The key difference is the relative size of the end of the femur or thigh bone. For a given front-to-back or anterior-posterior (AP) width, the femurs of females tend to be narrower in the medial-lateral (ML) or side-to-side direction compared to males. An implant which is proportioned based on male anatomy would, therefore, be too wide on a similarly sized female. The result is that when a surgeon sizes the implant based on the AP width of the femur, the implant could “overhang” the narrower female bone and cause irritation of the surrounding soft-tissue. Faced with this situation, orthopaedic surgeons are sometimes forced to “downsize” the femoral component during total knee replacement surgery. In downsizing the femoral component, the doctor chooses a smaller implant, so that the implant does not overhang the sides of the bone.
There are, however, some potential disadvantages to downsizing depending upon the type of instrumentation system the surgeon is using during the knee replacement procedure. If the surgeon is using an implant system where the femoral component is placed at a particular distance relative to the anterior surface of the femur (anterior referencing), excessive laxity in flexion can occur with downsizing. Alternatively, if the doctor is using a system which references the posterior aspect of the femur, the resulting femoral preparation could “notch” or undercut the anterior aspect of the femur. This could, in turn, lead to fracture of the femur.
Recognizing that neither of the above scenarios is desirable, the makers of implants specifically designed for females have sought to remedy this by making the implants more narrow in the ML direction. On a male knee, the implant could “underhang” leaving some distal femoral bone exposed, but this is generally not a problem.
In the company announcements, both manufacturers noted that their new “Female” knee replacements are designed to address the more narrow female femur. Zimmer also noted that differences extend beyond femur width to things such as the angle of the femur in relation to the tibia. It will be interesting to see how these new implants fair clinically and whether the potential advantages are realized.
