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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
Structural properties
Tissue Density(g/cm3)
E(GPa)
Comp Strength (MPa)
Thermal Expansion Coefficient (1/C)
Enamel 2.2 48 241 11.4x10-6
Dentin 1.9 13.8 138 8.3x10-6
*Park and Lakes, Biomaterials, 1992.
Dental Biomaterials
Amalgams/FillingsImplants /Dental screws
Adhesives/CementsOrthodontics
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
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
• Ni-Ti, gold, acrylic resins
Thermal expansion concerns
• Thermal expansion coefficient = ∆L/(Lo∆T)
= ∆T • Volumetric Thermal expansion coefficientV= 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 A = E (∆T ) ∆(amalgam/resin - enamel ) x πDh• F amalgam = 420 N• F resin = 228 N• Although the resin expands 4x greater than the amalgam, the reduced
stiffness (modulus) results in a lower force
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
Structural Requirements
• Fatigue resistance• Fracture resistance• Wear resistance• Corrosion resistance
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
Titanium Biocompatibility
• Bioinert• Low corrosion• Osseointegration
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.
• Critical crack sizes at threshold are ~1.5 mm (detectable).
Fatigue Properties of Ti6Al4V
Structural failures
• Stress Cracking• Fretting• 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
Regulatory Issues
• Class III– Requires PMA or 510K
• Requirements for PMA– Overall device specification– Manufacturing methods– Sterilization– Mechanical testing– Biocompatibility – Clinical Studies