The evolution of total knee arthroplasty. Part III: surface replacement

This approach, whichwas to survive the test of time and establish theprincipleswhichunderpin the success of today’s knee replacements, first emerged in the late1960s. From the start, it encompassed two main concepts; the first is, in our opinion, best termed the‘articular surface replacement’, as it sought to replace solely the articular surface of the femoral and tibial condyles, leaving the intercondylar zone and the cruciate ligaments intact. In essence, it built on the MacIntosh prosthesis and its descendents today are the unicompartmental replacements.The other concept replacednot just the articular surface of the condyles of both bones, but also included replacement of the femoral groove and excision of one or both cruciates with a tibial component which covered the complete tibial surface. Whilst these implants are generally termed ‘condylar replacements’ theyof coursereplacemuchmore than just the condyles. Interestingly, the patello-femoral joint has been and still remains their main weakness. For many surgeons, their initial attractionwas that they avoided themassive bone excision associated with the hinges and the awful problems posed by salvage when the latter failed. This was somewhat quaintly referred to as the ‘trumpet or daffodil disaster’, for often only a thin peripheral rim of femoral and tibial cortex remained. In this sense, the forerunner of the condylar replacement was the arthrodesis since the bone excision was essentially the same in both operations. This ‘condylar’ type accounts for thebulkof today’smarket, since it is technically easier to perform and the results are more consistent and less surgeon-dependent than the pure articular surface replacements. Both of these generic types had much of crucial importance in common. In principle, neither constrained rotation though clearly this is influenced in detail by the shape of the tibial surface. (Interestingly, the fact that the knee joint rotated was first discovered by the Weber brothers one of whomwas a Professor of physics in Gottingen and the other an anatomist in Leipzig as early as1836.) Both types were dependent on a proper understanding of limb alignment and ligament balancing, which was discovered relatively early on and the flexion--extension gap problem, which became apparent to the majority of surgeons only in the late 1980s andearly1990s.Finally, both requiredbioengineers not just for implant design but also for reliable instrumentation. Surgeons, on their part, would need to develop not only three-dimensional abilities, but also an excellent appreciation of angles of, for example, the tibial slope.