Visual acuity, lens flexure, and residual astigmatism of keratoconic eyes as a function of back optic zone radius of rigid lenses

Comparison of Flat and Steep Rigid Contact Lens Fitting Methods in Keratoconus

“…In previous work applying flat-and steep-fitting lenses to keratoconic eyes and measuring visual acuity in a single test session without allowing days to weeks of adaptation to the lenses, Zadnik and Mutti found that visual acuity was variable and that neither lens fitting strategy had a clear advantage, 27 whereas Sorbara et al found that the steepest base curve provided the worst visual acuity compared with flatter lenses and that lenses fitted 0.2 to 0.3 mm flatter than their subjects' habitual rigid lenses resulted in the best vision. 28 The data in this article show that eyes with mild to moderate keratoconus wearing steep-fitting lenses tended to have better visual acuity, on the order of half a line for entrance visual acuity for high-and low-contrast visual acuity, respectively, and 0.5 to one line better for best-corrected visual acuity. Thus, the clinician's bias toward fitting flat-fitting rigid lenses to improve visual acuity may be challenged by the data presented here.…”

Visual Acuity and Ocular Aberrations With Different Rigid Gas Permeable Lens Fittings in Keratoconus

“…26 The tear volume is believed to bridge the gap between the back surface of the RGP lens and the anterior corneal surface, but any manifest residual corneal astigmatism would still degrade vision. 27 Sorbara et al 27 suggested that uncorrected residual astigmatism may arise with steep-fitting RGP lenses because of lens flexure, the cylindrical nature of the cone under the lens, and because of the difference between the refractive indices of the tear film and the rigid lens.…”

“…Krachmer et al 1 were the first to suggest that steep-fitting RGP lenses give poorer acuity than flat-fitting lenses. In support of this, Sorbara et al, 27 in their study of nine keratoconic eyes, sequentially fitted five RGP lenses that ranged from 0.10 to 0.30 mm steeper and flatter than the patient's habitual lenses. Their results showed that the steepest base curve provided the worst acuity compared with flatter-fitting lenses and that lenses fitted 0.20 to 0.30 mm flatter than the subject's habitual RGP lenses resulted in the best visual acuity achieved.…”

The Biomechanics of Keratoconus and Rigid Contact Lenses

“…90 The presence of the bulky device, which was 2.5 mm thick at the center of the cornea, may have increased lid tension. 89 Absolute pressure values may have been increased accordingly, but the relative change between light and hard squeeze blinks was 18 times (2.8 to 51 mm Hg). When direct manometric pressure readings on a single subject were made with a pressure-sensing device connected to a needle in the anterior chamber, similar pressure ranges were found.…”

“…89 A double haptic contact lens device linked to a pressure transducer was used to measure changes in lid tension with different types of blinking. 90 This device measured any increase in IOP with blinking in addition to any pressure exerted by the lids when they are in a passive open-eye position.…”