How many times have you witnessed a hapless individual crawling around a dance floor or basketball court occur looking for a lost contact lens? Or how often have you seen a room full of people crawling around looking for a lost contact lens? The odds are that they are all searching for a hard lens, or more accurately, a rigid lens. Despite their tendency to become dislodged more often than soft lenses, the newer generations of rigid contact lenses have several advantages over soft lenses. This issue of Eye Facts will tell you about new rigid lens materials and who can benefit from this type of contact lens.
Rigid and soft contact lenses are the two main types of corneal contact lenses available to day. Rigid contact lenses get their name because they are made from inflexible or minimally flexible plastic. Of the 15 to 19 million contact wearers in the United States, more than 75% use soft lenses and less than 25 percent wear rigid lenses. However, rigid lenses are being prescribed in increasing numbers because of recent improvements in their development.
The first rigid corneal contact lens – commonly called the “hard lens” – was introduced in the late 1940’s. It was made from polymethyl methacrylate or PMMA (Lucite), a plastic developed for use in industry and by the military. Rigid contact lenses were made from this material from 1948 to 1980. Before the introduction of soft lenses, the PMMA rigid lens was the standard contact lens.
A major disadvantage of PMMA rigid contact lenses was their inability to transmit oxygen from the air to the cornea. Without this essential oxygen flow to the cornea, there was a risk or corneal swelling, warpage and other complication. Thousands of people work these rigid lenses for many years without significant problems. However, because a small number of patients developed serious corneal complications, scientists sought better and safer materials for rigid lenses.
Soft contact lenses, made from newer types of plastics, became available in the 1970’s. Although these lenses offered greater oxygen transmission and comfort, their nature was to mold to the shape of the cornea. Thus, they were unable to correct large degrees of astigmatism (unequal curvature of the cornea). Therefore, the search continued for a contact lens that could correct astigmatism (i.e., a rigid lens) and that was also more permeable to oxygen than the PMMA lens.
Modern development of rigid contact lenses
The earliest type of gas-permeable rigid contact lens was introduced in the late 1970’s. It was made from silicone, a material that – compared to PMA – allowed much more oxygen to pass directly through the contact lens and reach the cornea.
Since 1980, improved types of gas-permeable rigid lenses have arrived on the market almost yearly. The newest lenses are made from sophisticated fluorocarbon compounds and are commonly misnamed “Teflon” lenses. These lenses have better surface characteristics, which make them very comfortable. In addition, they have even greater permeability to oxygen. In fact, their oxygen permeability is so high that extended-wear versions of rigid lenses are now available. Today, gas-permeable lenses account for about 98% of rigid lens prescriptions; PMMA is rarely used anymore for contact lenses.
Advantages of rigid lenses
Gas-permeable rigid lenses have several advantages over soft lenses:
Visual acuity is better, especially in the persons with corneal astigmatism.
Allergic or toxic reactions to lens care solutions are less frequent. Unlike soft contact lenses, gas-permeablelenses do not absorb the disinfecting agents and preservatives.
Gas-permeable lenses last longer because they are more durable than soft lenses.
The cost of these lenses is usually less because of the longer lens life and lower cost of the material.
However, gas-permeable rigid lenses are not without risk of complications. As with any contact lens, they require proper care to minimize the risk of infection and other problems.
Reasons for choosing rigid lenses
The most common reason for prescribing rigid corneal lenses is for the correction of refractive errors such as myopia (nearsightedness) that are complicated by astigmatism. Individuals with astigmatism who want the best possible vision often prefer rigid lenses to soft lenses, even thought soft lenses may be somewhat more comfortable.
Rigid lenses also are commonly prescribed in patients who have suffered trauma to the cornea or who have corneal diseases such as keratoconus. Many such diseases leave corneal scarring and an irregular or bumpy surface, making it impossible to fully correct vision with glasses or soft contact lenses. Rigid corneal lenses have allowed many people with these types of visually disabling eye conditions to see normally.
The rigid corneal contact lens of today is greatly improved from its earlier counterparts. The new versions are more comfortable and pose far fewer risks to corneal health. Because of continued improvements in both lens materials and fitting techniques, rigid contact lenses are being prescribed more frequently today.
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