Dental Care

Different kind of contact lens

Contact lenses became popular after the Second World War with the introduction of Perspex. This material called PMMA by the contact lens industry became the first choice for producing contact lenses.

At first the lenses were made rather large in diameter, but soon contact lens practitioners realized that this caused the cornea to be starved of oxygen. The result was a corneal oedema which usually caused increased myopia. Also many patients started to complain about a halo effect around lights. Some of the halos consisted of a rainbow colors.

By ignoring these symptoms contact lens wearers would develop cell damage to the corneal surface and this made the eyes easy target for viral and bacterial infections. So the logical step was reducing the average diameter of the contact lenses to around 8 mm. Any smaller size was not practical because under low light conditions the pupils would become larger than the contact lenses and the lenses edge reflections started to interfere with the vision.

Another problem usually would be that the smaller contact lenses are more sensitive to the eyelids. This resulted in producing thinner and therefore lighter contact lenses.  Unfortunately these small contact lenses would shift more easily from the cornea to the sclera which is the white part of the eye. Because the sclera has a flatter curve the contact lenses tended difficult to be pushed back on the cornea. In some instances the contact lens wearers thought they had lost their lens only to be located deep under the eyelids by the optometrist.

In order to make it easier to locate the contact lenses the manufacturers started to produce tinted contact lenses. Through using dyes to make the tear flow under the contact lens more visible the optometrist got a better understanding of how the cornea received its oxygen. Because corneas do not possess blood vessels all the oxygen enters through the tear film into the cornea membrane.

Soon it became clear that a good movement of tear flow between the lenses and the corneas were essential to keep the cornea in good condition. A contact lenses fitted too tight would feel comfortable for the eyelids but would have an adverse effect on the health of the cornea. At the other hand a contact lens fitted too loose would move around too much and could easily fall out or constantly shift onto the sclera. So a fine balance was needed to obtain a perfect fitting contact lens. In this stage of contact lens development the fitting of lenses were more art than science and it took years of experience to fine tune these skills.