The humerus is the single long bone of the upper arm, extending from the shoulder to the elbow. The humeral head is the large, rounded structure located at the proximal end of this bone. It serves as the attachment point that links the arm to the torso. Understanding its structure is fundamental to grasping how the shoulder functions.
Anatomical Location and Structure
The humeral head sits at the proximal end of the humerus. Its smooth, hemispherical shape allows for articulation within a joint. The head is separated from the main shaft of the humerus by a slight constriction known as the anatomical neck.
The entire articulating surface of the head is encased in hyaline articular cartilage. This tissue is smooth and slick, providing a low-friction surface necessary for fluid movement. The cartilage on the humeral head is relatively thin compared to other large joints, and is thickest in the central portion of the dome.
Just below the head are two bony prominences: the greater and lesser tuberosities. These areas serve as attachment sites for the muscles and tendons of the rotator cuff. The tuberosities are separated from the main shaft of the humerus by the surgical neck, a common site for fractures due to its relative weakness.
Function in Shoulder Movement
The humeral head’s primary function is to form the ball component of the shoulder’s ball-and-socket joint, the glenohumeral joint. It articulates with a shallow depression on the shoulder blade (scapula) called the glenoid cavity. The large size of the humeral head compared to the small glenoid allows for an extensive range of motion.
This configuration makes the shoulder the most mobile joint in the human body. The movement is complex and multi-axial, enabling the arm to perform actions like flexion, extension, abduction (moving away from the body), adduction (moving toward the body), and internal and external rotation.
The joint’s stability depends on the surrounding soft tissues, including the glenoid labrum, the joint capsule, and the rotator cuff muscles. These structures stabilize the humeral head within the glenoid cavity, preventing it from slipping out during movement.
Common Issues Affecting the Humeral Head
Because of its role in a mobile joint, the humeral head is susceptible to various traumatic and degenerative conditions. One common acute injury is a proximal humeral fracture, a break in the upper portion of the humerus near the head. These fractures frequently occur in older individuals following a low-energy fall onto an outstretched arm, often due to underlying osteoporosis.
Another issue is dislocation, which happens when the humeral head is forced out of the glenoid socket. Anterior dislocations, where the head slips forward, are the most common type and can be caused by excessive extension and rotation of the arm. This event can damage the joint capsule and surrounding soft tissue structures.
Over time, the joint can suffer from glenohumeral osteoarthritis, which involves the breakdown of the articular cartilage covering the humeral head. As the cartilage erodes, the smooth surface is lost, leading to increased friction, pain, and stiffness in the shoulder joint. This degenerative process is a frequent cause of chronic shoulder pain.