The shoulder complex is built for mobility, however this ability to move comes with the designation of being the most unstable joint in the body. The shoulder is composed of three osseous joints and one articulation, with stability provided by muscles, ligaments, the glenoid labrum and joint capsule.
The shoulder has three osseous, or bone-to-bone, joints.
- The glenohumeral joint (shoulder joint)
- The acromioclavicular joint
- The sternoclavicular joint
The shoulder also contains the scapulothoracic thoracic articulation, which provides stability between the scapula (shoulder blade) and ribs of the posterior aspect of the thoracic (chest) cage.
Joint and Bone
The glenohumeral joint (shoulder joint) is considered a ball and socket joint. The joint brings together the head of the humerus bone (the ball) and the glenoid fossa of the scapula (the socket.) The head of the humerus is large and the glenoid fossa is small. This gives the joint mobility, but causes it to be more unstable.
The acromioclavicular joint is formed by the meeting of the distal end of the clavicle (collar bone,) and the distal end of the acromion, a part of the scapula (shoulder blade.)
The sternoclavicular joint is formed but the juncture of the proximal end of the clavicle (collar bone) and the sternum.
The osseous joint surfaces all have articular cartilage, which covers the ends of the bones. The articular cartilage has a smooth and shiny surface that allows the ends of the bones in the joint to slide freely over each other.
Common injuries of the shoulder joints include shoulder osteoarthritis, humeral fractures, shoulder dislocation, shoulder subluxation, shoulder impingement and acromioclavicular arthritis.
Muscles play a vital role in giving the shoulder both mobility and stability. The muscles that control the shoulder originate at the scapula and thoracic cage, and attach to the humerus and scapula by tendons.
Tendons are part of the muscle, whose function is to attach muscle to bone. Like the instruments in an orchestra, the interaction of joints and muscles in the shoulder is complex – with each component playing an important role in overall function. If there is restriction or weakness in any one of the structures of the shoulder, it can set up a cycle of pain and dysfunction.
In the shoulder, there are muscles that move the arm, muscles that stabilize the arm, and some muscles that do both. The rotator cuff muscles are a group of four muscles (supraspinatous, infraspinatous, teres major and subscapularis) that originate in the scapular region and attach to the humerus. Their function is to move the shoulder while stabilizing the humerus in the glenoid fossa. Each of the muscles of the rotator cuff has a specific job to do, while they work together to ensure normal shoulder function.
Other movers in the shoulder include the deltoid, pectoralis major, teres minor and the long head of the biceps.
Important shoulder stabilizers are the rhomboids, serratus anterior, trapezius, and levator scapular.
Common injuries to the muscles surrounding the shoulder include rotator cuff tendonitis, bicep tendonitis, shoulder impingement, calcific tendonitis and rotator cuff tear.
Ligaments and Labrum
The shoulder joint is protected and surrounded by a soft tissue sleeve called the shoulder capsule. Ligaments, soft tissue structures that connect bone to bone, reinforce the capsule. The capsule and ligaments provide passive stability to the shoulder while still allowing movements in different planes.
The labrum is a fibrocartilagenous structure located along the rim of the glenoid fossa. It provides added depth and stability to the joint.
Common injuries to these soft tissue structures include adhesive capsulitis, frozen shoulder, shoulder dislocation, shoulder instability and torn labrum or labral tear.
A burse is a fluid filled sac that decreases friction between two tissues. The subacromial burse is located between the rotator cuff muscles and the acromion. In addition to decreasing friction, the burse protects the rotator cuff from injury when moving under the acromion. The burse can become inflamed causing shoulder bursitis and shoulder impingement.
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