orthodontic appliances Fixed or re-
movable devices, commonly known as
braces, worn to correct
to reposition overcrowded or
Usually fitted during childhood and
adolescence, they move teeth using
sustained gentle pressure. A fixed appli-
ance has brackets attached to the teeth
through which an arch wire is threaded
and tightened to exert pressure. These
are usually kept in place for about a year,
after which time a retainer plate may be
needed to hold the teeth in place until
tooth and jaw growth has finished.
Removable appliances, consisting of a
plastic plate with
anchor over the back teeth, are used
when only one or a few teeth need cor-
recting. They apply force by means of
springs, wire bows, screws, or rubber
bands fitted to the plate.
concerned with preventing and treating
The procedures are usually
performed while teeth are developing
and still relatively manoeuvrable, but
can also be of benefit in adulthood.
An orthodontist may first make mod-
els of the teeth (see
of the head and jaws.
Certain teeth, often premolars, may be
extracted to make room for the remain-
ing teeth. Poorly positioned teeth are
then moved by gentle pressure exerted
orthognathic surgery An operation to
correct deformity of the jaw and the
that is invariably
associated with it. The bones of the jaw
are repositioned under general anaesthe-
sia, and often require splinting (see
) until they heal.
orthopaedics The branch of surgery con-
cerned with disorders of the
and their associated
. Procedures include
setting broken bones and applying casts;
arthritis, and back problems; treating
bone tumours and birth defects of the
skeleton; and repairing or replacing hip,
knee, or finger joints.
orthopnoea Difficulty in breathing when
lying flat. Orthopnoea is a symptom of
and also occurs with
obstructive pulmonary disease (see
monary disease, chronic obstructive
orthoptics Techniques used mainly in
children to measure and evaluate
including eye exercises, assessment of
monocular and binocular vision, and
measures to combat
orthotics Use of appliances to support
or correct weakened or deformed joints.
os An anatomical term for a bone; also
refers to an opening in the body, as in
the cervical os (entrance to the
Osgood-Schlatter disease Painful en-
largement and tenderness of the tibial
tuberosity (the bony prominence of the
), which occurs most commonly in
boys aged between 10 and 14. It results
from excessive, repetitive pulling of the
, due to repeated exer-
cise. The disorder often clears up without
treatment; severe pain may require
or immobilization of the
knee in a plaster
osmosis The passage of a solvent from a
weaker solution to a more concentrated
one through a semipermeable membrane.
All body cells are surrounded by such
membranes, which allow water, salts, sim-
ple sugars (such as
(but not proteins) to pass through.
Therefore, osmosis plays an important
part in regulating the distribution of water
and other substances in body tissues.
ossicle A small bone, especially any of the
3 tiny bones in the middle
incus, and stapes) that conduct sound
from the eardrum to the inner ear.