Rotational mal-alignment of the patella-femoral interface will result in increased wear. Highly cross-linked polyethylene will decrease wear even if mal-aligned.

A biomechanical model based on high load and flexion was used to measure wear of rotationally aligned and mal-aligned all-polyethylene patellae. The components were articulated against “aligned” and “mal-aligned” (60 internally rotated) femoral components. The patella were subjected to a constant 2224 N force and the femoral components rotated from 600 to 1200 at 1.33 Hz. Patellae of identical geometry made of conventional UHMWPE and highly crosslinked UHMWPE were tested to 1 000 000 cycles. Wear was determined by gravimetric measurement relative to cemented soak controls.

Conventional UHMWPE: All samples demonstrated damage (burnishing and scratching) of the articulating surfaces. There was a significant increase in wear (p< .05) in the mal-aligned patella.

Highly cross-linked patellae: All components fractured in the mal-aligned construct (gamma irradiated remelted n=6, gamma irradiated and annealed n=2). Failure first occurred at the cement interface then at the posts.

Correct femoral rotation is important during TKA. The intertrochlear line, tibial cut, epicondylar axis and posterior condyles are helpful landmarks, but there is still eyeball control of rotation. It is clear from this study that rotational mal-alignment will result in increased polyethylene wear.

Highly cross-linked polyethylene has decreased wear in THA. Unfortunately, the decrease in ductility and toughness may make the use of these materials unsuitable for TKA. Based on this study model, patellar components would need to be redesigned if highly cross-linked polyethylene were to be applied.

The wear rate of conventional UHMWPE patellae is increased by rotational mal-alignment. Highly cross-linked components were a poor solution to problem. Use of highly cross-linked polyethylene resulted in component fracture.