Every partial knee replacement will involve an implant, but not all implants are the same. Nick London looks at the differences between implants and shares the hugely impressive results of the Persona Partial Knee, an implant he’s been using for six years.
Surgeons carry out knee replacements when the knee joint wears to such a degree that bone is left to grind on bone, causing the pain of osteoarthritis.
The knee has three compartments and when two or more are severely worn a total knee replacement may be the likeliest surgical route (once non-surgical avenues are exhausted). But when just one of the compartments is affected by arthritis (wear), a partial knee replacement may be appropriate. You can find more about the benefits of a knee replacement – and the typical advantages of a partial knee replacement over a total – here.
Not every patient is suitable for a partial knee replacement. Yet currently in England and Wales, and according to the latest National Joint Registry report – just 14% of patients are receiving partial knees when, in my opinion, that figure could and should be closer to 50-70%.
The partial knee implant
Most partial knee implants contain three components: a metal plate used to resurface top of the tibia (shinbone), a second metal component to resurface the end of the femur (thighbone), and a polyethylene between the two to allow the metal surface on the femur to glide smoothly durinf knee movements as a healthy knee joint would.
All partial knee implants contain these elements, but it’s at this point that some implants begin to differ from others. The tibial and femoral components are usually cemented in place, but some implants don’t use cement. The PPK, in common with most successful knee implants, uses cement and we have not seen any issues with this tried and tested approach.
Many implants used fixed bearings, where the polyethylene bearing is physically clipped into the tibial plate. Others use a mobile bearing which is free to move and which, so the argument goes, may enable a wider range of movement while reducing long-term wear (although the evidence in support of that latter point has never been particularly strong).
It’s hugely important that we have an understanding of which knee implants perform best because the best implants give the patient the best outcomes and require the fewest revisions (that is, rework). We want to avoid revision surgery because every surgery carries risk, because we want patients to have the best possible outcome first time, and because revisions are costly in terms of time and expense for an industry with stretched resources.
That’s why every implant has its performance tracked in the National Joint Registry. Right now, no partial knee implants are performing better than Yorkshire Knee Clinic’s regular implants of choice, the Persona Partial Knee (PPK), produced by Zimmer Biomet, and the Physica ZUK, produce by Lima Corporate. We use the PPK for replacements of the inner — medial — compartment of the knee, and Physica ZUK for replacements on the outer — lateral — side of the knee.
The Success of the Persona Partial Knee
We’ve been implanting the PPK since it was launched in 2017, following rigorous testing. At that point, the clock began ticking on a series of reporting milestones against which we would be able to track long-term performance. The two-year results were exceptional. In early 2024 we will report our five-year results based on the experiences of 240 consecutive (that is, not selected or sifted) patients. They will be similarly impressive.
These results will be supported by two further (and very strong) pieces of evidence:
1. ODEP rating
ODEP is the Orthopaedic Data Evaluation Panel. They objectively assess medical implants to arrive at a globally acknowledged rating. The PPK’s ODEP rating is currently 5A*, which breaks down as follows:
- 5 – the number of years of evidence ODEP has been able to review. As years pass, this figure will increase
- A* – reflects the strength of evidence supporting an implant. A* is the highest possible rating.
The Physica ZUK, our other partial implant of choice, is closely related to the PPK and has been around rather longer. Its ODEP rating is 13A*.
2. Revision rate
The PPK has the lowest five-year revision rate of any partial knee ever monitored by the National Joint Registry. In fact, its results are so good that the implant is no longer required to be part of the Beyond Compliance programme, another implant performance measure for new devices.
These are spectacular results achieved sooner than we imagined might be possible. Zimmer Biomet and Lima Corporate can feel rightfully proud of what is a tremendous achievement in respect of both implants.
But if the PPK and Physica ZUK represent the best of implant performance, what about the rest?
Comparing the PPK
The PPK is a fixed bearing, cemented partial knee implant. Let’s compare it with one of the most frequently used mobile bearing implants, the Oxford partial knee.
- The PPK’s revision rate is about half that of the Oxford.
- Currently, no mobile bearing implants (including the Oxford) have ODEP star ratings.
Additionally, you might be interested to learn that the PPK revision rate is substantially lower than the best performing robot-assisted partial knee.
Not so long ago, supporters of mobile bearing implants cried foul over these figures. The PPK, they claimed, was only being implanted by high volume surgeons (that is, experienced surgeons carrying out lots of procedures every year). Mobile bearing implants, they said, were being carried out by a spread of high and low volume surgeons, which increased the risk of poorer results.
Yet the PPK also includes a mix of high and low volume surgeons in its clinical data.
It is worth noting that ten-year revision rates for the latest mobile bearing implants are better than their predecessors, though they still pale in comparison with the PPK.
Given these results, you might wonder why more surgeons aren’t adopting the fixed bearing approach. The evidence suggests they now are. Between 2014 and 2022, NJR data show a complete reversal in implant type choice. Where mobile bearings were the preferred option in 59% of partial knee replacements in 2014, now 60% are fixed.
In April 2024, I’ll be part of an event aimed at supporting and training even more surgeons to adopt fixed bearing partial knee replacements.
The best partial knee implant is…
Every partial knee surgeon, no matter their implant preference, agrees on the considerable advantages of partial knee replacement over total replacement. The risk of infection or blood clot is about a third that of total knee replacements. Patient dissatisfaction is less than 5% with partial replacements, compared with 10%+ for total.
Yet some partial knees are better than others.
So to answer the question posed at the outset of this article, the best partial knee implants, based on current evidence, are the Persona Partial Knee and the Physica ZUK.
What Is A Partial Knee Replacement?
As we explore here, your knee has three major compartments
What Is Osteoarthritis?
What does osteoarthritis look like? What are the symptoms? And short of surgery, how do you treat it?