In 2006, Zimmer, Inc. received clearance from the Food and Drug Administration (FDA) to market a Gender Solutions version of their NexGen prosthesis for use in total knee arthroplasty (TKA). Some orthopedic surgeons saw the gender-specific knee prosthesis (GSK) as an interesting new adjunct to traditional methods. Others saw it as a solution in search of a problem.
“Gender issues in medicine are currently hot issues,” says Matthew J. Kraay, MD, professor of orthopaedic surgery at Case Western Reserve University in Cleveland. “Most of the traditional implants were designed from studies that included both men and women.”
Anatomical Differences
Studies have outlined some of the anatomical differences that exist between men and women. Men have been shown to have an increase in contact area in the patello-femoral joint at higher flexion angles.1 There are also indications that women tend to have a narrower medial-lateral dimension than men.2
Traditional femoral implants are wider and may overhang the bone, possibly rubbing against other tissues and causing pain. For women, studies have found statistically significant associations between the size of the femoral component and the amount of overhang, with larger sizes having a greater amount of overhang.3 However, some question if the amount of overhang is really a cause for concern.
Zimmer’s GSK was developed after analyzing the knee anatomy of 800 women. The developers noted that, for a given femur height, women’s knees tend to be narrower than men’s, on average 0.8 mm less on the lateral condyle and 1.3 mm less on the medial. Also, because women’s hips are wider relative to their height, their limbs are set at a different angle than men’s are.
“Ideally, what we are trying to do with TKA is reproduce a patient’s anatomy as closely as possible,” says Scott M. Sporer, MD, assistant professor of orthopaedic surgery at Rush University Medical Center in Chicago.
On the other side of the decision, using a GSK can have a downside. If a female patient’s anatomy more closely resembles that of a male, there may be an underhang condition where the prosthesis doesn’t cover the entire bone. This situation can cause the implant to settle into the end of the femur, especially in those with bone loss from rheumatoid arthritis or long-term steroid use.
Do Differences Affect Outcomes?
“Zimmer made a number of changes to make their GSK a little more specific for women, but the question is whether these changes had a significant impact on outcomes,” says Friedrich Boettner, MD, assistant attending orthopaedic surgeon at the Hospital for Special Surgery in New York City. “That would imply that, before the advent of the GSK, female patients had measurably worse outcomes following TKA, and that is just not true.”
Even in those few studies showing that there might potentially be an outcome difference in women undergoing TKA, Dr. Boettner says that many other factors in addition to the size differentials of the implant would have a greater impact on outcomes. To the extent that these outcome disparities may exist, differences in activity levels and ways that outcomes are measured may play a more significant role than those addressed with a GSK.
Indeed, Dr. Boettner notes that the advent of GSK may be related more to marketing than clinical need. “With women comprising 60% of the patient base of TKA and the GSK being sold for a higher price than traditional implants, it was a smart idea that made Zimmer a lot of money and added market share.”
Other surgeons, however, see usefulness in the GSK concept. The differences in the sizes of the prostheses allow the surgeon to select the artificial knee that best fits the patient, instead of having to shape the bone surfaces to better fit the prosthesis. “If you think about it, just about everything works better if it fits correctly,” says Dr. Kraay. “Whether it is slacks, golf clubs, or shoes, if they fit you, you would expect a better outcome. However, there hasn’t yet been scientific data to back this up.”
Dr. Boettner, however, questions the reasoning behind such precision. “Both the gender-specific and standard implants have the same shape of the articulating surfaces and therefore move exactly the same. Both designs alter the natural kinematics of the knee significantly and the knee no longer rolls back naturally,” he says. “In addition, most of the changes they made in the prosthesis design are really minor changes and it is hard to imagine they have an impact on function in a knee with an altered kinematics and change in joint line. One should also not forget that other current implants like the Genesis II made by Smith and Nephew have received the FDA clearance for gender-specificity product requirements.”
Match to Patient and Not Gender
Selection of the implant should not be solely based on matching the “gender” of the knee to the gender of the patient. Rather, the implant used is most often based on decisions made by the surgeon during the operation. In some cases, the anatomy of a male knee may suggest that the use of a “female” implant is appropriate.
“I don’t use this on every female I see,” says Dr. Sporer. “About 70% of the time, I’ll use the GSK on female patients. There are also some men whose knees are a better fit for the GSK. For me, the decision to use a specific implant is based on the individual patient’s anatomical geometry and using the one that more closely reproduces the patient’s original knee.”
Dr. Kraay notes that in some ways this is the continuation of a process that began soon after the introduction of the first total knee prosthesis. “When I started doing knee replacement surgery, there were four different sizes of implants compared to the eight to 10 from each company that we have now,” he said. “We have a lot more ability to accommodate differences in anatomy than we used to, and GSK adds to that ability.”
In a similar vein, Dr. Sporer sees the release of the GSK more evolutionary than revolutionary. “By no means do I think that the older generation of implants was detrimental to women. As time goes on, the natural response of most of us in medicine is trying to fine tune things and optimize treatments to the individual patient.”
Surgeon Determines Outcome
All three experts agree that it is important that patients and physicians do not focus on the kind of prosthesis used in the procedure. Other aspects, especially the skill of the surgeon, have more impact on outcomes. As Dr. Kraay notes: “It is the magician, not the wand, that performs the magic.”
Dr. Boettner agrees. “Outcome is determined by the experience of the surgeon as well as their training and the number of cases they do,” he says. “There are a lot of very good implant surgeons who never have used a gender-specific implant and still have better outcomes than the one who does 20 GSKs a year.”
Another area of expert agreement is that the GSK is still a very new and untested technology. Randomized studies are currently ongoing, although any definitive results are still years away.
Gender-Specific Hip on the Way
Zimmer is also preparing to introduce a gender-specific hip (GSH) implant, the Zimmer M/L Taper Hip Prosthesis with Kinectiv Technology. A woman has smaller bones and shorter hips. In some cases, the stem of the implant may be too long, causing the hip to push out and the leg to be slightly longer than normal. The stress can cause the new hip to fail or can lead to significant pain and disability, requiring a revision.
The GSH is a system of modular stem and neck components designed to help the surgeon restore the hip joint center intraoperatively by addressing leg length, offset, and version independently. The many neck options available are said by the company to target a broad range of patient anatomies.
“The currently available approximately 20 standard and high-offset stems and four to six different head sizes available with most current implant designs should, however, enable most surgeons to restore leg length, offset, and anteversion of female osteoarthritic hips,” points out Dr. Boettner. Again, this is largely seen as an incremental change in technology that has been available for many years.
“These are very similar to the modular-neck implants that have been manufactured by Wright Medical Products for the last four or five years,” says Dr. Kraay. “Even though there are proportionality and anatomical issues in women’s hips, I see just as wide of a variation in those of men. Like knees, the concern in total hip arthroplasty is getting a ‘patient-specific’ implant, without regard to the gender labeling.”
Since the hip technology is even newer than the GSK, good studies on outcomes remain in the future.
“It is important to keep in mind during these discussions that both TKA and THA are very reliable, durable, satisfying, and in general very remarkable operations,” says Dr. Kraay. “The gender-specific implant is one company’s attempt to improve outcomes. Will it prove to be in the long run? Who knows?”
Kurt Ullman is a freelance writer based in Indiana.
References
- Csintalan RP, Schultz MM, Woo J, et al. Gender differences in patellofemoral joint biomechanics. Clin Orthop. 2002;402:260-269.
- Chin KR, Dalury DF, Zurakowski D, et al. Intraoperative measurements of male and female distal femurs during primary total knee arthroplasty. J Knee Surg. 2002;15:213-217.
- Hitt K, Shurman JR, Greene K, et al. Anthropometric measurements of the human knee: correlation to the sizing of current knee arthroplasty systems. J Bone Joint Surg Am. 2003;85: 115-122.
