The carpometacarpal (CMC) joints are found between the bones of the hand and those of the wrist. Specifically, these are the articulations between the clustered carpal bones in the base of the hand and the metacarpals, the five long bones stretching across the hand to the base of each finger. The carpometacarpal joints below the four fingers are known as arthrodial or gliding joints, a synovial, or movable, joint type that allows the bones to slide against one another. The joint at the base of the thumb is another type of synovial joint called a saddle joint, and it allows a much greater range of motion than the other four joints.
While the carpals are arranged in a sort of cluster like a small pile of irregularly shaped rocks, they can be divided into two rows. The proximal row articulates with one of the long bones of the forearm, the radius, to form the radiocarpal or wrist joint. Beyond the proximal row, the distal row of carpals meets with the proximal or near ends of the five metacarpals to form the five carpometacarpal joints. Four carpals are found in the distal row, with the hamate on the pinky finger side, the capitate next to it, then the trapezoid, and finally the trapezium on the thumb side.
The fifth or pinky-finger metacarpal aligns with the hamate. The fourth or ring-finger metacarpal articulates with the hamate and the near corner of the capitate. The third or middle-finger metacarpal meets mostly with the capitate. The second or index-finger metacarpal links to the trapezoid, though it touches the capitate and trapezium on either side, and the first or thumb metacarpal forms a joint with the trapezium bone.
As the second through fifth carpometacarpal joints do not permit a wide range of motion at the joints themselves. Instead, they make the wrist joint more flexible by allowing the metacarpal and carpal bones to move past each other in several directions as the hand moves about on the end of the radius bone. In addition, these four carpometacarpal joints allow movement about the palmar arch. This is the concave curve at the base of the palm formed when the hand curls around an object like a baseball and created by an inward rotation of the fifth metacarpal. It makes grasping small objects possible.
The first CMC joint is situated at the base of the thumb. It differs from the last four carpometacarpal joints in that it permits a wide range of motion, which distinguishes the human thumb as opposable or able to grasp and hold things. As a sellar, or saddle, joint, which gets its name because the ends of the two adjoining bones resemble a pair of saddles arranged perpendicular to and curved about each other, the first CMC joint allows front-to-back and side-to-side movements. It also allows circumduction, a circling motion, and opposition, which is the drawing inward of the thumb to touch it to the tips of the other fingers.