FIXATION FOR THE MILLENNIUM – THE KNEE

Abstract

Total knee replacement (TKR) has always been one decade behind total hip replacement (THR). Successful changes have been slower to develop and less creative. The response to failures of the 1980s was to reduce risks with TKR, while in hip surgery the response was to raise the bar one or two notches higher in risk-taking. Part of the risks taken by hip surgeons who promoted non-cemented fixation during the decades of the 1990s was certainly different than that by proponents of non-cemented knee surgery and the resultant confidence level in the fixation in these two different joints is evident. Furthermore, data on results with total knee replacement, both cemented and non-cemented, has been less available in the decade of the 1990s for TKR as opposed to THR. Part of the reason for decreased creativity and risk taking with TKR is that cemented fixation of the knee has performed remarkably well for tota1 knee replacement. Almost all of the long-term results of success with total knee replacement, including the Total Condylar and the Insall-Burstein knee, have been with knee replacements that were performed with cement including cemented patella replacement. The survivorship of these cemented implants at 15–20 years remains nearly 90%.

Several factors influenced both the increased number of failures with non-cemented implants and the loss of confidence among orthopaedic surgeons for the use of non-cemented fixation with TKR. Firstly, the PCA knee had a high number of failures because of the bad plastic inserts that were used for the tibia. The heat pressed polyethylene delaminated fairly rapidly and created a significant number of failures that were associated with severe osteolysis which created a good deal of fear for non-cemented fixation. In addition, all the non-cemented implants were using metal-backed patellae, which resulted in 20–25% revision rates for at least the patella in PCA, Miller-Galante, and Ortholoc prostheses. Finally, the initial design of non-cemented implants including the PCA and the original Ortholoc knee were too flat-on-flat and this contributed to the high amount of wear and failures from instability over time with these knees. Combination of complications confounded the evaluation of the fixation of these non-cemented implants and the risks needed to improve the fixation method were not taken.

Presently, cemented fixation still dominates as the fixation of preference for orthopaedic surgeons performing total knee replacement. The confidence level with cemented fixation is very high and the results continue to be outstanding with the use of cemented fixation. One of the reasons for this is that the average age of patients who have a total knee rep1acement is 68 and therefore most patients that are 70 years of age or older can easily achieve the durability with cemented fixation that is necessary for them to have only one operation during their lifetime. At the present time cemented fixation is recommended for patients older than 70 years and the use of an all-polyethylene tibia is just as effective as a metal tray in these patients. In fact, with the data from Engh on particle formation with metal trays, the use of an all-poly tibia may be preferable.

In the last two to three years of the 1990s there was a resurgence of investigation into total knee replacement without using cemented implants. One of the most prominent of these was the treatment of the patella. There were several studies, including those of Whiteside, Barrack, and others that showed that the use of no patella button at all did give satisfactory clinical results, which were difficult to differentiate from patients with patella replacement. Secondly, the LCS mobile bearing knee reported results at 15 years that showed that non-cemented fixation actually had some areas of superiority to cemented fixation. These results did demonstrate to the orthopaedic community that a knee design which had a good articulation surface that did not cause accelerated wear and osteolysis could perform as well as knees with cemented fixation. Results with the Natural Knee also demonstrated that except for failures by wear in those patients who had thin polyethylene, the fixation of the implants was universally excellent at 10 years postoperative. Finally. Leo Whiteside settled on a central fluted grit blasted stem for fixation of the tibia and achieved excellent and immediate fixation with his Profix implants. The recommendation by me at this time is that non-cemented fixation is preferable for patients under the age of 60.

In revision knees the most common technique has been the use of non-cemented stems with cemented metaphyseal fixation. However, the problem with this fixation method is that the metaphysis often is very weak bone or has an absent bone and this results in poor rotational support for the metaphyseal implant. The non-cemented stem of the knee is not as stable in the diaphysis of the femur as is the proximal femoral stem in a press-fit situation. Therefore, if rotational constraint is lost at the metaphysis, the entire femoral implant has a significantly increased risk of being loose. Therefore in older patients I believe that a cemented stem is much more preferable. Older is defined as patients over the age of 70. The press-fit stem in the tibia provides more fixation than it does in the femur so that tibial fixation has more expectation of durabi1ity. However, again I believe the surgeon can use cemented fixation of the stem and the tibia in patients over the age of 70 with an expectation that it will be as durable as the non-cemented stem.

In the future, there will be an increased used of mobile bearing knees because of the theoretical benefit that in more active patients the wear would be better. Since these knees will be initially directed towards more active patients, the use of non-cemented fixation would also be desirable. The LCS knee has demonstrated that with a mobile bearing design, non-cemented fixation is durable. The use of fixed bearing knees is being expanded to a younger population also. Younger patients are asking for total knee replacements because of the successful use of this operation during the 1990s. In these younger patients, non-cemented fixation should be just as beneficial as it is felt to be for total hip replacement. The Natural Knee has proven that the fixed bearing designs can indeed be durable with non-cemented fixation. Finally, the success with hip replacement and the yearly increasing numbers of hip replacements that are performed by non-cemented fixation demonstrate that the orthopaedic community is gaining more and more confidence with non-cemented fixation. The increased knowledge of bone and the increased knowledge of the causes of osteolysis are helping to provide more and more success with non-cemented fixation. For these reasons, I believe that in the future non-cemented fixation will become the standard for total knee replacement.