Requirements for successful total knee replacements. Design considerations.

Historical review up to 1969 showed the emergence of conservative hemiarthroplasty resurfacings and rigid all-metal hinges. The first cemented metal-on-plastic design, introduced in 1969, began a decade in which a large number of different condylar designs were introduced. Variations of these condylar replacements in terms of femoral-tibial surface conformity, the fixation elements, and the provision for the patellofemoral joint produced wide variations of clinical results. In parallel with these condylar replacement designs, unicompartmental and both lax and fixed hinges were still used for particular indications. A review of all of the different design types showed that the most successful results were obtained with partially conforming condylar replacements with provision for the patellofemoral joint. Although cemented condylar replacement components composed the majority of the clinical reports, press-fit uncemented designs also had significant promise. The results of unicompartmental designs depended dramatically upon the indications. Present hinged replacements, whether lax or fixed-axis, produced problems that were sufficiently serious to restrict their use to cases in which condylar replacements were distinctly inadequate. Despite the success of condylar replacements at up to 10 years follow-up, a number of problems remain. These include optimizing the femoral-tibial geometry for maximum range of motion, minimum loosening, and minimum wear. Longer-term problems include improved design for long-term fixation and reduction of wear of polyethylene by optimum surface geometry, by better material quality control, or by improved materials. Whichever instrumentation system was used, surgical parameters such as component placement and sizing were shown to affect the potential range of motion. Considerations of design were given for revision cases in which condylar replacements were inadequate. Certain design suggestions were made for minimizing the serious problems used with lax or fixed-axis hinged replacements.