Rigid lenses with a spherical rear surface form a tear lens in the space between the lens and the cornea. When rigid (and toric soft) contact lenses neutralize astigmatism at the corneal surface, the meridional aniseikonia created with toric-surfaced spectacle lenses is avoided. For this reason, people whose astigmatism has been corrected with contact lenses often experience an annoying change in spatial orientation when they switch to spectacles. However, refractive astigmatism is the sum of corneal and lenticular astigmatism. Lenticular astigmatism, if present, is not corrected by spherical contact lenses, so it persists as “residual” astigmatism when the corneal astigmatism component is neutralized by spherical rigid contact lenses. This finding is more common among older patients and often explains why their hard contact lenses fail to provide full correction. These cases can be identified by refracting while the contact lenses are in place. If it happens that the eye has lenticular astigmatism against the rule and a similar amount of corneal astigmatism with the rule, a soft contact lens may be used, so that the astigmatism of the cornea and of the lens continue to compensate for each other.
Figure 5-2 Effect of spectacle lenses on convergence demands. A, Lenses for correction of hyperopia create induced base-out prism with convergence, which increases the convergence demand. B, Lenses for correction of myopia create induced base-in prism, which decreases the convergence demand.
(Illustrations developed by Thomas F. Mauger, MD.)
For example, consider a patient whose refraction is −3.50 −0.50 × 180 and keratometry measurements of the affected eye are 42.50 D (7.94 mm) horizontal and 44.00 D (7.67 mm) vertical. Would a soft or rigid contact lens provide better vision (ie, less residual astigmatism)? The disparity between the corneal astigmatism of 1.50 D and the refractive astigmatism of 0.50 D reveals 1.00 D of against-the-rule lenticular astigmatism that neutralizes a similar amount of with-the-rule corneal astigmatism. Neutralizing the corneal component of the refractive astigmatism with a rigid contact lens exposes the 1.00 D of lenticular residual astigmatism. Therefore, a spherical soft contact lens would, in this case, provide better vision than a spherical RGP lens.
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.