What is posterior shoulder instability?
Posterior shoulder instability is an injury to the shoulder joint, causing the upper arm (humerus) to be displaced from its normal position in the center of the socket (glenoid) and the joint surfaces to no longer touch each other. This uncommon dislocation is posterior, where the humerus is behind the glenoid. Because the shoulder has more motion than any other large joint in the body, it is the most commonly dislocated large joint.
The shoulder is like a golf ball on a golf tee. A few of the many structures that provide shoulder stability include the cartilage rim (labrum), which helps provide depth to the socket; the capsule, with thickenings that are the ligaments of the shoulder; and the muscles of the rotator cuff, which surround the shoulder. To dislocate the shoulder, the rotator cuff muscles need to be stretched or torn, the capsule and ligaments need to be stretched, and often the labrum is pulled off the glenoid. Posterior subluxation of the shoulder is more common than dislocation, but both are much less common than anterior dislocation. Subluxation is when the ball of the humerus does not stay centered in the socket with shoulder motion and feels like it will slip out of place.
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How does posterior shoulder instability occur?
- Direct blow to the shoulder or backward force on an outstretched arm (such as blocking in football)
- End result of a severe shoulder sprain
- Congenital abnormality (you are born with it), such as a shallow or malformed joint surface
- Powerful muscle twisting or violent muscle contraction (such as with an epileptic seizure or electrocution)
Some people can willfully produce a recurrent dislocation.
What increases the risk?
- Contact sports (football, wrestling and basketball)
- Sports that require forceful lifting, hitting, or twisting, such as swimming, volleyball or and softball
- Previous shoulder dislocations or sprains
- Shoulder fracture
- Repeated shoulder injury of any kind
- Poor physical conditioning (strength and flexibility)
What are the symptoms of posterior shoulder instability?
- Severe pain in the shoulder at the time of injury
- Loss of shoulder function and severe pain when attempting to move the shoulder
- Feeling like the shoulder will slip out of place
- Tenderness in the back of the shoulder, deformity (fullness in the back of the shoulder), and swelling
- Inability to turn arm outward
- Numbness or paralysis in the upper arm and deltoid muscle from pinching, stretching, or pressure on the blood vessels or nerves
- Decreased or absent pulse at the wrist because of blood vessel damage (rare)
How is posterior shoulder instability treated?
Non-operative treatment: After reduction (repositioning of the bones of the joint) by trained medical personnel, treatment consists of ice and medications to relieve pain. Immobilization by brace, cast or immobilizer for six to eight weeks is recommended to protect the joint while the ligaments heal. After immobilization, stretching and strengthening of the stiff, injured and weakened joint and surrounding muscles (due to the injury and the immobilization) are necessary. These may be done with or without the assistance of a physical therapist or athletic trainer. Non-operative treatment provides relief in 70 percent to 90 percent of patients with posterior subluxation.
Operative treatment: Reduction usually can be performed without surgery; surgery may rarely be necessary to restore the joint to its normal position, as well as to repair ligaments. Surgery is uncommonly recommended after the first dislocation to tighten the shoulder ligaments and repair the labrum. Surgery is usually reserved for those who have recurrent dislocations despite 6 months of appropriate rehabilitation. This can be done arthroscopically or through a standard incision. Surgery is not as successful as for anterior dislocations.
Rarely, surgery is recommended for some individuals after the first dislocation. Posterior shoulder dislocations have up to a 10 percent to 30 percent likelihood of recurrent dislocations, particularly in young patients. The likelihood of success of a rehabilitation program is 70 percent to 90 percent in this group. Surgery may also be performed in a patient who has had a prior failed operation for posterior shoulder instability. The goal of surgery is to stabilize the shoulder to prevent further subluxations or dislocations.
One of the reasons the shoulder is the most commonly dislocated major joint is that it has more motion than any other major joint. Tightening the shoulder joint may reduce some shoulder motion. Stabilizing the shoulder is done by tightening the capsule. Less often, other structures may be moved or used to replace or give additional support to the capsule of the shoulder. Recurrent dislocations or subluxations without fracture are rarely associated with arthritis. Thus, the timing of surgery in relation to the injury is not critical. The results of posterior shoulder stabilization are not as good as for anterior stabilization, and this surgery has a higher complication rate. Further, the return to high-level sports, particularly overhead activity, rarely is improved by surgery.
Different techniques are in use at this time. There are arthroscopic techniques and open-incision techniques. The overall goal is to tighten the capsule and ligaments.
One of the most popular open techniques involves going through the deltoid fibers or removing part of the deltoid attachment from the roof of the shoulder. The infraspinatus muscle, which covers and is partially attached to the capsule of the shoulder, is either split in line with its fibers or all or part of it is removed from the arm bone. The capsule is removed from the underside of the infraspinatus tendon. The capsule is then cut and the stretched capsule and ligaments are tightened by folding the excess capsule underneath itself and stitching it together with sutures (threads), with or without surgical anchors, which are inserted into the glenoid rim.
Arthroscopic techniques involve using small incisions (arthroscopy portals) to tighten the stretched capsule, either by folding the excess capsule underneath itself and sewing it together with sutures (threads), with or without surgical anchors, which are inserted into the glenoid rim, or by using heat to shrink it.
Other techniques do not try to replicate the normal anatomy of the shoulder capsule and ligaments. These include moving muscle to reduce shoulder motion, moving bone from another area and using the bone to block shoulder dislocations, or cutting the below the glenoid, angling it, and inserting bone to maintain the increased angulation of the glenoid.
What are the complications of treatment?
Possible complications of operative treatment include:
- Surgical complications not specifically associated with this injury, such as pain, bleeding (uncommon), infection (<1percent), nerve injury (uncommon), stiffness, problems with anesthesia, and inability to return to previous level of pre-injury activity.
When can you return to your sport/activity?
With appropriate reduction (repositioning of the joint) and immobilization for three to six weeks, healing of ligaments can be expected in six weeks. Repeated shoulder dislocations depend on the amount of trauma necessary to cause the first dislocation, age at the time of injury, and associated shoulder injury (bony defect). Recurrent dislocation is less common than after anterior dislocation. If customary treatment does not prevent a recurrence, athletic activities should be modified until surgery can be performed to cure the problem. Non-operative treatment is successful in 70 percent to 90 percent of patients. Return to sports depends on the type of sport and the position played, as well as the quality of ligaments at the time of repair. Usually six to nine months of recovery is necessary after surgery before return to sports. Full shoulder motion and strength are necessary before returning to sports.
How can posterior shoulder instability be prevented?
- Appropriately warm up and stretch before practice or competition
- Maintain appropriate conditioning:
- Cardiovascular fitness
- Shoulder strength
- Flexibility and endurance
- For contact sports, wear protective shoulder pads
- Use proper technique