biomechanics of the spine and implant fixation...• modulus matching –peek vs.titanium – myth -...
TRANSCRIPT
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ORTHOKINETIC TECHNOLOGIES & ORTHOKINETIC TESTING TECHNOLOGIES
Lisa A. Ferrara, Ph.D.
Voice: 910.253.9883
Email: [email protected]
Website: www.orthokintech.com
BIOMECHANICS OF THE SPINE AND IMPLANT FIXATION
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DISCLOSURES– OKT – Strategic Planning & Regulatory Consulting
– OKT2 – ISO 17025 A2LA Accredited Test Facility
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MECHANICS 101
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Spinal Motion
• Motion along 3 planes
• 6 Freedom
• Dynamic IAR
• Bending in Disc: Compression & tension on annular
fibers
Displacement of nucleus• Flexion – dorsal
• Extension - ventral
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BIOTENSEGRITY• Application of tensegrity principles to biologic
structures
– muscles, bones, fascia, ligaments, tendons, cell
membranes
• Superior strength from unison tension +
compression of tissue structures
• Muscular-skeletal system
– Muscles & connective = continuous TENSION
– Bones = discontinuous COMPRESSION
• Maintains homeostasis
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• Intrinsic & Extrinsic Muscles serve as pulleys to spine for stabilization
• Apply constant tension for mobilization
• Transfer stress to surrounding structures – load sharing
MUSCULATURE
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STRESS & DEGENERATION
• Stress Transfer
• How does the stress flow
through the tissue?
Soft tissue injury, disc dessication
Eccentric stress transfer through
tissuesTissue
adaptation
Continued Degeneration,
progressive instability
Wolff’s Law
Inefficient
adaptations
(osteophytes)
DEGENERATIVE
CASCADE
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LOAD BALANCE AND CENTER
OF ROTATION FOR FSU
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LOAD BALANCE (STRESS TRANSFER) – HEALTHY
• Vertebra + Disc = 55 - 60%
• Cortical shell = 10%
• Posterior Ligaments = 10-15%
• Facets = 20-25% Load transmitted uniformly across endplates & disc
• Uniform load transmits to annulus
• Isotropic stress profile
• Ligaments pull stress away from disc
• 80% Anterior : 20% Posterior
Shirazi-Adl, Farfan, McBroom
}80%20%
80%
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LOAD BALANCE - DEGENERATIVE
60%40%
• Less Efficient
• Vertebra + Disc = 40%
• Cortical shell = 10%
• Posterior Ligaments = 10%
• Facets = 40%
• Facets > load transmission
• Stress profile anisotropic
• Excessive stress & COR transmitted posteriorly
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CURRENT CHALLENGES
FOR BIOFIDELIC IMPLANT
DESIGN
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FACET JOINT – COMPLEX JOINT
Interdependence of disc + facetsSoft + Hard tissue
DISC
FACET FACET
Spinal Joint COMPLEX
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Challenges with Designing Intervertebral Implants
• Unique disc structure & endplate
anatomy
• Bone Integrity
• Fit of implant within interspace
– Cortical Margining
– Avoid centrum = stress riser
– Endplate viscoelastic
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VISCOELASTICITY
• Behaves like fluid & solid
• Stress relaxation
• Load rate dependent
– Intervertebral Disc
– Bone
– Soft tissue
• Ex: Silly Putty
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ELASTIC MODULUS
• Defines compliance ‘elasticity’ of material
• EM = Stress / Strain
• Material Modulus – Ti = Stress/strain of bulk
material
• Global modulus – Stress/Strain of implant
• Localized modulus – Stress/strain of strut
member
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STRESS• Force / Area
• Units: N/mm2, Pascals
• = Force / Area
Z
100N
1mm2
100 Pa
100N
2mm2
50Pa
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STRAIN• Linear: length / original length
• Rotational: angle / original angle
• = L / L0
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• Modulus Matching – PEEK vs.Titanium
– Myth - Believed to be necessary to provide improved
load sharing, reduce incidence of stress risers/shielding
– Fact – Behavior at the bone-implant interface affected
by many factors; bone quality, implant footprint, design,
geometry, edges.
– Large footprint will decrease stress to surrounding bone
and capture stronger peripheral bone = less risk to
subsidence
BIOMECHANICAL MYTHS
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BULK ELASTIC MODULUS (GPa)
0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400
Cancellous Bone
Allograft Cortical Bone
Vertebral Endplate
Femur Cortical Bone
UHMWPE
Polymethylmethacry…
PEEK
OXPEKK®-IG
CFRP
Titanium Ti-6Al-4V
316L Stainless Steel
Si3N4
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Biomechanical Challenges for Implants
• Matching elastic moduli (??)
– Increase implant footprint
– Engage cortical margin
– Avoid implant stress risers
– Understand implant EM
• Matching endplate shape (curvature)
– Increase contact surface area
– Reduce stress (or stress risers) to endplate
– Avoid abrupt interfaces
• Avoid concentrated stress at centrum
– Endplate weakest at center
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• Settling
– Natural occurrence of the human tissue to creep under compressive loads due
to the viscoelasticity of the tissue
– Natural settling 1- 2mm - cervical fusion / 1.5-2.5mm lumbar / viscoelastic
endplates
• Subsidence
– Exceeds the natural viscoelastic limit of the tissue
– Enters the plastic deformation stage
– Potential ‘penetration’ into the weaker substrate
– Footprint greatest influence on subsidence
– But there is balance between footprint & other factors (EM, design, shape)
– STRESS TRANSFER
SETTLING VS. SUBSIDENCE
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IMPLANT FOOTPRINT & FUSION STABILITY
• Multi-directional bone exchange & incorporation = better biomechanical stability
• Improved stability allows for further ingrowth towards center- throughout implant
• Greater fusion footprint – better stress distribution throughout implant
• Surface texture can aid with attachment – earlier stability
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Surface Finish / Biomechanics / Mechanotransduction
• Surface finish / texture correlates with grip strength of implant component
• Improves bone attachment
• Rough = high coef. of fiction – reduces sliding of substrate on material
• Process where mechanical energy is converted -electrical & biochemical.
• Mechanical stimuli Tissue individual Cells
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Cellular Mechanotransduction
• All eukaryotic cells - mechanosensitive
• Forces (gravity, tension, compression, shear,) influence
cell behavior (growth and tissue remodeling).
• Cellular Mechanotransduction – the mechanism by
which cells convert mechanical signals into biochemical
responses.
• Cells respond to applied force stimuli to tissue --- AND---
• Internal forces generated from cell cytoskeleton respond
by generating contractile forces for motility, matrix
formation
• Continuous, dynamic cyclical process
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• Stiffness – Ex: PEEK vs. Titanium
– Myth - A stiff implant will have increased risk to
stress shielding & subsidence
– Fact – stiffness is related to combination of
material properties + implant design (footprint) +
stress transfer through implant
BIOMECHANICAL MYTHS
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STIFFNESS & STRESSLoading Mode Ti ALIF PEEK ALIF Ti Cylinder
Axial Compression (KN/mm) 6.3 4.1 3.8
Flexion (Nm/Deg) 8.3 5.5 2.7
Lateral Bending (Nm/Deg) 15.5 7.1 8.3
Axial Rotation (Nm/Deg) 48.7 36.2 24.6
The larger Ti ALIF highest stiffness, largest bone volume containment =
less stress at endplate, less stress than Ti cylinders
FEA of STRESS
PROFILE FOR
ENDPLATE
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• Understand your biomechanical environment
– Bone & soft tissue integrity (tissue stiffens with
dessication)
– Implant design & position within space
– Stress transfer to surrounding structures• Bigger IBF footprint = less stress to endplates = less risk to subsidence
– Too rigid can be as bad as not rigid enough
– Implantation - More is not always better
– Strive for load balance
– Implants should load share with tissue – prolonged load
bearing will = IMPLANT FAILURE
SUMMARY
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THANK YOU
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CONTACT INFORMATION:
Lisa Ferrara, Ph.D.
Voice: 910.253.9883
www.orthokintech.com
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IMPLANT DESIGN CHALLENGES
• Biocompatibility
• Optimized material & design for osteoblast recruitment
• Balance – re-establish a healthy loading pathway along
the spine to slow the degenerative cascade.– Ideal stress transfer to surrounding tissues within biomechanical thresholds
• Implant mechanics should match the kinetics of spine for
site specific performance
• Ease of Fabrication in reproducible, consistent manner – Adequate Sterilization, surface finish, cost of production
• Ease of Repair / Revision for failed implants
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THE GOOD, THE BAD, THE UGLY
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OPTIMIZING OSSEOINTEGRATION
• RESPECT & UNDERSTAND WOLFF’S LAW– Bone growth responds to stress and strain
– Region specific – different bone responds to different stress/strain
• MATERIAL + DESIGN = OSSEOINTEGRATION– Cascade of events for bone integration
– Immobilize/stabilize – optimal strain for osteoblast
differentiation/recruitment
– Material favored by osteoblasts
– Design provides uniform distribution of optimal strain + allows for
multiplanar bone ingrowth = early anchoring for stability and
continued
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BULK MODULUS (MATERIAL)
Material Elastic Modulus (GPa)
316L Stainless Steel 200
Titanium Ti-6Al-4V 115
Allograft Cortical Bone 4.0-8.0
PEEK 4.0
PEKK 4.0
Polymethylmethacrylate 1.0
Cancellous Bone 0.1
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FORCE DISTRIBUTION AT BONE INTERFACE
- GEOMETRICAL INFLUENCES (vs. modulus
matching)
Anterior PEEK ALIFTi ALIF
Stress risers – small geometry at bone interfaceResult in subsidence, pistoning through boneStress risers at bone and implant interface
Ti Cylindrical
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Stiffness of the Implant:
A stiff implant will have increased risk to
stress shielding and subsidence
Fact – stiffness is related to the combination
of the material properties + implant design
BIOMECHANICAL MYTH 2
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ELASTIC MODULUS (GPa)
Si3N4
100X stiffer than Cancellous Bone
60X stiffer than PEEK
0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400
Cancellous Bone
Allograft Cortical Bone
Vertebral Endplate
Femur Cortical Bone
UHMWPE
Polymethylmethacrylate
PEEK
OXPEKK®-IG
CFRP
Titanium Ti-6Al-4V
316L Stainless Steel
Si3N4
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MODULI COMPARISON
Material Elastic Modulus (GPa)
316L Stainless Steel 200
Titanium Ti-6Al-4V 115
Allograft Cortical Bone 4.0-8.0
PEEK 4.0
PEKK 4.0
Polymethylmethacrylate 1.0
Cancellous Bone 0.1
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7 WORDS YOU CAN’T SAY TO THE FDA
1. Micromotion
2. Subsidence
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Mechanotransduction
• Process where mechanical energy is converted into electrical
and biochemical signals.
• Mechanical stimuli Tissue individual Cells
• Cellular Mechanotransduction – the mechanism by which cells
convert mechanical signals into biochemical responses.
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Cellular Mechanotransduction
• All eukaryotic cells - mechanosensitive
• Forces (gravity, tension, compression, shear,) influence
cell behavior (growth and tissue remodeling).
• Cells respond to applied force stimuli to tissue --- AND---
• Internal forces generated from cell cytoskeleton respond
by generating contractile forces
• Continuous, dynamic cyclical process
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FORCE CLOSURE• Forces created by contraction of
stabilizing muscles
• produce a “self-locking” mechanism
for joints,
• resulting in increased joint stability.
• For SIJ - occurs through contraction
of deep muscles in low back, hip,
pelvis
• controls translation & shear by
compressing joint
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FORCE DISTRIBUTION AT BONE INTERFACE
- GEOMETRICAL INFLUENCES (vs. modulus
matching)
Anterior PEEK ALIFTi ALIF
Stress risers – small geometry at bone interfaceResult in subsidence, pistoning through boneStress risers at bone and implant interface
Ti Cylindrical
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WOLFF’s LAWForm Follows Function• Bone will remodel along lines of
stress / resorb in lack of stress-
strain
• Function – Walking
• Form – Results in Bone
remodeling along lines of
greatest stress during function
• Dynamic System – Bone
remodeling is a constant process
responding to constant stimuli
Principle Compressive
TrabeculaePrinciple Tensile
Trabeculae