, durability and in vivo “worst case” modelsin vivo “worst case” models

All results have exceeded design All results have exceeded design requirements with adequate requirements with adequate factor of safetyfactor of safety

Based on these results, clinical Based on these results, clinical evaluation was initiated evaluation was initiated

The Prodisc-C: The Prodisc-C: Concept for Concept for Cervical Disk Cervical Disk Arthroplasty Arthroplasty

Biomechanical choicesBiomechanical choices

The mechanical design of a prosthesis must fulfill The mechanical design of a prosthesis must fulfill several criterseveral criteriiaa

TThe mechanical construct must be adapted to the he mechanical construct must be adapted to the cervcerviical biomechanics when the disk and the cal biomechanics when the disk and the anterior longituanterior longituddinalinal lliigament, and often the gament, and often the posterιor one, have been resectedposterιor one, have been resected

The reconstruction must combine the capacity for The reconstruction must combine the capacity for stability with that for motion, with neutralization of stability with that for motion, with neutralization of the shear forcesthe shear forces

This motion must also be compatίble with the only This 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, and r 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 rotation with a radius of motion and a center of rotation compatible with those remaining posterior compatible with those remaining posterior structures and a tolerance of settstructures and a tolerance of settiing which ng which generallv adapts to local situatgenerallv adapts to local situatiionsons

ThisThis semiconstrained concept is the only one semiconstrained concept is the only one acceptable after the anterior release that removes acceptable after the anterior release that removes ΑLL, ΡLL, and diskΑLL, ΡLL, and disk..

The The primary anchorage is provided by primary anchorage is provided by

a keel that stabilizes the a keel that stabilizes the iimplant; mplant; secondary anchorage will be provίded secondary anchorage will be provίded by osteointegration. All of those by osteointegration. All of those solutions have been tested and used solutions have been tested and used in thousands of cases in the in thousands of cases in the fieldfield ofof diskdisk arthroplasty with the lumbar arthroplasty with the lumbar PPrοdίsc-L experience, rοdίsc-L experience, whichwhich starte started d 15 years ago15 years ago

The The range of motion covers 20° range of motion covers 20° inin flexion-extension (physiologically flexion-extension (physiologically around 17°),20° around 17°),20° inin lateral inclinations lateral inclinations (1(111°), and unl°), and unliimited rotation (12°). The mited rotation (12°). The posterior elements retain as much posterior elements retain as much physiological control over the range physiological control over the range of the mobility as possible.of the mobility as possible.

SizesSizes Different footprints are avaίlable: Different footprints are avaίlable:

– medium (15 mm width) with two depths (12 and medium (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 of The different cores allow a global height of 5, 6, and 7 mm (the physiological maximum 5, 6, and 7 mm (the physiological maximum measured is 7.5 mm).measured is 7.5 mm).

TestingTesting The tests were performed The tests were performed inin a laboratory to a laboratory to

evaluate evaluate ::– 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 milΙiiοη οη

cycyclcles, multes, multi-i-directional motion), directional motion), – the the ssnap-locking of the nap-locking of the inlinlay, ay, – the creep (slow change of the creep (slow change of dimdimensions under ensions under

stress).stress).

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

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

Mobi–C Cervical Disk Mobi–C Cervical Disk ProsthesesProstheses

Pearsall Elastomeric Textile Pearsall Elastomeric Textile ProsthesisProsthesis

Biointegration testingBiointegration testing

ConclusionsConclusions

The amount of clinical The amount of clinical

data available does not data available does not allow a detailed allow a detailed evaluation of the evaluation of the benefits and risk of benefits and risk of these implants, but the these implants, but the early success rates, the early success rates, the perioperative morbidity, perioperative morbidity, and the complications and the complications and adverse side effects, and adverse side effects, as well as the patients' as well as the patients' satisfaction, seem to satisfaction, seem to support taking an support taking an optimistic view of this optimistic view of this new technology .new technology .

Is the implantation Is the implantation procedure less invasive procedure less invasive than interbody fusion with than interbody fusion with a cage?a cage?

Can segmental mobility be Can segmental mobility be achieved and/or achieved and/or maintained?maintained?

Can the physiological Can the physiological curvature be restored and curvature be restored and retained? retained?

What will be the rate of What will be the rate of spontaneous fusions?spontaneous fusions?

How does the implant How does the implant behave behave iiηη the long term? the long term?