Biomechanics of Biomechanics of Cervical Disk Cervical Disk Replacement Replacement GEORGE SAPKAS GEORGE SAPKAS Associate Professor Associate Professor 1 1 st st Orthopaedic Department Orthopaedic Department Medical School Athens University Medical School Athens University Athens Greece Athens Greece
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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
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
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
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
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
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
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
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
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
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
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
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?