Shoulder Anatomy and Instability
The shoulder is called a ball-and-socket joint and is the most flexible joint in our body. It is made up of three bones; the upper arm bone (humerus), the shoulder blade (scapula) and the collarbone (clavicle). The head of the humerus rests in a shallow socket in the shoulder called the glenoid. The labrum is a cuff of cartilage that circles the shallow socket or glenoid, to make the socket deeper, to help keep the head centered in the socket. A thin sheet of fibers called the joint capsule surround the shoulder joint and provide for both range of motion and stability.
The shoulder is capable of incredible range of motion. We are dependent on both flexibility and stability of the shoulder joint to perform our daily activities such as reaching above, in front, to the side and behind us. Instability of the shoulder occurs when the head of the humerus moves out of the shallow shoulder socket. There are two types of shoulder instability: subluxations and dislocations.
Subluxation: A subluxation occurs when the ball of the humeral head slips to the edge of the shoulder joint. When a subluxation occurs, the humeral head will usually go back into the socket on its own.
Dislocation: A dislocation occurs when the head of the humerus is completely out of the shoulder joint.
What causes shoulder instability?
Genetic or congenital factors: Some people are born with ligaments that are more loose (elastic) than what is typical. When ligaments stretch too easily, they may not be able to hold (stabilize) the joint in place. With ligament laxity, the shoulder can often slip out of the socket in more than one direction and is referred to as “multi-directional instability.”
Repetitive Motion or Overuse
Individuals who perform an activity or sport that requires repetitive shoulder motion are more prone to gradually stretch the fibers of the capsule joint, causing the ball of the humeral head to slip around the socket. This condition is seen in baseball and softball pitchers, volleyball players, tennis players and swimmers.
Trauma
Shoulder instability may be the result of trauma to the shoulder joint. With a significant insult to the upper extremity, such as may occur in football, the ligaments and labrum that hold the shoulder in the socket may become stretched or torn. When the ligaments and labrum cannot stabilize the humeral head in the socket, the humeral head comes completely out of the socket. This is called a dislocation.
Symptoms of Shoulder Instability
- The shoulder may subluxate in certain positions, causing an abrupt feeling of pain or abnormal motion in the shoulder. The shoulder may feel loose, as if something is pinching or slipping in the shoulder.
- A shoulder dislocation is usually obvious. There is significant pain which is exacerbated by any movement of the shoulder. The shoulder will look abnormal.
- With both a subluxation and a dislocation, the nerves surrounding the shoulder may be stretched, causing numbness on the outside of the arm. There may also be a feeling of weakness in the affected arm.
Diagnosis
Your physician will take a detailed history and will address what activity you were doing when the episode of instability occurred. He will determine if this is the first episode of instability or a recurrence. Your doctor will then conduct a thorough clinical exam of your shoulder, assessing strength and mobility. He may obtain x-rays to assess the structure of the shoulder joint. In some cases, he may request an MRI at the Imaging Center to further look at surrounding ligaments and stabilizing structures.
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