The elbow is a complex junction between your upper arm and your forearm. This joint connects the bone of the upper arm to the two bones of the forearm, allowing for precise control over hand positioning. Its primary function is to provide the leverage and mobility that enables the hand to perform daily tasks.
The Elbow’s Structural Classification
The elbow is classified as a synovial joint, the most common and mobile type of joint in the human body. Synovial joints are characterized by a joint capsule containing synovial fluid, which reduces friction between the articulating bones. This structure allows the joint a wide degree of motion.
More specifically, the elbow is functionally defined as a hinge joint, an anatomical classification that describes the shape and movement of the articulation. Like the hinge on a door, this joint allows movement primarily in a single plane, permitting the forearm to swing toward or away from the body. Because the elbow complex also includes a rotational element, it is sometimes described as a trochoginglymus joint, indicating it possesses both hinge and pivot characteristics.
The Bones that Form the Joint
Three distinct bones converge to create the bony architecture of the elbow complex. The humerus is the single long bone of the upper arm. Below the humerus are the two bones of the forearm: the ulna, which is on the pinky-finger side, and the radius, which is on the thumb side.
The elbow is a complex of three joints housed within one capsule. The humeroulnar joint is the primary articulation, where the trochlea of the humerus meets the trochlear notch of the ulna, forming the hinge mechanism. The humeroradial joint involves the head of the radius meeting the capitulum of the humerus, allowing for both hinge and rotational actions.
The third component is the proximal radioulnar joint, where the head of the radius articulates with the ulna, which is the site of forearm rotation. The ends of all three bones are covered in hyaline cartilage, a tissue that cushions the bones and facilitates smooth gliding during movement. A fibrous joint capsule surrounds the entire area, and ligaments connect bone to bone, providing static stability to the joint.
Range of Motion and Movement
The primary movements of the elbow joint are flexion and extension, which occur at the humeroulnar joint. Flexion is the action of bending the elbow, bringing the forearm closer to the upper arm, and can achieve a range of about 146 degrees in healthy adults. Extension is the act of straightening the arm, moving the forearm away from the upper arm, and may extend a few degrees past a neutral straight line.
The elbow complex also enables the rotational movements of pronation and supination. These actions occur at the proximal radioulnar joint, which shares the elbow’s joint capsule. Pronation is the rotation that turns the palm of the hand to face downward or backward, allowing rotation of about 80 degrees.
Supination is the movement that rotates the palm to face upward or forward. This motion works in opposition to pronation, allowing for about 87 degrees of rotation. The combination of the hinge movement and these rotational movements makes the elbow a versatile joint, capable of positioning the hand for a wide variety of functional tasks.