ADVANCES IN TOTAL KNEE ARTHROPLASTY: A SECOND LOOK AT THE PAST 30 YEARS.

Abstract

Condylar knee prostheses were designed over 25 years ago, as it became evident that the hinge prostheses previously developed were unsatisfactory because of their marked potential for loosening and femoropatellar problems.

There are currently several hundreds of different types and subtypes of condylar prostheses available and more are being introduced on the market place every year. Continuous technical development has become a familiar feature with high technology products; does this also apply to knee prostheses?

The basic designs of condylar knee prostheses already existed 25 to 30 years ago, with the Freeman-Samuelson knee implanted in 1970, the Total Condylar in 1974 and the Insall-Burstein posterior stabilized knee in 1978. Long term results are now available for these prostheses as well as for others that came next: overall, survival rates between 84% and 98% have been reported with follow-up from 10 to 25 years. Will newer implants do any better? It will take that long to know, as it has been repeatedly demonstrated that theoretical studies or tests on knee simulators are not really predictive of the in vivo behavior of an implant.

The wide variety of current implants evidently reflects commercial interests. Each of the current prosthetic knees represents a more or less unique combination of a number of features related to the geometry of the articulating surfaces, including the trochlea and patella, the resection, preservation or substitution of the posterior cruciate ligament, the type of fixation to bone; most have a modular fixed tibial bearing, with various types of fixation to the underlying metal baseplate, and an increasing number of others have a mobile bearing with restrained or unrestrained mobility in rotation and/or in AP translation. The introduction of new knee prosthesis should hopefully provide some improvement in terms of function, range of motion, and implant survival as compared with the existing knees. Such improvements have proved very difficult to demonstrate on an objective basis, all the more as the results achieved with a specific implant do not only reflect its design, but also a number of manufacturing and processing features, and also the way it has been implanted.

We have learned a number of lessons from past failures such as

• - the importance of selecting adequate materials

• - the importance of an optimal manufacturing of implants (surface macro- and microgeometry, rugosity,

• - the importance of an adequate sterilization technique for polyethylene

• - the importance of a good trochlear design

• - the dangers of resurfacing the patella using a metal-backed component.

Surgical technique has substantially improved over the years, not only because better instrumentations have been developed, but also because we have come to a better understanding of the anatomy and physiology of the arthritic knee before and after arthroplasty.

The rotational positioning of the femoral component has been extensively studied, and is now a regular feature of the operative technique. This, together with other technical factors such as improved design of the prosthetic trochlea, has to a large extent cleared the femoro-patellar complications which marred the results of some early designs. Techniques for ligament balancing in the varus or valgus knee have become systematized.

The issue of preservation or substitution of the posterior cruciate ligament is still a matter for debate, but we now know that it is unrealistic to expect restoring normal knee kinematics with prosthesis when both cruciate ligaments are not intact. Posterior stabilized prostheses have been shown to provide more predictable kinematics than others intended to be closer to normal physiology.

To summarize, advances have been made over the past 30 years regarding a number of elements among which implant design was probably of lesser importance than better understanding of the anatomy and physiology of the knee, better understanding of ligament balancing and operative technique in general, as well as improved manufacturing and processing of implants.