The joints between bones give us our full range of body movements, but the
various joints differ in the amount of flexibility they allow and in their
vulnerability to injury.
Joints -- the points where two bones meet -- vary a great deal in type, from
those that are fixed, such as the sutures between the bony plates of the skull,
to the most flexible ones like the shoulder joint. In between these two extremes
are other types of joint that allow you only limited amounts of movement.
Finger joints are a good example of a
hinge joint, a simple joint that allows just two movements -- bending and
The knee and elbow joints are modified hinge joints which allow not only
bending and straightening but also a degree of rotation. The knee joint allows
only a limited amount of rotation, but the elbow, in conjunction with the
shoulder and wrist joints, lets the hand rotate through 360°.
OTHER TYPES OF JOINT
The wrist itself is an ellipsoidal
joint, which means that the oval-shaped wrist bone fits into a neat cavity at
the end of the arm which is elliptical in shape. This particular type of joint
allows side-to-side movement and up-and-down movement but no rotation.
Rotation is the only form of movement permitted by a pivot joint. The neck
joints are pivot joints, where a ring of bone rotates around a bony projection.
The most flexible of joints is a ball-and-socket joint. The shoulder joint and
the hip joint are both of this type and allow the widest range of movement --
backwards, forwards, up, down and rotation.
In structural terms these are the main types of joint, each suited for. a
particular application depending on their role in the body's movement. If, for
instance, our knee joints were ball-and-socket joints, able to bend every which
way, we might experience great difficulty in trying to walk. In this instance, a
hinge joint with a small degree of rotation is the best design for the job of
The bone surfaces of joints are coated with a very smooth cartilage to reduce
friction. This type of cartilage -- one of several in the body -- is known as
hyaline cartilage or articulating cartilage and provides an almost frictionless
layer over the bones.
To further smooth the working of the joint there is a lubricant that fills
the area around the joint including the space between the two bone-ends. This
sticky, lubricating liquid is known as synovial fluid and is produced by the
synovial membrane or synovium. The synovium is sometimes only one cell thick but
is very effective in oiling the joint. It lines the inside of the joint but does
not run over the articulating cartilage.
The whole joint is sealed within a tough fibrous capsule formed from strong
gristle, which prevents the synovial fluid from seeping away and holds the joint
in place, preventing abnormal movement.
Further steadying of the joint is provided by the strong ligaments that
surround it and prevent excessive movement. Muscles, which are attached by
tendons to the bone on either side of the joint, control the movement of the
Most joints are synovial joints of this type. Where the synovium becomes
diseased, as happens in rheumatoid arthritis, it may be removed without damaging
the joint in the short term. However, in the longer term, a healthy synovial
membrane is essential to prevent wear and tear.
The knee joint is surprisingly complicated and is commonly injured by
When you extend your knee beyond the straight position it locks. This locking
is brought about by a very slight but important twisting movement of the large
bone of the leg (the tibia) and the bones of the lower leg.
There are two pieces of gristle which
act as padding between the cartilage linings of the thigh bone (femur), the
body's largest bone.
One piece of cartilage wedges the outside of the joint and the other the
inside. These are known as sem-lunar cartilages because they are shaped rather
like a crescent moon with the points towards the inside of the joint. The
outside edge of each semi-lunar cartilage is attached to very strong ligaments,
one running down the outside of the leg and one down the inside. These form the
lateral and medial ligaments of the knee, with the cartilage forming a sandwich
Since the cartilages are attached at their points and at their edges, a
twisting movement of the knee which pulls these ligaments can tear the
cartilages. A torn cartilage -- common among footballers -- is an injury causing
severe pain and often the inability to bend the joint. If this persists, the
only treatment is to remove the cartilage. It may grow again, but in any event
the muscles strengthen to compensate for the missing cartilage.
Both the knee and the hip joint have ligaments in the centre of the joint for
added stability. In the case of the knee, there are two, and these can also be
The synovial membrane lining the
inside of the knee is very extensive and runs under the kneecap (patella) for
about 2.5cm (1in) down the front of the shin. There are several bursae (small
enclosed cavities ) the the knee which are lined with synovial membrane and
filled with synovial fluid.
Constant kneeling or a bang on the knee can inflame the synovium and cause
painful swelling due to increased production of synovial fluid. This is known as
bursitis or more usually 'housemaid's knee'.
Rheumatoid arthritis, a common disease of unknown cause, inflames the
synovium leading to an accumulation of fluid in the joints.
This disease is quite distinct from osteoarthritis, which is a result of
simple wear and tear.
Fibrous joints have no synovium, and
the bones are joined by tough, fibrous tissue, permitting very little movement.
This type of joint occurs in the back, the sacrum (the triangular bone between
the hip bones) and some of the joints in the ankle and the pelvis.
The joints of the spine are not only joined by fibrous tissue but are also
cushioned by a small bubble of jelly called a disc. This capsule of jelly acts
as a shock absorber and there is one between each of the spinal bones to prevent
damage to the spinal cord.
Some joints are formed between bone
and cartilage. Cartilage (or gristle) is a rubbery substance that is very
flexible and allows a good deal of movement without the need for synovium. The
joints between the ribs and breast bones at the front of the chest are
cartilaginous joints. Those attaching the ribs to the back are synovial joints.
Damage to the joints may give rise to
problems in later life. Sportsmen and other people who injure their joints
regularly are more prone to osteoarthritis. The articlage weas away from the end
of the bone so that bone rubs against bone. The exposed bone becomes stronger
and more dense but it cannot compare with the wearing properties of cartilage
and, in due course, the joint surface loses shape, resulting in pain, limited
movement and loss of mobility.
'Spare part' surgery has changed the
treatment of severe osteoarthritis. Once, the only relief from pain was with
analgesics. Now long-lasting replacement joints are made of plastic or metal.
Pain relief is dramatic and with physiotherapy a full range of movement can
be restored. Hip replacement is the most common operation but many other joints
can now be safely replaced.