The term “knuckle” is widely used in everyday language, but its anatomical meaning is often imprecise. While people commonly point to several bony prominences on the hand and fingers, the true knuckles are a distinct set of joints with a specialized function. Understanding their location helps define how the hand achieves its remarkable dexterity and strength.
Defining the Main Knuckles of the Hand
The joints that form the prominent lumps when a person makes a fist are the primary knuckles, medically known as the metacarpophalangeal (MCP) joints. These joints are located where the long bones of the hand (metacarpals) meet the first bones of the fingers (proximal phalanges). There are five MCP joints in each hand, forming the transition between the palm and the digits. They are classified as condyloid joints, meaning the rounded head of the metacarpal bone fits into a shallow socket on the finger bone.
This structure allows for a broad range of movement essential for complex hand actions. The MCP joints of the four fingers permit motion in multiple directions. The MCP joint of the thumb is structurally distinct, acting more like a hinge to support the grasping function.
Lesser Knuckles and Common Misnomers
While the MCP joints are the true knuckles, the term is often used to describe the smaller joints located further along the fingers. The two joints found within each finger are the interphalangeal (IP) joints, which connect the various phalanges to one another.
The proximal interphalangeal (PIP) joint is the middle joint, situated between the proximal and middle phalanges. Closer to the tip is the distal interphalangeal (DIP) joint, which connects the middle phalanx to the distal phalanx. Both IP joints function as simple hinge joints, allowing the finger to bend and straighten.
Why Knuckles Are Important for Hand Function
The unique structure of the MCP joints provides the mechanical foundation for the hand’s functional capabilities, enabling both powerful gripping and precise manipulation. These joints allow the fingers to flex (bend forward) up to 90 to 100 degrees, necessary for closing the hand around objects. They also permit extension (straightening), and the ability to move the fingers away from (abduction) and toward (adduction) the midline of the hand.
This combination of movements is possible because the MCP joints are supported by a network of ligaments and tendons that provide stability without sacrificing mobility. The ability to spread and bring the fingers together, along with flexion and extension, makes it possible to perform actions like grasping a baseball or manipulating a small coin.