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.