Knee Replacement is a surgical procedure that aims to replace the weight-bearing surfaces of the knee joint. The proximal part of the tibia and the distal part of the femur may turn out to be damaged, either because of rheumatoid arthritis, osteoarthritis, or traumatic injury. Such suboptimal conditions of the knee articulation (the largest joint in the human body) lead to pain and disability for the patient. In order to restore perfect mobility conditions, after a bone cut procedure the orthopaedic surgeon removes the damaged bone parts and most of the cartilages and knee soft tissues. The geometry of the bones, which plays a key role for the joint stability and mobility, is completely restored by means of the installed prosthetic components. A common knee prosthesis consists of a femoral and a tibial component, a patellar cap and a polyethylene insert (that replaces the menisci). Components are designed so that smooth movements and minimal wear are always ensured. In order to make the components better join to the cut bones, a specific acrylic cement can be employed during the installation.
After the surgical operation the patient normally undergoes a rehabilitation period of about 6 months. The patient slowly “learns” how to perform daily activities with their brand new knee. The ideal surgical outcome consists, in the long term, in the possibility to live a normal life without being limited by the prosthesis for most daily activities.
There are many different types of implants. When Knee Replacement is found to be necessary, the surgeon discusses with the patient about the clinical needs and all the possible implants. The basic principle is to reduce as much as possible the bone volume interested by the bone cut stage. Some people can benefit from just a partial (or unicompartmental) knee replacement, that is when the prosthesis replaces only one knee compartment (medial or lateral).
Concerning the material which prosthetic components are made of, there are many constraints. As already said, the set up of a proper compromise between stability and mobility of the prosthetic joint is a great surgical challenge. Thus, each component must be at the same time strong (enough to take weight-bearing loads) and flexible (enough to avoid undesired deformations and mechanical failure). But the most important constraint is represented by biocompatibility: the risk of rejection must be avoided. Biocompatibility strongly reduces the choice of usable materials (nowadays, prostheses are mainly in titanium or cobalt-chrome alloy).
In normal conditions, knee implants ensure a 15-20 years lifespan. When pathologies or suboptimal balance conditions occur on a prosthetic knee, this value is no longer ensured and risks being strongly reduced. In such cases, the only solution is represented by revision surgery. The patient undergoes a second Knee Replacement surgery in order to remove the suboptimal prosthesis and install a new one. This leads to a new hospitalisation period, a further bone cut procedure (revision components usually have longer stems that insert into the bones, as shown by the figure) and, of course, to a second rehabilitation period. Revision surgery always turns out to be more stressful than the first one: human body doesn’t appreciate when someone is playing with its parts, thus the probability of rejection increases and the recovery period gets harder.
My PhD project aims to develop a new generation of knee implants, able to compensate for suboptimal balance conditions without the need for revision surgery 🙂