Dr. Ben Wedro practices emergency medicine at Gundersen Clinic, a regional trauma center in La Crosse, Wisconsin. His background includes undergraduate and medical studies at the University of Alberta, a Family Practice internship at Queen's University in Kingston, Ontario and residency training in Emergency Medicine at the University of Oklahoma Health Sciences Center.
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
What is dislocation of the shoulder? What causes a shoulder dislocation?
The shoulder joint is the most mobile joint in the body and allows the arm to
move in many directions. This ability to move makes the joint inherently
unstable and also makes the shoulder the most often dislocated joint in the
The head of the humerus (upper arm bone) sits in the glenoid fossa, an extension of the scapula, or shoulder blade. Because the glenoid fossa (fossa = shallow depression) is so shallow, other structures within and surrounding the shoulder joint are needed to maintain its stability. Within the joint, the labrum (a fibrous ring of cartilage) extends from the glenoid fossa and provides a deeper receptacle for the humeral head. The capsule tissue that surrounds the joint also helps maintain stability. The rotator cuff muscles and the tendons that move the shoulder provide a significant amount of protection for the shoulder joint.
Dislocations of the shoulder occur when the head of the humerus is forcibly removed from its socket in the glenoid fossa. The shoulder can
be dislocated in many different directions, and a dislocated shoulder is described by the location where the humeral head ends up after it has been dislocated. Ninety percent or more of shoulder dislocations are anterior dislocations, meaning that the humeral head has been moved to a position in front of the joint. Posterior dislocations are those in which the humeral head has moved backward toward the shoulder blade. Other rare types of dislocations include luxatio erecta, an inferior dislocation below the joint, and intrathoracic, in which the humeral head gets stuck between the ribs.
Picture of the shoulder joint
Dislocations in younger people tend to arise from trauma and are often
associated with sports or falls. Older patients are prone to dislocations
because of gradual weakening of the ligaments and cartilage that supports the shoulder. Even in these cases, however, there still needs to be some force applied to the shoulder joint to make it dislocate.
Anterior dislocations often occur when the shoulder is in a vulnerable
position. A common example is when the arm is held over the head with the elbow
bent, and a force is applied that pushes the elbow backward and levers the
humeral head out of the glenoid fossa. This scenario can occur with throwing a
ball or hitting a volleyball. Anterior dislocations also occur during falls on
an outstretched hand. An anterior dislocation involves external rotation of the
shoulder; that is, the shoulder rotates away from the body.
Posterior dislocations are uncommon and are often associated with specific
injuries like lightning strikes, electrical injuries, and seizures. On occasion, this type of dislocation can occur with minimal injury in the elderly, and often the diagnosis is missed the first time the patient presents for evaluation of shoulder pain.
Medical Author: Benjamin C. Wedro, MD, FACEP, FAAEM
Medical Editor: Melissa Conrad Stöppler, MD
According to Cubs third baseman Aramis Ramirez, he had dived for infield
balls "3,000 times like that and didn't feel anything" but on May 8, 2009, he
landed on his shoulder and it dislocated. It had been nine years since the last
time it happened; the trainers tried to put it back in place on the field but
failed. Team doctors relocated the joint in the locker room.
The shoulder is the most mobile and least stable joint in the body. Its
ability to move in many directions makes it prone to dislocation and in younger
people, sports injuries are a common reason. Throwing or reaching for a ball
puts the shoulder at risk because there is little that stabilizes the shoulder
joint. The glenoid fossa, the small cup that holds the humeral head (the end of
the upper arm bone) in place is shallow and needs the help of the labrum, or
cartilage, to deepen the receptacle for the arm bone. The rotator cuff, a group
of four tendons, also helps keep the bones where they belong, but when those
muscles are being stretched in a throwing or reaching motion, any excess force
can pop the shoulder out of joint.