The less common direction
Most shoulder instability occurs at the front of the joint: the ball comes forward out of the socket, and the labrum and ligaments at the front are damaged. Posterior instability, where the shoulder moves or dislocates backwards, is considerably less common, and it brings its own set of considerations.
Why it is easily missed
Posterior instability often does not present in the way people expect. Rather than a dramatic dislocation with the arm stuck in an obvious position, it more commonly presents as pain at the back of the shoulder, a sense of the shoulder slipping or giving way when pushing with the arm in front of the body, and discomfort with activities such as bench pressing, push-ups or blocking in contact sport. Because the symptoms can resemble other causes of posterior shoulder pain, the diagnosis is sometimes made late.
What the imaging shows
An MRI scan can show a tear of the posterior labrum, the rim of cartilage around the back of the socket, where the shoulder has moved backwards out of joint. Other features that may be present include stretching of the posterior capsule, loss of bone from the back of the glenoid, and in some shoulders a glenoid that is naturally oriented in a way that predisposes to posterior instability. A CT scan is often used where the bony anatomy needs to be assessed properly, because the shape and orientation of the socket matters more in posterior instability than in anterior instability.
Treatment options
Not every posteriorly unstable shoulder needs surgery. A structured physiotherapy programme directed at the rotator cuff and the muscles controlling the shoulder blade is the usual starting point, and for a proportion of people it is sufficient.
Where surgery is required, the options broadly divide into repairing the torn posterior labrum and tightening the capsule, or, where there is significant bone loss or an unfavourable socket shape, a posterior bone block procedure. A bone block adds bone to the rim of the socket to restore its depth and shape, with the aim of restoring stability and preventing further dislocations. Which option is appropriate depends on the bone, the soft tissues and the demands placed on the shoulder.
Related information
Our shoulder instability page covers instability in all directions, and the shoulder anatomy page explains the structures involved. See also shoulder injuries, shoulder surgery and rehabilitation. To arrange an assessment, see the consultation page.