The best fixed-bearing partial knee implants outperform the best mobile-bearing implants. Nick London explains more.
One of the major components of any knee implant is the bearing. This is a polyethylene insert which enables the free movement of the joint. Bearings can be fixed (that is, physically clipped into the plate inserted at the top of the shinbone). Or they can be mobile, where they have the freedom to move within the joint.
For a long time, the mobile bearing was the dominant one. It was heavily marketed to surgeons and, in some countries, direct to patients. Compared with the fixed bearing, many claimed the mobile bearing suffered less wear (although there has been no clear evidence to support this — the best fixed-bearing implants last longer than their mobile-bearing counterparts in all national joint registries worldwide). It was also claimed to offer a slightly greater range of motion within the joint and to be more suitable for younger or more active patients. Both claims are unproven.
Small wonder, then, that in 2014 almost two thirds of knee replacement implants in England and Wales used mobile bearings*.
The risk of overcorrection
Things have changed. Over the past 25 years the advances in polyethylene have been so great that any historic potential advantage of mobile bearings in terms of polyethylene wear is no longer relevant.
At the same time, the mobile nature of the bearing has always meant there is a risk of dislocation (a complication not seen with the fixed bearing). The risk of dislocation has led some surgeons to ‘overstuff’ the affected compartment with the aim of tightening the knee unnaturally to try to ‘capture’ the mobile-bearing. This has the unfortunate effect of changing the alignment of the leg, increasing the likelihood of wear on the other side of the joint.
This is supported by evidence from national joint registries worldwide which indicates that mobile-bearing implants are more likely to fail as a result of wear on the other side of the knee between 7 and 15 years after the initial knee replacement.
Wear is most common on the inner side of the knee, which causes a slight bowing of the leg. The greater the wear, the greater the bowing. That’s why, when Yorkshire Knee Clinic surgeons perform partial knee surgery, we aim to almost fully correct the leg to straight, but we leave it subtly, slightly bowed, so the load goes through the new implant rather than being thrown onto the opposite side of the joint. As a consequence, failure or wear on the opposite side of the knee is exceptionally rare.
I am pleased to see there have been advances in the mobile-bearing implant, which do appear to be having a positive effect on the implant’s survival rate (although this remains poorer than the best fixed-bearing implants). Yet I still have grave concerns about the tendency of some surgeons to overcorrect bowing when performing mobile-bearing rather than fixed-bearing knee replacements because of dislocation fears.
Training surgeons to convert from mobile bearings
The risk of overcorrection — and how to avoid it — is something we have long taught trainee surgeons. Next April, we’ll be presenting a course on fixed-bearing partial knee and patella femoral replacement for all surgeons interested in increasing the proportion of partial knees they offer to their patients, or who wish to convert from a mobile-bearing practice to a fixed bearing.
Fixed or mobile bearing?
Today, the best fixed-bearing partial knee implants outperform the best mobile-bearing implants. We’re seeing the evidence of that in the complete switch from almost two thirds of knee replacements having a mobile bearing a decade ago, to almost two thirds using a fixed bearing today*.
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