A dislocation is an injury where two (or more) parts of a joint are separated and can no longer move in their normal fashion. Commonly dislocated joints include:

• shoulder (gleno-humeral) joint – which is between the upper arm and the shoulder blade
  
• acromio-clavicular joint (AC joint or collarbone) – which lies between the outer end of the collarbone and the top of the shoulder-blade, just above the true shoulder joint
  
• kneecap (patello-femoral) joint – often mistakenly called a “knee dislocation”, but actual fact the kneecap (patella) slips out towards the outer side of the thighbone (femur). It is much less common to dislocate the femur from the tibia, which is the correct meaning of a true “knee dislocation”
  
• small joints of the fingers (interphalangeal joints)

These are fairly common injuries and most can frequently be treated by manipulating the the dislocated joint back into normal alignment initially, also called a “closed reduction”. Any other joint in the body can dislocate, although they tend to be less common.

There are a few structural features of any joint that help keep all the components moving together and preventing most peoples’ joint from dislocating. This includes:

• the conforming shapes of the two bones involved in forming the joint
  
• the ligaments that hold the bones in the joint together
  
• the strength and direction of force of the muscles that move the joint

In general terms, there will be an abnormality of one or more of these structural components that will lead a joint to dislocate. Moreover, once a joint has dislocated there is often irreversible damage to either the muscles or ligaments that normally control the joint and this makes repeated dislocations much more likely. This is especially the case in the shoulder and patella, and also in the case of young children or adolescents, resulting in recurrent instability of the joint, more so than adults. While recurrent instability of a joint itself may be disabling, there will often be associated damage to the cartilage surfaces that can occur with the initial or subsequent dislocations and this may lead to later arthritis, as small chunks or flakes of cartilage can be sheared off during the abnormal movement of the joint surfaces or as they are put back into place during a closed reduction.

Very rarely it may be impossible to reduce the dislocation with closed techniques as muscle, tendon or nerves may have been trapped between the joint surfaces. In these uncommon cases an operation will be required to perform an “open reduction”, which allows the surgeon to remove the interposing structure and return the bones to their correct position.