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San Antonio Orthopaedic Specialists

I read it on the Internet/saw it on TV

Thirty Year Knee from Smith & Nephew

Smith & Nephew recently received FDA approval to market one of their knee with a claim not made by any other manufactorer, namely that the knee survived 30 years of simulated activity. The brand name of this knee is the Legion. This is quite an accomplishement and the advertisements are at least mostly true. I will discuss what has really been achieved here, but also some caveats that may be limits on the assumptions that can be made on the basis of the data available.

First, all artificial joints are mechanical devices. Like parts in an automobile, they can wear out. Early designs of hip and knee replacements (as in those from the 60's and early 70's) wore out pretty quickly, often in ten years or less. The wear debris can cause all sorts of problems. It is this experience that has led and still leads some surgeons to insist that a patient be 65 years of age before having a joint replacement. It also has led to research on new or different interfaces and materials. Perhaps the biggest recent advance in materials is cross linked polyethylene. These plastic parts reduced wear in many setting by 90%.

The thirty year knee uses one such advanced polyethylene. Smith & Nephew goes one step further. The femoral component's surface is oxidized zirconium. They literally bake the implant at 5000 degrees in an oxygen enriched environment, and the componet is black. The result though, is a surface with the smoothness of a ceramic, the wettability of a ceramic, and the durability of metal. There are other advantages of this surface, but they are beyond the scope of this web page. The cobination of oxydized zirconium and crosslinked polyethylene goes by the brand name of "Verilast".

Probably because of some of the early, and sometimes "sensational" early failures of older designs and materials, the FDA currently requires simulator testing of new designs before they can be released for general use. The simulators themselves have also improved over the years, so that current protocols yeild results in the simulators very close to the clinical results observed. The machines are not however inexpensive, and the company must test a stastically significant number of samples. However, running the simulator for 24 hours a day, several days of simulated walking can be accomplished in one calendar day.

It takes about 2.5 years to simulate 30 years of ordinary activity. To the best of my knowledge, the FDA likes to see between 5 and 10 million cycles (roughly "steps") for bearing appliances such as total knee replacements. Smith and Nephew submitted data for this claim for 45 million cycles. Smith & Nephew is the first, and to my knowledge, so far only company to even try this simulation. They have also tested this technology in other adverse situations such as simulated retained debris, and in each setting the Verilast has outperformed other material combinations. So again, well done Smith & Nephew.

So what are the limits of this claim? First and foremost, it is only simulator data. While most surgeons and the industry believe that the simulators are good, all agree that they are not perfect. Primarily, there are many more factors that can affect the longevity of an implant than the materials and design of the parts. There are patient derrived factors and surgeon derrived factors. The patient's weight is, at least in this community, the biggest factor. Heavier patients place more stress on the implant and on the interface with the native bone. When the BMI goes over 50, the risks of doing knee replacement surgery start to exceed the potential benefits, primarily on the bais of loosening. Activity level is another factor. While I personally believe that the Journey II knee which also uses the Verilast technology is so well designed that one "could" run on it regularly, there have been no simulator tests done yet. (In part because there is not enough data to set parameters for the machine.) Therefore, my personal recommendation is against running on a treadmill for exercise .

Surgeon factors are a bit more difficult to quantify. Some thoughts about implant position that have been well accepted for decades are now being questioned. For example, perfect mechanical alignment has been the goal for decades. Now, some surgeons believe that a patient who starts out "bow legged", should be left just a little "bow legged". The same is debated for "knocked knees". Significant malposition of implants though, tends to lead to poor results early but also can lead to early failure of the devices.

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This page represents the opinion of Dr. Adam I. Harris