There are many causes of painful symptoms associated with the extensor mechanism of the knee and the patellofemoral joint. Problems occurring with the knee which do not settle with simple treatment and cause persistent and significant trouble will require investigation by a specialist.  It is, however, helpful to understand something about the type of conditions that can cause these problems.

Mechanical Alignment and Stability Problems with the Kneecap

The kneecap is a small  piece of bone the under-surface of which is shaped like a shallow valley and covered with a thick layer of articular cartilage.  This articulates with a similarly shaped groove at the lower end of the femur.  The shape of the bones varies considerably and the kneecap can only be kept in the correct position by a delicate balance of the strong muscles at the front of the knee.  One of the commonest causes of problems with the kneecap is when the balance is disturbed.  The most severe form of this condition is when the kneecap actually dislocates out of its groove and then has to be physically put back into position. This condition is easy to recognise.

MR scan of the Patellofemoral joint showing the thick articular cartilage of the patella.

A CT scan showing the patella slipping out ot its groove.

There are many lesser forms of this condition, the most minor of which being when the kneecap is very slightly tilted and does not run properly in its groove.  This causes excessive pressure on one side of the joint leading to painful symptoms and damage to the articular cartilage surfaces.  This condition is very difficult to diagnose and often causes troublesome symptoms which persist for many years.

Problems with the Bearing Surfaces

The front of the knee is very vulnerable to injury and this can cause damage to the articular surfaces of the patellofemoral joint.  In spite of its vulnerability, sudden injury from a fall is relatively rare.  The precise causes of damage to the articular surfaces are not fully understood.  While injury remains a likely explanation, malalignment of the kneecap in its groove is probably a common and more insidious cause of damage.  This only occurs slowly over a number of years.

The kneecap is just under the skin and large muscles control the knee.

Secondary Effects of problems with the Kneecap

The underlying causes of damage to the kneecap are often difficult to diagnose and understand.  Secondary effects as a result of an underlying cause are, however, common and make the condition worse.  Principal amongst these is wasting of the quadriceps muscle at the front of the knee.  This controls the movement and the strength of the joint.  Any painful condition of the knee will rapidly lead to wasting and weakness of this muscle, which will then cause further problems of control and stability.  

Any damage to the inside of the joint will lead to the release of harmful substances which will cause inflammation, swelling and thereby more pain.  Thus a cycle of pain, swelling, inflammation and weakness develop which can lead to a gradual deterioration in the function of the joint.

Prolonged Damage Leading to Arthritis

If problems with the patella continue for a number of years this will eventually lead to established arthritis.  The strange thing is that the development of this condition can occur in the absence of any significant symptoms of pain or swelling and only becomes symptomatic when the condition is well established.  Arthritis of the patellofemoral joint is rare before the age of 40, but then becomes increasingly common.  After the age of 50 between 10-20% of people with painful knees have isolated arthritis at the patellofemoral joint.

X rays of typical Patellofemoral arthritis