- Email: [email protected]
Design compatibility and material stability: Prerequisites for fitting RGP corneal lenses successfully
Photoatlas
Joe B. Goldberg, OD, FAA0
It may be erroneous to assume that one cornea1 lens design can be expected to fit the majority of eyes successful1y.l In fact, how a contact lens fits on the eye is determined by several parameters that are dependent on each other.2 We cannot rely exclusively on the Dk values of rigid gas permeable materials to ensure successful wear. A good lens-cornea fit must be established first. Cornea1 lenses are fitted successfully when they allow a fresh supply of tears to move under them after each blink, and the frequency of blinking furnishes a volume of tears that satisfies the cornea’s oxygen requirements. However, because the blink rate varies among individuals, a rigid gas permeable cornea1 lens must also have a good lens-cornea fitting relationship. Thus, a successful cornea1 lens fit depends on the practitioner’s ability and skill in fitting and the judicious derivation of a cornea1 lens design. A rigid gas permeable cornea1 lens should fit on the cornea with a uniform pressure gradient and retain its dimensional stability and shape. Shape retention precludes flexing and resultant power changes when on the eye. It also allows the cornea to retain a normal physiological state. Unfortunately, rigid gas permeable materials have been found to flex on the eye and change the shape of a cornea1 lens.3 Flexing in the periphery causes a negative pressure effect that interferes with lacrimal interchange and allows cornea1 metabolic wastes to remain under a lens with resultant cornea1 edema (see Figures l-3). Making the base curve design compatible with the cornea1 topography and increasing the center thickness 302
are ways to control flexing and dimensional changes of rigid gas permeable cornea1 lenses. In fact, the amount of cornea1 astigmatism can be used as a guide to determine a base curve design (see Table 1). Although increasing center thickness values is a way to preserve dimensional stability and assure oxygen transmission in the amount expected, thicker gas permeable lens designs have also been found to flex significantly.4 Maximum performance still depends on alensdesigntocreateagood lens-corneafittingrelationship.5 The search for a contact lens material that will preserve the integrity of the cornea1 epithelium began with
Figure 1. A rigid gas permeable cornea1 lens has flexed when worn and adheres to the cornea. Flexing causes the lens to have a fixed, low, cornea1 position. ICLC
r TABLE 1 Guidelines for Base Curve Selection Cornea1 Astigmatism
Base Curve Design
O.OO-0.75D
Spherical or aspheric
.75D
Spherical or aspheric
1.00-l
2.00D and more
Figure 2. There is an impression of the lens on the cornea when it is removed.
Toric or aspheric
polymethylmethacrylate (PMMA) and is continuing.6 So far, we have found idiosyncrasies for all rigid gas permeable materials. The efficacy of rigid gas permeable cornea1 lenses has been established for daily and extended wear, but the choice of a specific material for an individual patient is unreliable.738 Wearing rigid gas permeable cornea1 lenses successfully for daily or extended wear confirms that there must be a meaningful relationship between design compatibility and material stability. References 1. Forst, G. Aspheric contact lenses. /CLC 12(2):93-102, 1985. 2. Ghormley, N.R. Clinical implications comment made in response
Figure 3. Vascular changes caused by a rigid gas permeable corneat lens that has flexed on the eye. These changes probably are created when lens flexure interferes with lens movement and lacrimal interchange on blinking and precludes removal of metabolic debris.
Volume 16, Numbers 9 & 10 September/October
1989
to Oxygen Permeable Hard Lenses-Is Permeability Enough? by G.E. Rich. /CLC 8(2):25-32, 1981. Larke. J. Cornea1 oxygen availability, in Larke, J. (Ed): The Eye in Contact Lens Wear. Butterworth, 1985, chap 7, pp 102-l 12. Egan, D.J., Bennett, E.S. Trouble-shooting rigid contact lens flexure-A case report. /CLC 12(3):147-149, 1985. Hill, R.M. Gas permeable perspectives. J BCU 8(1):27-29, 1985. Larke. J. Cornea1 swelling and its clinical sequelae, in Larke, J. (Ed): The Eye in Contact Lens Wear, Butterworth, 1985, chap 10, pp. 1477163. 7. Cordrey, P. What the papers say. J BCU 7(1):48-51, 1983. 8. Edwards, K H. Experience with XL20 and XL30. J BCLA 4(4):142144, 1981.
303
Joe B. Goldberg, OD, FAA0
It may be erroneous to assume that one cornea1 lens design can be expected to fit the majority of eyes successful1y.l In fact, how a contact lens fits on the eye is determined by several parameters that are dependent on each other.2 We cannot rely exclusively on the Dk values of rigid gas permeable materials to ensure successful wear. A good lens-cornea fit must be established first. Cornea1 lenses are fitted successfully when they allow a fresh supply of tears to move under them after each blink, and the frequency of blinking furnishes a volume of tears that satisfies the cornea’s oxygen requirements. However, because the blink rate varies among individuals, a rigid gas permeable cornea1 lens must also have a good lens-cornea fitting relationship. Thus, a successful cornea1 lens fit depends on the practitioner’s ability and skill in fitting and the judicious derivation of a cornea1 lens design. A rigid gas permeable cornea1 lens should fit on the cornea with a uniform pressure gradient and retain its dimensional stability and shape. Shape retention precludes flexing and resultant power changes when on the eye. It also allows the cornea to retain a normal physiological state. Unfortunately, rigid gas permeable materials have been found to flex on the eye and change the shape of a cornea1 lens.3 Flexing in the periphery causes a negative pressure effect that interferes with lacrimal interchange and allows cornea1 metabolic wastes to remain under a lens with resultant cornea1 edema (see Figures l-3). Making the base curve design compatible with the cornea1 topography and increasing the center thickness 302
are ways to control flexing and dimensional changes of rigid gas permeable cornea1 lenses. In fact, the amount of cornea1 astigmatism can be used as a guide to determine a base curve design (see Table 1). Although increasing center thickness values is a way to preserve dimensional stability and assure oxygen transmission in the amount expected, thicker gas permeable lens designs have also been found to flex significantly.4 Maximum performance still depends on alensdesigntocreateagood lens-corneafittingrelationship.5 The search for a contact lens material that will preserve the integrity of the cornea1 epithelium began with
Figure 1. A rigid gas permeable cornea1 lens has flexed when worn and adheres to the cornea. Flexing causes the lens to have a fixed, low, cornea1 position. ICLC
r TABLE 1 Guidelines for Base Curve Selection Cornea1 Astigmatism
Base Curve Design
O.OO-0.75D
Spherical or aspheric
.75D
Spherical or aspheric
1.00-l
2.00D and more
Figure 2. There is an impression of the lens on the cornea when it is removed.
Toric or aspheric
polymethylmethacrylate (PMMA) and is continuing.6 So far, we have found idiosyncrasies for all rigid gas permeable materials. The efficacy of rigid gas permeable cornea1 lenses has been established for daily and extended wear, but the choice of a specific material for an individual patient is unreliable.738 Wearing rigid gas permeable cornea1 lenses successfully for daily or extended wear confirms that there must be a meaningful relationship between design compatibility and material stability. References 1. Forst, G. Aspheric contact lenses. /CLC 12(2):93-102, 1985. 2. Ghormley, N.R. Clinical implications comment made in response
Figure 3. Vascular changes caused by a rigid gas permeable corneat lens that has flexed on the eye. These changes probably are created when lens flexure interferes with lens movement and lacrimal interchange on blinking and precludes removal of metabolic debris.
Volume 16, Numbers 9 & 10 September/October
1989
to Oxygen Permeable Hard Lenses-Is Permeability Enough? by G.E. Rich. /CLC 8(2):25-32, 1981. Larke. J. Cornea1 oxygen availability, in Larke, J. (Ed): The Eye in Contact Lens Wear. Butterworth, 1985, chap 7, pp 102-l 12. Egan, D.J., Bennett, E.S. Trouble-shooting rigid contact lens flexure-A case report. /CLC 12(3):147-149, 1985. Hill, R.M. Gas permeable perspectives. J BCU 8(1):27-29, 1985. Larke. J. Cornea1 swelling and its clinical sequelae, in Larke, J. (Ed): The Eye in Contact Lens Wear, Butterworth, 1985, chap 10, pp. 1477163. 7. Cordrey, P. What the papers say. J BCU 7(1):48-51, 1983. 8. Edwards, K H. Experience with XL20 and XL30. J BCLA 4(4):142144, 1981.
303