Little by little, total knee implants have been improved over the years. But it hasn't happened by magic. Makers of the implants sponsor studies like this one to evaluate what works, what doesn't, and what changes are needed to improve the results. Better quality of life and longer lasting implants are two main goals with total knee replacements (TKRs).

In this study, 116 surgeons in 34 centers in the United Kingdom performed total knee replacements on over 2350 patients. Three specific design features were the focus of attention: 1) metal backing on the tibial portion of the implant, 2) patella resurfacing, and 3) a metal bearing between the tibia (lower leg bone) and the femur (thighbone).

Surgeons and patients were selected carefully. In the case of the surgeons, only those who routinely did knee replacements were part of the trial. Each patient received one of the three components being studied. Anyone who needed a particular type of replacement for any reason received the required implant and was not included in this study.

What did they know about these components before heading into this study? Metal backing spreads the load out and reduces overall stress on the implant. But it takes up space in the joint. And there is also a polyethylene (plastic) lining for the new joint surface. So when there's a metal backing, the lining has to be thinner.

A thinner lining puts more pressure and shear force on the weight-bearing surface of the new joint. But there haven't been any studies comparing metal-backed with nonmetal-backed implants to see if there's a difference in performance.

Patellar resurfacing involves placing a smooth metal surface on the back of the patella (knee cap). This feature allows for smooth motion of the patella as it glides over the femur. No studies have been done to show the benefits of this feature.

And finally, the third design studied was the mobile bearing. This rotating-platform between the tibial and femoral components is supposed to reduce polyethylene wear and shearing that occur between the implant and the bone. The hope is that the bearing will help prevent loosening of the implant. But there have been reports that the bearing dislocates.

Each of these design features was compared between two groups of total knee replacement patients. One group received the particular component. The other group did not. Surgeons performed the procedures according to their own standard methods. Outcomes were measured in terms of cost, effectiveness, complications, function, and quality of life. The study had a follow-up period of two years.

There were many findings from the study such as complication rate (equal among all groups), operative time (longer in the metal backed group), and hospitalization (average was nine days for all groups). In addition, 95 per cent of all patients went home directly from the hospital. Pain was reduced and function improved within the first three months. Gradual improvement continued after that initial recovery period.

But the bottom-line was that the overall results were the same from group to group. Everyone improved equally by the end of two years. Rates of complications during that time were the same and functional recovery was no different from group to group. Outcomes were measured according to results important to the patients (not the surgeons). The results are all considered short-term, so patients will be followed for another eight years for a better look at long-term effects.

The authors could not recommend one total knee replacement design over another from the results of this one study. But the hope is that with long-term results, patterns of design wear and the pros and cons of each will be more obvious. The wide range of centers and surgical techniques may have some effects on the results. These factors will be taken into consideration over time as well.

The KAT Trial Group. The Knee Arthroplasty Trial (KAT). In The Journal of Bone & Joint Surgery. January 2009. Vol. 91A. No. 1. Pp. 134-141.