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Dental Tissues and their Replacements
Issues <ul><li>Dental decay </li></ul><ul><li>Periodontal disease </li></ul><ul><li>Movement of teeth (orthodontics) </li>...
 
Marshall et al., J. Dentistry, 25,441, 1997.
Tissue Constituents <ul><li>Enamel -hardest substance in body-calcium phosphate salts-large apatite crystals </li></ul><ul...
ENAMEL <ul><li>96%mineral, 1% protein &lipid, remainder is water (weight %) </li></ul><ul><li>Minerals form Long crystals-...
DENTIN <ul><li>Type-I collagen fibrils and nanocrystalline apatite </li></ul><ul><li>Dentinal tubules from dentin-enamel a...
Structural properties Park and Lakes, Biomaterials, 1992 and Handbook of Biomaterials, 1998 8.3x10 -6 35-52 138 13.8 1.9 D...
Structural properties Park and Lakes, Biomaterials, 1992 and Handbook of Biomaterials, 1998 Note: remodeling is primarily ...
Dental Biomaterials Amalgams/Fillings Implants /Dental screws Adhesives/Cements Orthodontics
Materials used in dental applications <ul><li>Fillings: amalgams, acrylic resins </li></ul><ul><li>Titanium: Ti6Al4V domin...
Motivation to replace teeth <ul><li>Prevent loss in root support and chewing efficiency </li></ul><ul><li>Prevent bone res...
Amalgams/Fillings <ul><li>An amalgam is an alloy in which one component is mercury (Hg) </li></ul><ul><li>Hg is liquid at ...
Thermal expansion concerns <ul><li>Thermal expansion coefficient  </li></ul><ul><li>  = ∆L/(L o ∆T) </li></ul><ul><li>  ...
Volume Changes and Forces in Fillings <ul><li>Consider a 2mm diameter hole which is 4mm in length in a molar tooth, with t...
Volume Changes and Forces in Fillings (cont.) <ul><li>F  amalgam  = 52 N ; S = F/A shear =2.1 MPa </li></ul><ul><li>F  res...
Environment for implants <ul><li>Chewing force can be up to 900 N </li></ul><ul><ul><li>Cyclic loading Large temperature d...
Structural Requirements <ul><li>Fatigue resistance </li></ul><ul><li>Fracture resistance </li></ul><ul><li>Wear resistance...
Titanium implants <ul><li>Titanium is the most successful implant/fixation material </li></ul><ul><li>Good bone in-growth ...
Titanium Implants  <ul><li>Implanted into jawbone </li></ul><ul><li>Ti6Al4V is dominant implant </li></ul><ul><li>Surface ...
Titanium Biocompatibility <ul><li>Bioinert </li></ul><ul><li>Low corrosion </li></ul><ul><li>Osseointegration </li></ul><u...
Fatigue <ul><li>Fatigue is a concern for human teeth (~1 million  cycles annually, typical stresses of 5-20 MPa) </li></ul...
Fatigue Properties of Ti6Al4V
 
Structural failures <ul><li>Stress (Corrosion) Cracking </li></ul><ul><li>Fretting (and corrosion) </li></ul><ul><li>Low w...
 
Design Issues <ul><li>Internal taper for easy “fitting” </li></ul><ul><li>Careful design to avoid stress concentrations </...
Surgical Process for Implantation <ul><li>Drill a hole with reamer appropriate to dimensions of the selected implant at  l...
<ul><li>Place temporary abutment into implant </li></ul>Temporary Abutment                        
Insertion <ul><li>Insert implant  </li></ul><ul><li>with temporary abutment attached into prepared socket </li></ul>
Healing <ul><li>View of temporary abutment after the healing period (about 10 weeks) </li></ul>
Temporary Abutment Removal <ul><li>Temporary abutment removal after healing period </li></ul><ul><li>Implant is fully osse...
Healed tissue <ul><li>View of soft tissue before insertion of permanent abutment </li></ul>
Permanent Crown Attached <ul><li>Abutment with all-ceramic crown integrated </li></ul><ul><li>Adhesive is dental cement </...
Permanent Abutment <ul><li>Insert permanent abutment with integrated crown into the well of the implant </li></ul>
Completed implant <ul><li>View of  completed implantation procedure </li></ul><ul><li>Compare aesthetic results of all-cer...
Post-op <ul><li>Post-operative radiograph with integrated abutment crown  in vivo </li></ul>
Clinical (service) Issues <ul><li>The space for the implant is small, dependent on patient anatomy/ pathology </li></ul><u...
Clinical Issues <ul><li>Stress is a function of diameter, or remaining bone in ridge </li></ul><ul><li>Values for perfect ...
Clinical Issues <ul><li>Full dentures may use several implants </li></ul><ul><ul><li>Bending of bridge, implants </li></ul...
Clinical Issues <ul><li>Outstanding issues </li></ul><ul><li>Threads or not? </li></ul><ul><ul><li>More surface area, not ...
Comparison with THR <ul><li>Compare </li></ul>Contrast
Comparison with THR <ul><li>Compare </li></ul><ul><li>Stress shielding </li></ul><ul><li>Graded stiffness/ integration </l...
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  1. 1. Dental Tissues and their Replacements
  2. 2. Issues <ul><li>Dental decay </li></ul><ul><li>Periodontal disease </li></ul><ul><li>Movement of teeth (orthodontics) </li></ul><ul><li>Restorative treatments </li></ul><ul><li>Thermal expansion issues related to fillings </li></ul><ul><li>Fatigue and fracture of teeth and implants </li></ul>
  3. 4. Marshall et al., J. Dentistry, 25,441, 1997.
  4. 5. Tissue Constituents <ul><li>Enamel -hardest substance in body-calcium phosphate salts-large apatite crystals </li></ul><ul><li>Dentin -composed largely of type-I collagen fibrils and nanocrystalline apatite mineral-similar to bone </li></ul><ul><li>Dentinal tubules -radiate from pulp </li></ul><ul><li>Pulp -richly vascularized connnective tissue </li></ul><ul><li>Cementum -coarsely fibrillated bonelike substance devoid of canaliculi </li></ul><ul><li>Periodontal Membrane -anchors the root into alveolar bone </li></ul>
  5. 6. ENAMEL <ul><li>96%mineral, 1% protein &lipid, remainder is water (weight %) </li></ul><ul><li>Minerals form Long crystals-hexagonal shape </li></ul><ul><li>Flourine- renders enamel much less soluble and increases hardness </li></ul><ul><li>HA= Ca 10 (PO 4 ) 6 (OH) 2 </li></ul>40 nm 1000 nm in length
  6. 7. DENTIN <ul><li>Type-I collagen fibrils and nanocrystalline apatite </li></ul><ul><li>Dentinal tubules from dentin-enamel and cementum-enamel junctions to pulp </li></ul><ul><li>Channels are paths for odontoblasts (dentin-forming cells) during the process of dentin formation </li></ul><ul><li>Mineralized collagen fibrils (50-100 nm in diameter) are arranged orthogonal to the tubules </li></ul><ul><li>Inter-tubular dentin matrix with nanocrystalline hydroxyapatite mineral- planar structure </li></ul><ul><li>Highly oriented microstructure causes anisotropy </li></ul><ul><li>Hollow tubules responsible for high toughness </li></ul>
  7. 8. Structural properties Park and Lakes, Biomaterials, 1992 and Handbook of Biomaterials, 1998 8.3x10 -6 35-52 138 13.8 1.9 Dentin 11.4x10 -6 10 (ish) 241 48 2.2 Enamel Thermal Expansion Coefficient (1/C) Tensile Stren. (MPa) Comp Stren. (MPa) E (GPa) Density (g/cm 3 ) Tissue
  8. 9. Structural properties Park and Lakes, Biomaterials, 1992 and Handbook of Biomaterials, 1998 Note: remodeling is primarily strain driven 1.5-7.4 23-450 MPa Trabec. Bone (various) 133 (long.) 205 (long.) 10-20 GPa 1.9 (wet) Cortical Bone Tensile Stren. (MPa) Comp Stren. (MPa) E Density (g/cm 3 ) Tissue
  9. 10. Dental Biomaterials Amalgams/Fillings Implants /Dental screws Adhesives/Cements Orthodontics
  10. 11. Materials used in dental applications <ul><li>Fillings: amalgams, acrylic resins </li></ul><ul><li>Titanium: Ti6Al4V dominates in root implants and fracture fixation </li></ul><ul><li>Teeth: Porcelain, resins, ceramics </li></ul><ul><li>Braces: Stainless steel, Nitinol </li></ul><ul><li>Cements/resins: acrylate based polymers </li></ul><ul><li>Bridges: Resin, composite, metal (Au, CoCr) </li></ul>
  11. 12. Motivation to replace teeth <ul><li>Prevent loss in root support and chewing efficiency </li></ul><ul><li>Prevent bone resorption </li></ul><ul><li>Maintain healthy teeth </li></ul><ul><li>Cosmetic </li></ul>
  12. 13. Amalgams/Fillings <ul><li>An amalgam is an alloy in which one component is mercury (Hg) </li></ul><ul><li>Hg is liquid at RT- reacts with silver and tin- forms plastic mass that sets with time </li></ul><ul><ul><li>Takes 24 hours for full set (30 min for initial set). </li></ul></ul>
  13. 14. Thermal expansion concerns <ul><li>Thermal expansion coefficient </li></ul><ul><li> = ∆L/(L o ∆T) </li></ul><ul><li> =  ∆T </li></ul><ul><li>Volumetric Thermal expansion coefficient </li></ul><ul><li>V= 3  </li></ul>
  14. 15. Volume Changes and Forces in Fillings <ul><li>Consider a 2mm diameter hole which is 4mm in length in a molar tooth, with thermal variation of ∆T = 50C </li></ul><ul><li> amalgam = 25x10 -6 /C  resin = 81x10 -6 /C  enamel = 8.3 x10 -6 /C </li></ul><ul><li>E amalgam = 20 GPa E resin = 2.5 GPa </li></ul><ul><li>∆ V = V o x 3  x ∆T </li></ul><ul><li>∆ V amalgam = π (1mm) 2 x 4mm x 3 (25-8.3) x10 -6 x 50 </li></ul><ul><li>= 0.03 mm 3 </li></ul><ul><li>∆ V resin = 0.14 mm 3 </li></ul><ul><li>(1-d) F = E x ∆  x A filling </li></ul><ul><li>F = E (∆T ) ∆(  amalgam/resin -  enamel ) x π/4D 2 </li></ul><ul><li>F amalgam = 52 N ; S = F/A shear =2.1 MPa </li></ul><ul><li>F resin = 29 N ; S = 1.15 MPa </li></ul><ul><li>Although the resin “expands” 4x greater than the amalgam, the reduced stiffness (modulus) results in a lower force </li></ul>
  15. 16. Volume Changes and Forces in Fillings (cont.) <ul><li>F amalgam = 52 N ; S = F/A shear =2.1 MPa </li></ul><ul><li>F resin = 29 N ; S = 1.15 MPa </li></ul><ul><li>Recall that tensile strength of enamel and dentin are </li></ul><ul><ul><li>σ f,dentin =35 MPa (worst case) </li></ul></ul><ul><ul><li>σ f,enamel =7 MPa (distribution) </li></ul></ul><ul><li>From Mohr’s circle, max. principal stress =S </li></ul><ul><li>->SF=3.5! (What is SF for 3mm diameter?) </li></ul><ul><li>-> Is the change to resin based fillings advisable? What are the trade-offs? </li></ul><ul><li>-> We haven’t considered the hoop effect, is it likely to make this worse? </li></ul><ul><li>-> If K Ic =1 MPa*m 1/2 , is fracture likely? </li></ul>
  16. 17. Environment for implants <ul><li>Chewing force can be up to 900 N </li></ul><ul><ul><li>Cyclic loading Large temperature differences (50 C) </li></ul></ul><ul><li>Large pH differences (saliva, foods) </li></ul><ul><li>Large variety of chemical compositions from food </li></ul><ul><li>Crevices (natural and artificial) likely sites for stress corrosion </li></ul>
  17. 18. Structural Requirements <ul><li>Fatigue resistance </li></ul><ul><li>Fracture resistance </li></ul><ul><li>Wear resistance** </li></ul><ul><li>Corrosion resistance** </li></ul><ul><ul><li>While many dental fixtures are not “inside” the body, the environment (loading, pH) is quite severe </li></ul></ul>
  18. 19. Titanium implants <ul><li>Titanium is the most successful implant/fixation material </li></ul><ul><li>Good bone in-growth </li></ul><ul><li>Stability </li></ul><ul><li>Biocompatibility </li></ul>
  19. 20. Titanium Implants <ul><li>Implanted into jawbone </li></ul><ul><li>Ti6Al4V is dominant implant </li></ul><ul><li>Surface treatments/ion implantation improve fretting resistance </li></ul><ul><li>“ Osseointegration” was coined by Br å nemark, a periodontic professor/surgeon </li></ul><ul><li>First Ti integrating implants were dental (1962-1965) </li></ul>
  20. 21. Titanium Biocompatibility <ul><li>Bioinert </li></ul><ul><li>Low corrosion </li></ul><ul><li>Osseointegration </li></ul><ul><ul><li>Roughness, HA </li></ul></ul>
  21. 22. Fatigue <ul><li>Fatigue is a concern for human teeth (~1 million cycles annually, typical stresses of 5-20 MPa) </li></ul><ul><li>The critical crack sizes for typical masticatory stresses (20 MPa) of the order of 1.9 meters . </li></ul><ul><li>For the Total Life Approach , stresses (even after accounting for stress “concentrations”) well below the fatigue limit (~600 MPa) </li></ul><ul><li>For the Defect Tolerant Approach , the Paris equation of d a /d N (m/cycle) = 1x10 -11 (D K ) 3.9 used for lifetime prediction. </li></ul><ul><li>Crack sizes at threshold are ~1.5 mm (detectable). </li></ul>
  22. 23. Fatigue Properties of Ti6Al4V
  23. 25. Structural failures <ul><li>Stress (Corrosion) Cracking </li></ul><ul><li>Fretting (and corrosion) </li></ul><ul><li>Low wear resistance on surface </li></ul><ul><li>Loosening </li></ul><ul><li>Third Body Wear </li></ul>
  24. 27. Design Issues <ul><li>Internal taper for easy “fitting” </li></ul><ul><li>Careful design to avoid stress concentrations </li></ul><ul><li>Smooth external finish on the healing cap and abutment </li></ul><ul><li>Healing cap to assist in easy removal </li></ul>
  25. 28. Surgical Process for Implantation <ul><li>Drill a hole with reamer appropriate to dimensions of the selected implant at location of extraction site </li></ul>
  26. 29. <ul><li>Place temporary abutment into implant </li></ul>Temporary Abutment                        
  27. 30. Insertion <ul><li>Insert implant </li></ul><ul><li>with temporary abutment attached into prepared socket </li></ul>
  28. 31. Healing <ul><li>View of temporary abutment after the healing period (about 10 weeks) </li></ul>
  29. 32. Temporary Abutment Removal <ul><li>Temporary abutment removal after healing period </li></ul><ul><li>Implant is fully osseointegrated </li></ul>
  30. 33. Healed tissue <ul><li>View of soft tissue before insertion of permanent abutment </li></ul>
  31. 34. Permanent Crown Attached <ul><li>Abutment with all-ceramic crown integrated </li></ul><ul><li>Adhesive is dental cement </li></ul>
  32. 35. Permanent Abutment <ul><li>Insert permanent abutment with integrated crown into the well of the implant </li></ul>
  33. 36. Completed implant <ul><li>View of completed implantation procedure </li></ul><ul><li>Compare aesthetic results of all-ceramic submerged implant with adjacent protruding metal lining of non-submerged implant </li></ul>
  34. 37. Post-op <ul><li>Post-operative radiograph with integrated abutment crown in vivo </li></ul>
  35. 38. Clinical (service) Issues <ul><li>The space for the implant is small, dependent on patient anatomy/ pathology </li></ul><ul><li>Fixation dependent on </li></ul><ul><ul><li>Surface </li></ul></ul><ul><ul><li>Stress (atrophy) </li></ul></ul><ul><ul><li>Bone/implant geometry </li></ul></ul><ul><li>Simulation shows partial fixation due to design </li></ul><ul><ul><li>(Atrophy below ~1.5 MPa) </li></ul></ul>Vallaincourt et al., Appl. Biomat . 6 (267-282) 1995
  36. 39. Clinical Issues <ul><li>Stress is a function of diameter, or remaining bone in ridge </li></ul><ul><li>Values for perfect bond </li></ul><ul><li>Areas small </li></ul><ul><li>Fretting </li></ul><ul><li>Bending </li></ul>
  37. 40. Clinical Issues <ul><li>Full dentures may use several implants </li></ul><ul><ul><li>Bending of bridge, implants </li></ul></ul><ul><ul><li>Large moments </li></ul></ul><ul><ul><li>Fatigue! </li></ul></ul><ul><ul><li>Complex combined stress </li></ul></ul><ul><ul><li>FEA! </li></ul></ul>FBD
  38. 41. Clinical Issues <ul><li>Outstanding issues </li></ul><ul><li>Threads or not? </li></ul><ul><ul><li>More surface area, not universal </li></ul></ul><ul><li>Immediately loaded** </li></ul><ul><li>Drilling temperature: necrosis </li></ul><ul><li>Graded stiffness </li></ul><ul><ul><li>Material or geometry </li></ul></ul><ul><li>Outcomes: 80-95% success at 10-15 yrs.* </li></ul><ul><ul><li>Many patient-specific and design-specific problems </li></ul></ul>
  39. 42. Comparison with THR <ul><li>Compare </li></ul>Contrast
  40. 43. Comparison with THR <ul><li>Compare </li></ul><ul><li>Stress shielding </li></ul><ul><li>Graded stiffness/ integration </li></ul><ul><li>Small bone section about implant </li></ul><ul><li>Modular Ti design </li></ul><ul><li>Morbidity </li></ul><ul><li>Contrast </li></ul><ul><li>Small surface area </li></ul><ul><li>Acidic environment </li></ul><ul><li>Exposure to bacteria </li></ul><ul><li>Multiple implants </li></ul><ul><li>Variable anatomy </li></ul><ul><li>Complicated forces </li></ul><ul><li>Cortical/ trabecular </li></ul><ul><li>Optional </li></ul>

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