This is a joint in which a small space called a
synovial cavity or joint cavity is present between
the articulating surfaces of the bones. The bone ends are covered
with a smooth hyaline cartilage which is lubricated by the
synovial fluid. This is a clear, yellowish, viscous fluid
which contains mucin. Its function is to nourish the hyaline cartilage
and to lubricate the joints. The fluid prevents the friction between
the solid surfaces in contact with each other. The synovial fluid
is secreted by the synovial membrane which lines the joint
capsules. The membrane contains fat cells which may form fat
pads forming cushions to the joint. The synovial joints are
freely movable and are made stronger by ligaments. The ligaments
may be within or partly outside the joint cavity or may be inside
the joint cavity (e.g., hip and knee of mammals).
On the basis of variations in the shape of articulating
surfaces the synovial joints are sub divided into following types.
(i) Gliding joint. Articulating surfaces
are usually flat, with gliding or non axial movement (e.g., between
the carpal bones or vertebrae).
(ii) Hinge joint. The spool-shaped surface
fits into the concave surface and allow limited movement in one
plane about a single axis (Uniaxial), like a hinge-door movement
(e.g., knee and elbow joints). This resists dislocation.
(iii) Pivot joint. In this arch-shaped joint
the surface rotates about a rounded or peg-like pivot, and allows
rotation of one bone around its own long axis (uniaxial), (e.g.
between atlas and axis and between radius and ulna).
(iv) Condyloid joint. An oval-shaped condyle
fits into an elliptical cavity. The movement is in two planes
(biaxial), at right angles to each other, back and forth and side
(v) Saddle joint. A saddle-shaped bone fits
into a socket that is concave-convex in opposite directions . It
is a modified condyloid joint. Movement is biaxial (e.g. thumb,
between metacarpal and carpal bones, heterocoelous cervical vertebrae
(vi) Ball-and-socket joint. Ball-shaped
head fits into a concave socket. It allows movement in many directions
including rotation (triaxial) (e.g., hip joint and shoulder joint).
These joints are prone to sudden-strain and are easily separated
(vii) Snap joints are made stable by their
ligaments which are lateral in position. Snap joints are found at
the elbow and ankle of large mammals. Snap joints provide passive
support in the standing position.
(viii) Plane joints have more or less flat
surfaces and allow various motion. These joints are found between
the carpal bones of large mammals and between pre and post-zygapophysis.
(ix) Radial joints are those in which one
bone turns across another, as and when the radius of the
forearm turns upon its own axis across the ulna in rotating
the arm (Figure 21.5) to turn the palm upwards (supination)
or downwards (pronation).
Arthritis, the inflammation of a joint
makes the joint painful and may immobilize movement at the
joint. This results from a lack of synovial fluid, ossification
of cartilage, deposition of uric acid crystals in the joint
cavity or other changes due to age. A slipped disc is displacement
of vertebrae from their normal positions. This results in
pressing the nerves between the vertebrae, therefore it is