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 to that of a cowboy's saddle is formed. This type of synovial joint
is called a saddle joint. In the human body, the saddle joint is located at the base of
the thumb.
c. Multi-Axial Synovial Joints. In multi-axial joints, motion is
possible in all three planes of space.
(1) The ball-and-socket-type synovial joint has the freest motion in all
directions. A spherically rounded head (ball-like) fits into a receiving concavity
(socket). The hip joint is an example of the ball-and-socket type, with the spherical head
of the femur fitting into the cup or socket (acetabulum) of the pelvic bone.
(2) In the plane joint, the contact surfaces of the bones are essentially
flat. These flat surfaces slide on one another (also called translatory motion). The
acromioclavicular joint of the shoulder region is an example of a plane joint.
Primary Content Providers: The U. S. Army, The
U.S. Navy
Ancillary Content and Online Version: David L. Heiserman
Publisher: SweetHaven Publishing Services
