SHOULDER ANATOMY

What is the shoulder joint made of, and how important are these structures?

The shoulder joint, also referred to as the glenohumeral joint, comprises a ball-and-socket that rests between the humerus (long arm bone) and scapula (shoulder blade). The shoulder joint is considered an extremely mobile point that attaches the upper limb to the trunk. The glenoid cavity, also called the fossa, and the humeral head form the shoulder joint; hence the name, the glenohumeral joint. Like most synovial joints, hyaline cartilage coats the articular surfaces. However, the humeral head is extremely large compared to the glenoid fossa, enabling a much more diverse range of motion at the expense of the joint’s stability. However, to balance the unequal surface, a fibrocartilage rim, referred to as the glenoid labrum, cushions the glenoid fossa.

A sheath made of fibrous tissue forms the joint capsule, which safeguards the contents of the joint. The joint capsule extends from the humeral neck to the glenoid fossa rim. The capsule is loose, which allows for optimal mobility. Inside the capsule, covering the surfaces rests the synovial membrane, which produces synovial fluid to lubricate and manage friction against the articular surfaces. Similarly, the bursa minimises friction within the shoulder joint. The bursae are sacs full of liquid that act like pillows, protecting the tendons and other structures within the joint.

Different types of bursae exist, such as:

• Subacromial bursae are found near the acromion and deltoid, near the joint capsule and supraspinatus These types of bursae minimise friction underneath the deltoid and enable smooth, uninterrupted motion of the rotator cuff tendons.
• Subscapular bursae can be found in the middle of the scapula and subscapularis tendon to lower the risk of a worn-out tendon when the shoulder is active.

Ligaments play a significant role in increasing the stability of the bony components of the joint.

Several ligaments which ensure the shoulder moves efficiently include the following:

• The glenohumeral ligament consists of the inferior, superior and middle ligaments that attach the glenoid fossa to the humerus and form the joint capsule.
• The transverse humeral ligament is the length of the distance between the humeral tubercles.
• The coracoclavicular ligament works together with the acromioclavicular ligament to align the clavicle to the scapula.
• The coracoacromial ligament is a significant ligament that forms the coracoacromial arch, which reduces the risk of the humeral head being displaced.

A ball-and-socket lubricated (synovial) joint enables a wide range of motion, such as extension, flexion, abduction, adduction, internal and external rotation and circumduction (moving the shoulder/upper limb circularly).

The rotator cuff muscles, specifically the supraspinatus and infraspinatus, engulf the shoulder joint and attach to the greater tuberosity of the humerus. The muscles act cohesively to anchor the humeral head within the glenoid cavity.