Rigid Gas-Permeable Contact Lenses
Rigid gas-permeable lenses have smaller chord diameters than soft lenses, and allow more circulation of fresh tears under the lens with each blink. Central and peripheral curves, edge shape, and central thickness can be tailored for stable vision, comfort, and tear exchange.
Unlike a soft contact lens, an RGP lens maintains its shape when placed on the cornea, and a tear lens fills the space between the lens and the cornea. The power of the tear lens is determined by the difference between the curvature of the cornea (K) and that of the base curve of the contact lens (Fig 5-6). The fit is described as apical alignment (on K), when the base curve matches that of the cornea; apical clearance (steeper than K), when the base curve has a steeper fit, with radius shorter than that of the cornea; and apical bearing (flatter than K), when the base curve is flatter than the cornea.
To fit an RGP lens of a particular design and material, one can consult the laboratory fashioning the lens, a fitting guide for the lens, or use trial lenses. In designing the lens, consideration is given to pupil size and lid tension, as well as refractive error, keratometry, and possibly topography.
The most common type of RGP lens fit is an apical alignment fit with the upper edge of the lens under the upper eyelid (Fig 5-7). This “lid attachment” fit allows the lens to move with each blink, enhances tear exchange, and decreases lens sensation, because the upper eyelid does not strike the lens edge with each blink.
Figure 5-6 A rigid contact lens creates a tear (or fluid) lens whose power is determined by the difference between the curvature of the cornea (K) and that of the base curve of the contact lens.
(Courtesy of Perry Rosenthal, MD. Redrawn by Christine Gralapp.)
Figure 5-7 The most common and most comfortable type of rigid gas permeable lens fit is apical alignment, in which the upper edge of the lens fits under the upper eyelid.
(Modified with permission from Albert DM, Jakobiec FA, eds. Principles and Practice of Ophthalmology. Philadelphia: Saunders; 1994;5:3630. Redrawn by Christine Gralapp.)
Alternatively, a central or interpalpebral fit is achieved when the lens rests between the upper and lower eyelids. The diameter of the lens is smaller than with a lid-attachment fit. There may be greater lens sensation, because the eyelid strikes the lens edge with each blink. This type of fit may be preferable for patients who have high upper eyelids, hyperopia, or Asian eyes.
To evaluate the fit of a contact lens, the clinician looks for stability of vision and appropriate lens movement. The fluorescein pattern is evaluated at the slit lamp (Fig 5-8). If there is apical clearing of the cornea, pooling or a bright green area will be observed centrally; if the RGP lens is touching the cornea, this area will appear dark. Over-refraction determines whether a power change is needed, when the fit is satisfactory. Table 5-7 summarizes RGP lens parameters and the range of normal values.
Figure 5-8 Examples of fluorescein patterns in contact lens fitting.
(Courtesy of Perry Rosenthal, MD.)
Table 5-7 Rigid Gas-Permeable Lens Parameters
The power of the tear lens is approximately 0.25 D for every 0.05-mm radius-of-curvature difference between the base curve of the contact lens and the central curvature of the cornea (K), and this power becomes somewhat greater for corneas steeper than 7.00 mm. Tear lenses created by rigid contact lenses with base curves that are steeper than K (smaller radius of curvature) have plus power, whereas tear lenses formed by base curves that are flatter than K (larger radius of curvature) have minus power (Figs 5-9, 5-10). Therefore, the power of a rigid contact lens must account for both the eye’s refractive error and the power introduced by the tear lens. An easy way of remembering this is to use the rules SAM (steeper add minus) and FAP (flatter add plus). Clinical Example 5-3 illustrates these calculations.
Because the refractive index of the tear lens (1.336) is almost identical to that of a cornea (1.3765), it masks the optical effect of the corneal surface. If the back surface of a contact lens is spherical, then the anterior surface of the tear lens is also spherical, regardless of the corneal topography. Thus, the tear layer created by a spherical rigid contact lens neutralizes more than 90% of regular and irregular corneal astigmatism. This principle simplifies the calculation of the tear lens power on astigmatic corneas: the powers of the steeper corneal meridians can be ignored, and only the flattest meridians need to be considered. The refractive error along the flattest meridian is represented by the spherical component of refractive errors expressed in minus cylinder form. For this reason, it is simpler to use the minus cylinder format when dealing with rigid contact lenses (Clinical Example 5-4). When a spherical contact lens is in place, there may still be “residual” astigmatism, which may be caused by the eye’s natural lens and the shape of the posterior surface of the cornea. There are “back surface toric” gas permeable lenses with toric back surfaces, chosen to match the shape of the cornea, stabilizing the lens position, with a spherical front surface. There are also “bitoric” RGP lenses, whose back surface matches that of the cornea, while the front toric surface is chosen to correct the residual refractive error (Clinical Example 5-5).
Figure 5-9 Determining the power of a contact lens using the FAP-SAM rules.
(Illustration developed by Thomas F. Mauger, MD.)
Figure 5-10 Determining the power of a contact lens using the FAP-SAM rules.
(Illustration developed by Thomas F. Mauger, MD.)
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.