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Cultural and gender differences begin to impact new knee prosthesis designs

FDA clears Zimmer’s gender-specific knee as interest in cultural-specific total joints grows.

Issue: June 2006

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Female patients captured the spotlight in the orthopedic industry recently when a few implant manufacturers introduced new gender-specific knee implants or repositioned existing implants as being well-suited for a woman’s joint replacement needs.

This also generated a lot of enthusiasm at this year’s meeting of the American Academy of Orthopaedic Surgeons (AAOS). But instead of touting new gender-specific implants, some companies, like DePuy, a Johnson & Johnson company, simply devoted parts of their exhibit hall booths to female-related orthopedic solutions.

“I took away from that they felt like they didn’t have a problem. But, in reality, if they look at the study we did in 2003, they have the same problem as everybody else … No company is immune to this issue,” said Kirby D. Hitt, MD, director, adult reconstructive surgery, Scott and White Memorial Hospital, Temple, Texas.

In Hitt’s study, one of the first of its kind, investigators identified distinct anatomical differences between men and women’s knees from bone cuts made during total knee arthroplasty (TKA). One difference: Women’s knees are narrower mediolaterally.

More studies have since elucidated knee differences related to gender and also culture.

FDA clearance

In perhaps the biggest news to date in this area, Zimmer recently received FDA 510(k) clearance for its Gender Solutions High-Flex knee prosthesis, which is shaped to fit a woman’s anatomy.

“I think this knee is going to create a new level of sophistication,” Robert E. Booth Jr., MD, said at a press conference at Pennsylvania Hospital, Philadelphia, where he is head of orthopedic surgery. Booth was the first U.S. surgeon to implant Zimmer’s Gender Solutions prosthesis. The new TKA device has since been used elsewhere in the United States and France.

Cultural differences

Many surgeons have long felt that something about fitting femoral TKA components in women just wasn’t right. “But, it was never proven,” Hitt explained. Once the data became available, drastic product changes followed.

Women aren’t the only group getting special attention, either. Implant companies have rolled out more knee and hip arthroplasty devices designed with cultural-specific differences in mind, such as options that provide for alternative sizing and more extreme range of motion (ROM).

It’s more than fit

Hitt, who performs 300 TKA surgeries annually, welcomed the change because he often encountered problems fitting femoral knee implants in women, like overhang >5 mm. To adjust, he often downsized implants in the OR, but that produced knee balancing issues, he told Orthopedics Today “We were also concerned about the overhang creating some soft tissue irritation and about decreasing the ROM.”

After bringing these concerns to Stryker, Hitt conducted his study, which separated out anatomical differences by gender. “That’s the trend you’re seeing now that people are following suit,” he said.

Two camps

The major orthopedic firms fall into two camps, according to approach. In one camp is Zimmer, which emphasizes the unmet need in the marketplace for specially shaped, gender-specific knee implants.

Those in the other camp — Biomet, DePuy, and Stryker — contend that when they designed their current TKA products, and launched them two or three years ago, they took into account a woman’s unique anatomical needs.

DePuy has adopted more of an educational, awareness-raising approach, supporting it with appropriate products, like its PFC Sigma RPF knee system, said Sarah Colamarino, vice president, WW Communications, DePuy Franchise.

“I think what we’re dealing with … is some interesting marketing,” said Bill Kolter, president of Biomet Orthopedics. “If you look at most of the changes that the ‘female knee’ represents, we introduced those with the Vanguard [implant] in 2004. These are probably good changes for Zimmer to make, but they’re certainly not novel.”

Zimmer based its new Gender Solutions knee design on research in conjunction with Mohamed Mahfouz, PhD, director of computational bioengineering, University of Tennessee, Knoxville. He compiled a three-dimensional bone atlas from computed tomography scans of more than 800 knees and from it measured anatomic differences between male and female knees.

The results are reflected in three characteristics Zimmer incorporated into the shape of its Gender Solutions High Flex knee implant. First, the ratio of implant height to width, or aspect ratio, differs from other knee implants. “These new knees have an aspect ratio that makes them taller and less wide, and that’s a significant benefit in making them fit better,” Booth explained. “It eliminates the overhang and it accommodates the unique shape of the female knee.”

A second feature — a much thinner anterior flange — resulted from a better understanding of the shape of a woman’s femur and the less-prominent anterior condyle commonly found in a woman’s knee. The slimmer design helps prevent overstuffing a woman’s patellofemoral joint with a standard prosthesis.

Zimmer also took into account that a woman’s Q-angle (the angle the quadriceps and patella form with the knee joint) is greater than a man’s by increasing the sulcus angle of the new Gender Solution High-Flex component by 3°. The male’s angle gets built into standard TKA implants. Increasing this angle should lead to more normal patella tracking and less patellofemoral (PF) problems, Booth said.

Been there, done that

Officials at Stryker contend their Triathlon implant, introduced in 2004, meets women’s needs. “For a given AP dimension, the mediolateral (ML) dimension has been narrowed to accommodate the female anatomy,” Hitt said.

According to Michael P. Mogul, president, Stryker Orthopaedics, the Triathlon development process varied significantly from that of earlier designs. “It was the first time we looked at a woman’s anatomy and, in our smaller sizes, made the components more narrow so the surgeons could better fit the woman’s knee.

“The reason that we really want to start talking about it is that we hit our 30,000th implantation back at the end of February,” Mogul told Orthopedics Today.

Surgeons who use the Triathlon knee implant find it easier to fit women, he added. The system, instrumentation and number of femoral sizes remain unchanged, but the sizes get narrower faster.

Not a problem

“The conclusion that I come to is the female knee is a solution to a problem that has not been substantiated in the peer review literature,” Kolter said. “Our 15-year results show no gender-specific differences in survivorship or complications. Granted, others may not have a similar track record.”

Instead, with each major knee system it launched, Biomet consistently used anthropomorphic data to look at variations between men and women and between Americans and people from other cultures, he said.

“Our objective is to address as much of the population as possible with a single system. It’s interesting that the ‘female knee’ is, to my knowledge, just one of the three knee components. If you truly want to have a knee that addresses custom fit, it makes sense to have complete interchangeability between the tibial and femoral components. We’ve had that since 1983,” he added.

Features built into Biomet’s Vanguard knee prosthesis line that cater to women include better PF function, a modified femoral profile, better compatibility with the soft tissue envelope and nine sizes of femoral components, with a tenth being added soon. This helps the surgeon re-establish the correct joint line and reduces notching.

“When we designed the Vanguard knee, we inspected data from over 3000 knees, approximately 1800 of which were women. We’ve always designed with women and men of all statures in mind, and we’re going to continue to do that,” Kolter said.

Cost issues

From a cost containment standpoint, Biomet officials decided having a female-only knee with unique instrumentation would likely represent new technology that could mean higher costs to payers or that required hospitals to stock a lot more inventory. So, they steered clear of that approach.

“At the end of the day, every individual patient has to have something that’s as close to custom for their particular situation as possible. And, you have to do that in a fashion that does not create a huge burden on the health care system,” Kolter said.

Biomet did, however, recently introduce a gender-specific product: the Vanguard PFR PF replacement system, which addresses the increased 8% incidence of PF arthritis that women have compared to 2% in men.

Beyond the implant

In its advertising and promotions during the past 18 months, DePuy focused heavily on why women do not come in for treatment as early as their male counterparts. They are three times more likely to continue living with knee pain instead of having needed TKA surgery, according to statistics in the New England Journal of Medicine and Arthritis and Rheumatism.

Primary research DePuy conducted via a Harris interactive survey in 2004 supports its belief that their new high-flexion knees, which have a rotating platform and various sizing options, dovetail perfectly with their philosophy: “Get women in so that we all have an opportunity to treat them,” Colamarino said.

“We recognize the issue is not just the implant.”

Increased ROM

Greater ROM is one feature. “What we’re seeing is … we’re achieving up to 15% or slightly more ROM with this knee than any other knee that I’ve used. That’s been consistent,” said Hitt, who has done about 500 cases with the Triathlon.

Another goal of these new gender-specific implants: reduced pain. Hitt said Knee Society pain scores with the Triathlon implant were better than those with other implants with similar follow-up, and recovery was faster. “We’re seeing better results at six weeks than we’ve ever seen before. Again, is it a reflection of the design or the sizing issue?”

Increasing ROM is also touted as especially important in implants designed and marketed to address cultural-specific joint replacement needs. Greater ROM ranges can better support such activities of daily living (ADL) as kneeling and sitting, proponents say [See related story].

One example: Stryker’s CentPillar hip prosthesis was designed for the Japanese anatomy and is sold successfully and exclusively in Japan.

High flexion

“We have done exactly the same thing on the knee side, where we know that Japanese need to get very high flexion,” Mogul said. The company’s solution to different anatomy and ADL: Stryker’s Scorpio NRG knee implant with very high ROM and increased stability.

Biomet addressed the need for a high-flexion knee in its Asian markets by making all of its knees high flexion. They chose 145° of flexion for the Vanguard product because “we wanted to hit that proper balance between high flexion and bone preservation,” Kolter said. “We do it without creating an economic penalty for our hospital customers.”

For total hip arthroplasty in cultural-specific patients, Biomet, Stryker and Zimmer offer various offsets, neck lengths and angles that help surgeons address any hip anomalies they encounter intraoperatively.

DePuy’s high-flexion knee was first marketed in Asia to patients who do a lot of deep knee bends, but the firm launched it a bit more aggressively in the United States at the recent AAOS meeting.

Sheryl L. Conley, Zimmer group president, Americas and Global Marketing, and chief marketing officer, said Zimmer was the first company to market and sell a high-flex knee in Asia (in 1999) that could safely accommodate flexion up to 155º. Zimmer, she added, was also the first to sell a high-flex design in the United States and Europe (in 2000).

For now, Gender Solutions will be a key platform going forward for Zimmer, and they expect to apply the bone atlas research to different populations.

“It’s fair to say we’re looking at both gender and cultural differences everywhere,” Conley said.

For more information:

• Hitt KD, Sherman JR, Greene K, et al. Anthropometric measurement of the human knee: correlation to the sizing of current knee arthroplasty systems. J Bone Joint Surg Am. 2003;85-A:Suppl.4:115-122.