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Dental Tissues and their Replacements
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Issues
• Dental decay• Periodontal disease• Movement of teeth
(orthodontics)• Restorative treatments• Thermal expansion
issues related to fillings• Fatigue and fracture of
teeth and implants
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Marshall et al., J. Dentistry, 25,441, 1997.
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Tissue Constituents
• Enamel-hardest substance in body-calcium phosphate salts-large apatite crystals
• Dentin-composed largely of type-I collagen fibrils and nanocrystalline apatite mineral-similar to bone
• Dentinal tubules-radiate from pulp• Pulp-richly vascularized connnective tissue• Cementum-coarsely fibrillated bonelike
substance devoid of canaliculi• Periodontal Membrane-anchors the root into
alveolar bone
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ENAMEL
• 96%mineral, 1% protein &lipid, remainder is water (weight %)
• Minerals form Long crystals-hexagonal shape
• Flourine- renders enamel much less soluble and increases hardness
• HA= Ca10(PO4)6(OH)2
40 nm1000 nm in length
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DENTIN
• Type-I collagen fibrils and nanocrystalline apatite• Dentinal tubules from dentin-enamel and
cementum-enamel junctions to pulp • Channels are paths for odontoblasts (dentin-
forming cells) during the process of dentin formation
• Mineralized collagen fibrils (50-100 nm in diameter) are arranged orthogonal to the tubules
• Inter-tubular dentin matrix with nanocrystalline hydroxyapatite mineral- planar structure
• Highly oriented microstructure causes anisotropy• Hollow tubules responsible for high toughness
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Structural properties
Tissue Density(g/cm3)
E(GPa)
Comp Stren. (MPa)
Tensile Stren. (MPa)
Thermal Expansion Coefficient (1/C)
Enamel 2.2 48 241 10 (ish) 11.4x10-6
Dentin 1.9 13.8 138 35-52 8.3x10-6
Park and Lakes, Biomaterials, 1992 and Handbook of Biomaterials, 1998
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Structural properties
Tissue Density(g/cm3)
E Comp Stren. (MPa)
Tensile Stren. (MPa)
CorticalBone
1.9 (wet) 10-20GPa
205(long.)
133(long.)
Trabec. Bone(various)
23-450MPa
1.5-7.4
Park and Lakes, Biomaterials, 1992 and Handbook of Biomaterials, 1998
Note: remodeling is primarily strain driven
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Dental Biomaterials
Amalgams/Fillings
Implants /Dental screws
Adhesives/Cements
Orthodontics
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Materials used in dental applications
• Fillings: amalgams, acrylic resins
• Titanium: Ti6Al4V dominates in root implants and fracture fixation
• Teeth: Porcelain, resins, ceramics
• Braces: Stainless steel, Nitinol
• Cements/resins: acrylate based polymers
• Bridges: Resin, composite, metal (Au, CoCr)
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Motivation to replace teeth
• Prevent loss in root support and chewing efficiency
• Prevent bone resorption
• Maintain healthy teeth
• Cosmetic
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Amalgams/Fillings
• An amalgam is an alloy in which one component is mercury (Hg)
• Hg is liquid at RT- reacts with silver and tin- forms plastic mass that sets with time– Takes 24 hours for full set (30 min for initial set).
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Thermal expansion concerns
• Thermal expansion coefficient
= ∆L/(Lo∆T)
= ∆T
• Volumetric Thermal expansion coefficient
V= 3
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Volume Changes and Forces in Fillings
• Consider a 2mm diameter hole which is 4mm in length in a molar tooth, with thermal variation of ∆T = 50C
amalgam= 25x10-6/C resin= 81x10-6 /C enamel
= 8.3 x10-6 /C• E amalgam = 20 GPa E resin = 2.5 GPa• ∆V = Vo x 3 x ∆T • ∆Vamalgam= π (1mm) 2 x 4mm x 3 (25-8.3) x10-6 x 50 = 0.03 mm3
∆Vresin = 0.14 mm3
• (1-d) F = E x ∆ x Afilling
F = E (∆T ) ∆(amalgam/resin - enamel ) x π/4D2
• F amalgam = 52 N ; S = F/Ashear=2.1 MPa• F resin = 29 N ; S = 1.15 MPa• Although the resin “expands” 4x greater than the amalgam, the
reduced stiffness (modulus) results in a lower force
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Volume Changes and Forces in Fillings(cont.)
• F amalgam = 52 N ; S = F/Ashear=2.1 MPa• F resin = 29 N ; S = 1.15 MPa
• Recall that tensile strength of enamel and dentin are– σf,dentin=35 MPa (worst case)– σf,enamel=7 MPa (distribution)
• From Mohr’s circle, max. principal stress =S• ->SF=3.5! (What is SF for 3mm diameter?)• -> Is the change to resin based fillings advisable? What
are the trade-offs?• -> We haven’t considered the hoop effect, is it likely to
make this worse?• -> If KIc=1 MPa*m1/2 , is fracture likely?
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Environment for implants
• Chewing force can be up to 900 N– Cyclic loading Large temperature differences (50 C)
• Large pH differences (saliva, foods)
• Large variety of chemical compositions from food
• Crevices (natural and artificial) likely sites for stress corrosion
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Structural Requirements
• Fatigue resistance
• Fracture resistance
• Wear resistance**
• Corrosion resistance**– While many dental fixtures are not “inside” the body,
the environment (loading, pH) is quite severe
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Titanium implants
• Titanium is the most successful implant/fixation material
• Good bone in-growth
• Stability
• Biocompatibility
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Titanium Implants
• Implanted into jawbone• Ti6Al4V is dominant implant• Surface treatments/ion
implantation improve fretting resistance
• “Osseointegration” was coined by Brånemark, a periodontic professor/surgeon
• First Ti integrating implants were dental (1962-1965)
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Titanium Biocompatibility
• Bioinert
• Low corrosion
• Osseointegration– Roughness, HA
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Fatigue
• Fatigue is a concern for human teeth (~1 million cycles annually, typical stresses of 5-20 MPa)
• The critical crack sizes for typical masticatory stresses (20 MPa) of the order of 1.9 meters.
• For the Total Life Approach, stresses (even after accounting for stress “concentrations”) well below the fatigue limit (~600 MPa)
• For the Defect Tolerant Approach, the Paris equation of da/dN (m/cycle) = 1x10-11(DK)3.9 used for lifetime prediction.
• Crack sizes at threshold are ~1.5 mm (detectable).
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Fatigue Properties of Ti6Al4V
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0.0001 0.0010 0.0100 0.1000IN ITIAL CRACK LENG TH (m )
PR
ED
ICTED
FATIG
UE L
IFETIM
E (cy
cles
)
0.01 0.10 1.00IN ITIAL CRACK LENG TH (inches)
0
1
10
100
1000
YEARS O
F USE
Ti-6A l-2Sn-4Zr-6M oM ax. S tress=20M Pa
0.1105
106
107
108
109
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Structural failures
• Stress (Corrosion) Cracking• Fretting (and corrosion)• Low wear resistance on surface• Loosening• Third Body Wear
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• Internal taper for easy “fitting”
• Careful design to avoid stress concentrations
• Smooth external finish on the healing cap and abutment
• Healing cap to assist in easy removal
Design Issues
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Surgical Process for Implantation
• Drill a hole with reamer appropriate to dimensions of the selected implant at location of extraction site
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• Place temporary abutment into implant
Temporary Abutment
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Insertion
• Insert implant
with temporary abutment attached into prepared socket
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Healing
• View of temporary abutment after the healing period (about 10 weeks)
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Temporary Abutment Removal
• Temporary abutment removal after healing period
• Implant is fully osseointegrated
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Healed tissue
• View of soft tissue before insertion of permanent abutment
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Permanent Crown Attached
• Abutment with all-ceramic crown integrated
• Adhesive is dental cement
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Permanent Abutment
• Insert permanent abutment with integrated crown into the well of the implant
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Completed implant
• View of completed implantation procedure
• Compare aesthetic results of all-ceramic submerged implant with adjacent protruding metal lining of non-submerged implant
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Post-op
• Post-operative radiograph with integrated abutment crown in vivo
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Clinical (service) Issues
• The space for the implant is small, dependent on patient anatomy/ pathology
• Fixation dependent on– Surface– Stress (atrophy)– Bone/implant geometry
• Simulation shows partial fixation due to design– (Atrophy below ~1.5 MPa)
Vallaincourt et al., Appl. Biomat. 6 (267-282) 1995
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Clinical Issues
• Stress is a function of diameter, or remaining bone in ridge
• Values for perfect bond
• Areas small
• Fretting
• Bending
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Clinical Issues
• Full dentures may use several implants– Bending of bridge, implants– Large moments– Fatigue!– Complex combined stress– FEA!
FBD
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Clinical Issues
Outstanding issues• Threads or not?
– More surface area, not universal
• Immediately loaded**• Drilling temperature: necrosis• Graded stiffness
– Material or geometry
• Outcomes: 80-95% success at 10-15 yrs.*– Many patient-specific and design-specific
problems
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Comparison with THR
Compare Contrast
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Comparison with THR
Compare
• Stress shielding
• Graded stiffness/ integration
• Small bone section about implant
• Modular Ti design
• Morbidity
Contrast• Small surface area• Acidic environment• Exposure to bacteria• Multiple implants• Variable anatomy• Complicated forces• Cortical/ trabecular• Optional