btt (01/28/83)
Some recent net.med inquiries re soft contacts prompts the
following which may be of general interest.
From THE MEDICAL LETTER on Drugs and Therapeutics, Dec 10, 1982:
SOFT CONTANCT LENSES
More than ten million people in the USA use contact lenses; about
half now wear soft lenses, and the percentage of soft-lens users
appears to be increasing.
PHYSICAL CHARACTERISTICS - Soft lenses are generally made from
hydroyxy-ethylmethacrylate or other water-soluable monomers, combined
with a cross-linking agent to form a hydrophilic gel. Brittle when
dehydrated, they become flexible after immersion in water, and conform
to the shape of the cornea. Oxygen is transmitted mainly by fluid
exchange though the lens. Soft lenses made of silicone (Dow Corning;
Danker Labs) are pliable when dry; they have excellent optical
properties and oxygen transmission.
ADVANTAGES OVER HARD LENSES - Soft lenses are more comfortable
than hard lenses and more permeable to oxygen; they do not require a
long time to adjust to their use, and are less likely to cause corneal
irritation and edema. Silcone soft lenses are not generally as
comfortable as standard soft lenses, but they are more comfortable
than hard lenses. Since soft lenses are less easily displaced and
therefore less likely to be lost, they are usually superior to hard
lenses for wear during exercise. Hard-lens wearers are often unable
to switch quickly back to eyeglasses because lens-induced changes in
the cornea cause temporary "spectacle blur"; hard-lens wearers
generally suffer a loss of tolerance if they do not wear the lens for
at least a few hours each day.
DISADVANTAGES - The main disadvantage of soft lenses is that they
sometimes do no provide the same visual acuity as hard lenses,
especially in patients with astigmatism; even without astigmatism, the
curvature and optical properties of the soft lens may change with
variations in the mechanical force of lid movements. Another
disadvantage of standard soft lenses is that they may require more
care.
MAINTENANCE - Most soft lenses require daily cleaning and
disinfection. Lens cleaning solutions may cause eye irritation. due
either to allergy or poor rinsing. After cleaning, either thermal of
chemical disinfection is required. Many users prefer chemical
disinfection, but almost all disinfecting solutions contain
chlorhexidine (Hibiclens), thimersol (Merthiolate; and others), or
other ingredients that can cause eye irritation. Even when recommend
cleaning devices and solutions are used, soft lenses sometime tear
easily or become coated with debris and irreversibly discolored.
Although soft lenses are advertises as lasting one to three
years, their useful life is often much shorted, and the cost of
replacement lenses can vary from $40 and $75. Hard lenses often last
for five years.
EXTENDED-WEAR LENSES - Recently extended-wear soft lenses, which
can be left in place for weeks, have become available in the USA.
Extended-wear lenses for myopia and hyperopia are designed to be
removed and cleaned by the patient about every two weeks - or when
necessary, which may be every few days; extended-wear contact lenses
for patients who have had cataracts removed can sometimes be worn for
month, but they generally have to be removed and cleaned (or replaced)
by a professional. Extended-wear soft lenses are either extremely
thin or more highly hydrated than other soft lenses, allowing
sufficient oxygen to reach the cornea even during sleep, but some are
not quite as comfortable as standard extended-wear soft lenses.
Deposits of various types are probably the most frequent problem
encounted with extend-wear soft lenses. A Medical Letter consultant
reports, however, that one aphakic patient wore a Dankler Labs
silicone lens, day and night, for four years without removing it, and
it remained crystal-clear.
CONCLUSION - Soft contact lenses are more comfortable and more
convenient than hard lenses, but they may improved visual acuity less
and are more difficult to maintain. Extend-wear soft lenses that can
be worn continuously for weeks or months are now available.
Todd Troost MD
decvax!cwruecmp!medman!btt
Department of Neurology
CWRU, Cleveland OH
(216) 444-3190