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Total Knee Replacement at HSS

Armen Hareyan's picture

Implants for total knee replacement

When total knee replacement was in its infancy in the early 1970s, the manufacturers of the implants offered the prostheses in very limited sizes. At surgery the patient's bone had to be sculptured so as to fit the available sizes of implants. It rapidly became obvious, however, that a better scenario would be to have a variety of implant sizes that would come close to matching the patient's anatomical situation. Also, it was obvious that sizing was more than just designing implants for tall or short patients, male or female patients, or even for thin or heavy patients.

Differences in implant size and shape

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At present, all of the prosthetic implant companies offer knee prostheses in a variety of sizes. How were these sizes determined? Initially, designers studied the anatomy of normal femoral and tibial bones in the pathology laboratory. Subsequently, with the advent of CT scans, these were used to compile a database of the "standard" sizes and shapes of the bone of patients who were going to undergo knee replacement.

As a broad generality, women are smaller than men. Consequently, more female bones were studied for the smaller sizes of implants, while male bones were studied for the larger sizes. Obviously this was not an absolute: there are many short men, and likewise many tall women. There were also evaluations of the width and height of the joint surfaces that were used to determine implant sizes.

This method of designing implants is a good one as long as the "database" of CT scans or bones that are evaluated is representative of the patients that will be operated upon. There are subtle variations between various ethnic groups throughout the world. Therefore, the more diverse patient bones that are studied, the more valid is the database.

The result has been that all modern, reputable manufacturers of prostheses offer implants that cover over 98% of the sizes that we see in clinical practice. For those in the remaining 2%, a custom implant may be required.