SYNOVIAL JOINTS
The six types of synovial joints are plane, hinge, pivot, condyloid, saddle, and balland-socket. A plane joint is formed by the flat surfaces of two bones in which the
surfaces are about the same size and shape. The surfaces glide across each other
in one or more directions. The number of directions in which a plane joint moves
depends on the how tightly constrained the joint is by ligaments and nearby bones.
Plane joints are found between the carpal bones of the wrist, the tarsal bones of the
foot, the clavicle and acromion of the scapula, and the superior and inferior
processes of adjacent vertebrae.
A hinge joint is formed by the convex, cylinder-shaped end of one bone and the
concave, trough-shaped end of a second bone. The cylinder-shaped bone fits into
the trough-shaped bone and rotates along the joint’s long axis. The elbow joint
between the humerus and ulna is a good example of a hinge joint. Other hinge joints
include the knee, ankle, and interphalangeal joints between the phalanx of the
fingers and toes.
A pivot joint is formed by the round end of one bone fitted within a ring-shaped
structure on a second bone. The ring-shaped structure is formed partially by bone
and partially by ligament. The round bone rotates within the ring along the round
bone’s long axis. The joint between the C1 and C2 vertebrae is a pivot joint that
enables you to turn your head from side to side. The proximal radioulnar joint is a
pivot joint enables you to rotate your forearm.
A condyloid joint is formed by the oval-shaped convex surface of one bone fitted into
the oval-shaped concave surface of a second bone. The convex bone can rotate
within the concave bone in the anterior-posterior axis and the medial-lateral axis.
The metacarpophalangeal joints at the base of the fingers are condyloid joints that
allow you to bend and straighten your fingers, as well as spread your fingers apart
and bring them back together. The radiocarpal joint in the wrist is another condyloid
joint that allows similar movement.
A saddle joint is formed by the concave surface of one saddle-shaped bone fitting
perpendicularly over the concave surface of a second saddle-shaped bone. This
arrangement gives saddle joints the appearance of a rider sitting in a saddle. Saddle
joints allow movement in two planes. For example, the carpometacarpal joint at the
base of the thumb is a saddle joint that allows you to move your thumb laterally
away from the side edge of the hand. It also allows you to move your thumb to lie
against the palm of your hand. This saddle joint is what gives humans the distinctive
“opposable thumb” trait.
A ball-and-socket joint is formed by the rounded head (ball) of a long bone fitting into
the concave surface (socket) of a second bone. Ball-and-socket joints allow rotation
in many different planes, thus providing the greatest range of motion of all the joint
types. The hip and shoulder joints are the only ball-and-socket joints in the body. At
both of these joints, the head are able to rotate within the socket in the medial-lateral
direction, the anterior-posterior direction, and around the long axis of the long bone.