Knee replacement was introduced many years after hip replacement became fairly mainstream. Because the knee as a joint is more complex than the hip there have been many more issues to resolve, and there has been a steady flow of innovation that is continuing. A knee replacement is expected to last for many years, despite being subjected to repetitive motion and demanding stresses.
Knee replacement continues to advance, with trends towards greater precision both in placement and in match to the patient's body type.
The materials of knee replacements are sophisticated and the manufacture is precise. Materials and designs are still evolving. Gender-specific prostheses are new. Computer-assisted positioning during surgery will soon be mainstream as incisions get smaller. Concepts to understand include the rationale behind the use or avoidance of cements, the preservation or destruction of the cruciate ligaments and the presence or absence of a 'meniscal' function.
Still under evaluation is the Unispacer and variants of it. This is an implant that does not involve removal of bone but it lies free within the joint and relieves the joint cartilage of abnormal stresses when there is for example a missing meniscus.
Computer-assisted surgery allows precise placement of total knee replacement, minimising abnormal stresses through the joint and implant. Smaller incisions are the trend, although there are surgeons who remain sceptical about the safety of this.
Unrecognised chronic infection as well as aseptic loosening are among the causes of chronic pain and eventual failure of knee replacement.