Hip resurfacing has been the subject of much controversy in the large-joint reconstructive implant field over the last few years due to its high revision rate. Interestingly, though, partial (unicondylar and patellofemoral) knee implant procedures which are generally performed on similar patients, namely those who are younger and have isolated osteoarthritis have not been subjected to the same scrutiny. We might immediately assume that this is because revision rates have not been such a problem for these knee replacements however, from looking at historical data, this doesn't seem to be the case.
According to data from the England and Wales National Joint Registry, by the second year postsurgery, both unicondylar and patellofemoral knee implants already have a higher revision rate than hip resurfacing procedures, and this disparity continues to grow considerably through the seventh year postsurgery. By that time, revision rates for patellofemoral knee implants are reported at 17%, unicondylar knee implants at 15%, and hip resurfacing implants at 10%.
So again, why hasn't this come down as hard on the knee implant companies as on those manufacturing hip resurfacing implants? A couple of theories have been put forth exploring this issue. For example, a few sources have suggested that the conversion from patellofemoral or unicondylar implants to a total knee replacement is relatively straightforward, which may have encouraged surgeons to perform a revision sooner, even when not strictly clinically necessary.
Another theory suggests that partial knee replacements are too often being performed on patients with the wrong profile. This trend may have to do with the fact that performing a total knee replacement outright is viewed as very technically demanding, which may lead more surgeons to opt for a partial knee replacement, even on patients who do not have true isolated osteoarthritis. However, there are some dangers to this approach because patients will often soon require a revision anyway. Additionally, performing a total knee replacement immediately instead of a partial replacement is associated with fewer secondary surgical procedures and a lower incidence of progressive patellofemoral joint disease. As a result, even though these partial procedures preserve bone, they still might not be the best option for all patients.
It's interesting that in the case of hip implants, the device manufacturers have taken a lot of blame for the perceived inadequacies of their devices that have led to high failure rates, while the questions in the knee implant space have revolved around physician preference and proper patient selection. Given that revision rates for partial knee implants are, however, higher than for hip resurfacing, this may be deserving of more attention than it has received so far.