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Dental tissues and their replacements/ oral surgery courses  

The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and
offering a wide range of dental certified courses in different formats.for more details please visit 
www.indiandentalacademy.com

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Dental tissues and their replacements/ oral surgery courses  

  1. 1. Dental Tissues and their Replacements INDIAN DENTAL ACADEMY Leader in continuing Dental Education
  2. 2. Issues • Dental decay • Periodontal disease • Movement of teeth (orthodontics) • Restorative treatments • Thermal expansion issues related to fillings • Fatigue and fracture of teeth and implants
  3. 3. Marshall et al., J. Dentistry, 25,441, 1997.
  4. 4. 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
  5. 5. 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 nm 1000 nm in length
  6. 6. 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
  7. 7. 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
  8. 8. Structural properties Tissue Density (g/cm3 ) E Comp Stren. (MPa) Tensile Stren. (MPa) Cortical Bone 1.9 (wet) 10-20 GPa 205 (long.) 133 (long.) Trabec. Bone (various) 23-450 MPa 1.5-7.4 Park and Lakes, Biomaterials, 1992 and Handbook of Biomaterials, 1998 Note: remodeling is primarily strain driven
  9. 9. Dental Biomaterials Amalgams/Fillings Implants /Dental screws Adhesives/Cements Orthodontics
  10. 10. 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)
  11. 11. Motivation to replace teeth • Prevent loss in root support and chewing efficiency • Prevent bone resorption • Maintain healthy teeth • Cosmetic
  12. 12. 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).
  13. 13. Thermal expansion concerns • Thermal expansion coefficient α = ∆L/(Lo∆T) ε = α ∆T • Volumetric Thermal expansion coefficient V= 3α
  14. 14. 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 ∀ αamalgam= 25x10-6 /C αresin= 81x10-6 /C αenamel = 8.3 x10-6 /C • E amalgam = 20 GPa E resin = 2.5 GPa • ∆V = Vo x 3α 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 ∆ε x Afilling F = E (∆T ) ∆(αamalgam/resin -αenamel ) 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,
  15. 15. 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?
  16. 16. 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
  17. 17. 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
  18. 18. Titanium implants • Titanium is the most successful implant/fixation material • Good bone in-growth • Stability • Biocompatibility
  19. 19. 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)
  20. 20. Titanium Biocompatibility • Bioinert • Low corrosion • Osseointegration – Roughness, HA
  21. 21. 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).
  22. 22. Fatigue Properties of Ti6Al4V
  23. 23. 0 . 0 0 0 1 0 . 0 0 1 0 0 . 0 1 0 0 0 . 1 0 0 0 I N I T I A L C R A C K L E N G T H ( m ) PREDICTEDFATIGUELIFETIME(cycles) 0 . 0 1 0 . 1 0 1 . 0 0 I N I T I A L C R A C K L E N G T H ( i n c h e s ) 0 1 1 0 1 0 0 1 0 0 0 YEARSOFUSE T i - 6 A l - 2 S n - 4 Z r - 6 M o M a x . S t r e s s = 2 0 M P a 0.110 5 10 6 10 7 10 8 10 9
  24. 24. Structural failures • Stress (Corrosion) Cracking • Fretting (and corrosion) • Low wear resistance on surface • Loosening • Third Body Wear
  25. 25. • 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
  26. 26. Surgical Process for Implantation • Drill a hole with reamer appropriate to dimensions of the selected implant at location of extraction site
  27. 27. • Place temporary abutment into implant Temporary Abutment
  28. 28. Insertion • Insert implant with temporary abutment attached into prepared socket
  29. 29. Healing • View of temporary abutment after the healing period (about 10 weeks)
  30. 30. Temporary Abutment Removal • Temporary abutment removal after healing period • Implant is fully osseointegrated
  31. 31. Healed tissue • View of soft tissue before insertion of permanent abutment
  32. 32. Permanent Crown Attached • Abutment with all-ceramic crown integrated • Adhesive is dental cement
  33. 33. Permanent Abutment • Insert permanent abutment with integrated crown into the well of the implant
  34. 34. Completed implant • View of completed implantation procedure • Compare aesthetic results of all-ceramic submerged implant with adjacent protruding metal lining of non- submerged implant
  35. 35. Post-op • Post-operative radiograph with integrated abutment crown in vivo
  36. 36. 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 Vallaincourt et al., Appl. Biomat. 6 (267-282) 1995
  37. 37. Clinical Issues • Stress is a function of diameter, or remaining bone in ridge • Values for perfect bond • Areas small • Fretting • Bending
  38. 38. Clinical Issues • Full dentures may use several implants – Bending of bridge, implants – Large moments – Fatigue! – Complex combined stress – FEA! FBD
  39. 39. 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
  40. 40. Comparison with THR Compare Contrast
  41. 41. 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

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