Approved for Ohio Credit by the Ohio Optical Dispensers Board

Anatomy and Physiology for the Contact Lens Fitter

By Julie Harp, CPOT, ABO, NCLE

Having a thorough working knowledge of ocular anatomy is not only helpful but also essential for successful contact lens fitting. Keeping the patients eye healthy is, as always, the practitioners 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 dont 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 cornea
• Removal of debris
• Oxygenation of the cornea
• Provides a smooth surface of refraction
• Protection of the eye from infection.

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.

Refractive 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.

LIDS

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 cobblestone-like texture.

THE CORNEA

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 Bowmans layer. Sometimes referred to as Bowmans 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 Bowmans 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 equilibrium.

The fourth layer is Descemets 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 Descemets 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.

CORNEAL TOPOGRAPHY

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 Bowmens 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.

CONJUNCTIVA

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 conjunctiva.

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 and inflamed.

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.

IRIS/PUPILS

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 misjudged.

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.