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  • Dental Tissues and their Replacements

    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 corrosionTi 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 wearMechanism 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 corrosionIf 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.

  • IssuesDental decayPeriodontal diseaseMovement of teeth (orthodontics)Restorative treatmentsThermal expansion issues related to fillingsFatigue and fracture of teeth and implants

  • Marshall et al., J. Dentistry, 25,441, 1997.

  • Tissue ConstituentsEnamel-hardest substance in body-calcium phosphate salts-large apatite crystalsDentin-composed largely of type-I collagen fibrils and nanocrystalline apatite mineral-similar to boneDentinal tubules-radiate from pulpPulp-richly vascularized connnective tissueCementum-coarsely fibrillated bonelike substance devoid of canaliculiPeriodontal Membrane-anchors the root into alveolar bone

  • ENAMEL96%mineral, 1% protein &lipid, remainder is water (weight %)Minerals form Long crystals-hexagonal shapeFlourine- renders enamel much less soluble and increases hardnessHA= Ca10(PO4)6(OH)2

    40 nm1000 nm in length

  • DENTINType-I collagen fibrils and nanocrystalline apatiteDentinal tubules from dentin-enamel and cementum-enamel junctions to pulp Channels are paths for odontoblasts (dentin-forming cells) during the process of dentin formationMineralized collagen fibrils (50-100 nm in diameter) are arranged orthogonal to the tubulesInter-tubular dentin matrix with nanocrystalline hydroxyapatite mineral- planar structure Highly oriented microstructure causes anisotropy

  • Structural properties

    *Park and Lakes, Biomaterials, 1992.

    TissueDensity(g/cm3)E(GPa)Comp Strength (MPa)Thermal Expansion Coefficient (1/C)Enamel2.24824111.4x10-6Dentin1.913.81388.3x10-6

  • Dental BiomaterialsAmalgams/FillingsImplants /Dental screwsAdhesives/CementsOrthodontics

  • Materials used in dental applications

    Fillings: amalgams, acrylic resinsTitanium: Ti6Al4V dominates in root implants and fracture fixationTeeth:Porcelain, resins, ceramicsBraces:Stainless steel, NitinolCements/resins: acrylate based polymers

  • Motivation to replace teethPrevent loss in root support and chewing efficiencyPrevent bone resorptionMaintain healthy teeth Cosmetic

  • Amalgams/FillingsAn 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 timeNi-Ti, gold, acrylic resins

  • Thermal expansion concernsThermal expansion coefficient

    = L/(LoT) = T Volumetric Thermal expansion coefficient

    V= 3

  • Volume Changes and Forces in FillingsConsider a 2mm diameter hole which is 4mm in length in a molar tooth, with thermal variation of T = 50Camalgam= 25x10-6/C resin= 81x10-6 /C enamel = 8.3 x10-6 /CE amalgam = 20 GPa E resin = 2.5 GPaV = Vo x 3 x T Vamalgam= (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 A

    = E (T ) (amalgam/resin - enamel ) x DhF amalgam = 420 NF resin = 228 NAlthough the resin expands 4x greater than the amalgam, the reduced stiffness (modulus) results in a lower force

  • Environment for implantsChewing force can be up to 900 NCyclic loadingLarge temperature differences (50 C)Large pH differences (saliva, foods)Large variety of chemical compositions from food

  • Structural RequirementsFatigue resistanceFracture resistanceWear resistanceCorrosion resistance

  • Titanium implantsTitanium is the most successful implant/fixation materialGood bone in-growthStabilityBiocompatibility

  • Titanium Implants Implanted into jawboneTi6Al4V is dominant implantSurface treatments/ion implantation improve fretting resistance

  • Titanium BiocompatibilityBioinertLow corrosionOsseointegration

  • FatigueFatigue 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. Critical crack sizes at threshold are ~1.5 mm (detectable).

  • Fatigue Properties of Ti6Al4V

    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.

  • Structural failuresStress CrackingFrettingLow wear resistance on surfaceLooseningThird Body Wear

    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.

  • Design Issues Internal taper for easy fittingCareful design to avoid stress concentrationsSmooth external finish on the healing cap and abutmentHealing cap to assist in easy removal

    157.psd

    158.psd

    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.

  • Surgical Process for ImplantationDrill a hole with reamer appropriate to dimensions of the selected implant at location of extraction site

    You then remove the temporary abutment from the implant by applying a pulling force with a simultaneous torque.

  • Temporary AbutmentPlace temporary abutment into implant

    159.psd

    160.psd

    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.

  • InsertionInsert implant

    with temporary abutment attached into prepared socket

    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.

  • HealingView of temporary abutment after the healing period (about 10 weeks)

    In the meanwhile, an all-ceramic crown is cooked and shaped onto the permanent abutment and the final result looks like this.

  • Temporary Abutment RemovalTemporary abutment removal after healing periodImplant is fully osseointegrated

    Finally, you insert and tap the permanent implant stem into the well of the implant and

  • Healed tissueView of soft tissue before insertion of permanent abutment

    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 wont with any of our implants because the crown margin is below the gumline.

  • Permanent Crown Attached

    Abutment with all-ceramic crown integratedAdhesive is dental cement

    And this is just a radiograph of our implant happily in vivo.

  • Permanent AbutmentInsert permanent abutment with integrated crown into the well of the implant

  • Completed implantView of completed implantation procedureCompare aesthetic results of all-ceramic submerged implant with adjacent protruding metal lining of non-submerged implant

  • Post-opPost-operative radiograph with integrated abutment crown in vivo

  • Regulatory IssuesClass IIIRequires PMA or 510KRequirements for PMAOverall device specificationManufacturing methodsSterilizationMechanical testingBiocompatibility Clinical Studies

    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 corrosionTi 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 wearMechanism 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 beca