PHYSICAL MATHEMATICS SEMINAR


TOPIC:		OXIDATION MODEL FOR RADIATION-STERILIZED POLYETHYLENE JOINT
		REPLACEMENT BEARINGS


SPEAKER:	THIERRY A. BLANCHET
		Department of Mechanical Engineering,
		Aeronautical Engineering, & Mechanics
		Rensselaer Polytechnic Institute


ABSTRACT:

Gamma irradiation, the conventional sterilization technique for ultra high
molecular weight polyethylene (UHMWPE) bearing surfaces of hip and knee
joint replacements, secondarily strips hydrogen atoms from these
molecules, leaving reactive free radicals along their carbon backbones.
Though crosslinking between radicals may improve mechanical properties,
typical post-irradiation handling (air shelf-storage or in vivo service)
favors instead scission reactions with oxygen from the surrounding
environment.  As oxidative aging of radiation-sterilized ultra high
molecular weight (UHMWPE) joint replacement components especially
diminishes wear resistance, numerical models of this oxidation process
have recently been developed.  However, these models neglect dose-depth
profile, an aspect whose importance has to date only been discussed
qualitatively.  The quantitative model presented here extends from the
presentation of Crank for moving boundaries (fronts) of molecules (oxygen)
diffusing through a medium (UHMWPE) containing immobile holes (allyl
radicals) with which they react (oxidative scission), modified to account
for hole concentration which varies with position and time due to,
respectively, radical concentration-depth profile and radical decay
(crosslinking).  This quantitative model's capabilities are illustrated in
two predictions of irradiated UHMWPE aging behavior: in atmospheres of
augmented oxygen partial pressure and temperature commonly used to
accelerate the process while otherwise attempting to remain simulative;
following post-irradiation vacuum storage period (a process intended to
consume free radicals through crosslinking alone, but typically performed
to an incomplete extent).

DATE:		TUESDAY, NOVEMBER 28, 2000

TIME:		2:30 PM

LOCATION:	Building 2, Room 338


Refreshments will be served at 3:30 PM in Room 2-349


Massachusetts Institute of Technology
Department of Mathematics
Cambridge, MA  02139