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All Knees Are Not Created Equal

by Jennifer Chan

Since female knees are thinner, the gender-specific knee replacement allows for a more natural movement, said a professor at the Keck School of Medicine of USC.

Photo/Lauren Walser

Each year, nearly half a million Americans receive knee replacements to relieve pain and disability.

According to a recent survey, two-thirds of the primary recipients are women because they tend to live longer than men, thereby incurring more joint wear.

And with the number of knee replacement surgeries increasing, surgeons have realized that the unique differences between the female and male anatomy of the knee were not being addressed with traditional knee replacement options.

“The size and shape of the female knee bones, including the femur, tibia and patella, are significantly different from men,” said Donald Longjohn, assistant professor of orthopaedic surgery at the Keck School of Medicine of USC.

The differences were not lost on Zimmer, a company that is a worldwide leader in joint replacement solutions. Zimmer recently developed a gender-specific implant to better meld with the female anatomy.

Extensive analysis of computed tomography scans from both female and male patients were used to help determine appropriate design features of the implant � named the Zimmer Gender Solutions Knee.

Longjohn and his colleague Kelly G. Vince, associate professor of orthopaedic surgery at the Keck School, were consultants at Zimmer, which allowed them to be among the first physicians to bring the implant to their female patients, long before other physicians in Southern California.

An important change was made in the gender-specific implant, Longjohn explained. Instead of the traditional design of same size medial-lateral (ML) dimension and anteroposterior (AP) dimension, the ML dimension is smaller.

“Implant size is based on the measurement of the AP dimension of the patient’s femur. The AP size is important for proper kinematics � motion and stability � of the knee,” he said. The problem with many traditional implants for women is that when the size is correct in the AP dimension, it is too large in the ML dimension.

“If we use that implant, it will overhang the bone on the inside, outside or both sides of the knee and soft tissue may rub over it and cause pain,” he said. “If we go to a smaller ML dimension to avoid overhang, then the AP dimension will be too small and that could cause instability. To compensate we would have to use a thicker plastic insert, except then the knee would be too tight to extend straight.”

Longjohn said that to correct the extension tightness, more bone would be cut from the end of the femur so the ligaments would function properly.

When the Gender Solutions Knee is used in this situation, all these steps are avoided, he said.

The Gender Solutions Knee has a unique shape and size that has three distinct differences from all other knee implants: It has a thinner profile to eliminate bulkiness, allows for more natural movement of the kneecap and has a shape specially contoured for women that may allow additional flexion and extension movement.

“Female knees are thinner and have a slight difference in angle between the femur and tibia than male knees, so the gender-specific knee ensures precise articulation that allows for a more natural movement,” Longjohn said.

He said that knee replacement in general improves quality of life � often dramatically in patients with severe arthritis. Overall, he said, “I like the concept of making the implants better fit the patients rather than making the patients fit the implants. The Gender Knee does that.”

The U.S. Food and Drug Administration approved the Zimmer Gender Solutions Knee in May 2006.

All Knees Are Not Created Equal

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