Implant Alloys

• 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 side - shear; 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