Corneal lens flexure

Photoatkts Cornea1 Lens Flexure Joe B. Goldberg, OD, FAA0 Cornea1 lenses of siloxane-methacrylate materials will flex when worn when they are too thin and there is a poor lens-to-cornea fitting relationship. The incidence of flexure can be reduced by making the lenses thicker and/or fitting them with design compatibility. Flexing is an inherent problem with siloxane-methacrylate cornea1 lenses. Flexing occurs when a cornea1 lens warps when it is worn. It is detected by examining the front surface of a cornea1 lens on the eye with an ophthalmom- eter (keratometer), doing an overrefraction and with biomicroscopy (see Figures I and 2). Flexing appears to be more prevalent when a cornea1 lens rests on the cornea in an immobile position regardless of whether it is above or below the central cornea1 area. Pearson’ states that in vivo flexure is influenced by the nature of the fitting. Because siloxaneemethacrylate corneal lenses have a low modulus of rigidity, they will flex when they are too thin and when there is a poor lens- to-cornea fitting relationship. Figure 1. For the OD, the lens rests on the cornea in a high, immobile position. There is cornea1 bearing and restricted lacri- mal interchange under it. The lens diameter and center thickness affect flexure. A cornea1 lens begins to flex when the lid presses it to the cornea. Flexing causes the lens to lift at the 3-9 positions, a bellows effect, with a subsequent breaking of the tear film meniscus and cornea1 dessication at these positions.’ The incidence of flexing can be reduced by design compatibility.3 Spherical base curves should be fit from 0.25D to 0.50D flatter than the flatter cornea1 meridian when the cornea1 astigmatism is from l.OOD to 1.75D. Cornea1 lenses with aspheric ocular surfaces should be prescribed when the cornea1 astigmatism is less than l.OOD. Bitoric and aspheric designs should be prescribed for corneal astigmatism more than 2.00D. Figure 2. For the OS, the lens is displaced superiorly and nasally. There is arcuate bearing in the limbal area. References Address reprint requests to Dr. Goldberg at 4217 Virginia Beach Blvd., Virginia Beach, VA 23452. Accepted for publication August 1990. 1. 2. 3. Pearson RM: Choosing the best gas permeable hard materials. Optician 1989;5185:197(21-24). Fatt I: Flexure of RGP permeable lenses in the eye. Optician 1989;5185:197(33-37). Goldberg JB: Design compatibility and material stability: Pre- requisites for fitting RGP cornea1 lenses successfully. KXC 1989;16(9,10):302. 0 1990 Butterworth-Heinemann /CUT, Vol. 17, September/October 1990 249

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Transcript of Corneal lens flexure

Photoatkts

Cornea1 Lens Flexure

Joe B. Goldberg, OD, FAA0

Cornea1 lenses of siloxane-methacrylate materials will flex when worn when they are too thin and there is a poor lens-to-cornea fitting relationship. The incidence of flexure can be reduced by making the lenses thicker and/or fitting them with design compatibility.

Flexing is an inherent problem with siloxane-methacrylate cornea1 lenses. Flexing occurs when a cornea1 lens warps when it is worn. It is detected by examining the front surface of a cornea1 lens on the eye with an ophthalmom- eter (keratometer), doing an overrefraction and with biomicroscopy (see Figures I and 2).

Flexing appears to be more prevalent when a cornea1 lens rests on the cornea in an immobile position regardless of whether it is above or below the central cornea1 area.

Pearson’ states that in vivo flexure is influenced by the nature of the fitting. Because siloxaneemethacrylate corneal lenses have a low modulus of rigidity, they will flex when they are too thin and when there is a poor lens- to-cornea fitting relationship.

Figure 1. For the OD, the lens rests on the cornea in a high, immobile position. There is cornea1 bearing and restricted lacri- mal interchange under it.

The lens diameter and center thickness affect flexure. A cornea1 lens begins to flex when the lid presses it to the cornea. Flexing causes the lens to lift at the 3-9 positions, a bellows effect, with a subsequent breaking of the tear film meniscus and cornea1 dessication at these positions.’

The incidence of flexing can be reduced by design compatibility.3 Spherical base curves should be fit from 0.25D to 0.50D flatter than the flatter cornea1 meridian when the cornea1 astigmatism is from l.OOD to 1.75D.

Cornea1 lenses with aspheric ocular surfaces should be prescribed when the cornea1 astigmatism is less than l.OOD. Bitoric and aspheric designs should be prescribed for corneal astigmatism more than 2.00D.

Figure 2. For the OS, the lens is displaced superiorly and nasally. There is arcuate bearing in the limbal area.

References

Address reprint requests to Dr. Goldberg at 4217 Virginia Beach Blvd., Virginia Beach, VA 23452.

Accepted for publication August 1990.

Pearson RM: Choosing the best gas permeable hard materials. Optician 1989;5185:197(21-24). Fatt I: Flexure of RGP permeable lenses in the eye. Optician 1989;5185:197(33-37). Goldberg JB: Design compatibility and material stability: Pre- requisites for fitting RGP cornea1 lenses successfully. KXC 1989;16(9,10):302.

0 1990 Butterworth-Heinemann /CUT, Vol. 17, September/October 1990 249