A joint dislocation occurs when the two joint surfaces, which normally sit in intimate contact with each other, are wrenched away from each other to lie apart without any relationship. Joints have a surrounding ligamentous bag called a joint capsule and this can be typically injured as the surfaces force their way past each other. The surfaces of the joints themselves can be damaged as they hit each other on the way to becoming dislocated. Other injuries which can occur include damage to the local nerves and ligaments.
Shoulder dislocation is the most common type of joint dislocation, accounting for nearly half of all such joint injuries. The shoulder dislocates frontwards, an anterior dislocation, in the vast majority of cases. The most common type of injury is one which forces the head of the arm bone forwards with the arm in a position of abduction, outward rotation and extension, the vulnerable position of the joint. Other mechanisms of injury can include a forceful abduction and outward rotation movement of the arm, a blow to the back of the upper arm and a fall onto the outstretched hand (FOOSH).
A posterior dislocation is uncommon and secondary to a stress on the arm when it is inwards across the body and inwardly rotated, with the large back and chest muscles sometimes pulling the joint out of its socket. This can occur if someone is electrocuted or if they have epileptic seizures, both of which can cause muscle spasms. The shoulder can dislocate downwards if there is a very forceful movement of the shoulder outwards and sideways, with the joint being levered out over part of the scapula above. This sort of dislocation should be closely monitored as complications of the injury are common with nerve damage, blood vessels injury and rotator cuff tears.
There may be no trauma in some cases of shoulder dislocation and instability of the shoulder may occur in all joint directions, typical presenting in patients who have hypermobile joints. This condition is called multidirectional instability and tends to happen in both shoulders, run in the family and be in younger people under thirty. A joint subluxation is often the start of these problems, where the joint slips partly off its partner to an amount and then clicks back into place. An ability to voluntarily dislocate the shoulder can occur, perhaps related to psychiatric difficulties in this group of people.
The presentation of anterior dislocation of the shoulder is for the patient to hold their arm rotated outwards and slightly to the side, the arm bone head easily felt at the front of the joint. The shoulder muscles may be in a powerful spasm and trying to move the shoulder results in high levels of pain. A dislocation of the shoulder posteriorly shows itself by the patient keeping the arm close to the body and turned inwards, the head of the humerus being palpable at the rear of the joint, although this condition has been misdiagnosed as frozen shoulder.
Several techniques are used to reduce a shoulder dislocation and the time it takes for the reduction to be performed is important. The muscle spasm can increase in severity and make the restoration of the normal joint alignment increasingly difficult. The oldest technique is to pull firmly on the arm whilst putting the foot in the armpit to give counter pressure. A more modern technique which is less traumatic is for the surgeon to move the arm bone outwards whilst pressuring the humeral head with their hand. Once the arm is at a right angle out to the side the arm can be tractioned and turned outwards, often leading to the joint being relocated.
Pain is a major presentation problem in shoulder dislocation and there are many alternatives that the medical staff can apply to give good pain relief and ease the process of reduction. A recent reduction can be moderately easily relocated in the absence of strong painkillers or muscle relaxants. The most useful sedative drug will have a quick onset of action, be able to supply good muscular relaxation and with an action which goes off quickly to allow rapid recovery. After the joint is back in place a sling is used for up to three weeks to allow the capsular damage to heal.