The human body uses joints as connections where two or more bones meet, providing both stability and the ability to move. The saddle joint is one of the more specialized types of these connections, characterized by a unique shape that permits movement in multiple directions. It is classified as a synovial joint, meaning it is surrounded by a fluid-filled capsule that lubricates the articulating surfaces for smooth function.
Structure and Classification
The anatomical structure of a saddle joint is defined by the reciprocal shape of the two articulating bones. One bone surface is concave in one direction and convex in the other, resembling a saddle seat. The opposing bone surface has the exact opposite configuration, allowing the two bones to fit together perfectly. This interlocking fit is what lends the joint its name, also known by the anatomical term “sellar joint.”
This specialized joint is categorized as one of the six main types of synovial joints, alongside types like hinge, pivot, and ball-and-socket joints. Functionally, the saddle joint is considered biaxial, meaning it allows movement around two main axes that are at right angles to each other. The joint surfaces are covered with slick articular cartilage, and the entire structure is encased in a joint capsule containing synovial fluid, which reduces friction and ensures smooth movement.
Location and Specific Examples
The most prominent and functionally significant location of a saddle joint in the human body is at the base of the thumb. This joint is formally known as the first carpometacarpal joint, or the trapeziometacarpal joint. It connects the trapezium, one of the small carpal bones in the wrist, with the first metacarpal bone, which extends into the thumb.
Another example of a saddle joint is the sternoclavicular joint, which is located in the upper chest. This connection is formed where the clavicle, or collarbone, meets the manubrium, the upper part of the sternum or breastbone. The sternoclavicular joint is the only direct bony attachment point that connects the upper limb to the axial skeleton, providing support and allowing the shoulder to move.
Less commonly discussed examples include the incudomalleolar joint, found in the middle ear, where it connects the malleus and incus bones to transmit sound vibrations.
Range of Motion
The biaxial nature of the saddle joint means that movement is possible in two perpendicular planes. The first pair of movements involves flexion, which decreases the angle between the bones, and extension, which increases the angle, such as when bending and straightening the thumb. The second pair of movements includes abduction, which moves the bone away from the midline of the hand, and adduction, which moves it back toward the midline.
The unique structure of the saddle joint also enables a complex, combined movement known as circumduction, where the thumb can move in a circle. Most significantly, the thumb’s saddle joint allows the distinct motion of opposition, which is the act of bringing the tip of the thumb across the palm to touch the tips of the other fingers.