The human elbow is a complex and highly functional joint central to countless daily activities. It enables a wide range of movements, from lifting and carrying objects to intricate hand-eye coordination tasks. Understanding its components clarifies how it facilitates these diverse functions. The elbow’s design allows for both strength and precision in arm movements.
The Bones of the Elbow
Three bones form the elbow joint, enabling its varied movements. The humerus, the long bone of the upper arm, makes up the upper portion of the joint. Its lower end features two structures: the spool-shaped trochlea and the rounded capitulum, located more laterally.
The ulna, a forearm bone, is positioned on the medial side, aligning with the little finger. At its proximal end, the ulna has a prominent, hook-like projection called the olecranon, which forms the bony point of the elbow. It also features a curved indentation, the trochlear notch, which fits with the humerus’s trochlea.
The radius, the other forearm bone, lies on the lateral side, corresponding to the thumb side. Its proximal end, or head, is cylindrical and disc-shaped, articulating with the humerus’s capitulum. The radius is generally shorter and thicker than the ulna, and its unique shape allows for rotational movements important for forearm and hand positioning.
Anatomy of the Elbow Joint
Beyond the bones, several non-bony structures are integral to the elbow’s stability and smooth operation. The ends of the humerus, ulna, and radius that meet within the joint are covered by smooth, slippery hyaline cartilage. This articular cartilage reduces friction between the bones as they move and helps absorb shock during impact.
A fibrous joint capsule encloses the elbow joint, containing lubricating synovial fluid. This fluid minimizes friction and nourishes the cartilage, contributing to effortless movement. The capsule is reinforced by ligaments that connect the bones and prevent excessive motion.
The medial collateral ligament (ulnar collateral ligament) is on the inner side of the elbow, connecting the humerus to the ulna. This ligament stabilizes against outward-bending forces. On the outer side, the lateral collateral ligament (radial collateral ligament) connects the humerus to the radius and blends with the annular ligament. The annular ligament forms a ring around the head of the radius, holding it securely against the ulna while still allowing it to rotate freely.
Elbow Movement and Function
The elbow performs two main types of movement. Flexion involves bending the arm, bringing the forearm closer to the upper arm, while extension is the action of straightening the arm. These hinge-like movements primarily occur at the humeroulnar joint, where the humerus’s trochlea articulates with the ulna’s trochlear notch. Normal flexion can reach approximately 140 degrees, and extension extends to 0 degrees or slightly beyond.
Beyond bending and straightening, the elbow also facilitates rotational movements of the forearm. Pronation is turning the palm downward or backward, while supination is turning the palm upward or forward. These movements occur at the proximal radioulnar joint, where the head of the radius rotates within the annular ligament and against the ulna. Pronation and supination are important for tasks like turning a doorknob, using a screwdriver, or bringing food to the mouth.