In addition, the ideal knee replacement material should also be smooth and resist abrasion to minimize generation of wear particles. This combination of properties has been difficult to find. While a monolithic ceramic knee implant provides a smooth, abrasion-resistant articular surface, it has poor fracture toughness and may shatter when impacted. On the other hand, metal implants have a high fracture toughness but tend to roughen and scratch over time, gouging the polyethylene and producing particles. Efforts to reduce the wear rate of metal implants through surface modifications have had difficulty with durability.
Coatings can crack, chip or peel, especially when damaged.