· introduced by Charnley in 1962
· first tested by Craven
· given to him by salesman of plastic gears
· choice of materials for bearing surfaces
· replacement developed in 1970’s
· polyacetal compound called Delrin
· significantly higher failure rates reported
· withdrawn from used shortly after
· manufactured by only 2 companies
· Hoechst
· Himont
· 40 000 tonnes manufactured in 1993
· majority used for
· ski bottoms
· cutting boards
· coal-chute liners
· medical use was only 0.1% of this (40 tonnes)
· little attention paid to medical uses by manufacturers
· produced as fine granular white powder
· two methods of device manufacture
· powder placed in mould
· compressed into final shape
· heated to consolidate it
· item machined from bulk form
· polyethylene heated during machining
· two bulk forms
Rod
· powder ram-extruded into cylindrical bar
Sheet
· powder moulded into sheets
Annealing
· bars may be annealed to remove residual stress
· alters its physical properties
· is 2-6 x 106
· determined by measurement of relative viscosity
· has crystalline and non-crystalline (amorphous) phase
· crystallinity is 60%
· crystallinity irreversibly decreased when heated beyond melting point
· poorly understood
· resistance increases with increased crystallinity
· contains defects or asperities
· evenly distributed or localised
· are unconsolidated UHMWPE particles
· accelerated oxidation after gamma sterilisation
· leads to fragmentation of polymer chains and loss of molecular weight
· increases brittleness and rate of wear
· may be avoided by oxygen-free environment during sterilisation
· other method of sterilisation is low-dose gamma irradiation
· process for production of higher crystallinity forms developed
· leads to increased elastic modulus and yield strength
· commercial product is Hylamer (DePuy Dupont)