Another type of ocular prosthesis is a scleral
shell, which covers a dysfunctional and disfigured
eye that remains in place. The scleral shell is
somewhat like an oversize contact lens, fitted to
rest on the eye as does a contact lens. The ocular-
ist designs the front of the shell to match the
appearance of the other eye. Because the scleral
shell rests on the surface of the eye, it moves in
synchronization with the other eye for a natural
appearance.
Over time the orbital structures change and the
materials of the prosthetic eye experience some
natural deterioration. Most people need to replace
the prosthetic eye every two years, though the
implant is permanent. Children may need more
frequent replacements to keep pace with their
growth. The prosthesis requires regular care and
cleaning.
See also ENUCLEATION.
pterygium A growth arising from the conjuncti-
val tissue around the perimeter of the EYE, usually
on the side near the NOSE. A pterygium character-
istically grows in a triangular shape, and has its
own BLOODsupply to support its growth. Growth
generally is slow. Often a pterygium remains
innocuous, though some people experience irrita-
tion and a sensation of grittiness in the affected
eye. Occasionally the growth encroaches on the
CORNEA, applying pressure or growing into the
corneal region. When this occurs, surgery to
remove the pterygium is necessary to preserve
vision. Pterygia tend to recur following surgery,
though it may take a number of years to reach a
size that interferes with vision.
See also CONJUNCTIVITIS; PINGUECULA; SCLERITIS.ptosis Drooping of the upper eyelid. Ptosis often
is a consequence of MUSCLEweakness or neuro-
logic damage, and is sometimes a symptom of a
neurologic condition such as MYASTHENIA GRAVISor
MUSCULAR DYSTROPHY. Ptosis may be congenital
(present at birth), the result of underdevelopment
or absence of the levator muscle that raises the
eyelid. Ptosis also may develop with advanced age,
reflecting weakening of the muscles that control
the eyelid.
When the drooping obscures vision, surgery to
raise the eyelid is necessary to prevent AMBLYOPIA.
Surgery may result in a slight asymmetry in the
movements of the upper eyelids, particularly in
congenital ptosis, when the levator muscle is miss-
ing and the surgeon must configure eyelid move-
ment to make use of other muscles. Generally no
treatment is necessary when the ptosis does not
interfere with vision, except as desired for cos-
metic purposes.
See also BLEPHARITIS; BLEPHAROPLASTY; ECTROPION.110 The Eyes