Primary Knee Symposium: Biomechanical Considerations in the Knee

Unintended Wear Surfaces – Thomas P. Schmalzried, MD

Dr. Schmalzried reviewed different types of wear: adhesion (transfer films), abrasion (surface roughness), fatigue, and third body. In a complicated total joint replacement, there can be a combination of wear mechanisms involved. The type of wear determines the type of wear particles.

He then reviewed the four wear modes (1995 Charnley Award). PSlide A PSlide B PSlide C PSlide D In a retrieval analysis of mode 4 wear, Dr. Schmalzried could discern the types of wear particles, the amount of relative motion between the insert and baseplate, and other fundamental variables such as contact area, counter surface characteristics, and containment. The relative number of particles was a function of the component design. Loosening at the metal-cement interface (also mode 4) showed debonding—metal and cement particles. Dr. Schmalzried showed the role design plays in loosening. PSlide E

Depending upon the component geometry, rotational constraint can be reduced. Dr. Schmalzried believes a single-radius design is the best way to go because it has a high contact area with less rotational constraint. Testing reported by Bijimi et. al. (2007) showed Triathlon, a single-radius design, has the lowest rotational constraint and the tightest locking mechanism. PSlide F

Dr. Schmalzried concluded with a discussion on minimizing backside wear PSlide G and minimizing unintended wear overall. PSlide H