The recent advertising campaign by the implant manufacturer Stryker irks me in particular. It is to my way of thinking, a classic example of “Figures don’t lie, but liars figure.” Here I’ll attempt to prove that to you.
The first issue is that “designed for women” and “designed with women in mind” are not exactly the same. The “Gender solutions knee” released by Zimmer in early 2006 represents the former. It is possible that Stryker’s knee was in fact “designed with women in mind”, but the knee is not substantially different in many key ways when compared to the “standard” knee implant. The Stryker implant is not brand new. Only the advertising campaign is new.
The anatomy of a woman’s knee differs in three key ways that impact the design of the implant. All of these relate to the anatomy of the end of the femur, or thigh bone. The first is that for a given height, the woman’s knee is less wide than a man’s knee. The second is that if one were to look at the end of the femur from the side, the surface of the joint on the top of the knee (as opposed to the end) is less prominent than that of a man. Lastly, the kneecap travels in a groove as it contacts the end of the femur. The angle of this groove relative to the shaft of the femur is higher in women than it is in men.
The Stryker knee only addresses the first of these issues, and only partially. The graph on the left is taken from the Stryker web site. The Triathelon knee is compared to the “anthropometric mean” only. They don’t compare their knee to any other brand. The graph on the right is taken from the Zimmer web site. The line representing the Stryker Triathelon knee is not really different from the average knee, the Zimmer Gender solutions knee sits apart clearly from the others. Some of Stryker’s advertising to physicians does compare the Triathelon knee to a “Brand X” whose proportions lie above and to the left of the Stryker Triathelon implant. Clearly, “Brand X” is not the Zimmer Gender solutions knee. Figures don’t lie, but liars figure.
The Stryker web site tries to claim attention to the height of the “anterior flange” with a very nice picture of an angled cut made to put in the knee. The angle of the cut means something only to the surgeon putting in the artificial knee. The angle of the cut is not related in any fashion to the height of the flange above the rest of the femur. All of the Zimmer knees have an angled anterior cut. This not new. This feature has been present in Zimmer knees at least since the mid 80’s, when I was in training. It is also a common feature of most modern total knee arthroplasties. The Gender solutions knee also has a less prominent anterior flange, matching the female anatomy more closely.
Lastly, if one reads the Stryker web site carefully one finds many “interesting” comments. For example, they state that “The Triathelon design incorporates a variable aspect ratio to adequately fit the female anatomy while still accommodating the male population.” (emphasis added) In fact, all knee systems incorporate a variable aspect ratio. Look at the graph on the right. None of the lines are exactly straight! Conversely, Stryker’s own graph shows no variation in aspect ratio. The line is absolutely straight.
Given Stryker’s example, all shoes also incorporate a variable aspect ratio. A “C” width shoe for a size ten foot is wider than a “C” width for a size seven foot. It is patently ridiculous to try to make a shoe designed to “adequately” fit both men and women. If somebody were to advertise a shoe that “adequately” fit both men and women, would anybody buy it?
Around the end of the year 2006, Stryker dropped from their web site the comment that the Triathelon system “...closely match[es] both the female and male anatomy.” Like shoes, it really can’t be done, at least not well.
For my family, and for my patients, I don’t want “adequate”, I want “optimal”. In choosing a prosthesis, the shoe analogy works well. Some shoes are very fancy, and very attractive. However, at a party, often one finds that the women have removed their shoes and are dancing in stocking feet. If the knee does not fit well, then this overpowers, in Dr. Harris’ opinion, all other design factors. For women with smaller bone structure, knees made by Smith and Nephew or Wright Medical may function better. (see also comments under the DePuy Knee) For women with larger bone structure, Zimmer seems to have hit this particular nail right on the head. Stryker appears to be spending a great deal of advertising money trying to fool you.
