Transcript
Dental Tissues and their Replacements
Issues
• Dental decay• Periodontal disease• Movement of teeth
(orthodontics)• Restorative treatments• Thermal expansion
issues related to fillings• Fatigue and fracture of
teeth and implants
Marshall et al., J. Dentistry, 25,441, 1997.
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
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
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
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
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
Dental Biomaterials
Amalgams/Fillings
Implants /Dental screws
Adhesives/Cements
Orthodontics
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)
Motivation to replace teeth
• Prevent loss in root support and chewing efficiency
• Prevent bone resorption
• Maintain healthy teeth
• Cosmetic
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).
Thermal expansion concerns
• Thermal expansion coefficient
= ∆L/(Lo∆T)
= ∆T
• Volumetric Thermal expansion coefficient
V= 3
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
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?
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
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
Titanium implants
• Titanium is the most successful implant/fixation material
• Good bone in-growth
• Stability
• Biocompatibility
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)
Titanium Biocompatibility
• Bioinert
• Low corrosion
• Osseointegration– Roughness, HA
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).
Fatigue Properties of Ti6Al4V
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
Structural failures
• Stress (Corrosion) Cracking• Fretting (and corrosion)• Low wear resistance on surface• Loosening• Third Body Wear
• 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
Surgical Process for Implantation
• Drill a hole with reamer appropriate to dimensions of the selected implant at location of extraction site
• Place temporary abutment into implant
Temporary Abutment
Insertion
• Insert implant
with temporary abutment attached into prepared socket
Healing
• View of temporary abutment after the healing period (about 10 weeks)
Temporary Abutment Removal
• Temporary abutment removal after healing period
• Implant is fully osseointegrated
Healed tissue
• View of soft tissue before insertion of permanent abutment
Permanent Crown Attached
• Abutment with all-ceramic crown integrated
• Adhesive is dental cement
Permanent Abutment
• Insert permanent abutment with integrated crown into the well of the implant
Completed implant
• View of completed implantation procedure
• Compare aesthetic results of all-ceramic submerged implant with adjacent protruding metal lining of non-submerged implant
Post-op
• Post-operative radiograph with integrated abutment crown in vivo
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
Clinical Issues
• Stress is a function of diameter, or remaining bone in ridge
• Values for perfect bond
• Areas small
• Fretting
• Bending
Clinical Issues
• Full dentures may use several implants– Bending of bridge, implants– Large moments– Fatigue!– Complex combined stress– FEA!
FBD
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
Comparison with THR
Compare Contrast
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
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