In studying a series of synovial biopsy specimens from patients with
loose joint replacements, we were able to differentiate reliably between
polymethylmethacrylate and ultra high-molecular-weight polyethylene by
observing alterations in their birefringence with changes in temperature.
The glass transition temperature of polymethylmethacrylate (when it softens
from a glass-like to a rubber-like state) is about 105 degrees centigrade.
Polyethylene melts at 135 degrees centigrade. The birefringence in
polyethylene returns after it has been melted and then cooled; this is not
true of methylmethacrylate fragments heated above their glass transition
temperature. When cooled, methacrylate fails to regain its birefringence.
In addition, free particles of the two plastics were studied in vitro to
validate this method of differentiation. We recommend its use routinely in
histological studies of patients with loosening.