Knee Arthritis

Arthritis is a word of Latin origin.  “Arthro” means joint and “itis” means inflammation.The “joint” part of the word is clear for all to understand.  The “inflammation” part is a little more difficult.  Inflammation is a specific pathological process that the body uses typically in a pattern of defence.  There are many causes of inflammation.  They include trauma, chemical insults, burns, infection and even cancers.

Most forms of arthritis are of a degenerative nature. This is so-called osteoarthritis. Other forms include rheumatoid arthritis, psoriatic arthritis and even ankylosing spondylitis. The joint is made up of several important tissues. There is a bony skeleton that forms the base. The part of the bone that rubs against the neighbouring bone (the joint surface) is covered by cartilage. This is a white, smooth, low friction material made up of molecules and cells. It is called hyaline cartilage. The joint is surrounded by a capsule which is lined with synovium. This is a soft, fluffy type of material that secretes nutrients and enzymes into the joint space. The joint also has a small volume of fluid. When the arthritic process starts and progresses, the synovium becomes swollen and reddened, the volume of fluid is increased, the enzymatic distribution is changed and the whole joint becomes swollen, hot, painful and stiff. Whilst the cause of the inflammation can vary, the pathological process is the same albeit to a greater or lesser extent. As the process progresses (if it does), the joint surfaces (and their cartilage coverings) are gradually worn and deformed. Ultimately, with osteoarthritis, the cartilage is lost completely and a so-called bone on bone joint surface results. This is the end stage and is usually associated with quite marked pain, stiffness and deformity.

Meniscal Tears

In addition to the bone, cartilage, synovium and capsule, there are other structures within the joint.  They include the menisci.  These structures are made of fibrocartilage and when looked down upon (in plan view), have a “C” shape.  When they are looked on in end elevation, they have a wedge shape.  The peripheral parts of the menisci are thick and the parts nearer the centre of the joint (both medially and laterally) are quite thin.

The menisci are sometimes referred to as the “cartilage”. You may recall friends of yours sustaining a “cartilage tear” many years ago whilst playing football and undergoing an operative removal of the “cartilage”. It would have been the meniscus that they were talking about. Meniscal tears are usually of a traumatic origin. Occasionally, tears can occur through a degenerate region in the process of osteoarthritis. Some meniscal tears are of no great significance. Others can give rise to considerable pain and impingement between the lower end of the thigh bone and the upper end of the shin bone (the two bones that form the knee joint). The tears can be so large on occasions that they will actually jam the joint. This is so-called “locking”. It typically occurs when the knee joint is flexed (say at about 90°) and the patient is unable to extend beyond that position. Then the joint will suddenly “unlock” with a click or a clunk and a sudden sharp pain.

Osteochondral Lesions

You will recall that the joint is made up of bone which is covered by hyaline cartilage. Occasionally, a segment of the underlying bone can become detached, taking its cartilage cap with it. This would be a so-called “osteochondral lesion”. They can occur developmentally (ie be present from birth or thereabouts) or alternatively, can be of a post-traumatic origin. Because part of the joint surface is involved, these lesions assume very great importance. It is preferable to reattached the bony base of the osteochondral loose body and allow is to regrow. Sometimes, the osteochondral lesion has to be removed. This can leave a defect in or near a weight bearing surface and predispose the patient to the onset and progression of osteoarthritis in quite a rapid manner. This requires careful assessment and treatment by your Orthopaedic Surgeon.

Anterior Cruciate Ligament (ACL) Ruptures

Along with the bone, the hyaline cartilage, the synovium, the capsule and the menisci, the joint also has two special ligaments within. These are called the “cruciate ligaments”. There is an anterior cruciate (at the front) and a posterior cruciate (at the back). The term “cruciate” comes from “cross” in Latin and refers to the fact that the anterior cruciate ligament runs upwards and backwards and the posterior cruciate ligament runs upwards and forwards. They cross in the middle of the knee as you would note on a crucifix. Thus the name.

These ligaments are very important. They provide a large part of the stability enjoyed by a knee joint. They are responsible for limiting and controlling forwards and backwards movement of the shin bone on the thigh bone, and also rotary movement of the shin bone on the thigh bone. If the anterior cruciate ligament is ruptured, patients often feel quite unstable whilst loading bearing on the knee. This can occur with normal walking but is also exacerbated by ascending and descending steps or slopes, running and squatting.

Patellar Dislocations

There are three bones that make up the knee joint. The lower end of the thigh bone, the upper end of the shin bone and the kneecap all inter-relate. The kneecap sits at the front and rubs against the lower end of the front part of the thigh bone. The kneecap is a so-called “sesamoid bone”. That means that it lies within a tendon structure. It is designed by nature to stop the tendon wearing excessively. The tendon above the kneecap is the quadriceps muscle and its tendon and the tendon below is the part of the tendon that runs from the kneecap itself down to the shin bone. As the quadriceps muscle contracts, it pulls on the kneecap which in turn pulls on the patellar tendon.

This is how you can extend your knee. As the quadriceps muscles relaxes, so does the patella move downwards and so does the knee joint bend. It is this upwards and downwards movement (flexion and extension) that would wear the quadriceps/patellar tendon mechanism excessively if the kneecap did not exist. The kneecap is a “V” shaped structure, again covered with this special hyaline cartilage. It sits in a “V” shaped groove on the front of the lower end of the thigh bone. There is a combination of factors which allows the kneecap to run true and safely in this groove. Sometimes those factors are disturbed, diminished or lacking. Sometimes excessive force is applied. Whatever the cause, there are occasions when the patella will jump outside that groove. If it does do so, this is a so-called “patellar dislocation”. Dislocations always occur laterally. Sometimes, the patient may experience a partial dislocation or so-called “subluxation”. This is obviously not as severe but is usually just as painful. The condition requires very careful analysis and special treatment afforded only by a competent Orthopaedic Surgeon.