The elbow joint is a complex structure located between the upper arm and the forearm, enabling a variety of arm movements. It functions as a hinge joint, allowing the arm to bend and straighten, and also contributes to the rotation of the forearm. This intricate arrangement of bones and their connections is essential for performing everyday tasks, from lifting objects to reaching for items. The elbow’s design provides both stability and a wide range of motion.
The Three Key Bones
The elbow joint involves the articulation of three distinct bones: the humerus, the ulna, and the radius. The humerus is the long bone of the upper arm, and its lower end contributes to the elbow joint. Specifically, the distal humerus features a spool-shaped trochlea on its medial side and a rounded capitulum on its lateral side, both designed for articulation.
The ulna is positioned on the pinky finger side of the forearm. Its proximal end has a large, C-shaped trochlear notch that fits with the trochlea of the humerus. Additionally, the ulna has an olecranon process, which forms the bony point of the elbow, and a coronoid process that articulates with the humerus during flexion.
The radius is located on the thumb side. Its proximal end features a disc-shaped head that articulates with the capitulum of the humerus. This radial head also articulates with the ulna at a small indentation called the radial notch.
How the Bones Connect
The elbow joint comprises three separate articulations enclosed within a common joint capsule. The primary connection is the humeroulnar joint, formed between the trochlea of the humerus and the trochlear notch of the ulna. This articulation functions as a hinge joint, allowing for bending and straightening movements of the arm. The precise fit of these surfaces provides significant stability to the elbow.
The humeroradial joint involves the articulation between the capitulum of the humerus and the head of the radius. It also contributes to the hinge-like motion of the elbow. The head of the radius rotates against the capitulum during forearm rotation. Both the humeroulnar and humeroradial joints are lined with hyaline cartilage, which reduces friction and allows for smooth movement.
The third articulation is the proximal radioulnar joint, sharing the same joint capsule. This joint is formed by the head of the radius rotating within the radial notch of the ulna. It is classified as a pivot joint, allowing rotational movement of the forearm. An annular ligament forms a collar around the radial head, holding it securely against the ulna and enabling smooth rotation.
Elbow Joint Movement
The elbow joint enables two primary types of movement: flexion/extension and pronation/supination. Flexion involves bending the arm, bringing the forearm closer to the upper arm, while extension is the act of straightening the arm. These movements occur predominantly at the humeroulnar joint. A normal range for flexion can be up to 140-150 degrees, and extension reaches 0 degrees, meaning a straight arm.
Forearm rotation, known as pronation and supination, occurs primarily at the proximal radioulnar joint, with the radial head pivoting within the ulna. Pronation involves turning the palm downward or backward. Conversely, supination is the movement of turning the palm upward or forward. These rotational movements allow for fine manipulation and positioning of the hand in space.