Toric lenses are specially designed to treat astigmatism. Astigmatism is a refractive error in which an irregularly-shaped cornea fails to properly focus light on the retina. This causes a blurry and distorted image. Most toric lenses are soft. However, some toric contact lenses come in rigid gas permeable (RGP or GP) form, also called bifocal gas permeable lenses.
Multifocal contact lenses are specially designed for the treatment of astigmatism and presbyopia. Presbyopia is a refractive error in which the lens starts losing its strong focusing ability due to age. Multifocal contact lenses generally come in three primary forms; soft multifocal lenses, RGP multifocal lenses and hybrid multifocal lenses. RGP multifocal lenses provide sharper vision than soft multifocal lenses.
Toric Contact Lenses
Most toric contact lenses for treating astigmatism are made of a conventional hydrogel substance or silicone hydrogel. Hydrogel refers to plastic material which displays rigidity when dry and softness and flexibility when it is hydrated. Hydrogel consists of 38-75 percent of water when fully hydrated. Silicone hydrogel is more permeable than the regular hydrogel material. This allows up to five times more oxygen into the cornea. Silicone is a more advanced material, containing silicon, carbon, oxygen and other chemicals. It is used to make both soft and hard lenses.
Advantages of of Toric Lenses
Toric lenses carry several advantages. These include:
- Toric lenses have a wider diameter than gas permeable lenses. This advantage makes it hard for the lens to dislodge from the eye as often happens in GP lenses.
- Because toric lenses are made from silicone hydrogel, they help in decreasing incidents of hypoxia. Hypoxia occurs when the amount of oxygen entering the eye is significantly reduced. Less oxygen could bring about eye discomfort, dry eye, corneal inflammation and a blurry vision. It also increases the risk of eye infection.
- Due to its ability to lower incidents of hypoxia, toric contact lenses may increase the safety of both daily and extended toric contact lens wearers.
Disadvantages of Toric Lenses
Toric lenses have some limitations such as:
- Astigmatism comes in various forms, meaning it may not appear the exact way in every eye. Therefore, more than a pair of these lenses may be required to find the perfect fit in terms of brand, design, comfort and visual acuity.
- A lot more expertise is required to fit toric lenses compared to regular lenses. This factor can push the cost up.
- The replacement cost of toric lenses is higher than in conventional soft contact lenses depending on the lens design, material and retailer.
Multifocal Contact Lenses
Multifocal contact lenses can be used in the treatment of unusual types of astigmatism. They can correct astigmatism without a toric design because they are rigid and keep their spherical shape on the eye. With multifocal contact lenses, the uniform front surface of the GP lens takes the place of the irregular cornea as the main light-bending surface of the eye. This corrects astigmatism without having to control the rotation of the lens as happens in a toric design.
Some multifocal lenses come in a bifocal design with two distinct lens power – for distance and near vision. Others work like progressive lenses with gradual alterations in vision power. Disposable soft multifocal contact lenses are also available.
Types of Multifocal Contact Lenses
Multifocal lenses come in various forms:
- Toric multifocal lenses can correct astigmatism. The lenses can also provide presbyopia patients clear vision at all distances. Soft toric multifocal lenses come with a stabilization system that keeps the astigmatism correction appropriately positioned in front of the eye.
- GP multifocal lenses provide better optic outcomes. However, they need more adaptation mechanisms and daily wear before one can attain comfort.
- Multifocal hybrid contact lenses can treat astigmatism and presbyopia. These lenses have a central zone that is made of RGP plastic, with the surrounding area made of soft lens material. They offer more comfort than conventional gas permeable lenses.
Multifocal Lens Designs
There are two types of designs for multifocal contacts:
- Lenses with a progressive power design that functions the same way progressive eyeglass lenses do.
- Lenses that have concentric rings of different powers for different viewing distances. The part of the lens that covers the central area of the pupil has the power for near vision. The power for distance vision is in the central zone of the lens.
Advantages of Multifocal Contact Lenses
- These lenses offer comfort.
- Multifocal lenses are custom-made according to the needs of the individual.
- They provide sharper vision than toric soft contact lenses. For people over 40 years who are switching to multifocal contact lenses, clear vision is restored at all distances.
Disadvantages of Multifocal Lenses
- Some tasks may require additional vision enhancements. For example, reading glasses can be worn over multifocal contact lenses to read the very fine print or do fine detail work like threading a needle.
- It's advisable to wear glasses instead of contacts in case of prolonged computer work or extensive reading. People usually blink less during such tasks causing dryness in the multifocal contact lenses. Glasses provide greater clarity and comfort. Computer glasses can be worn instead of conventional bifocal or progressive lenses.