Implant Alloys

Cards (24)

  • Replacement of missing single teeth - treatment options:
    • Removable partial denture
    • Pts don't seem keen to opt for this nowadays, particularly for anterior cases
    • Fixed bridge
    • Device is fitted over a standing tooth next to where there is the missing tooth
    • Requires a lot of destructive preparation
    • Pts less keen on this too because of destructive tendencies
    • Implant
    • Pts like this option because doesn't involve the destruction of any other teeth
  • Replacement of missing single teeth:
    • Pts less accepting
    • Of removable partial denture
    • Preparation of intact teeth for fixed partial
    • Particularly true anteriorly
    • Implants have become more acceptable
    • Other applications now common eg implant-retained dentures
    • Implants have 3 components (typically)
    • Crown - often made from a ceramic
    • Abutment - links to crown to screw implanted in maxilla/mandible
    • Screw - focus of this lecture - provides retention for the implant
  • Majority of dental implants now made from Ti and Ti alloys - most successful option.
  • Titanium:
    • Commercially Pure Titanium (cpTi)
    • 99%pure titanium, trace amounts (<0.25 wt%) of Fe, C, H, N
    • Concentration of oxygen important
    • Increase oxygen: increase strength, decrease ductility
    • Forms a stable oxide layer that is well tolerated by the body (passivation)
  • Passivation - formation of stable oxide layers:
    • Iron oxide is an unstable oxide layer - weak bond between iron oxide and iron
    • Stainless steel - chromium oxide forms - stable layer
    • Titanium oxide - stable layer - body tolerates it better than chromium oxide on stainless steel 
  • For passivation the oxide layer must be:
    • Coherent - must bond strongly to underlaying metal
    • Isovolumetric - must not swell relative to metal
    • Continuous - must cover the whole surface - if not then oxidation may occur at the uncovered sites
    • Impermeable - must stop O₂ and H₂O penetration
  • Titanium crystal structure:
    • Ti above 883°C in α phase (HCP)
    • HCP has close packed atoms
    • HCP = hexagonal close packed
    • Ti below 883°C in β phase
    • BCC atoms less well packed
    • BCC = body-centred cubic
    • Strength increases as atom packing increases
    • Alloys produce better mechanical properties
    • Solution hardening
    • Certain metals stabilise the α phase - force some of the Ti to stay in the HCP structure
  • Titanium alloys (Ti-6Al-4V):
    • Most commonly used titanium alloy
    • Greater than 89% titanium, trace amounts (<0.25 wt%) of Fe, C, H, N
    • Concentration of oxygen less important than for cpTi - more interested in the added aluminium
    • Aluminium - stabilises Ti α-phase (get a stronger but less ductile material)
    • Vanadium - reduces chance of TiAl₃ forming (improves corrosion resistance)
  • cpTi (commercially pure titanium) and Ti-6Al-4V (titanium alloy) both form stable oxide layers
    • Oxides well-tolerated by the body
    • Don't elicit a fibrous response
    • Soft & hard tissues grow close to the implant - termed osseointegration
    Synthetic material placed into the body:
    • Body attempts to destroy/remove synthetic material
    • If too big, body attempts to wall off the synthetic material
    • Fibrous case will be loosely bound to surrounding tissue
    • Implant movement likely to cause failure
    • Need to prevent early implant movement
    • Most of the time, let screw heal in bone before pacing crown on top
  • Osseointegration - close approximation of the bone to an implant:
    • Space between bone and implant must be less than 10 nm
    • Space must contain no fibrous tissue
    • Interface must survive normal loading
  • To achieve osseointegration:
    • Bone preparation must not cause necrosis or inflammation - don't drill too fast
    • Inflammation may disrupt healing process and cause fibrous encapsulation
    • Implant must be allowed time to heal without load
    • Material selection must be correct
    • Not all materials promote osseointegration - cpTi (commercially pure titanium) and Ti-6Al-4V (titanium alloy) do
  • Attempts to improve osseointegration include:
    • Maximising load transfer
    • Minimising relative motion between implant and tissue; will cause loosening
    • Developing materials that allow accelerated tissue application to the tissue surface
    • Optimising roughness
    • Using growth factors
    • Coating with ceramics (bioactive ceramics)
  • Mechanical factors influencing implants:
    • Magnitude of force
    • Depends on location - more force extended through molars and premolars than incisors
    • Stress
    • Depends on applied load and area
    • Loss of teeth previously can lead to increase in stress
    Type of force:
    • Bone 30% weaker in tension
    • Bone 70% weaker in shear
    • So must be careful designing occlusion
  • Geometric factors influencing implants:
    • Need to transfer load to surrounding tissue
    • Loads must be correct direction and magnitude
    • Needed to maintain tissue viability
    • Natural tooth: PDL transforms occlusal forces into tensile forces
    • Implant: no-PDL, occlusal forces transform into compressive forces
    • Must stabilise implant-bone interface quickly
    • Stability must remain over life-course
  • Bone resorption:
    • Need to apply approximately physiological stress levels or bone resorbs
    • Stiffness governs stress and strain transfer to bone
    • Inversely proportional to the strain transferred to the bone
    • Titanium is 5x stiffness of bone (100 GPa vs 20 GPa)
    • Significantly less stress and strain are transferred to bone
    • Tensile forces stimulate bone formation
    • Compressive forces lead to bone resorption
    • A high stiffness leads to stress-shielding of the bone
    • Leads to bone resorption around the implant
  • Implant design factors:
    • Smooth sideshear; didn't get sufficient retention in bone
    • Screw thread combine compression + tension
    • Sharp threads - high stress
    • Round threads - less shear
    • Implant diameter
    • Greater diameter -> greater area of force distribution (lower stress)
    • But, can cause stress shielding
    • Implant length
    • Increase in length -> increased surface area (lower bone stress)
    • Bone heating during drilling -> osteonecrosis, major cause of failure
    • Greater implant length - potential more drilling, potential higher heat
    • Most stress concentrated around upper cortical plate
  • Methods to improve implant longevity (roughening):
    • Cells react to roughness and surface features at macro, micro and nanoscale
    • Add micro- and nano-scale features on macro- features increases bone-implant contact
    • Rough surfaces are better than smooth
    • Better bone apposition on rough surface (increases bond strength)
    • Smooth surfaces tend to produce fibrous tissue (lowers bond strength)
    • Grift-blasting followed by acid-etching or coating currently seems best
    • Some contradictory results but:
    • Smooth < textured < screw < plasma sprayed < porous coated < porous body design
  • Methods to improve implant longevity (coating):
    • Coating Ti with ceramics to promote better apposition of tissue
    • Bone is a ceramic
    • Some ceramics can be considered biologically inert, ie do not elicit fibrous response
    • Bioactive ceramics
    • Don't elicit fibrous response
    • Form direct bond to bone
    • Designed to resorb or degrade in the body
  • Methods to improve implant longevity (coating):
    • Calcium phosphates common
    • Calcium hydroxyapatite (similar to cortical bone)
    • Well-tolerated by osteoblasts - can start to get bone cells deposited on them
    • β-tri-calcium phosphate (β-TCP) resorbs over time (dissolves and goes into solution)
    • If timed right, we can get new bone to grow into where the β-TCP has resorbed - gives v strong bond
  • Methods to improve implant longevity (coating):
    • Hydroxyapatite (HA) has similar mechanical and physical properties to bone
    • HA is thermally unstable and can transform during processing - calcium phosphate ceramics (CPC) different properties and degradation rates
    • Too stiff and brittle to use as stand alone implant material, so used as coating
    • Other bioactive ceramic coatings
    • Bioglass (silica, phosphate, calcia, soda)
    • Other apatite glasses
  • Surface coatings:
    • Applied using plasma spraying
    • Weak, mechanical bond between coating & surface
    • Rapid cooling -> coating cracking -> failure
    • Alloy implant supplies most mechanical properties
    • Difficult to predict lifetime coating bond to implant
    • As coating resorbs, implant-bone interface becomes unstable -> micro-motion & loosening
    • Ceramic-alloy bond weaker than ceramic-bone bond
    • Important variables: powder particle size/shape, pore size/shape, pore size distribution, specific surface area, phases present, crystal structure, density, coating thickness, hardness, roughness
  • Surface coatings:
    • If the coating does lead to the implant being completely fused to surrounding bone
    • Biointegration
    • No intervening space between bone and implant
  • Surfaces and biocompatibility - ion release:
    • Wear from implant
    • Debris particles - leads to bio response
    • Corrosion
    • Metals and alloys can corrode, so need to choose alloy that does not elicit adverse response
    • Ti and Ti alloys passivate and are well tolerated but large scale break down is a problem
    • Implant surface (if alloy) is likely to be heterogenous oxide layer
    • Oxide can be effected by processing and cleaning
  • Causes of failure:
    • Early loosening = most often due to lack of initial osseointegration
    • Late loosening = aseptic loosening or loss of osseointegration
    • Bone resorption - stress shielding
    • Infection
    • Fracture of the implant or abutment
    • Coating delamination
    • Wear debris from implant
    • Debris particles reported in lymph nodes
    • Possible long term health implicatons