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There are two usual methods for contact lens fitting. The first
is the procedure in which taking measurements is involved in order
to determine the parameters. The curve of the cornea is measured
using keratometry or videometry. The diameter of the iris is measured
horizontally, the palpebral aperture is measured vertically and
the diameter of the pupil is also determined. All these findings
are taken together to determine the contact lens parameters to
be ordered.
The second type of fitting is the diagnostic lens procedure where,
after the aforementioned measurements are taken, corresponding
and appropriate lenses are chosen from the eye specialists’ contact
lens trial set and tested on the patient’s eye. The set
is kept on the eyes for 15-20 minutes so that the lacrimation
decreases and, in the case of soft lenses, this test period is
very important. The temperature on the eye is greater than the
surrounding temperature thereby forcing fluid out of the lenses,
so a point of equilibrium must be reached. The relationship between
the back surface of the contact lens and the front surface of
the eye must be perfected to result in a good lens fit.
Positioning of Contact Lenses
Corneal lens fitting is necessary for superior positional or
intrapalpebral aperture positioning of the lenses. For either,
the lens should be situated at the centre of the eye in a horizontal
manner. The lower edge of the lens should be 1-2mm above the lower
eyelid. In case of superior positioning, the upper edge of the
contact lens must be under the upper eyelid and not above the
superior limbus. In case of intrapalpebral positioning, the upper
edge of the lens should be immediately beneath the upper eyelid.
After positioning, while blinking the lens should ideally shift
1-2mm and then come back to its original position and remain there.
In case of rigid lenses to check how well the lens adjusts to
the cornea, sodium flourescin is used which mixes with the film
between the lens and the cornea and, if illuminated with a black
light, it shows various patterns. The more perfect the correspondence
between the curves of the cornea and lens, the fluorescence is
bluish black while less than perfect adjustment will give off
a brighter yellowish green fluorescence.
Determining Flat and Steep Lenses
The ideal is to use lenses that are not too flat and not too
steeply curved so they fit the curvature of the eye and don’t
irritate the eye. The sides of the lenses give information about
the sag of the lens. If the lens sides turn away from the eyes
after blinking, it is a case of a flat lens whereas if the lens
sides press heavily on the cornea or buliar conjunctiva, this
points to a steep lens. A lens, which moves excessively away from
the centre after blinking, is found to be too flat whereas a steep
curved lens will never move.
In case the position movement or back surface fitting relationship
does not match the required standards, trial lenses with several
parameters are tried. After the appropriate fit is obtained, the
lens power is calculated by computation or directly by a method
of refraction over the trial lenses. The quality of the vertex
distance also needs to be considered following the calculation
of the spherical power of the contact lenses.
When rigid lenses that have a spherical base curve are fitted,
the lachrymal tear film occupies the space between the cornea
and back surface of the lens so the toricity of the cornea filled
by the lacrimal layer neutralizes the corneal cylinder. Thus,
the flattest corneal meridian or flat K becomes the point of reference
for all calculations. However, in soft contact lenses with a spherical
base curve there is no lacrimal film as the anterior of the lens
drapes the cornea allowing little neutralization of corneal cylinder
to occur.
Follow up Care
Since the contact lens is an external device made of plastic
situated over the cornea, appropriate care of the lenses and subsequent
follow up checks become imperative to maintaining lenses as well
as health of the eyes. Though after the compulsory initial frequent
check ups when someone first starts to use contact lenses, one
visit a year after that is still necessary as problems can occur
any time over the years. The shape of the eye can change and the
prescription may need to be changed as well. Check ups include
examination of history, eyesight, lens fit, tissue integrity,
patient compliance and physical structure of lenses.
A contact lens acts as an optical patch or bandage. While acting
as the former it reduces the passage of oxygen and carbon dioxide
to and from the cornea. The patch creates hypoxia and disrupts
the corneas natural aerobic metabolic cycle. To check these follow
up care becomes a must.
As a bandage, contact lenses create stress on the tissues underneath
and this reduces the moistness of the eye surface. It can also
become infected with organic and inorganic deposits and can also
be scratched, clipped or torn. To counter all these dangers taking
advice from the eye doctor is necessary.
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