Follow up on Female Knee Replacement

In one of our first posts, we discussed what was at the time a new type of knee replacement implant specifically designed for female anatomy.  These “gender specific” knee replacement implants have a shape tailored to fit the narrower distal femur or thigh bone in women. (see New Total Knee Replacements Specially Designed for Female Anatomy).  In that original post it was stated that “It will be interesting to see how these new implants fair clinically and whether the potential advantages are realized.”  Well, in a recent study published in the Journal of Bone and Joint Surgery1, these exact issues were investigated.

In the study, a group of female patients who received gender specific knee replacements were observed. Eighty five (85) women received a “gender specific” implant in one knee and a “standard” implant design in the other knee.  The group was followed for two years.  The results showed virtually no difference in outcome. Knee scores, range of motion and patient satisfaction were similar.   Interestingly, the only significant difference noted between the two types of implants was that at the time of surgery, the surgeons observed a better implant fit for the standard implant group vs. the gender specific group.  This is not what one would have assumed since the gender specific knee replacements are specifically designed to better-fit female anatomy. Although there are several different brands of gender specific knee replacements available and only one particular brand was studied, the results do suggest that the proposed advantages of gender specific or female knee replacement implants may be overstated.  However, the authors acknowledge that the 2 year follow up period may not be indicative of more long term results..  Stay tuned as more information becomes available on this controversial topic in total knee replacement.

1 Young-Hoo Kim, Yoowang Choi, and Jun-Shik Kim. “Comparison of a Standard and a Gender-Specific Posterior Cruciate-Substituting High-Flexion Knee Prosthesis: A Prospective, Randomized, Short-Term Outcome Study”. JBJS, 2010; 92: 1911-1920

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New Total Knee Replacements Specially Designed for Female Anatomy

Recently, two major orthopaedic implant manufacturers, (Stryker Corp.  and Zimmer Holdings Inc.) announced the availability of implants specifically designed for women.  We all know that men and women are different, but how does this relate to knee implants?

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.

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