Friday February 23, 1996

Surface stresses may lead to polymer damage

A scientific exhibit (S 25) on factors influencing the longevity of ultra high molecular weight polyethylene (UHMWPE) tibial components shows high surface stresses increase the potential for polymer damage and represent an etiological mechanism which catalyzes component wear and debris generation.

A series of laboratory evaluations demonstrates that gamma irradiation, nonconforming geometrics, decreasing polymer thickness and imprecise femoral component tolerances increase peak surface stresses in knee designs and decrease component longevity.

Contemporary total knee components were articulated at the position of maximum, interface force for walking gait (0 degree) in a materials testing system. Pressure sensitive film, in conjunction with interactive computer software, was utilized to determine the surface stress distributions.

Decreasing UHMWPE tibial plateau thickness resulted in a progressive increase in surface stresses and a decrease in contact area.

A comparison between UHMWPE and Hylamer-M demonstrated that the increase in polymer stiffness produced higher surface stresses over a smaller contact area.

EtO had minimal effect on surface stress distributions when compared to unsterilized components. In contrast, gamma irradiated UHMWPE tibial plateaus produced surface stress distributions with increased potential for polymer damage.

A comparison of femoral component finishing techniques demonstrated that a precision finishing technique produced components of improved tolerances resulting in larger contact areas with a reduction in the peak surface stress.

Decisions on material processing and design for total knee components influence tibial-femoral surface stress distributions and thus, the potential for polymer damage and component longevity. An understanding of these variables by the practicing orthopaedic surgeon provides a rationale for component selection.

Co-authors of the study are A. Seth Greenwald, D.Phil. (Oxon.), Christine S. Heim, B.Sc. and Paul D. Postak, BSc, all of the Orthopaedic Research Laboratory, Mt. Sinai Medical Center, Cleveland, Ohio.

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