Unicompartment Osteoarthritis

What is Osteoarthritis?

Osteoarthritis is really not a very accurate term. It is a degenerative condition which happens to everyone’s joint surfaces with time to a greater or lesser extent. The suffix ‘itis’ usually indicates an inflammatory condition.This is why the term osteoarthritis (OA) is inaccurate. A much better term is Degenerative Joint Disease (DJD) or osteoarthrosis.

In contrast rheumatoid arthritis and its many variants belong to a different group of true inflammatory disease, which affects multiple joints usually and does not spare any particular part of the joint. The end result of both groups is similar; that is destruction of the ‘articular surfaces’ coating the bone ends in the joint.

Anatomy of the knee

The knee joint is the articulation between the thigh bone (femur) and the shin bone (tibia).
We can conveniently think of the knee as formed of 3 compartments:

  • The medial compartment, on the inside of the knee (i.e. nearest to the other knee!)
  • The lateral compartment on the outer side of the knee and the patellofemoral joint (the joint between the kneecap – patella – and the front of the lower end of the femur).
  • The first two are weight bearing while the patella acts as a ‘pulley’ in the front of the knee to increase the efficiency and power of the quadriceps muscle, but is not directly involved in weight bearing.In each of the two weight bearing compartments, there is another kind of cartilage almost unique to the knee (menisci or ‘Footballer’s cartilages’). On the medial side this is called the medial meniscus and likewise in the lateral compartment, there is the lateral meniscus.

What is unicompartmental OA?

As OA is not a systemic disease but a degenerative condition, which can be affected by mechanical problems or focal injury, it can possibly affect a single compartment sparing the other two. In such situations, usually affecting the medial compartment particularly in males, the patient may notice a progressive deformity in the knee over a number of years, typically the patient could become gradually more bowlegged due to loss of some or all of the thickness of articular surface covering on the weight-bearing areas.

Inflammatory arthritis such as rheumatoid variants, blood tests are not helpful for OA.

X rays will show some signs of OA. Cartilage however, cannot be seen by plane X-rays. In order to determine the extent of damage to the cartilage, it is essential to take the frontal X-ray views in a weight bearing position. The importance of standing X-rays is often over-looked and can give wrong information. In fact, it has been shown that the most accurate assessment includes weight-bearing X-rays taken with the knees slightly bent (20 – 450)

X-rays for the kneecap

There are also special x-rays for the kneecap not taken routinely at most institutions which give valuable information (skyline views). I find stress X-rays taken with a special device (Telos) very useful in assessing the opposite side of the joint when only one compartment is apparently damaged. By transferring stress to the more normal looking side of the joint, you can tell whether or not there is significant cartilage damage to that side by virtue of whether or not that side becomes narrow with stress.

Degenerate meniscal tears

Although the meniscus is the kind of cartilage that young athletes tend to tear, what a lot of people don’t realise is that just like articular cartilage, meniscal tissue also degenerates and you can get a degenerative tear of the meniscus on one side of the joint with little or no injury. If that is the main problem, then relief is rapid and success rates are very high after arthroscopic partial menisectomy (taking out the torn bit though keyholes).

Cartilage Repair Techniques

Articular cartilage has no blood supply. It receives its nutrition from joint fluid. It therefore has no regenerative power on its own. Whereas the underlying bone is rich in blood supply. By drilling little channels into the bone (microfracture), or abrading the bare bone surface with a burr, you bring some new blood supply to the surface. Trickles of blood coming through these drill holes, form a blood clot, rich in cells which seals the defect.

High Tibial Osteotomy (HTO)

When OA damage is restricted to only one of the two weight-bearing compartments, and provided we have confirmed that the opposite side is virtually pristine and that the patellofemoral joint is reasonable, then a re-alignment procedure can be very useful. This involves cutting the bone (osteotomy) incompletely on one side and opening up a wedge under the damaged side, thereby off-loading the damaged compartment. The gap created is then either held open by internal fixation using a plate and screws for example, in which case bone grafting from the hip is necessary to fill the gap, or by an external fixator with gradual correction, so that the patient’s own biology is harnessed to create new bone (callus), by a process called callus distraction.

The external fixator is applied with screws through the skin above and below. The advantage of the latter technique is that it uses the patient’s own biology to form new bone at the osteotomy site, therefore not requiring bone grafting. Using an external fixator also respects the patient’s biology by allowing for a minimally invasive tiny incision for the osteotomy. Although, due to the tiny minimally invasive approach, the chances of deep infection are smaller, the chances of superficial pin site infection are higher. This is seldom a problem to the patient and usually settles with either pin site care or oral antibiotics for a few days. This operation (High Tibial Osteotomy) is useful for the younger more active usually male patient who is keen to remain active particularly at sports. A typical patient is in his forties although it can be performed on patients up to the age of 65. We look at physiological rather than chronological age; i.e. we are not ageist at the Knee Clinic! This procedure usually keeps the patient going for around ten years before he needs a knee replacement.

Although this has been around for some time, the results have not been good until the more recent models had been developed. Again we do not have significant information about how well they will do in the long term, but we have very promising short and medium term results, you can always convert this to a Total Knee replacement when it has done its time in the future.As you can see from the above, there are a large number of options for patients who have OA limited to a single compartment. The clinician has to spend time considering the best option for his patient, taking on board the patient’s main symptoms, history, clinical examination and investigations. At the Knee clinic the patient is very much involved with every step of these investigations and shares in the decision making process throughout.