Degrees of freedom of joints

This refers to the imposition of one of a pair of articulating bones, in one, or more planes. The axis of a joint is basically the fulcrum (pivot) about which the bones that make up the joint move. The movement occurs within a plane.

• Uni-axial: one degree of freedom – movement of a bone at a joint is limited to rotation about a single axis.
• Bi-axial: two degrees of freedom – completely independent movements can occur about two distinct axes.
• Tri-axial: three degrees of freedom – movement of a bone occurs about the three main axes