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The metacarpophalangeal (MCP) joints are diarthrodial joints where the large convex heads of the distal aspect of the metacarpals articulate with the concave-shaped proximal aspect of each phalange. The articulating surface of each metacarpal head and proximal phalange is composed of hyaline cartilage. There are five separate MCP joints in each hand and these joints serve as transitions between the palm and the fingers. In layman's terms, the MCP joints are known as the “knuckles,” and the metacarpal heads are especially prominent dorsally when making a fist. These joints provide a combination of stability and flexibility which allows for the dexterity required by the hand. Similar to other joints in the body, the MCP joints are acted upon by muscles to allow for specific joint movements. These movements include flexion, extension, abduction, adduction, and limited circumduction. Clinically, arthritis involving the MCP joints is a classic and differentiating feature of rheumatoid arthritis (RA) from osteoarthritis (OA), which typically involves the distal interphalangeal (DIP) joints.