Reasons for and modes of failure of unicompartmental knee arthroplasty

Improper patient selection

The “classic” and updated guidelines for appropriate patient selection are given in Chapter 2 . ^, Some controversy still exists regarding patient selection and it is expected that these criteria will slowly continue to evolve.

The original patient selection criteria as espoused by Kozinn and Scott recommended that patients weighing over 80 to 90 kg (176–198 pounds) were not good candidates for UKA for fear of component loosening or subsidence. The criteria were developed when component design was early in its evolution before components of more sizes were made available to better distribute forces across the fixation interface. Failure due to loosening in obese patients has become less frequent, ^, although some recent studies still report higher failure rates in the obese population. ^, Perhaps more relevant than actual patient body weight predisposing failure is their specific age, gender, and activity level. Mesomorphic middle-aged males appear to be at higher risk for component loosening (see Chapter 9 ).

Poor prosthetic design

The effects of poor prosthetic design on the propensity for failure are discussed in Chapter 3 . They are mainly due to poor sizing and capping of the available bone, poor fixation, poor articular contact area, poor polyethylene, and excessive articular congruency in fixed bearings. Instrumentation that is insufficient to allow for accurate component implantation can also be a cause of subsequent failure. These effects, of course, will be a less prevalent cause of failure with modern designs and materials.