source (copy-paste): this website
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.
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. 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. This is very much akin to sizing for shoes. If your size is somewhere between 5A and 14D, you can probably buy a pair of shoes over the counter. If you lie beyond these “norms” (not meant in any pejorative sense), then you may need a custom product. Is there a downside to having a custom implant for each and every patient? Cost is obviously a consideration, but more than that, there is the potential error involved in fabricating any custom product.
how to determine the proper implant size?
Traditionally, the surgeon has used plastic overlay templates that are placed over the pre-operative x-rays. The templates are corrected for the magnification that can occur when the x-ray is taken. By comparing a variety of template sizes and shapes, the surgeon can get a strong suggestion of the implant that is needed. The final determination of size is made at surgery. The size and shape of the bones are measured during the surgical procedure and from these measurements the surgeon can judge how much bone to remove. When the manufacturers of the implants provide surgeons with a series of trial prostheses, these can be placed on the bone so as to assess for proper fit and soft tissue balance. At that point small changes to the bony cuts can be made, if necessary, and soft tissue releases performed to assure that the implants fit well and that the soft tissue (the capsular sleeve) is well balanced. Finally, permanent implants the same size as the appropriate trial implants are chosen, and these are then affixed to the bone ends.
Pre-operative templating is possible electronically. Using the digital images from radiology with digitized templates, the surgeon can assess implant sizes and perform a computer simulation of the insertion of the prosthesis. The surgeon can assess effects of changing implant sizes, changing implant designs (posterior stabilized vs. posterior cruciate ligament retaining prostheses), and even implants from different manufacturers. Thus, it is possible to identify in advance which patient will require a specially-sized – or even a custom – implant, and these can then be fabricated by the manufacturer.