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