During total knee replacement surgery, or arthroplasty, surfaces of the thighbone (femur), the shinbone (tibia) and the kneecap (patella) are replaced with a metal-and-plastic implant.

“Traditionally, the implants have been designed based on men’s knees,” says Dr. Robert Bucholz, a professor of orthopedic surgery at UT Southwestern Medical Center (Dallas). “For women, smaller implants were used, but size did not resolve the anatomical differences, which sometimes accounted for pain and discomfort and an ‘awkward’ feeling.”

Orthopedic device manufacturers have developed “gender knees,” which accommodate numerous shape differences between the average male and female knee, such as:

• Narrower shape. Knee implants are typically sized by measuring the end of the femur from front to back and from side to side. Most women’s knees are shaped like a trapezoid and are narrower from side to side; men’s knees are more rectangular. Implant size is typically based on the front-to-back measurement to allow the knee to move and flex correctly. In women, however, an implant that fits from front to back can be too wide from side to side, sometimes causing it to overhang the bone.
  
  
• Thinner density. Typically, the bone in the front of a woman’s knee is less prominent than in a man’s. After traditional implantations, some women describe a “bulky” feeling in the joint despite better function. The front of the new implant is thinner so the replacement more closely matches the female anatomy.
  
  
• Tracking. A woman’s femur attaches to the tibia at a slightly different angle than a man’s, because most women have wider hips. The angle difference between the pelvis and the knee joint, which is taken into account with the gender-specific implants, affects how the kneecap tracks over the end of the femur as the knee moves through a range of motion.