Shoulder Joint Dislocation Injury and Nerve Pain

Structure of the Glenohumeral Joint Makes it Unstable

Nov 23, 2009
Noreen Kassem

Shoulder Pain Common in Baseball Players - Bartleys

Shoulder joint pain and dislocation are common patient complaints. This joint is prone to nerve injury and wear and tear especially from repetitive movements.

The shoulder or glenohumeral (GH) joint has very little support inferiorly (or underneath it) and is particularly at risk of dislocation and injury in the abducted position (when the arm is raised). Nerve injury is also common in this area because the axillary nerve (which runs from the underarm) comes in close contact with the joint. This nerve damage can cause paralysis or numbness in the deltoid muscle (at the back of the shoulder), making it difficult to raise the arm up towards the head and causing a loss of sensation over the outer part of the arm.

The Shoulder Joint Is Prone to Dislocation and Injury

The GH joint is one of the most commonly dislocated and injured large joint. It is also one of the most unstable joints as it has the greatest range of movement of any synovial joint in the body. The most common and moveable joint, a synovial joint contains a capsule filled with lubricating synovial fluid that help the articulating bones glide against each other. The greater the contact between the two articular surfaces (the bones that allow movement in a synovial ball and socket joint), the more stable the joint is.

In the shoulder, the GH joint between the head of the humerus (the large bone in the upper arm) and the glenoid fossa on the scapula (the large fan-like bone at the back of the shoulder) is anatomically more like an egg and spoon joint due to the shape of the articulating bones. The surface of the glenoid fossa is very shallow and can only contain about a third of the humeral head at a time even though it is brimmed with a fibrocartilaginous collar called the glenoid labrum, which provides more support. The shoulder joint is more susceptible to dislocation if this fibrocartilaginous collar is damaged by injury or wear and tear.

The Collar Bone Is Easily Fractured in Injury Due to Strong Ligaments

Ligaments provide further support anteriorly (at the front) and superiorly (above) the shoulder joint. Superiorly, there are three important ligaments that secure the joint and allow it to move extensively. The broad coracohumeral ligament stretches between the edge of the coracoid process (a bony hook jutting from the back of scapula bone) to the front of the humerus bone.

The tendon of the long head of the biceps (the large muscle in the upper arm) also extends from the humerus bone through the articular synovial capsule to the attach to the top part of the glenoid fossa, to help stabilize the joint.

Thirdly, the coracoacromial ligament forms a triangular band joining the coracoids process to the acromion, which is also a process on the scapula that extends on the front of the joint. The acromion is also where the scapula articulates with the clavicle or collar bone. The ligaments attaching this joint are so strong that the bones are more likely to fracture in an injury than the ligaments are to rupture.

Reference:

Gray, Henry: Gray's Anatomy, Anatomy of the Human Body 40th Edition, London, UK. 2008

Noreen Kassem - Noreen Kassem is a hospital doctor based in London, UK. She has extensive experience in clinical research and an undergraduate honors ...