Although the first knee replacements were attempted just after the second world war, modern knee replacements were really only developed in the 1960s, some ten years or so after hip replacements were fairly routine.
The mechanics of the knee are complex, and in that early phase the subtleties were poorly understood. By the 1980s that first generation of knee replacements had started to fail from wear-and-tear and loosening, and after removal many of the failed prostheses (implants) were examined carefully by development engineers and the surgeons working with these engineers. Design advances resulted (eg. cementless knee replacements) - and surgeons started to implant the second generation of prostheses.
Currently hybrid knee replacements are being tried, with cement on the tibia side and no cement on the femur side. Again, when a significant number of these have failed and been studied, the next cycle will begin. The current generation of replacements have been lasting longer than the first, and the signs suggest that these knee replacement will last about twenty years before wearing out.
So what does this dynamic moving process mean to you?
Age at first knee replacement
Well, it relates to the age at which the first knee replacement is practicable.
Revision surgery, that is having the previous knee prosthesis removed and replaced by a new one, is a much bigger procedure than primary (the first) knee replacement. There will have been wear and tear of the original one. Debris (mostly from the plastic component) will have triggered some irritation. Old cement may be hard to remove. All the old cement has to be drilled out. The bone may be fragile from osteoporosis, and may break - requiring plating and long-stemmed prostheses. This is why surgeons generally press patients to wait until they are older before having their first surgery.
It's a matter of mathematics.
Say you are forty at the time of the first knee replacement. So a revision is likely by 60. And another may be necessary at 80. People are living longer these days. With each revision the risks of a major complication increase. The bone gets more and more fragile, and the patient less able to meet the demands of surgery and rehabilitation.
To try and be objective, surgeons often use scoring systems to gauge whether the level of disability is really worth the risk of knee replacement in the younger person. These usually include an assessment of the level of disability during 'activities of daily living'.
The extent of the arthritic process is also important to consider, as the younger patient may be a good candidate for a 'uni-compartmental' procedure, thus delaying the age of eventual total knee replacement.
The Concept of Knee Compartments
The term 'uni-compartmental osteoarthritis' refers to joint destruction limited to one of the three compartments of the knee. The knee is not actually physically divided into compartments, but it is helpful to think of the three separate areas of joint articulation as if they were separate compartments. These three areas are the inner part of the joint between femur and tibia (the 'medial compartment'), the outer part of the joint between femur and tibia (the 'lateral compartment') and the joint between the patella and the femur the 'patello-femoral compartment').
The medial and lateral compartments exist because the femur has two 'knuckles' or condyles, which each has its own contact surface with the tibia below. These surfaces called the 'tibio-femoral' joints.
Joint destruction - arthritis - frequently commences just in one compartment. For example, a damaged shock-absorber (meniscus) on one side between tibia and femur may trigger joint damage just in that compartment. While the arthritis is still confined to the one compartment, it is generally an option that this one joint compartment can be replaced with a prosthesis (artificial joint) - the so-called uni-compartmental or partial arthroplasty
Limited damage - uni-compartmental (partial) knee replacement
A uni-compartmental arthroplasty (knee replacement) replaces the joint surfaces of the single damaged compartment. Thus this may be a tibio-femoral arthroplasty (which could be medial or lateral) or it may be a patello-femoral arthroplasty. The latter is commonly called resurfacing.
Uni-compartmental arthroplasty is still in its infancy, particularly with respect to the patello-femoral joint. The idea is that joint mechanics can be much improved by replacing this damaged portion, possibly deferring total joint replacement for many years, or even permanently.
Extensive damage - total knee replacement
Once the joint damage extends beyond a single compartment, there is generally little to be gained by doing a uni-compartmental procedure. One is then looking at total knee replacement, which in effect means replacing both tibio-femoral joints and possibly the patellofemoral joint as well
Design Considerations of knee prostheses
The design of knee prostheses depends on a number of factors -
- the number of compartments being replaced
- the incongrous joint surfaces
- the integrity of the cruciate ligaments
- cement fixation
Number of compartments being replaced
The femoral part of joint replacement is usually metal, curved into the shape of the lower end of the femur with its two condyles (the 'knuckles'). Usually the metal is titanium-based, but other metals (eg zirconium) are used as well as mixtures. Ceramic is being tried by a small number of manufacturers, as it reduces the wear of the polyethlylene component upon which the femoral part rides.
The tibio-femoral unicompartmental prosthesis has two components - the curved metal femoral component shaped much like the femoral condyle it replaces, and the almost flat tibial component - made of polyethylene - replacing the flat tibial plateau. This latter may be fixed to a metal backing plate or it may move freely on it.
The patello-femoral prosthesis has a v-shaped titanium plate shaped like the 'trochlea' - the gully between the rounded femoral condyles. The patellar component is made of polyethylene, with a kind of backng button to fix it to the remaining part of the patella.
The total knee prosthesis - like its cousin - has a metal femoral component, this time with two rounded condyles, sitting on a flattened polyethylene tibial surface, again fixed or free on a metal backing plate
Incongruous joint surfaces
The important thing to understand is that the condyle and underlying tibia are incongrous - one is rounded, while the other is flattened. Early designs had metal on both the femoral and tibial side, but there were major problems with wear - and the tibial contact surface was replaced with polyethylene. Where a flatter polyethylene plate was used, greater mobility was possible for the patient, but the wear of the polyethylene was greater. Constraining the femoral condyles in grooves within the polyethylene created less wear of the polyethylene, but reduced the patient's mobility, as the joint operated effectively like a hinge joint.
Polyethylene fatigue tend to occur below the surface, so when it breaks up it comes to pieces quite quickly and in chunks, so it is very important to try and reduce the stresses on this component.
The development teams came up with the solution of constraining the metal condylar surfaces in deeper grooves in the polyethlylene, but then allowing the polyethylene componenet to slide on its own metal plate, thus allowing greter mobility of the whole without particlarly stressing the polyethylene. This was called 'mobile bearing' as compared to the more rigid 'fixed bearing'. A later improvement was the concept of 'meniscal bearing' where the mobile polyethylene plate more closely resembled the meniscus (the original shockabsorber between femur and tibia), which of course is lost in this procedure.
Integrity of the Posterior Cruciate Ligament
The cruciate ligaments are attached in the middle of the knee to the femur above and to the tibia below. The are very important contributors to the stability of the knee. The one at the back - the posterior cruciate ligament (PCL) - is often damaged by the arthritic process, and in this case the surgeon will tend to use a posterior stabilised total knee prosthesis which helps to stabilise the knee in this circumstance. Some surgeons feel that it is best to remove the PCL, even if it is functional, and to use the same prosthesis. Others, however, feel it is important to retian a functional PCL, and instead they will use a special cruciate sparing prosthesis.
All older prostheses were fixed in place with bone cement, in reality a compound called polymethylmethacrylate which is prepared by mixing two compounds together during surgery just a few minutes before it is needed. The timing is fairly crucial as the compound goes rock hard after a short while and it becomes impossible to adjust the position of the new parts.
The cement has several other considerations -
- Some patients can have quite a severe reaction to the product, with a sudden drop in blood pressure, for which the surgical team is generally well prepared.
- The first generation of prostheses generally failed because of degradation of the cement, rather than of the prosthesis itself.
Cementless designs were produced (press-fit), coated with little bubbles which aid in fixing the metal to bone. The jury is still out as to which of the two - cemented or cementless - give better results, and meanwhile many surgeons are combining the two, cementing the tibial component but not the femoral one.