Structural Anatomy: Shoulder Structure
Introduction to the Shoulder: Part I Structural Anatomy
The shoulder joint has the greatest range of motion of all joints and is actually comprised of four separate bony and musculotendinous joints.
The glenohumoral joint is a ball and socket joint is where the ball end of the humerus meets the glenoid fossa hollow of the scapular
The acromioclavicular (AV) joint attaches the scapular to the clavicle
The sternoclavicular (SC) joint is where the clavicle and sternum (breastbone) connect and moves only slightly.
The scapulothoracic articulation, is not actually a joint at all, but rather is a muscular attachment of the scapula to the back and side of the chest via musculo-tendinous attachments that insert either onto the ribs or the edges of the spine.
These four joints of the shoulder work together to
attach the upper extremity to the thorax
provide a powerful and smooth mechanism for upper extremity motion
Healthy shoulders are capable of:
180º elevation (up and down movement) at the front
about 60º of up and down movement at the back
170º degrees of round-house swing when the arm is hanging at the side
180º degrees of round-house swing when the arm is held straight out to the side
Almost no round-house swing when the arm is fully extended overhead.
This level of movement is remarkable considering both the forces applied to the shoulder by activity and the minimal bony contact present in the joint. Not surprisingly, injuries to this joint are common in athletes, particularly in those who participate in sports in which additional shoulder stresses due to throwing or physical contact occur (baseball, tennis, swimming, golf, football) leading to either over-use or traumatic injuries, respectively.
STRUCTURE OF THE SHOULDER JOINT
The bony elements that make up the shoulder joint include the scapula (shoulder blade), the humerus (upper arm bone), the clavicle (collar bone) and somewhat indirectly, the sternum (breast bone).
Scapula
The scapula is a roughly triangular bone on the back of the rib cage that is oriented such that its most pointed edge faces downward toward the feet. The inner surface of the scapula (the side against the back of the chest cage) is generally smooth except at its upper-outer quadrant where a bony protrusion extends forward and hooks toward the shoulder to form the coracoid process (Figure 4). The outer surface of the scapula has a bony ridge that runs diagonally across its surface and ends by curving forward to form a bony knob called the acromion.
Nestled between the acromion and coracoid is a shallow hollow called the glenoid fossa, with which only about 33% of surface of the rounded head of the humerus bone fits (see below) to form the glenohumoral joint.
This minimal amount of bone-to-bone contact contributes to the great range of motion of the shoulder, but also requires extensive muscular and ligamentous connections at the shoulder to maintain joint structure and function.
Humerus
The humerus (upper arm bone) is a tubular bone whose head is composed of a structure approximating a third of a sphere that is angled 30º backward and 45º toward the chest. This forms the “ball” component of the ball and socket glenohumoral joint between the humerus and scapula (Figure 5).
The glenohumoral ball and socket joint is where the humerus meets the scapula in the glenoid fossa. The ligaments connecting the humerus and scapula are relatively loose, meaning the shoulder joint is very mobile but lacks stability.
Also of importance, the “bicipital groove” located below the humeral head on the front of the bone allows passage of the long tendon of the biceps muscle to its origin on the genoid labrum.
Clavicle
The clavicle is a flattened, double-curved bone that attaches to the scapula (at the acromion via the acromioclavicular joint) on its outside end. The AC joint lets the clavicle glide and rotate on the scapula during movements such as shrugging or raising the arm overhead. The sternoclavicular (SC) joint is where the clavicle and sternum (breastbone) connect and moves only slightly.
The sternoclavicular joint, although located some distance from the shoulder, serves an indirect role in maintaining the structure of the shoulder by keeping the far end of the clavicle in a fixed position next to the sternum.
The fourth “joint” of the shoulder, the scapulothoracic articulation, is not actually a joint at all, but rather is a muscular attachment of the scapula to the back and side of the chest via musculo-tendinous attachments that insert either onto the ribs or the edges of the spine. This joint allows free “sliding” movement of the scapula over the muscles covering the back of the chest cage, to facilitate movement of the shoulder to the extremes of its range of motion. This scapulothoracic region is important because the muscles surrounding the shoulder blade work together to keep the socket lined up during shoulder movements.