Dental Tissues and their Replacements

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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

• 96mineral, 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 nm 1000 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
Park and Lakes, Biomaterials, 1992 and Handbook
of Biomaterials, 1998
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Structural properties
Note remodeling is primarily strain driven
Park and Lakes, Biomaterials, 1992 and Handbook
of Biomaterials, 1998
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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 p (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 p/4D2
• F amalgam 52 N S F/Ashear2.1 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/Ashear2.1 MPa
• F resin 29 N S 1.15 MPa
• Recall that tensile strength of enamel and dentin
  are
• sf,dentin35 MPa (worst case)
• sf,enamel7 MPa (distribution)
• From Mohrs circle, max. principal stress S
• -SF3.5! (What is SF for 3mm diameter?)
• - Is the change to resin based fillings
  advisable? What are the trade-offs?
• - We havent considered the hoop effect, is it
  likely to make this worse?
• - If KIc1 MPam1/2 , 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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Structural failures

• Stress (Corrosion) Cracking
• Fretting (and corrosion)
• Low wear resistance on surface
• Loosening
• Third Body Wear

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Design Issues

• 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

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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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Temporary Abutment

• Place temporary abutment into implant

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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