Requirements for artificial knee joints
Three basic aspects are absolutely critical for the implantation of an artificial joint to be successful: 1) a kinematic mechanism that is as natural or physiological as possible; 2) stable anchoring of the components; 3) minimal wear and tear. The principle “rolling followed by sliding” is associated with a highly specific activation of the ligaments, muscles and sensors during the various forms of movement. The complex interplay between the joint, muscles and reflex systems should not be destroyed by the artificial knee joint. Walker (2000) defined the design criteria as follows: after implantation the forces in the remaining structures and the kinematics of the joint best be restored to normal.
These aims have previously never been achieved. In the early years of knee endoprosthetics, coupled prostheses with congruent sliding surfaces were the order of the day. As a result of the reduced numbers of degrees of freedom and the high level of friction inherent in such designs, the shearing and torsional forces would typically lead to a loosening of the tibial component shortly after the implantation. Later designs have tried to overcome these problems by reducing the congruence of the joint surfaces. While this modification did allow for more degrees of freedom in the knee joint, it did not affect the sliding motion. It instead effectively reduced the area of force transfer, thereby significantly increasing the strain on the polyethylene.
Subsequent designs started to include axial rotation and AP-translation by relying on auxiliary constructions, which would add complexity and complications, oftentimes diminishing the clinical results.
Despite the advances in design and component materials, the sliding mechanism remained unchanged. The femoral heads of the artificial joints slide inside more or less congruent bowls, which in turn are anchored rigidly or loosely, with or without an array of studs or screws. Taken together, progress in recent years has mostly come from improved combinations of materials with reduced levels of wear and tear.
The demands formulated by Walker have not been met and the ever more complex auxiliary constructions are a reflection of a profound lack of understanding of the natural kinematics of the knee joint.
Given the rather disappointing state of affairs at the time, with poor clinical results being the rule, Nägerl, Kubein-Meesenburg, Cotta and colleagues reexamined the kinematics of the human knee joint. They focused their attention on the incongruent geometry of the articulating surfaces, which turned out to hold the key to a new and fundamental understanding of knee joint function. The results of this breakthrough line of research forms the basis of a new generation of prostheses developed and marketed by
ÆQUOS Endoprothetik GmbH.