Having a thorough working
knowledge of ocular anatomy is not only helpful but also essential for
successful contact lens fitting. Keeping the patient�s eye healthy is, as
always, the practitioner�s primary concern. Familiarity with corneal metabolism
and structure will make the fitter more aware of how the process can break
down. Furthermore, knowing where and how refractive errors occur within the eye
can lend a more knowledgeable approach to proper lens selection.
The F.D.A. considers all contact
lenses controlled medical devices. This is because they come into contact with
the eye and have a direct impact on ocular health. Misuse of contact lenses can
have permanent and visually impairing consequences. Since the general public is
not expected to understand the physiological breakdown of the eye it must be
carefully controlled by those professionals that do. Thus the patient must
return for their appointed exams and cannot be allowed to have lenses on an
expired Rx. They may not be satisfied with this methodology. Some may even find
the practitioners motives suspect. This only reinforces the conclusion, though,
that the patient is not always knowledgeable enough to be responsible. Whatever
they might conclude it remains the duty of the eye care professional to protect
the patient from the dangers they don�t understand.
In this review we are going to
discuss those features of the eye that have a direct impact on the success of
contact lens wear. Whether it pertains to the continued health of the cornea or
precision of the refractive correction certain things must be kept in mind when
selecting the most appropriate lens.
THE TEAR FILM
The tear film is the first and
most directly influential factor in the fitting process. It represents the
initial point of interface between the contact lens and the eye. The tear film
covers the entire front surface of the eye and has a multitude of important
functions. Among these are:
Lubrication of the sclera and
Removal of debris
Oxygenation of the cornea
Provides a smooth surface of
Protection of the eye from
The tear film is made up of three
separate layers each playing its own part in maintaining the system. The
anterior-most layer is composed of lipids that slow evaporation, keeping the
cornea hydrated between blinks. The middle layer is aqueous containing salts
and antibacterial substances to fend off infections. The inner layer is mucoid.
Its primary function is to anchor the tear film. A breakdown in any one part of
this system can result in dry eyes. A patient with dry eyes represents a
challenge as a contact lens fit. Why is this? When a lens rests on the eye, be
it rigid or soft, it is actually floating on the tear film. This allows it to
sit and move comfortably during blinking. The cornea demands oxygen filtered
through the tear film to maintain clarity as well. If tears cannot flow under
the lens, metabolism will be compromised and corneal edema will result. For
this reason observing lens movement is an important step in the fitting
process. Too much movement may produce irritation and poor vision but too
little would create the above mentioned condition. The tear/lens relationship
is not only affected by the dimensions of the contact but also by the material
as well. A soft lens flexes to match the shape of the eye creating a uniform
thickness tear film beneath. A rigid contact will produce a different
circumstance. In a lens that is steeper than the corneal surface, more tears
will pool in the greater space beneath the central portion than the periphery.
This is popularly referred to as the �tear lens.� A tear lens that is thick
centrally and thin peripherally will effectively add plus power to the over all
correction. For this reason the power of the tear lens must be considered when
selecting the power of the lens.
Consider, for clarification, the
following example. A woman with -5.00 diopters of myopia is fit with a GP lens
that is slightly steeper than her cornea. The resulting tear lens produces
+0.50 diopters of refractive correction.
error- Tear lens = Power needed for contact lens
-5.00 � (+0.50) = -5.50 diopters
In this case the tear lens is
counteracting the correction somewhat and must be compensated for by increasing
the minus in the contact correction. The reverse situation may be true as well
though when a lens is fit flatter than the corneal curvature.
The most anterior structures
affecting contact lens fit are the eyelids. The eyelids have three main
functions. These are:
Protection of the globe
Distribution of the tear film
Production of tears
The eyelids contain many of the
structures for producing the tear film. Among these are the meibomian glands.
Opening at the lid margin they produce the sebaceous top layer. The lacrimal
glands located superior and temporal on the underside of the lid and produce
the aqueous component. As the lid blinks it spreads the tear film over the
surface of the eye toward the lacrimal ducts positioned on the upper and lower
lid margins. While some of the tears are lost to evaporation the majority drain
out of the eye through the openings, called puncta, into the nasolacrimal duct.
During the pre-fit evaluation it
is important to examine the lids and lid margins. They should be free of debris
and inflammation. The under-surface of the lid should be smooth and uniform in
color and texture. Patients that exhibit signs of chronic allergies or
blepharoconjunctivitis are unlikely to be successful contact lens wearers.
Follow up exams should monitor the lids as well. One common complication
observed with contact lenses is giant papillary conjunctivitis or GPC. GPC is a
hypersensitivity reaction most commonly diagnosed in soft contact lens wearers.
The underside of the lid becomes red and irritated developing a
Of utmost importance in the
fitting process is the preservation of corneal health. Any disruption of
metabolism can cause loss of tissue transparency and decreased visual acuity.
Since the cornea depends heavily on the tear film for nutrition and protection,
how a lens fits on the eye will have direct impact on this relationship.
The cornea itself is composed of
five separate layers. The most anterior layer is the epithelium. The
epithelium is five to six cells thick. Each cell begins in a cuboidal shape at
the base. Over the course of seven days it migrates to the surface, flattens
out and is ultimately sloughed off into the tear film to be removed from the
eye. This rapid turnover allows the eye to recover from minor injuries with
great speed. The epithelium will not scar if it is compromised though it may
take up to a month to recover if the full thickness is removed. The main
function of this layer lies in creating a smooth refractive surface of the eye.
The second layer is known as Bowman�s
layer. Sometimes referred to as Bowman�s membrane this is mainly a
transition area between the epithelium and the underlying stroma. Composed mostly of tough, densely packed collagen fibrils, this acellular
region does not regenerate if it is compromised. In such situation the
adjoining layers would grow into the area. Fortunately Bowman�s layer is highly
resistant to such intrusions.
The main thickness of the cornea
is represented by the third layer, the stroma.
The stroma is composed of collagen fibrils as well though these are larger and
arranged in uniformly straight, parallel patterns. This strict pattern of
fibril placement allows light penetration or transparency, of the cornea.
Stromal transparency is highly dependant on fluid pressure to maintain
uniformity. Any fluctuation in water concentration may result in loss of this
delicate balance. One situation that can represent compromise is over wearing
contacts. This can cause the hypoxic cornea to undergo anaerobic respiration, a
condition that results in corneal edema, an excessive intake of fluid. In an
attempt to chemically balance the system the eye has sacrificed optical
clarity. These patients may report reduced visual acuity and halos. The best
treatment is to get the lenses out and allow the cornea to recover its
The fourth layer is Descemet�s
membrane. It is composed of two thin layers of collagen fibrils
arranged in such a way that they exhibit elastic properties. It is very
resistant to trauma and continually regenerated throughout life by the
innermost layer or endothelium.
The endothelium is composed of a
single layer of cells. In addition to producing Descemet�s membrane, it acts as
a pump for the cornea transporting excess water out and into the anterior
chamber, the fluid filled space behind the cornea. A number of studies have
concluded the endothelium can be affected by long term contact lens wear.
Polymorphism (changes in shape) and polymegathism (changes in size) of the
cells can occur. These conditions can affect pump function but these moderate
changes do not usually compromise the barrier.
The cornea is avascular because
it must remain transparent. It derives its nutrition almost entirely from the
air with minor contributions from limbal blood vessels and the aqueous anterior
chamber. All contact lenses produced today have a certain level of gas
permeability called the dk value. Higher dk value and thinner lenses will allow
more oxygen to pass through. These two values are the determining factors for
wearing schedule. Some lenses with high oxygen transmissibility are approved
for extended wear of up to 30 days continuously. Others must be removed every
night without exception. Sleeping in a low dk or thick lens can cause
complications to the cornea. The immediate effect is hypoxia induced edema. If
bad habits continue neovascularization can occur. Commonly referred to as
�Neo,� this is when blood vessels begin to grow into the oxygen deprived cornea
in an effort to provide the missing sustenance (above). The progress of this
condition can be halted in one of two ways. Wear of the contacts must cease or
a lens of higher transmissibility must be employed. When the source of the
problem is removed the blood will withdraw from the newly formed blood vessels.
The empty vessels, however, will always remain.
In order for a contact lens to
fit appropriately it must approximate the curvature of the cornea. A thorough
knowledge of corneal topography is therefore a must in any contact lens
fitting. Only a very small central section of the cornea is said to be near
spherical. As it moves outward toward the limbus the curvature flattens out.
For this reasons gas permeable lenses will often have one or more peripheral
curves flatter than the central.
Simple refractive errors bear
mention here in that they can be partially caused by steeper/flatter than
normal corneal curvature. While the axial length of the eye (too short or long)
is the primary causal factor, it is important to recognize that both
possibilities exist. Corneal curvature may not always have a direct correlation
with refractive error.
Astigmatism presents an
additional challenge to the fitting process as well. Here the cornea is steeper
in one direction than the other. The rule of thumb when fitting a patient with
astigmatism is to fit the lens based on the flattest meridian. This is called
fitting �on K.� With GPs more than soft lenses, whose flexibility will
accommodate toric corneas, it is critical to be able to accurately identify
this parameter. In addition, it is important to recall the tear lens that was
described earlier. If we have a spherical lens equal to the flattest curvature
sitting on a toric surface there will be a toric tear lens nullifying the
astigmatic error. This will often correct for the patients� astigmatism without
it being necessary to put this correction into the lens. There are exceptions
of course. If the astigmatism is very high a spherical lens may be unstable and
rock too much. A toric back surface would improve the fit. Not all astigmatism
is corneal either. It can occur on the lens within the eye as well. This is
often referred to as residual or lenticular astigmatism. In such case a lens
may have toric correction on the front surface of the lens.
Worth special mention in the
contact lens fitting world is the condition known as keratoconus. Keratoconus
is a corneal condition involving degeneration in the basement membrane and
Bowmen�s layer. As it progresses the stroma thins and the internal pressure of
the eye causes the cornea to bulge out into a cone shape. This thinning can
cause irregular astigmatism to manifest as well. Irregular astigmatism is when
the axis is not 90 degrees from the point of greatest curvature. As these
symptoms worsen GPs
become necessary for smoothing out the refractive surface of the eye. In the
most extreme cases the cone may be so steep that a GP does not fit well. One
solution that has been successfully employed involves putting a soft lens on
first then piggybacking a hard lens on top. This would stabilize the fit as
well as make the GP more comfortable.
The conjunctiva is divided into
two main sections: the palpebral, lining the undersurface of the lids, and the
bulbar covering the sclera. It is a translucent mucous membrane that is not
easily observable unless irritation of the eye is present. The conjunctiva
provides a smooth interface between the lid and eye during blinking. It has two
layers: an epithelial and stromal layer. The epithelium crosses at the limbus
and is continuous with the corneal epithelium. Goblet cells, which produce the
mucoid bottom layer of the tear film, are also located in this layer of the
Contact lens wearers may
experience conjunctival injection if a lens does not fit properly. Excessive
mechanical irritation can cause the loosely attached tissue to become edemic
The conjunctiva is also
susceptible to inflammation of a viral, bacterial or allergic nature. These
conditions always contraindicate contact lens wear and must be carefully
screened for at every visit. Non-compliance with this rule would slow down the
healing process due to the limited oxygen supply reaching the eye. Furthermore,
soft lenses would need to be disposed of in cases of viral or bacterial insult.
Reusing the lens would risk re-infection of the eye.
The iris is a circular structure
located in the anterior chamber surrounding the pupil. It contains four layers
and two muscles: The sphincter muscle, which shrinks the pupil on contraction,
and the dilator muscle, which dilates it. Innervated by the parasympathetic and
sympathetic systems respectively pupil size can range from 1mm in bright light
to 8 mm in complete darkness.
Normal pupil size in average
lighting may vary from one individual to the next as well. Blue eyes tend to be
larger and dilate faster, for example. Knowing pupil diameter is especially
important with GP fits where the lens is smaller. If the lens is not big enough
a patient with large pupils may be able to see around the optic zone. This
would produce unacceptable blurring. With a cosmetically tinted lens this same
person may have trouble because they can see the edges of the tinted segment in
their field of view. This is a popular complaint with patients who wear their
tinted lenses at night when their eyes are more dilated.
Having pupils that are small
might be a concern as well. A presbyope being fit in a simultaneous vision
bifocal lens would need pupils large enough to take in the near and distance
sections of the optic zone.
IN REVIEW AND CONCLUSION
The tear flow, so important in
corneal metabolism, must never be impeded by a badly fitting lens. Trading one
type of blurred vision for another is certainly not the goal. Understanding how
the tears flow under the lens, namely how they can contribute to the refractive
correction will prevent needless errors in fitting RGPs.
Knowing normal lid and
conjunctival processes will not only assist in spotting complications but help
predict and prevent them as well. Certain lid conditions might contraindicate
contact lens wear entirely while others merely mitigate a schedule or material
change. Conjunctival reaction will readily reveal a complication of contact
lens origin. Most will preclude lens wear until the causal factors are
identified and removed.
The cornea with all its layers is
a system in strict and ultimately fragile balance. It can regenerate with
amazing speed or leave enduring scars. How far can the system be pressured
before it breaks down? And what must be done once it does? Knowing the answers
here can save a patients vision.
It is not difficult to see how
topography is an integral input to the fitting process. The cornea is after all
the stage upon which all our efforts are focused. Understanding the variety of
forms it may take removes much of the guesswork from fitting contacts and
prevents complications that may impact the health of the eye.
Being aware of pupil size and how
it can range is a critical factor in certain types of fittings. Specifically,
GP and bifocal lens selections can be problematic if the field of view is
The anatomy of the anterior
segment cannot be done proper justice with so few words. Books and more have
been dedicated to examining those processes that define successful fitting. The
emphasis here lies in beginning to understand how the eye affects and is
affected by contact wear. Only by understanding how the system functions can
one begin to safely fit contact lenses.