Posterior shoulder instability typically results from a dislocation injury to the shoulder joint when the humeral head (ball) of the humerus (upper arm bone) is forced from its normal position in the center of the glenoid (socket) and out the back.
Posterior shoulder dislocations most commonly occur with falls on an outstretched hand in front of the body.
What Is Posterior Shoulder Instability?
The shoulder has more range of motion than any other large joint in the body and is also the most commonly dislocated large joint. The shoulder is like a golf ball on a golf tee, with many structures contributing to its stability.
These structures include the bony contours of the humeral head (ball) and glenoid (socket), the labrum, capsule and ligaments, and rotator cuff muscles. The labrum surrounds the rim of the socket and helps keep the head of the humerus in place.
When the shoulder is dislocated, any of these stabilizing structures may be injured, including the humeral and glenoid bone, labrum, capsule and ligaments, and rotator cuff. In younger, active patients, the labrum may tear away from the glenoid along with the attached stabilizing capsule and ligaments. When this occurs, it is called a Bankart lesion.
Causes and Risk Factors
Causes and risk factors for posterior shoulder dislocation and instability can include:
- A direct blow to the shoulder
- Falling on an outstretched arm in front of the body
- Repetitive throwing motion or swimming
- A previous shoulder dislocation injury
- A congenital abnormality, such as a shallow or malformed joint surface
- Violent muscle contractions associated with seizures
- Contact sports, such as football, wrestling, and basketball
- Sports involving repetitive overhead activity, such as baseball, volleyball, and swimming
- Sports requiring forceful lifting, hitting, or twisting
- Previous shoulder dislocations or sprains
- Shoulder fracture
- Repeated shoulder injury
- Poor physical conditioning, including strength and flexibility
- Fixed dislocation
Symptoms
Frequent signs and symptoms can include:
- Severe shoulder pain at the time of the dislocation injury
- Loss of shoulder function and severe pain when attempting to move the shoulder
- Apprehension or the feeling that the shoulder wants to slip out of place with certain positions
- Tenderness, deformity, and swelling
- Pain with shoulder movement, especially when reaching overhead or with heavy lifting
- Pain that awakens you at night
- Loss of strength
- Numbness or paralysis in the upper arm and deltoid muscle from pinching, stretching, or pressure on blood vessels or nerves
- Crepitation (a “crackling” feeling or sound) when the injured area is touched or with shoulder motion
- Decreased or absent pulse at the wrist because of blood vessel damage, although this is rare
Diagnosis
Following a posterior shoulder dislocation, X-rays, MRI studies, and sometimes CT scans may be needed to determine the full extent of injury to the stabilizing structures of the shoulder joint.
Treatment
Shoulder dislocations should be reduced (put back in place) as soon as possible for the comfort of the patient and to relieve potentially damaging traction on nerves and blood vessels. Reduction may also help restore blood flow to the humeral head bone that may be obstructed with the dislocation.
Following reduction, slings should be discontinued and early range of motion and gentle strengthening should begin to restore shoulder function.
Younger patients may require arthroscopic surgery to repair the torn labrum, capsule, and ligamentous structures and sometimes fractures of the bone or tears in the rotator cuff.
Older patients often return to previous levels of function without surgery unless they sustain a fracture or rotator cuff tear that requires surgery. Undiagnosed chronic posterior dislocations are complex and may require different surgical approaches based on the individual aspects of the case.
Recovery and Outcomes
Following reduction of a posterior shoulder dislocation, functional outcomes can be excellent and re-dislocation rates are lower with proper treatment.
For younger, active patients who continue to experience instability or pain, arthroscopic repair of the injured stabilizing structures—primarily the torn labrum, capsule, and ligaments—can be beneficial.
Posterior shoulder dislocations may result in continued subtle instability that can cause asymmetric wear of the glenoid cartilage and bone and lead to early post-traumatic arthritis. Posterior dislocations that are discovered late and have not been reduced may have a less desirable outcome and often require more complex surgery to restore and maintain a stable shoulder.
When to See Dr. Chudik
If you experience a shoulder dislocation, continued pain or instability, or the feeling that your shoulder wants to slip out of place with certain positions, contact Dr. Chudik’s office for an evaluation.
Call 630-324-0402, email contactus@chudikmd.com,or request an appointment online.