What is shoulder instability?
Shoulder instability means that the shoulder joint is too loose and is able to slide around too much in the socket. In some cases, the unstable shoulder actually slips out of the socket. If the shoulder slips completely out of the socket, it has become dislocated.
What parts of the shoulder are involved?
The shoulder is made up of three bones: the scapula (shoulder blade), the humerus (upper arm bone,) and the clavicle (collarbone). The rotator cuff surrounds the shoulder joint. The rotator cuff is actually made up of the tendons of four muscles: the supraspinatus, infraspinatus, teres minor, and subscapularis. Tendons attach muscles to bones. Muscles move bones by pulling on tendons. The muscles of the rotator cuff also keep the humerus tightly in the socket.
A part of the scapula, called the glenoid, makes up the socket of the shoulder. The glenoid is very shallow and flat. A rim of soft tissue, called the labrum, surrounds the edge of the glenoid, making the socket more like a cup. The labrum turns the flat surface of the glenoid into a deeper socket that molds to fit the head of the humerus.
Surrounding the shoulder joint is a watertight sac called the joint capsule. The joint capsule holds fluids that lubricate the joint. The walls of the joint capsule are made up of ligaments. Ligaments are soft connective tissues that attach bones to bones. The joint capsule has a considerable amount of slack, loose tissue, so that the shoulder is unrestricted as it moves through its large range of motion.
If the shoulder moves too far, the ligaments become tight and stop any further motion, sort of like a dog coming to the end of its leash. Dislocations happen when a force overcomes the strength of the rotator cuff muscles and the ligaments of the shoulder. Nearly all dislocations are anterior dislocations, meaning that the humerus slips out of the front of the glenoid. Only three percent of dislocations are posterior dislocations, or out the back. Sometimes the shoulder does not come completely out of the socket. It slips only partially out and then returns to its normal position. This is called subluxation.
What causes shoulder instability?
Shoulder instability often follows an injury that caused the shoulder to dislocate. This initial injury is usually fairly significant, and the shoulder must be reduced. To reduce a shoulder means it must be manually put back into the socket. The shoulder may seem to return to normal, but the joint often remains unstable. The ligaments that hold the shoulder in the socket, along with the labrum (the cartilage rim around the glenoid), may have become stretched or torn. This makes them too loose to keep the shoulder in the socket when it moves in certain positions. An unstable shoulder can result in repeated episodes of dislocation, even during normal activities. Instability can also follow less severe shoulder injuries.
In some cases, shoulder instability can happen without a previous dislocation. People who do repeated shoulder motions may gradually stretch out the joint capsule. This is especially common in athletes such as volleyball players and swimmers. If the joint capsule gets stretched out and the shoulder muscles become weak, the ball of the humerus begins to slip around too much within the shoulder. Eventually this can cause irritation and pain in the shoulder.
A genetic problem with the connective tissues of the body can also lead to ligaments that are too elastic. When ligaments stretch too easily, they may not be able to hold the joints in place. All the joints of the body may be too loose. Some joints, such as the shoulder, may be easily dislocated. People with this condition are sometimes referred to as double-jointed.
What are the symptoms?
Chronic instability causes several symptoms. Frequent subluxation is one. In subluxation, the shoulder may slip (sublux) in certain positions, and the shoulder may actually feel loose. This commonly happens when the hand is raised above the head, for example while throwing. Subluxation of the shoulder usually causes a quick feeling of pain, like something is slipping or pinching in the shoulder. Over time, you may stop using the shoulder in ways that cause subluxation.
The shoulder may become so loose that it starts to dislocate frequently. This can be a real problem, especially if you can't get it back in the socket and must go to the emergency room every time. A shoulder dislocation is usually very obvious. The injury is very painful, and the shoulder looks abnormal. Any attempted shoulder movements cause extreme pain.
A dislocated shoulder can damage the nerves around the shoulder joint. If the nerves have been stretched, a numb spot may develop on the outside of the arm, just below the top point of the shoulder. Several of the shoulder muscles may become slightly weak until the nerve recovers. But the weakness is usually temporary.
How is shoulder instability diagnosed?
Your doctor will diagnose shoulder instability primarily through your medical history and physical exam. The medical history will include many questions about past shoulder injuries, your pain, and the ways your symptoms are affecting your activities. In the physical exam, your doctor will feel and move your shoulder, checking it for strength and mobility. Your doctor will stress the shoulder to test the ligaments. When the shoulder is stretched in certain directions, you may get the feeling that the shoulder is going to dislocate. This is a very important sign of instability. It is called an apprehension sign. (Don't worry. Unless your shoulder is extremely loose, it will not dislocate.)
Your doctor may order an X-ray. X-rays can help confirm that your shoulder was dislocated or injured in the past. You may also need to have a CT or MRI scan. A surgeon may need to examine your shoulder using an arthroscope while you are under general anesthesia. An arthroscope is a tiny TV camera inserted into the shoulder through a small incision. This allows a good look at the muscles and ligaments of the shoulder. When you are awake, it is hard to test the ligaments because you automatically tighten the muscles during the exam.
What is the treatment?
Your doctor's first goal will be to help you control your pain and inflammation. Initial treatment to control pain is usually rest and anti-inflammatory medication, such as aspirin or ibuprofen. Your doctor will probably refer you for physiotherapy. This is very important to retrain the muscles around the shoulder so that they are more effective in preventing excess motion of the joint.
If your therapy program doesn't stabilize your shoulder after a period of time, or you shoulder keeps dislocating, you may need surgery.
The most common method for surgically stabilizing a shoulder that is prone to anterior dislocations is the Bankart repair, which repairs the soft tissues that have been damaged. In the past, the Bankart repair was done through a large incision made in the front (anterior) shoulder joint. This required damage to a great deal of normal tissue in order for the surgeon to be able to see the damaged portion of the joint capsule. The arthroscope has changed all that. An arthroscope is a special type of camera designed to look inside the joint. Using the ability to see inside the joint, the surgeon can then place other instruments into the joint and perform surgery while watching what is happening on the TV screen. The arthroscope lets the surgeon work in the joint through a very small incision. This may result in less damage to the normal tissues surrounding the joint, leading to faster healing and recovery.
Sometimes that damage to the bone around the shoulder is so great that an open surgical procedure is still required. For example, in the Latarjet procedure, a piece of bone is transferred from one area of the shoulder (the coracoid) to another (the glenoid) to repair the damage.