A joint is the site where two or more bones meet, allowing movement or providing structural support. Synovial joints, or diarthroses, are categorized by the number of axes around which movement occurs. A multiaxial joint represents the highest level of mobility in this classification. It enables motion in three distinct planes of space, allowing for complex, multi-directional actions.
Understanding the Axes of Movement
The term “multiaxial” refers to a joint’s ability to move around multiple anatomical axes, corresponding to the body’s three primary planes of motion.
The sagittal plane involves movement around a mediolateral axis, enabling flexion and extension (bending forward or backward).
The frontal (coronal) plane involves motion around an anterior-posterior axis. This permits moving a limb side-to-side, away from the midline (abduction) or toward the midline (adduction).
The transverse (horizontal) plane involves movement around a vertical or longitudinal axis. This third axis permits rotational movements, such as turning the arm inward (medial rotation) or outward (lateral rotation). A multiaxial joint allows movement around all three of these axes, giving it the greatest possible range of motion in the body.
Classifying Joints by Range of Motion
Joints are functionally organized based on the number of planes they can move within.
A uniaxial joint is the most restricted, permitting movement around only one axis and limiting motion to a single plane. Hinge joints, like the elbow, are classic examples, allowing only flexion and extension.
Biaxial joints offer greater flexibility by allowing movement around two distinct axes. These joints enable motion in two planes, such as flexion/extension and abduction/adduction. The metacarpophalangeal joints (knuckles) are examples, permitting movement forward/backward and side-to-side.
A multiaxial joint, also called a triaxial or polyaxial joint, operates around all three axes. This full mobility allows for a combination of all movements, including the circular motion known as circumduction.
Key Multiaxial Joints in the Human Body
The primary structural classification that provides multiaxial movement is the ball-and-socket joint. This structure involves the rounded head of one bone fitting into a cup-shaped socket of an adjacent bone. The two most prominent examples are the glenohumeral (shoulder) joint and the acetabulofemoral (hip) joint.
The Shoulder Joint
The glenohumeral joint connects the head of the humerus to the shallow glenoid cavity of the scapula. It has the most expansive range of motion in the body, allowing all three plane movements, rotation, and circumduction. The relative shallowness of the socket contributes to this freedom but also makes the joint less stable.
The Hip Joint
The acetabulofemoral joint, or hip, is formed by the head of the femur fitting into the deep acetabulum of the pelvis. While multiaxial, the hip is more constrained than the shoulder due to its deeper socket and strong surrounding ligaments. This added constraint provides greater stability, which is necessary for supporting body weight and locomotion.