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Guide to contact lenses

If you would like to know more about the distinction between hard, RGP and soft contact lenses, this overview will give you basic information regarding their advantages and disadvantages. Furthermore, you can find out what are:

• extended wear contact lenses
• therapeutic contact lenses
• multifocal contact lenses
• colored contact lenses
• overnight orthoK lenses

1. Hard and RGP lenses

Conventional hard lenses are made of the PMMA material which does not transmit oxygen (Dk~0). In most cases they are now replaced by rigid gas permeable (RGP) contact lenses. Materials used for the manufacture of RGP lenses are more flexible and oxygen-permeable than the PMMA material for hard lenses. In comparison to the soft lenses, RGP lenses are smaller in size and considerably more mobile thus providing better tear film exchange and the additional amount of oxygen to the cornea. These lenses are in many ways beneficial for your eyes and their only downside is a light initial discomfort during the adaptation period.

The application of hard and RGP lenses demands better educated and more experienced personnel as well as more precise equipment than the application of soft lenses. For any additional questions you can contact the Opto Centar Eye Clinic.

ADVANTAGES of RGP contact lenses:

• 1. oxygen permeability; RGP materials permeate oxygen which is important for your eyes' health. The flow of oxygen to the cornea is additionally endorsed by the tear film exchange due to their mobility during blinking.
• 2. excellent sharpness of vision; since they retain their shape after blinking, the eye does not have to accommodate anew. Sharpness of vision obtained with RGP lenses is the exemplar in correction of refractive eye errors.
• 3. correction of astigmatism and eye aberrations; spherical RGP contact lenses can correct irregularities in corneal shape (astigmatism up to three diopters).
• 4. long life and durability; these lenses are made of tear and wear resistant materials. Depending on the used material, they should be replaced every 1 to 2.5 years.
• 5. resistance to deposits; this type of lens absorbs less tears than soft lenses, thus accumulating less protein and other surface deposits.
• 6. simple handling; since they do not alter their shape, lens handling is much simpler than in the case of soft contact lenses.
• 7. treatment of keratoconus; these lenses represent the treatment of choice for the patients with keratoconus.
• 8. prevention of the progression of nearsightedness in children; the majority of studies have confirmed that these lenses slow down the progression of minus diopter powers in nearsighted children and adolescents. At present there are no conclusive proofs in relation to the suppression of eye growth in nearsighted persons.
• 9. RGP lenses cause considerably fewer cases of eye inflammations than soft lenses which is why they are considered to be the healthiest type of contact lenses.

DISADVANTAGES of RGP contact lenses:

• 1. initial discomfort; these lenses require an adaptation period of 7 to 14 days.

Our recommendation: Comfort RGP design with Boston EO gas-permeable material.

2. Soft lenses

On account of the short adaptation period and option of an intermittent wear, soft contact lenses currently represent one of the most popular methods of correction of refractive errors. They can also be occasionally worn during sporting activities, such as skiing.

According to the replacement schedule we differentiate: conventional yearly and 6-month soft lenses, planned replacement lenses (3-month, monthly and 2-week) and daily soft lenses which are discarded at the end of the day

ADVANTAGES of yearly soft lenses:

• 1. very short adaptation period; most patients find these lenses comfortable the first time they put them on.
• 2. suitable for sporting activities; since they are larger than the RGP lenses, they tend to fall out less frequently.
• 3. longevity; in the contrast to the planned replacement soft lenses, these lenses can be worn for a whole year. That makes them the most cost-effective soft lenses.
• 4. less problems with dry eyes; the lens material contains less water than the ones used for the manufacture of planned replacement lenses, making them more appropriate for the milder cases of tear film disorders.

DISADVANTAGES of yearly soft lenses:

• 1. slightly poorer vision; since they alter their shape during blinking, the sharpness of vision is somewhat poorer than with RGP contact lenses.
• 2. eye dryness; although the water does not evaporate to such great extent as with other types of soft contact lenses, they still contain more water than RGP contact lenses, thus increasing the possibility of various tear film disorders.
• 3. more demanding maintenance; it is advisable to use peroxide and other systems with a strong antimicrobial effect, as well as additional enzymatic tablets against formation of deposits.
• 4. less resistant to external influences; soft lenses are more susceptible to mechanical damage and accumulation of organic deposits than the RGP lenses.

Our recommendation: OptoSoft U8 yearly soft lenses.

ADVANTAGES of planned replacement lenses:

• 1. short adaptation period; most patients find these lenses comfortable the first time they wear them. That makes them ideal for intermittent wearing.
• 2. high oxygen permeability in silicon hydrogel planned replacement lenses. Still, availability of oxygen through the tear film exchange under the lens is lower than with the RGP lenses.
• 3. suitable for sporting activities; they are bigger than RGP lenses and less prone to fall out during the rapid eye movements.
• 4. simpler maintenance; since they are being replaced relatively often, the so called "All in One" solutions can be used for their maintenance without having to regularly use enzymatic tablets.

DISADVANTAGES of planned replacement lenses:

• 1. slightly poorer vision; since they alter their shape during blinking, sharpness of vision is somewhat poorer than with RGP contact lenses. Conventional planned replacement soft lenses cannot correct astigmatism and eye aberrations. These aberration errors can however be corrected with the new generations of soft lenses (PRIMASOFT), while astigmatism can be corrected with somewhat more expensive toric soft lenses.
• 2. short life; soft lenses are very susceptible to mechanical traumas and accumulation of organic deposits. Hydrogel lenses are more prone to formation of protein deposits while silicon hydrogel lenses typically build up lipid deposits.
• 3. more demanding handling; the majority of these lenses are much thinner and they alter their shape much easier than yearly soft lenses.

Our recommendation: PRIMASOFT monthly soft lenses.

ADVANTAGES AND DISADVANTAGES of daily lenses:

are pretty similar to the ones of planned replacement soft lenses. Since these lenses are discarded at the end of the day, they are a bit more expensive than other contact lenses. This manner of lens wearing is proven to be the safest way of wearing soft lenses, apart from using RGP contact lenses.

According to the type of material used for the manufacture of soft contact lenses, we distinguish the conventional hydrogel lenses and the more recent silicon hydrogel lenses.

The conventional hydrogel contact lenses are made of hydrophilic materials with diverse water content. Lenses with less than 50% water are called low water-content lenses. They last longer than lenses with higher water content and they are more frequently used for the yearly wear regime. Soft contact lens wearers who have problems with the "dry eye" syndrome better respond to the lenses with the low water content because the tears do not evaporate that much from their surface. High water-content lenses (more than 50% water) have better oxygen permeability. Because of their poorer durability and low resistance to the accumulation of deposits they are ordinarily used within the planned replacement program.

Recently, the so called silicon hydrogel soft contact lenses have become increasingly popular. The silicon component of the lens material provides extremely good oxygen permeability, but its hydrophobic feature can cause poorer comfort and increase the accumulation of deposits. This is the reason why new SiHi materials and care solutions are being continually developed to improve lens comfort during extended wearing periods.

These lenses can also act as therapeutic lenses for the treatment of various non-infective corneal diseases.

3. Special types of contact lenses

EXTENDED WEAR CONTACT LENSES

Extended wear contact lenses are made of materials with very high oxygen permeability. They can be worn up to 30 days without removal. Currently there is a significant increase in the application of RGP lenses approved for the extended wear in America and Western European countries. There are many soft and RGP lenses that are already approved for different extended wear regimes.

This mode of lens wearing is connected with the increased risk of potentially dangerous corneal infections and it is being implemented only in specifically prescribed cases involving explicit occupational demands or in a treatment of non-infective corneal diseases (see: therapeutic lenses).

THERAPEUTIC CONTACT LENSES

Therapeutic contact lenses are used for the protection, pain control and faster healing of cornea in the cases of various non-infective diseases. Soft silicon hydrogel contact lenses are the most common type of lenses used in the medical therapy today. They can be worn overnight, but only, of course, under strict control of the ophthalmologist.

MULTIFOCAL CONTACT LENSES

History

Before the development of multifocal eyeglasses and contact lenses, presbyopia used to be corrected with the so called bifocal eyeglass lenses. Bifocals had two distinct lens powers – one for distance vision and the other for near vision, with the visible line of separation between them.

For the good medium-distance vision, which is important for the people working with computers, another, so called intermediary zone was added. Such eyeglass lenses were then called trifocal lenses, but a number of patients had problems in adapting to this type of lens.

With the development of technology, the firstto appear were multifocal eyeglass lenses. Soon after, bifocal and multifocal contact lenses followed. Multifocal lens comprise visual zones with different prescriptive powers blended across the lens. That is why this type of lens is also called a progressive addition lens (PAL).

MULTIFOCAL CONTACT LENSES

There are RGP and soft multifocal contact lenses. We distinguish two basic types of the multifocal contact lenses: simultaneous and translating.

• 1.Simultaneous multifocal contact lenses have two visual zones, one for the near and one for the distance vision, placed concentrically in front of the pupil at the same time. We differ center-near and center-distance simultaneous contact lenses, depending on which zone, the one for the near or the one for the distance vision, is situated in their center. The modus operandi of these lenses relies entirely on the capacity of the brain to select and "sharpen" the object situated at one distance and to neglect all other distances. This principle of simultaneity can be applied to all soft lenses because of their low mobility on the cornea.
• 2.Translating or alternating lenses usually have the visual zone for near vision placed in the lower half of the lens, while the distance vision zone is placed more centrally and above the near vision one. The necessary prerequisite for this translating lens effect is that the lens is mobile. That is why only the RGP contact lenses can act as translating lenses.
  
  One of the major disadvantages of this type of lens manifests itself when the lens rotates in the eye which subsequently causes undesirable changes in lens dioptric power. However, this shortcoming is now amended with the new design which combines both the translating and the simultaneous design. A fine example of such design is one of the currently leading multifocal contact lens: ESSential RGP Multifocal.

Our recommendation: ESSential RGP Multifocal lenses

As an alternative to bifocal lenses, many eye care practitioners today use a system called "monovision".  Through this method one of the patient's eyes, usually the dominant one, is fitted with a distance-vision contact lens and the other with a lens with a different power, the near-vision one. A large number of researchers have confirmed a success rate of around 50 % for the "monovision" fitting. It has been particularly successful for the persons with emmetropia and early cases of presbyopia. There is also an option of the modified "monovision" when a patient is fitted with the multifocal lens on one eye, and the so called hybrid "monovision" when a patient has the "centre-distance" multifocal contact lens on one eye and the "centre-near" lens on the other. The major disadvantages of this mode of contact lens wear are the problems during night driving, poorer depth perception and lower contrast sensitivity.

COLORED CONTACT LENSES (COSMETIC LENSES)

Colored contact lenses are soft contact lenses used for cosmetic purposes, for example for the change of the original eye color. They are usually available in plano form (without diopter), although there is a number of colored lenses with the visual correction. Opto Centar offers a wide range of colored lenses for their clients to choose from.

The so called clear pupil and black pupil soft lenses represent custom made cosmetic lenses used for a better aesthetic effect after more serious eye injuries or diseases (macula cornea, iris dialysis, fibrosis in the pupillary opening and others).

ORTHOKERATOLOGY OR OVERNIGHT LENSES

Orthokeratology represents a special approach to RGP lensfitting. The patient inserts lenses before going to sleep and removes them in the morning. This is done in order to reshape the cornea and the lenses are hence able to modify its surface. Once the lenses are removed, the cornea gradually acquires its original shape. For this particular method of lens wearing it is extremely important to obtain necessary expert knowledge regarding its application as well as to use only the tested and approved orthokeratology systems. Proper maintenance of the overnight lenses is here of utmost importance because the risk of keratitis is similar to the one associated with extended wear lenses.