The prominent joints visible when clenching a fist are commonly referred to as knuckles. These are synovial joints, characterized by a fluid-filled cavity allowing for smooth movement. The main knuckle joints of the hand are classified as condyloid joints, indicating their unique shape and range of motion.
Metacarpophalangeal Joints
The joints most commonly identified as knuckles are the metacarpophalangeal (MCP) joints. There are five such joints in each hand, connecting the long bones of the palm, called metacarpals, to the initial bones of the fingers, known as the proximal phalanges. These articulations are formed by the rounded, convex heads of the metacarpal bones fitting into the shallow, elliptical cavities at the bases of the proximal phalanges. This specific anatomical configuration defines them as condyloid joints. The articular surfaces of both the metacarpal heads and the proximal phalanges are covered with hyaline cartilage, which facilitates smooth, low-friction movement within the joint capsule.
Range of Motion
The condyloid nature of the metacarpophalangeal (MCP) joints allows for movement in two primary planes. These movements include flexion, which is bending the fingers towards the palm, and extension, which involves straightening the fingers away from the palm. Active flexion typically ranges around 90 degrees, while extension can vary from 10 to 30 degrees, depending on the finger.
Beyond these primary motions, MCP joints also permit abduction, the spreading of fingers away from the midline of the hand, and adduction, the bringing of fingers together. A combination of these movements allows for circumduction, a circular motion of the finger. While some limited rotation can occur, condyloid joints do not allow for full 360-degree rotation like a ball-and-socket joint.
Other Hand Joints Often Called Knuckles
While the metacarpophalangeal joints are the primary knuckles, other finger joints are sometimes colloquially referred to as knuckles. These include the proximal interphalangeal (PIP) joints, located in the middle of each finger between the proximal and middle phalanges, and the distal interphalangeal (DIP) joints, found closer to the fingertips, connecting the middle and distal phalanges.
Unlike the MCP joints, both PIP and DIP joints are classified as hinge joints. Hinge joints permit movement primarily in a single plane: flexion (bending) and extension (straightening). This limited range of motion distinguishes them from the more versatile MCP joints. Therefore, while all these joints contribute to finger articulation, “knuckle” most accurately refers to the metacarpophalangeal joints, which are notably prominent at the base of the fingers.