A Medical Device Daily

CHICAGO – At last week's American Academy of Orthopaedic Surgeons (Rosemont, Illinois) annual meeting here, Zimmer Holdings (Warsaw, Indiana) reported its intention to soon begin marketing what it said is the first knee replacement system designed specifically for women patients. But the need for such a product hasn't been universally accepted.

Ray Elliott, Zimmer president, CEO and chairman, said in a statement: “Our Gender Solutions knee femoral implants have been specifically designed for women, which distinguishes our system from those which are merely downsized versions of existing designs – women are clearly not little men.“ He said that the company would make 510(k) application for the device “as a means of addressing the specific anatomical differences typically seen in women. We believe we are the first company to do so.“

The company noted that women represent the majority of knee replacement patients with about 60% of all cases, with the company noting that a woman's knee is different from a man's in the ratio between the width and height of the femur, the angle of the femur in relation to the tibia, and a less prominent anterior surface of the femur, among other differences. The ligaments supporting the knee also tend to be more “lax“ in women.

“The new Gender Solutions implants will address a need that we as surgeons have realized for some time now – that compromises are made in matching implant sizes to a patient's anatomy,“ said Robert Booth, MD, chief of orthopedic surgery at University of Pennsylvania Hospital (Philadelphia). He said the design “is based on anatomical studies of the differences between men and women, meaning that we are able . . . to come closer to matching components to each patient. With women representing so much of our patient base, it only makes sense that we would take this step to try to improve implant fit through very specific design features.“

The company said that the Gender Solutions knee implant is based on the clinical success of Zimmer's flagship brand, the NexGen knee solution. The Gender Solutions implant will first be available in Zimmer's “flex“ design, which enables patients who are able to achieve high degrees of flexion for their specific lifestyle needs, such as bending and kneeling.

Elliott said the new development of gender-specific products might be “the advance of the decade,“ but not everyone in the sector agrees the need is compelling.

Rival Stryker (Kalamazoo, Michigan), commanding about 19% of the knee market, launched the Triathlon knee about a year ago and recently has been more vocal about marketing it to women.

The product has been well received and is the fastest-growing knee product line, Stephen MacMillan, Stryker chairman, said in an interview with Reuters. “It's appropriate for both men and women. This has been mostly a marketing challenge,“ he said, adding that the company may step up its advertising campaign this year.

The Triathlon knee comes in several sizes, has a slightly different shape than conventional knee replacements, and commands a slight premium over Stryker's other knee implants, MacMillan said.

Another competitor, Johnson & Johnson 's (New Brunswick, New Jersey) orthopedic business DePuy (Warsaw, Indiana), which last year commanded 23% of the knee market, recently hired Angela Lansbury – who became best known starring in the television mystery series “Murder, She Wrote“ – for its advertising campaign. The actress has had two hip implants and one knee implant.

DePuy spokeswoman Monika Gibson said the campaign would focus on the social and emotional differences between the sexes. “Our data doesn't really support that women need a different implant than a man,“ she told Reuters.

Dane Miller, CEO of Biomet (Warsaw, Indiana), the fourth-largest maker of knee implants, concurred, saying he did not believe the differences were great enough to warrant a different design for women.

According to Mohamed Mahfouz, MD, part of the research team for the Gender Solutions implant design and co-director of the Center for Musculoskeletal Research at the Oak Ridge National Laboratory (Oak Ridge, Tennessee) as well as assistant professor, mechanical, aerospace and biomedical engineering department at the University of Tennessee (Knoxville), anatomical studies do indicate some clear distinctions between female and male knee anatomy.

“We created what we call 'anatomical atlases,' or bone morphology atlases, which are huge collections of data sets that we can combine in a statistical way and then start using the atlases to highlight differences between males and females,“ said Mahfouz. He said that over five years and 800 femurs and patellae reviewed, “we found significant differences . . . including the narrower width of female femurs, reduced anterior condyle height and a tendency toward a more lateral patellar track. Zimmer addressed these anatomical differences in their Gender Solutions design.“

The company said it hopes to receive regulatory clearance to begin marketing the implants with specific claims regarding women patients in the second half of 2006.

Also at the AAOS meeting:

In a panel session, orthopedic experts discussed recent advancements in limb lengthening and deformity-correction techniques.

For a variety of reasons – previous injury, neurological conditions, bone infection or diseases or congenital conditions – a person's legs can grow to different lengths. This condition, commonly referred to as leg-length discrepancy (LLD), affects both adults and children.

John Birch, MD, orthopedic surgeon and assistant chief of staff at Texas Scottish Rite Hospital for Children (Dallas), said, “Minor discrepancies in lower extremity limb lengths – considered 1-1/2 cm or less – are actually extremely common. However, adults and children with more than 2 cm difference between legs may have an increased risk of wear and tear on the back, hip and knee.“

Trauma is seen as a common cause of LLD, but many problems stem from muscle weakness, birth defects or other deformities. To handle LLD of 2 cm or more, a non-surgical option is to wear a build up, or lift, in the shoe to make the legs feel the same length while standing or walking. However, most patients prefer not to wear a visible lift.

In children, the longer limb can sometimes be slowed through surgery, or by surgically shortening the longer limb. A final option is to lengthen the shorter limb.

Panel participants noted that compared to limb lengthening, limb shortening is less complicated and less painful, but there is a limit to how much the bone can shorten without permanent muscle weakness.

William Mackenzie, MD, chairman of the department of orthopedics at Alfred I. duPont Hospital for Children (Wilmington, Delaware), said, “Although different types of bone fixation are used today, the underlying method of limb lengthening and realignment is virtually the same throughout the world.“

After applying an external fixator – a device placed outside of the leg that holds the bone in position with pins and wires – the bone is divided in a non-traumatic fashion to preserve the surrounding tissues and is gradually lengthened. As the bone lengthens, new bony tissue fills the gap.

“Most people don't realize that if you pull bone apart slowly, it will stretch and grow,“ said Mackenzie. “However, since limbs can be lengthened only 1 mm per day, this is a very gradual process that takes a considerable amount of time.“