Shoulder instability is a condition where the ball of the ball-and-socket shoulder joint can come out of the socket. Sometimes the ball comes part of the way out of the socket, called a shoulder subluxation. Other times, the ball comes completely out of the socket, called a shoulder dislocation.
There are two general types of shoulder instability:
- Traumatic Instability
Traumatic shoulder instability occurs when there is an acute injury to the shoulder, such as a fall or sports injury. The shoulder is forcefully pulled out of its socket, and often must be placed back into position with special maneuvers, sometimes requiring anesthesia. A traumatic dislocation often damages the ligaments that hold the ball in the socket, and makes the shoulder prone to dislocating again in the future.
- Multi-Directional Instability
Multi-directional instability (sometimes abbreviated as MDI) occurs when the shoulder joint is loose within the socket. There is not a traumatic event that causes the instability, but rather the shoulder has a tendency to shift excessively, causing pain in the joint. Often, people who have symptoms of multi-directional instability complain of clunking, or shifting, of the shoulder with overhead movements.
Shoulder StabilityThere are three factors that contribute to the stability of any joint in the body. These include:
- Bony Anatomy
The bones of the shoulder contribute very little to the stability of this joint. The socket is very shallow, and without other structures to hold the shoulder in place, the ball would not stay in position. Compare this to the hip joint, which has a very deep socket and where it is hard to remove the ball from the socket.
- Static Stabilizers
The static stabilizers are the ligaments that surround the joint. Ligaments connect two bones together. Ligaments are flexible (they can bend), but not elastic (they do not stretch). People with traumatic shoulder instability often tear the ligaments of the shoulder joint. People with multi-directional instability often have loose ligaments. In fact, there are genetic ligament conditions that can cause severe multi-directional instability.
- Dynamic Stabilizers
The dynamic stabilizers are the muscles and tendons around the shoulder. These muscles include the rotator cuff, the group of muscles that surrounds the ball of the shoulder. Dynamic stabilizers are also flexible, and they are elastic. Many people with multi-directional instability can strengthen the dynamic stabilizers to compensate for loose ligaments.
Treatment of Multi-Directional InstabilityThe treatment of multi-directional instability is different from treatment for traumatic instability of the shoulder. Most often, people can recover from multi-directional instability with non-surgical treatments; this includes high-level, competitive athletes. Treatment should be focused on strengthening the dynamic stabilizers of the shoulder joint. In addition, many people with multi-directional instability are thought to have poor shoulder mechanics -- specifically, their scapular (shoulder blade) movements are not well-coordinated with their shoulder movements. By restoring normal scapular motion, and strengthening the dynamic stabilizers including the rotator cuff, shoulder joint function can often improve.
Numerous studies have shown that the vast majority of patients who are motivated can recover from multi-directional instability with a focused shoulder rehabilitation program. About 85% of patients who undergo such a program will report good results. There are some people who fail to improve, and may ultimately decide to have shoulder surgery.
Surgery for Multi-Directional InstabilitySurgical procedures for multi-directional instability are considered for patients who have persistent symptoms of the shoulder coming out of the socket, despite lengthy non-surgical treatments. Most often, the surgery involves tightening the ligaments that surround the shoulder. Some surgeons prefer to perform this arthroscopically, and others through standard surgical incisions. Not long ago, it was popular to perform a procedure called a thermal shrinkage, using heat probes to cauterize soft-tissue in the shoulder to tighten the joint capsule. This thermal shrinkage procedure proved to have very poor results, and often required further surgical treatment.
The best surgery for multi-directional instability is a form of a capsular shift or capsular plication, which are both procedures that tighten the shoulder capsule. In addition, some surgeons will perform a rotator interval closure, a procedure that closes the gap between two of the rotator cuff muscles.
Rehab following surgery for multi-directional instability usually lasts many months. Initially after surgery, the shoulder is immobilized to allow the tightened tissues to solidly heal, and then work is started to regain mobility, followed by strengthening. Most athletes are allowed to resume full activity within 6 months.
Gaskill TR, et al. "Management of Multidirectional Instability of the Shoulder" J Am Acad Orthop Surg December 2011 ; 19:758-767.