CLASSIFICATION OF SYNOVIAL JOINTS Synovial joints are further classified according to the kind of motion and the number of axes of motions used. a. Uni-Axial Synovial Joints . (1) In uni-axial synovial joints, motion occurs in only one plane. The joints of the fingers (interphalangeal) flex and extend in the sagittal plane. These are commonly referred to as hinge joints. (2) If a single rotatory (rotational) motion occurs around a post-like structure, the joint is a pivot joint. The atlas vertebra rotating around the dens (tooth like projection) of the axis vertebra at the top of the neck (base of the skull) is a pivot joint. b. Bi-Axial Synovial Joints . In bi-axial synovial joints, motion between the bones occurs in two planes. Here the surface in contact is curved or rounded in two directions. (1) The proximal phalanx of a finger can flex and extend and move from side to side on the rounded head of the metacarpal bone. This is the MP or metacarpophalangeal joint. (2) When the two surfaces are curved in directions at right angles to each other, a shape similar
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