Abstract
It is clear in 2013 that there is a substantial opportunity to improve patient outcomes after total knee replacement. Much attention in the last decade has focused on the apparent satisfaction gap between patients who have had total hip arthroplasty and those who have had total knee arthroplasty. Most authors note that a higher proportion of total hip patients claim to have complete satisfaction or note that they have forgotten that they had the joint replaced. The concept of “the forgotten joint replacement” is an interesting one because as surgeons and researchers we all recognise that neither total hip replacement nor total knee replacement will completely restore the native hip or knee joint's dynamic 3D biomechanics or kinematics. What the concept of the forgotten joint does tell us however is that there is a level of kinematic function above which humans cannot detect a difference with normal function. The inherent simplicity of the ball-and-socket design of the hip joint means we can achieve this level of function more reliably and reproducibly than we do in the knee joint. The knee joint presents a more difficult challenge.
Recent data suggests that there is a definable trade-off in total knee prosthesis design, and likely with component position and limb alignment, between those optimised for the best kinematics and those optimised for the best durability using contemporary biomaterials (namely metal, ceramic and ultra-high molecular weight polyethylene). Given this inherent trade-off then there will be an almost never-ending debate about what constitutes “the best” overall knee implant design because that will inevitably require an individual value-judgement about the relative merit of better kinematics or better durability. Currently, we have some insights into this trade-off when we consider the role of unicompartmental knee replacement in 2013. There is little debate that unicompartmental knee replacement results in closer-to-normal knee kinematics than does total knee replacement and that many patients seem to benefit from a quicker recovery and easier rehabilitation. Data from multiple national joint registries however shows that UKR is not quite as durable as total knee replacement (mean yearly failure rate 1.53% for UKR versus 1.26% for TKR). Different surgeons and different surgeons will look at that data however and come to markedly different conclusions about how to act — some will discount the difference in durability and favor the better function/quicker recovery of UKR while other equally intelligent persons will discount the difference in function and prefer the demonstrated better durability of TKR. Like any value-judgement there is no right answer or wrong answer.
As surgeons and researchers we do have opportunities in regard to surgical technique that remain unexplored. We have been limited over the past several decades by thinking primarily in terms of 2D static analyses of alignment, rotation and ligament balance. This is primarily because most assessments have been done using plain radiographs. The last decade however has seen a marked improvement in our capabilities for 3D imaging and dynamic assessment of knee joint function. The promise of computer-navigated and robotic-assisted surgery has largely remained unfulfilled as the limitations of 2D targets have come into focus. It is my belief that tomorrow's gains in total knee replacement will not involve dramatic changes in prosthesis design but instead on defining and then hitting more precise 3D targets for alignment, rotation and ligament balance in surgery. For surgeons and researchers this is an exciting time as there is a distinct opportunity to improve outcomes for millions of total knee replacement patients over the coming decades.
