Primary Knee Replacement: Management and Alternatives

Medscape Orthopaedics & Sports Medicine eJourn. 2000;4(2) 

In This Article

Primary Knee Replacement: Management and Alternatives

The knee absorbs joint reactive load over a wide area of thick cartilage. In degenerative arthritis, which is the condition most commonly necessitating total knee replacement, the quality and quantity of the articular cartilage is diminished. As wear continues, there is more bone-on-bone contact, resulting in increasing pain. At this Symposium, innovative treatments and management strategies for total knee arthroplasty (TKA) were discussed.

The session began with a review of several papers describing the nonoperative treatment of patients with osteoarthritis. Dr. Johan Bellemans, of the University of Leuven in Belgium, presented an overview of the use of chondroitin sulfate and glucosamine for patients with osteoarthritis. A review of the published data indicates that these oral substances were superior to placebo in relieving pain, but were less effective than the nonsteroidal anti-inflammatory drugs. The best results were seen in patients with very early osteoarthritis; the results diminished as the severity of the osteoarthritis increased.

The use of injectable hyaluronic acid substitutes in the treatment of osteoarthritis was also discussed. A comparison of the various types indicates that those with the highest molecular weight and highest amount of cross-linking seemed more efficacious, but they also had a higher incidence of inflammatory reactions when compared with lower-molecular-weight, less cross-linked drugs. The propensity for an inflammatory reaction after injection did not appear to be related to patient age or diagnosis. Again, as was the case with chondroitin sulfate and glucosamine, the best results were observed in patients with early osteoarthritis; the results in patients with more advanced arthritis are much less predictable.

Two alternatives proposed for formal total knee replacement are high tibial osteotomy and unicompartmental replacement.

High Tibial Osteotomy. Dr. Robert Bourne of the University of Western Ontario in London, Ontario, reported a long-term survivorship follow-up of patients who had undergone high tibial osteotomy. A total of 106 tibial osteotomies were performed by 2 surgeons between 1976 and 1988 in patients who had primary degenerative osteoarthritic changes in the medial compartment of the knee and a varus deformity. Survivorship analysis revealed that the probability of survival was 73% at 5 years, 51% at 10 years, 39% at 15 years, and 30% at 20 years. Complications such as delayed union, nonunion, intra-articular fracture, and infection occurred in up to 10% of patients. Of the 106 high tibial osteotomies, 61 required conversion to a total knee and, of those, 11 (18%) had already required a second total knee replacement. They found that those patients who preoperatively had a lateral thrust or range of motion less than 120 degrees did not do well postoperatively; neither did those with a small body mass. Neither did the thinner patients. Survivorship of those with more than 120 degrees of flexion, no lateral thrust, and age younger than 50 was 95% at 5 years, 80% at 10 years, and 65% at 15 years. These investigators recommended the ideal candidate as being younger than 50 years of age and having a body mass index greater than 25 kg per square meter, isolated medial compartment osteoarthritis, no lateral thrust, a varus deformity less than 10 degrees, and a range of motion greater than 120 degrees.

Dr. Cecil Rorabeck, also from the University of Western Ontario, presented the results of patients with unicompartmental replacement who were followed for 10 years. They found that failures, when they did occur, occurred early (≤ 5 years), and that, in order for this procedure to function successfully, patients required an anterior cruciate ligament that was intact. Ten-year survival in this study was 85%.

Total Knee Arthroplasty. Dr. Richard Laskin, of The Hospital for Special Surgery, discussed the key design criteria for a total knee replacement prosthesis for the year 2000. He cited the following as important factors in femoral component design: an anatomic right and left femoral component; a proper trochlear groove that is deep, long, and funneled; and prostheses that accommodate either retention or removal of the posterior cruciate ligament.

The tibial component should be asymmetrical to maximize tibial coverage and to provide a proper capture mechanism for the polyethylene to prevent cold flow and lower surface wear. Because of the unknown and various scenarios that may occur at the time of surgery, it is important to have polyethylene inserts available to accommodate PCL retention, posterior stabilization, and the use of a conforming implant. Both Dr. Laskin and Dr. Leo Whiteside of St. Louis, Missouri, discussed the crucial importance of proper configuration of the trochlear surface of the femoral component to aid in patella tracking and to minimize contact stresses in that compartment of the knee. Dr. Whiteside suggested that with the proper femoral component and proper trochlear groove, one could consider not resurfacing the patella. He substantiated this suggestion with data demonstrating that 90% of his cases had excellent results.

The concept of a medial pivot knee was also mentioned, and opinions were expressed as to whether it had any clinical relevance in the total knee patient in whom the ACL was removed. There was no consensus other than that further study is necessary to determine its clinical relevance.

Polyethylene. Dr. Kevin Weaver of Memphis, Tennessee, presented the current status of cross-linked polyethylene and its use in total hip and total knee replacement (Figure 1). He presented data demonstrating that polyethylene that was irradiated in air and then allowed to age had inferior wear properties compared with polyethylene that was nonsterilized or treated by ethylene oxide sterilization. When polyethylene was gamma-radiated in air, numerous free radicals were formed which could cause breaks in the polyethylene chains. When polyethylene was irradiated in an inert atmosphere, oxidation did not occur; however, some cross-linking did occur. The optimal method to obtain cross-linking would be first to irradiate the polyethylene and then melt it to eliminate the free radicals. The material is then machined into a final component form.

This comparison chart shows the amount of bone resected for the femoral box in various total knee PS prostheses. The image is used with permission of Dr. Laskin.

A major concern regarding cross-linked polyethylene was discussed: the trade-off between wear properties and strength. The more polyethylene is cross-linked, the greater its wear properties -- but its strength diminishes. Since the articulations of the hip and knee differ, one could not extrapolate results seen in the hip with cross-linked polyethylene to results in the knee. Theoretically, a weaker material might not be appropriate in the knee, because there is more localized loading and lesser congruity. Dr. Weaver concluded that all cross-linked polyethylenes are not the same and that trade-offs would have to be made between strengths and wear to find the optimal material.

Dr. Laskin discussed the configuration of the femoral box and the amount of bone removed with the different posterior stabilized femoral components presently available. The volume of bone removal varied, ranging from a low of 5.9 cm3 to a high of 13.5 cm3. With increasing amounts of bone removal, there is increasing concern that fracture of the distal femur may occur. To decrease the probability of fracture, the posterior stabilized post must be correctly placed at a proper height. A post that is too anterior impinges on the femoral component when the knee is fully extended or slightly hyperextended. A post of insufficient height and improper placement could allow the femoral component to jump over the post and dislocate.

A different problem, known as patella clunk, occurred with the use of the initial Insall-Burstein II, a posterior stabilized knee design. It has not been seen with any other design of this genre. The initial Insall-Burstein II had a very short trochlear flange and a sharp change in radius of curvature between the trochlear and the distal femur. Any soft tissue that developed near the junction of the patella and the quadriceps tendon could get caught in this area. This design problem was resolved in later versions of the implant.

Dr. Laskin discussed the results of a study using an ultra-conforming implant, rather than a posterior stabilized polyethylene. He presented data that demonstrated that range of motion, ability to reciprocate stairs, and stability were comparable. He noted that any posterior stabilized design must be used in conjunction with proper ligament balancing in order to have a stable knee in all degrees of flexion and extension.

Biomechanics of the Total Knee. Dr. Scott Banks of West Palm Beach, Florida, discussed the motion of total knee replacements during gait and presented the results of his study performed in conjunction with Dr. Steven Haas, Dr. Richard Laskin, and Dr. James Otis at The Hospital for Special Surgery. Twenty patients who had undergone total knee replacement and had excellent Knee Society scores were studied. He found that treadmill gait was characterized by a slightly prolonged stance phase and a slightly reduced loading response in knee flexion. The cruciate retaining knees exhibited some anterior femoral sliding and external rotation during knee flexion, while the cruciate substituting knees exhibited femoral rollback and internal rotation during flexion. Neither group exhibited large condylar translations or rotations during the stance phase of gait. Most interesting was the fact that there was no evidence of condylar liftoff in the fluoroscopic analysis of patients with this particular prosthesis (Genesis II).

The final paper was presented by Dr. Robert Bourne and discussed whether mobile bearing knees had a place in today's orthopaedic practice. Dr. Bourne drew upon his experience with the SAL prosthesis, which had a rotating and translating bearing, as well as results described with the LCS prosthesis, which had just a rotating bearing. In theory, the rotating movable bearings are highly congruent and therefore should have decreased wear and a decreased incidence of loosening. When he reviewed the results obtained with the SAL prosthesis, he found that one third of the patients had proper rotation and translation, one third tended to rotate only, and one third had a paradoxical translation.

He concluded that a fixed bearing was appropriate for most patients. A rotating platform could be considered in patients with a life expectancy of more than 15 years, a deformity less than 15 degrees, and in those who would not require a medial or lateral release. He also suggested that most patients would do best with a rotating or translating bearing, so he would save the rotating type of component only for patients with gross PCL deficiency, those with a prior patellectomy, or possibly those with severe inflammatory arthritis.

The final conclusion was that all rotating platform knees are not the same; they differ in terms of rotation and translation, axis of curvature, and contact stresses. Rotating platforms present a potentially new design for total knee replacements; they may increase longevity and function. However, before widespread use of these components occurs, more clinical studies are needed to determine who would benefit most from them and what those benefits are.

Made possible through an unrestricted educational grant from Smith & Nephew.

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