Unicompartmental Arthroplasty with the Oxford Knee. Buy the book here.
This chapter is available from Goodfellow Publishers as a PDF.
Introduction
Having demonstrated in Chapter 2 that a fully conforming mobile bearing can minimise polyethylene wear, in this chapter we show that a mobile bearing prosthesis, unconstrained in the sagittal plane, can restore natural mobility and stability.
For surgeon readers who are less interested in the theoretical background, it might be advisable to go straight to Chapter 4, Indications, or to start by reading the final section of this chapter, The Loaded Prosthetic Knee. If that proves interesting, the surgeon might attempt The Unloaded Prosthetic Knee. For the more research minded surgeon or engineer, it seems more logical to start with the Unloaded Natural Knee (the longest section of the chapter) and to read from there. The chapter may also be of interest to those surgeons embarking on the use of a bi-cruciate retaining total knee replacement.
The numerous writings on knee movement and the many methods used for its measurement and analysis over the past two centuries have been reviewed in detail by Pinskerova, Maquet and Freeman (2000) and by Freeman and Pinskerova (2005). We will not attempt to repeat such reviews. We present our own evidence as to how the passive soft tissues of the human knee interact to control the passive motion of the bones and how this motion is modified in activity in the presence of muscle force, external loads and consequent tissue deformation. This provides a base with which to compare the kinematics and mechanics of the Oxford Knee in cadaver specimens and in living patients. In designing these studies, we have used as our model the example of D’Arcy Thompson:
…. ligaments and membrane, muscle and tendon, run between bone and bone; and the beauty and strength of the mechanical construction lie not in one part or another, but in the harmonious concatenation which all the parts, soft and hard, rigid and flexible, tension-bearing and pressure-bearing, make up together.
D’Arcy Wentworth Thompson, On Growth and Form, 1945, Cambridge University Press, Macmillan Edition.
The shapes of the articular surfaces of the Oxford Knee components do not match those of either compartment of the natural joint and, even if they did, they could hardly be expected to match exactly the shapes of the surfaces of each individual patient. How is it possible for such an implant to restore normal mobility and stability, normal kinematics and mechanics?
The complex three-dimensional pattern of movement of the natural knee depends upon the following:
(1) the shapes of its articular surfaces which hold the bones apart;
(2) the design of the array of ligaments that hold the bones together;
(3) the deformation of the tissues dependent on the magnitude and direction of the forces applied by muscle contraction in response to gravity and ground reaction and other external loads.
In any particular joint, features (1) and (2) are constant and therefore the movements of the unloaded knee should be predictable and repeatable. However, the forces applied during activity are as infinitely variable as the uses to which the human limb is put, and the consequent patterns of movement of the loaded knee are also infinitely varied. Blankevoort et al.(1988) noted that “the basis for the understanding of the kinematics of the knee joint lies in the description of its passive motion characteristics”. Passive motion is what the surgeon observes on the operating table with the patient under anaesthetic.