Polymer Composites 5 Flashcards
Identifying a need
In the 1970s, metallic implants were recognized as not fully adequate for bone replacement due to high modulus (stress shielding)
- synthetic composite system may better match the mechanical properties of bone as bone is a composite material through the association of HA with collagen
Stress shielding
the reduction in local stresses in an adjacent bone due to a load bearing implant taking a portion of the tensile load or bending moment
- leads to an adaptive remodeling process of bone that is akin to osteoporosis
Device Design: bioactive composites as bone analogue devices
HA particulate reinforced HDPE (high density polyethylene)
- HA is chemically similar to inorganic component of bone, bioactive, stiff, strong and brittle
- HDPE is a non degradable thermoplastic, ductile, tough, bioinert
- path to clinic simplified as both materials are already used clinically
material synthesis and testing
range of composites of HA and HDPE with HA volume fractions of 0.1-0.6 for a modulus of 1-8GPA and failure strain of >100-3%
Fabrication - flow diagram of fabrication HA/HDPE composites
raw materials (HA and HDPE) –> mechanical mixing at HA Vf of 0.1-0.6 –> compounding (Betol BTS40L, co-rotating twin screw extruder, 210-250C) –> HA/HDPE composite wire formation, cooled in distilled water, room temp –> powdering, centrifugal mill, liquid nitrogen temp –> compression moulding, 205C, step wise increase of pressure –> composite plates
Action of compounding twin screw extruders
- blend components are def into the feed throat
- motion of screw against barrel wall heats polymer wile conveying it to higher shear section of screw
- mixing occurs within intermeshing screw elements
- vents downstream to allow moisture, air and degradation products to escape
co-rotating twin screw extruders
- barrel with a figure 8 profile encasing 2 screws that rotate in same direction
- mixing arises from high shear introduced by movement of the screw again barrel and by screws relative to one another
Sterilization
radiation, which is an air and surface disinfectant that kills bacteria (some viruses and fungi) and leaves no residue.
Device testing
- upper limit of incorporating HA was 0.4 Vf
- fracture toughness comparable to that of bone
HA particle size of 4um was optimum for mechanical fixation of filler and matrix - 0 toxicity in vitro
- less than 0.2 Vf were inert in vivo –> fibrous encapsulation was formed
- 0.4HA Vf was confirmed as bioactive
–> designated as HAPEX TM.
Regulatory
- HAPEX considered for non-load bearing applications
- 18 patients received implants with good outcome
- licensed to commercial manufacturer as a bone subtitute wih primary application in ear nose and throat surgery
Clinical Use
- hearing is transferred from the eardrum to the inner ear by three bones; the hammer, anvil and stirrup
- the middle ear devices can replace all three bones
- composite offers better advantage than bioactive glass as it can be shaped in the operating theater
explant analysis
implant extrusion (failure) was 5.3% - hearing success rate was 56.8% in 233 patients