The Total Condylar Prosthesis (Howmedica, Rutherford, NJ and Cintor, Braintree, MA) has been the gold standard against which results of other implants are compared. Rodriguez et al 17 recently reported 20-year results. Six of 220 knees (2.7%) had loosening, but no osteolysis was measured and no failures from wear were reported. Pavone et al 13 reported results with the Total Condylar knee prosthesis with survivorship of 91% at 23 years with three of 120 prostheses revised for loosening and no prostheses revised for osteolysis or wear. Six patients had osteolytic areas around the tibial component, but in no patients did this cause any failure. Clearly, wear is not an issue if the operation is done correctly with a CoCr femoral component, round-on-round articulation, a monoblock metal-backed tibia or an all-polyethylene tibia, and a cemented all-polyethylene patella.
The lessons from the Total Condylar design have been learned repeatedly since 1980 when significant alterations to total knee designs were made. 4,5,10,18 The PCA knee design (Howmedica) attempted to normalize rotational movements of the knee by using a flat-on-flat articulation surface. This theoretically attractive concept, in combination with the first truly good and user-friendly instrumentation, quickly dominated the knee replacement market. The response of competitive companies was to mimic this articulation surface and instrumentation. The PCA also introduced bone ingrowth fixation with knee replacement and this too was promoted by competitive designs. As a consequence of these changes, wear was a dramatic complication of the flat-on-flat articulations, especially with the heat-pressed polyethylene of the PCA prosthesis, which delaminated rapidly. 5 In a retrieval study, Benjamin et al 2 reported 78% of flat-on-flat inserts had eccentric and asymmetric wear patterns compared with 12% of curved inserts. Because so many of these flat-on-flat replacements were implanted with bone ingrowth fixation, wear became associated more with cementless fixation than with cemented fixation. 14,18
In the 1990s, results of the success with the AGC knee design (Anatomic Graduated Component, Biomet, Warsaw, IN) which is a flat-on-flat design, heightened the emphasis on the importance of polyethylene. 15 Ritter et al 15 credited the success of the AGC design to the use of direct compression molded polyethylene. The effects of polyethylene, and that of design, on the wear of polyethylene has been studied best by several authors. 12,14 They reported that catastrophic polyethylene failure of unicondylar implants occurred when the shelf life of the polyethylene was 4.5 years or more. This work was confirmed by Collier. (Collier J: Catastrophic polyethylene failure of unicondylar implants. Presented at the American Association of Hip and Knee Surgeons Meeting, Dallas, TX 2001.) These implants were sterilized with gamma irradiation in air and stored on the shelf in an oxygen environment.
Perhaps the most important research was a study of the motion of polyethylene inserts in modular metal trays. 12 Every tray design studied had excessive motion of the polyethylene in the tray that created backside wear with polyethylene debris-induced osteolysis as the consequence. Wasielewski et al 18 also suggested that the nonarticular surface could be the dominant source of polyethylene debris. Clinical confirmation has been observed with the IB (Zimmer, Warsaw, IN) monoblock versus modular tibial components. Colizza et al 7 reported no radiographic wear or osteolysis in the monoblock IB prosthesis at 10 years postoperative. Pagnano et al 10 reported a modular IB knee replacement with osteolysis that required revision at 6 years postoperative and correlated this to backside wear of the polyethylene. Rodriguez reported a 1.5% incidence of revision for osteolysis at 10 years postoperative. (Rodriguez JA: The results with the modular PFC. Presented at the American Association of Hip and Knee Surgeons Meeting, Dallas, TX, 2001.) In another report, the incidence of osteolysis and synovitis was 5% in the modular tibial group and 0% in the all-polyethylene tibial group. 16 The conclusion was that backside wear in modular tibial components offsets the benefits of modularity. Another example of wear-caused failures with modular tibial implants was reported by Parker et al 11 to be 10% with the Miller-Galante I knee replacement (Zimmer) at 10 years followup.
Fixation with total knee replacement between 10 and 20 years postoperative has remained excellent with cemented and noncemented implants. 6,9,13,17,19 Failure caused by loss of fixation for all reported implants was less than 5% at 15 to 20 years. Clearly, fixation of total knee replacement, absent excessive wear that can cause osteolysis and failure, could be expected to be durable and life-long for most patients having total knee replacement who are 65 years or older.
Wear is not an issue with total knee replacement if the lessons from the history of total knee replacement are understood by the surgeon and he or she makes correct choices. The first choice is that the total knee replacement design should not be one that promotes normal rotation and kinematics of the knee, but is one that promotes low wear of polyethylene. Because clinical results are not different, yet longevity clearly is, the choice is clear. In general, designs with the lowest prevalence of failure by wear or osteolysis are those with round-on-round articulation surfaces such as the Total Condylar and IB concepts. The quality of polyethylene used with any given design also must be known as is evidenced by the success of the AGC knee replacement with direct compression molded polyethylene and the failure in polyethylene with long shelf life as presented by Engh. (Engh G: Catastrophic polyethylene failure in unicondylar knee replacement. Presented at the American Association of Hip and Knee Surgeons Meeting, Dallas, TX 2001.)
The tibial component contributes to wear and osteolysis if it is modular because of polyethylene debris from the nonarticulating surface. Even mobile-bearing knee replacements had revision for polyethylene failure and osteolysis, which was reported by Beuchel et al 6 as 1.8% of the LCS rotating-platform (low contact stress, DePuy, Warsaw, IN) and 5% for meniscal-bearing designs. In contrast, the total condylar all-polyethylene tibia and monoblock IB and AGC knee replacements were not revised for these causes. 7,13,15,17 Although cementless fixation has been implicated as a cause of polyethylene failure, in fact, this only was because none of the knee replacements with this fixation had fixed-bearing tibial components. It is clear that wear is not an issue if the tibial component is all-polyethylene or monoblock with good polyethylene.
If the patella is resurfaced, this should be done with a cemented all-polyethylene patella. Berger et al 4 reported 35 of 112 (31%) metal-backed patellas with the MG-I knee replacement were revised because of wear through of the polyethylene. Berend et al 3 reported 180 of 4287 (4.2%) cemented AGC all-polyethylene patellas to be radiographically loose with 15 of 4287 (0.3%) requiring revision. None of these cemented all-polyethylene patellas were revised for patellar wear. For those surgeons who choose to resurface the patella, the choice should be a cemented all-polyethylene patella.
Finally, it is the responsibility of the surgeon to do this operation correctly. To prevent failures from catastrophic polyethylene wear, the knee replacement must have the components mated correctly so tracking of the femur on the polyethylene is not malaligned; the soft tissue balance must be correct for the medial and lateral compartments and flexion and extension so that lift-off and edge-loading of the femur on the polyethylene does not occur. Sharkey stated that instability was the predominant cause of failure. (Sharkey P: Causes for revision of total knee replacement. Presented at the American Association of Hip and Knee Surgeons Meeting, Dallas, TX 2001). Polyethylene wear was a cause in 22%, but was most commonly associated with malalignment and instability. Barrack et al 1 studied the influence of internal rotation of the tibial component on anterior knee pain and found a direct correlation to this malalignment of the tibial component and anterior knee pain postoperatively.
D’Lima et al 8 used a knee wear simulator to show that malalignment caused the highest wear rates. It is the responsibility of the surgeon to become very skilled at doing total knee replacement with his or her choice of design to avoid excessive wear. Wear is not an issue with failure of total knee replacement when correct choices are made by the surgeon.
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Richard S. Laskin, MD—Guest Editor