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