Dental tissues and their replacements/ oral surgery courses
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Dental tissues and their replacements/ oral surgery courses
- 1. Dental Tissues and their Replacements INDIAN DENTAL ACADEMY Leader in continuing Dental Education
- 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
- 4. Marshall et al., J. Dentistry, 25,441, 1997.
- 5. 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
- 6. 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
- 7. 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
- 8. 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
- 9. 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
- 10. Dental Biomaterials Amalgams/Fillings Implants /Dental screws Adhesives/Cements Orthodontics
- 11. 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)
- 12. Motivation to replace teeth • Prevent loss in root support and chewing efficiency • Prevent bone resorption • Maintain healthy teeth • Cosmetic
- 13. 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).
- 14. Thermal expansion concerns • Thermal expansion coefficient α = ∆L/(Lo∆T) ε = α ∆T • Volumetric Thermal expansion coefficient V= 3α
- 15. 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,
- 16. 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?
- 17. 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
- 18. 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
- 19. Titanium implants • Titanium is the most successful implant/fixation material • Good bone in-growth • Stability • Biocompatibility
- 20. 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)
- 21. Titanium Biocompatibility • Bioinert • Low corrosion • Osseointegration – Roughness, HA
- 22. 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).
- 23. Fatigue Properties of Ti6Al4V
- 24. 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
- 25. Structural failures • Stress (Corrosion) Cracking • Fretting (and corrosion) • Low wear resistance on surface • Loosening • Third Body Wear
- 27. • 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
- 28. Surgical Process for Implantation • Drill a hole with reamer appropriate to dimensions of the selected implant at location of extraction site
- 29. • Place temporary abutment into implant Temporary Abutment
- 30. Insertion • Insert implant with temporary abutment attached into prepared socket
- 31. Healing • View of temporary abutment after the healing period (about 10 weeks)
- 32. Temporary Abutment Removal • Temporary abutment removal after healing period • Implant is fully osseointegrated
- 33. Healed tissue • View of soft tissue before insertion of permanent abutment
- 34. Permanent Crown Attached • Abutment with all-ceramic crown integrated • Adhesive is dental cement
- 35. Permanent Abutment • Insert permanent abutment with integrated crown into the well of the implant
- 36. Completed implant • View of completed implantation procedure • Compare aesthetic results of all-ceramic submerged implant with adjacent protruding metal lining of non- submerged implant
- 37. Post-op • Post-operative radiograph with integrated abutment crown in vivo
- 38. 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
- 39. Clinical Issues • Stress is a function of diameter, or remaining bone in ridge • Values for perfect bond • Areas small • Fretting • Bending
- 40. Clinical Issues • Full dentures may use several implants – Bending of bridge, implants – Large moments – Fatigue! – Complex combined stress – FEA! FBD
- 41. 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
- 42. Comparison with THR Compare Contrast
- 43. 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
Editor's Notes
- Must be able to resist deformation and disperse loads
- Fracture Toughness: high ductility and low modulus allowing for crack blunting. Ti-6Al-4V has a fracture toughness of roughly 50MPam (11). This high fracture toughness is excellent for dental applications because of the cyclic loading in the mouth. Mean loading rate is 100 N/s for a mean masticatory frequency of 70 chews per minute (6). The elastic match at bone/implant interface is important for physiological stress transfer. Ti has an elastic modulus of roughly 27 GPa and bone has an elastic modulus of about 15.2 GPa Fatigue: high resistance to fatigue crack nucleation because of optimized manufacturing processes A high density of alpha grains resists crack nucleation, thus, the size of primary alpha can be decreased as far as possible, while also decreasing the beta grain size (BCC) (7). This high density of grains and small lamellae will prevent cracks from propagating easily through the material because of the obstacles of grain boundaries. It is important to lower Oxygen content to prevent the alpha case; stiff, brittle case which has high fatigue sensitivity. Cycles to failure: the loads are at maximum 900 N, which is much lower than the fatigue limit of 35280 N (assuming 400 mm2 surface area) for 0.085% weight oxygen (Ti-6Al-4V has a max of 0.13% oxygen (7)). As seen from the figure, the Ti-6Al-4V lasts for 107 cycles at loads of about 2.5e5 N which is 278 times larger than the maximum load in the mouth
- Corrosion: Not very susceptible to stress crack corrosion because the mouth is not a location for strong acids. Ionic strength data for K1scc in Ti was shown for harsh conditions such as ferric acid or 1M HCl which are usually not in the mouth. Very low ionic strength as in the mouth showed little effect on Ti. But susceptible to Fretting Corrosion which is mechanically assisted corrosion Ti is known to exhibit material transfer and adhesive wear from the asperities on the opposing contact surfaces, leading to a higher coefficient of friction. aggravated by the alpha cast which is susceptible to wear Mechanism by which cracks nucleate. The asperities make break off leading fretting debris, continued corrosion of the fresh surface, and accelerate the crack nucleation stage of fatigue. Wear: poor wear resistance because of the oxygen cast (lowered resistance to crack growth); fretting corrosion If the screw is poorly fitted and there is micromotion, causing rubbing against adjacent bone, much debris of vanadium and aluminum in surrounding tissues is present (11). Improvement to wear resistance has been attempted by surface treatments, through the introduction of nitrogen to surface layers or ion implantation.
- The patient should have come in with a new extraction site where a reamer is used to drill a hole into the subgingival bone that will accommodate the implant.
- The first thing that a dentist does is remove all the sterilization-maintaining containers and wrappings and pick out the temporary abutment and implant parts, and lock them together by simply tapping the abutment into the implant socket.
- Next you insert the implant with the temporary abutment attached into the hole prepared by the reamer. The implant is made to stay in its place by sutures that close up the flaps of the gums around the temporary abutment. The patient is then sent home for the soft tissue and bone tissue to heal and integrate the implant system.
- After about 10 weeks, the patient comes in for another appointment and this is what things looks like after the healing period where you see the temporary abutment protruding above the gumline.
- You then remove the temporary abutment from the implant by applying a pulling force with a simultaneous torque.
- You then clean out the well of the implant socket and this is what it looks like from below, and notice the round groove of the soft tissue shaped by the head of the temporary abutment.
- In the meanwhile, an all-ceramic crown is cooked and shaped onto the permanent abutment and the final result looks like this.
- Finally, you insert and tap the permanent implant stem into the well of the implant and…
- Presto, this is the final product. You can compare the aesthetic advantage of our system with a nonsubmerged screw system next to our implant. You can see metal at the gumline, whereas you won’t with any of our implants because the crown margin is below the gumline.
- And this is just a radiograph of our implant happily in vivo.