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040507.ppt - PowerPoint Presentation by liwenting

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									Dental Tissues and their
    Replacements
                         Issues

• Dental decay
• Periodontal disease
• Movement of teeth
  (orthodontics)
• Restorative treatments
• Thermal expansion
  issues related to fillings
• Fatigue and fracture of
  teeth and implants
Marshall et al., J. Dentistry, 25,441, 1997.
             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
                  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
                     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
                     Structural properties

Tissue          Density         E     Comp Tensile                       Thermal
                (g/cm3)         (GPa) Stren. Stren.                      Expansion
                                      (MPa) (MPa)                        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
                     Structural properties

Tissue               Density          E                Comp              Tensile
                     (g/cm3)                           Stren.            Stren.
                                                       (MPa)             (MPa)

Cortical             1.9 (wet) 10-20                   205               133
Bone                           GPa                     (long.)           (long.)


Trabec.                               23-450           1.5-7.4
Bone                                  MPa
(various)
Note: remodeling is primarily strain driven
 Park and Lakes, Biomaterials, 1992 and Handbook of Biomaterials, 1998
 Dental Biomaterials




   Amalgams/Fillings
Implants /Dental screws
  Adhesives/Cements
     Orthodontics
          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)
         Motivation to replace teeth

• Prevent loss in root support and chewing
  efficiency
• Prevent bone resorption
• Maintain healthy teeth
• Cosmetic
                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).
      Thermal expansion concerns

• Thermal expansion coefficient
 = ∆L/(Lo∆T)
 =  ∆T
• Volumetric Thermal expansion coefficient
V= 3
         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= 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 ∆ x Afilling
           F = E (∆T ) ∆(amalgam/resin - enamel ) 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,
  the reduced stiffness (modulus) results in a lower force
         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?
           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
          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
            Titanium implants

• Titanium is the most successful
  implant/fixation material
• Good bone in-growth
• Stability
• Biocompatibility
              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)
         Titanium Biocompatibility

• Bioinert
• Low corrosion
• Osseointegration
  – Roughness, HA
                      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).
Fatigue Properties of Ti6Al4V
                                                           INITIAL CRACK LENGTH (inches)
                                           9
                                                        0.01               0.10             1.00
                                      10                                                               1000
                                                                                  Ti-6Al-2Sn-4Zr-6Mo
                                                                                  Max. Stress=20MPa
PREDICTED FATIGUE LIFETIME (cycles)




                                           8
                                      10                                                               100




                                                                                                             YEARS OF USE
                                           7
                                      10                                                               10




                                           6
                                      10                                                               1




                                       5
                                      10                                                               0.1
                                                                                                       0
                                               0.0001             0.0010           0.0100          0.1000
                                                               INITIAL CRACK LENGTH (m)
            Structural failures

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


• Drill a hole with reamer
  appropriate to dimensions
  of the selected implant at
  location of extraction site
             Temporary Abutment


• Place temporary
  abutment into implant
                     Insertion

• Insert implant
  with temporary abutment
  attached into prepared
  socket
                      Healing

• View of temporary
  abutment after the
  healing period (about 10
  weeks)
       Temporary Abutment Removal

• Temporary abutment
  removal after healing
  period
• Implant is fully
  osseointegrated
             Healed tissue

• View of soft
  tissue before
  insertion of
  permanent
  abutment
    Permanent Crown Attached



• Abutment with
  all-ceramic
  crown integrated
• Adhesive is
  dental cement
              Permanent Abutment

• Insert permanent
  abutment with integrated
  crown into the well of the
  implant
             Completed implant

• View of completed
  implantation
  procedure
• Compare aesthetic
  results of all-ceramic
  submerged implant
  with adjacent
  protruding metal
  lining of non-
  submerged implant
                 Post-op

• Post-operative
  radiograph with
  integrated
  abutment crown in
  vivo
          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
                Clinical Issues

• Stress is a function of
  diameter, or remaining
  bone in ridge
• Values for perfect bond
• Areas small
• Fretting
• Bending
                 Clinical Issues

• Full dentures may use            FBD
  several implants
  – Bending of bridge, implants
  – Large moments
  – Fatigue!
  – Complex combined stress
  – FEA!
                 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
          Comparison with THR

Compare            Contrast
           Comparison with THR

Compare                Contrast
• Stress shielding     • Small surface area
• Graded stiffness/    • Acidic environment
  integration          • Exposure to bacteria
• Small bone section   • Multiple implants
  about implant        • Variable anatomy
• Modular Ti design    • Complicated forces
• Morbidity            • Cortical/ trabecular
                       • Optional

								
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