Contact Lens

History

The idea of contact lens was first conceived by Leonardo Da Vinci in 1508. He placed a glass cup containing water over the eye, eliminating the cornea as a refractive surface. After so many attempts made by different scientists, in 1920, Zeiss produced a fitting set used to correct keratoconus. This was the first set of trial contact lens. In 1929 Heine described a method of fitting contact lens by means of a trial set consisting of a large number of contact lenses.

Later, in 1937 there was a break through. William Feinbloom, an American, used plastic in the construction of contact lens. A year earlier in 1936 the Rohm and Hass company introduced transparent methyl methacrylate. The first plastic corneal contact lens was introduced in 1947 by Kevin Touhy. Then in 1960, Wichetrle developed the soft contact lens which is made up of hydrophilic material.

Advantages of contact lenses

There are several advantages of contact lenses and they are:

• Fewer magnification effects
• Decreased peripheral and chromatic aberrations
• Increase in the size of visual fields
• Marked decrease in aniseikonia and anisometropia
• Good cosmetic appearance
• Permits better correction for refractive errors that occur with keratoconus and irregular astigmatism
• Safer for athletes and other sportspeople

Indications

1. Optical: myopia, hypermetropia, astigmatism, presbyopia, aphakia, post keratoplasty, keratoconus
2. Orthoptic uses: anisekonia, anisometropia, amblyopia (occlusion)
3. Special uses: albinism, aniridia, nystagmus with refractive error, coloboma, symblepharon
4. Therapeutic uses: bullous keratopathy, corneal ulcers
5. Prosthetic uses: pthisical eye, corneal opacity, leukoma and corneal scars
6. Surgical uses: corneal protection after surgery

There are several contraindications:
- Dry eyes, lid problems such as active blepharitis, stye, chalazion, entropion.
- Acute and chronic conjunctivitis, corneal abrasions, hyphema, Vth cranial nerve paralysis, hypopyon uveitis and iritis
- Allergies, uncontrolled diabetes, pregnancy period and pterygium

Types and Materials

The modern system classifies contact lens into three major types :
- Soft hydrogels
- Gas permeable lenses (semi soft)
- Hard contact lens

Soft contact lens

Soft contact lens are flexible contact lenses that are composed of either hydrogel or silicon material. These soft contact lenses are made up of different polymers but basically hydroxy ethyl metha acrylate (HEMA) which is a stable, clear, nontoxic, non allergic and optically desired material. These lenses are usually larger in size than the cornea for optimum centering and stability. They are more comfortable than rigid lens because of their soft qualities and their ability to flex on blinking. Their larger size produces a fit with its edge lying under the upper and lower eye lids.

Advantages

• More comfortable because the lens fits under the eyelid margins, flexes with each blink and the softness permits more oxygen to reach the cornea
• Spectacle blur is uncommon
• Less chances of loss of lens, because of larger size and minimal movement
• Minimal over wear reaction, because of its soft nature and ability to stimulate the oxygen tear pump mechanism by flexing with each blink
• Less glare and photophobia
• Ideal for children because of comfort and less chances of losing the lens

Semi-soft contact lens

Semi-soft contact lens are gas permeable lenses which are made up of a unique plastic that has the ability to permit oxygen to diffuse into and carbondioxide to diffuse out of the lens.

Materials

• CAB (cellulose acetyl butyrate) lenses
• Silicon acrylate
• Butylstyrene

Advantages

• Increased comfort
• Longer wearing time
• Reduced corneal edema, spectacle blur and over wear syndrome
• Rapid adaptation
• Permeability of more oxygen than other lenses
• Larger optic zone offers increased visual field and fewer glares
• Gas permeable lenses in a spheric form can correct up to five diopters of astigmatism

Hard contact lens

Hard contact lens is made up of PMMA (poly methyl metha acrylate) which is a stable, clear, non toxic, non allergic, easily worked and optically desired material. It can be molded or lathed and the stability of PMMA is more than rigid gas permeable (RGP) lenses. The oxygen permeability of hard contact lens is almost nil. It provides oxygen only by means of the tear pump.

Fitting procedures

For soft contact lenses

Soft contact lenses are usually fitted larger than the corneal diameter to maintain good centration and stability.

Fitting steps include

• Record the keratometry values
• Measure the corneal diameter in mm
• Diameter: The initial lens diameter selected should be 1.0-2.0 mm larger than the corneal diameter
• Power: Determine the spherical power first, convert the refraction prescription into minus cylinder and use the spherical equivalent method.
• Add to the sphere to determine the lens power and compensate for the vertex distance
• Base curve: Select the base curve which is 0.4-0.6 mm flatter than the flattest ‘K’ for smaller lenses and 0.6-1.0 mm flatter for larger lenses
• Fit the contact lens to the respective eye and have the patient wait for 15-20 minutes to settle the lens well

For gas permeable and hard

• Record the keratometry readings
• Select the initial lens based on base curve
• The base curve of the initial lens should be slightly steeper than the flattest meridian
• Diameter selection is directly related to base curve. The flatter the cornea the larger the lens.
• The following table describes the selection of diameter:

  Base curve Power	Diameter
  40.0 - 43.0 D	9.4 mm
  43.25 - 45.0 D	9.2 mm
  Greater than 45.25 D	9.0 mm

• While fitting the trial lenses , leave the patient for 20-30 minutes to attain good centration and settlement of the lens
• For best results fit the lens according to its best position and comfort

Handling Methods

Insertion and Removal for soft contact lenses

1. Wash hands thoroughly with oil free soap and dry with lint - free towel
2. Take the lens out of the case , clean and rinse it well
3. Place the lens on the tip of the index finger
4. Look up, and retract the lower lid with the middle finger and gently apply the lens to the lower part of the eye
5. Remove the finger and then slowly release the lid
6. Close the eye and gently massage the lids
7. Cover the other eye and focus it to make the correct centration
8. Repeat the same procedure for the other eye
9. For removal, look upward and retract the lower lid with middle finger and place the index finger tip on the lower edge of the lens
10. Slide the lens down to the white portion of the eye
11. Compress the lens between the thumb and the index finger, so that the air breaks the suction under the lens
12. Remove the lens for cleaning and sterilizing

Insertion for hard and RGP

1. Wash your hands thoroughly with oil free soap
2. Take out the lens from the container
3. Keep in your palm, clean it well with the prescribed solution
4. Depress the lower lid with the middle finger while the index finger carrying the lens is gently applied on the cornea
5. Slowly release the lids to avoid accident ejection of the lens. Release the lower lid first and then the upper

Removal for hard and RGP

1. Look downward, open the lids wide so that the edge of the lid will engage the edge of the lens
2. Draw the lid tight by a lateral pull of the index finger and blink
3. The lid should dislodge the lens
4. Cup the other hand under the eye to catch the lens

Scissors technique

Hold the upper lid by the index finger and the lower lid by the middle finger. Apply lateral traction to the lids and squeeze the lens off with a scissors motion.

Contact lens care and maintenance

Contact lens care and maintenance are the most crucial aspects of contact lens wear. It can influence the success of contact lens wear and patient’s satisfaction.

The lens care and maintenance procedures have 4 steps (cleaning, rinsing, disinfecting, and storing the lenses).

They can prevent potentially sight destroying infections.

Cleaning

The cleaning agents usually contain surfactants and are used to remove most loosely bound foreign bodies on the lens which includes cell debris, mucus, lipid, protein and micro organisms. The mechanical action of rubbing reduces the amount of loose debris and also enhances the efficacy of the solutions surfactant properties.

Rinsing

After cleaning, the lenses should be rinsed. The rinsing procedure helps to remove the loosened deposits and some micro organisms.

Disinfecting and storage

The process of disinfecting helps to kill or deactivate the microorganisms. There are two types of disinfecting systems.

Thermal disinfection

The lenses should be placed in the case with saline solution and heated to 70 - 80 degree c for 10-20 minutes.

Chemical disinfecting

Hydrogen peroxide based solutions are used for chemical disinfection. This is reasonably effective within 10-15 minutes.

   These disinfecting solutions are also used for storage. They function as a hydrating medium which helps to maintain the stability of contact lens parameters and physical parameters.

Multi purpose solutions

The modern lens care systems use one solution to perform the functions of a number of components. For ease of use and patients convenience, multipurpose solutions are formulated to allow cleaning, rinsing soaking and disinfecting functions to be combined.

To avoid lens contaminations, the lens case should be rinsed after every use and the lenses should be stored in fresh solution. For better lens care, change the lens case monthly.

Complications

The complications of contact lens wear include:
- Hypoxic related problems such as corneal edema, superficial punctuate keratitis, decreased sensation, superficial and deep infiltrates, vascularisation, superior limbal kerato conjunctivitis, epithelial microcysts.
- Allergic related problems include hyperemia, sterile infiltrates and giant papillary conjunctivitis.

Care and Maintenace

• Wash your hands with oil free soap before handling the contact lenses
• Before wearing the lens it should be cleaned thoroughly in cleaning solution and rinsed well
• Place the lens which should look like a bowl on the tip of the index finger
• After removing, the lens should be placed properly in the lens case with solution
• The lens should be placed in the center of the lens case; it should not be placed at the side because of chances of tearing
• Solution should be changed daily. If the lens is not used daily; solution should be changed once in two days
• The lens should not be kept without solution because it will get dried and the lens will be ruined ,/li>
• While wearing the lens avoid sitting under the fan or in windy places
• While traveling plain glass or sun glass should be worn
• The lens case should be washed weekly and should be changed once in three months ,/li>
• Don’t change the solution brand without the advice of the practitioner
• Proper care and follow up is must
• Lens should be changed according to the recommendations of the manufacturer
• If any complaints persist in the eye the lens should be removed and consult medical practitioner immediately
• Alternative use of spectacles is advisable if any discomfort develops

Summary

Contact lenses are the ideal choice for refractive errors. They give better vision correction without any distortions. Proper lens care and regular follow up are very essential to maintain a good ocular health.

Student exercise

Answer the following

1. What is a contact lens?
2. What are the advantages of contact lens?
3. List the types of contact lens
4. What are the indications and contraindications of contact lens?
5. What is the fitting procedure for soft RGP lens?
6. Write short notes on care and maintenance of contact lens.