Contact Lens Principles Flashcards

A patient fitted with a rigid gas-permeable (RGP) lens shows fluorescein pooling in the midperiphery and bearing centrally. Which fitting relationship does this describe, and what is the likely consequence if left uncorrected?

Answer: Steep fit; risk of peripheral seal-off reducing tear exchange and causing central epithelial hypoxia

A steep RGP fit shows a central fluorescein pool (lens vaults the cornea) with midperipheral bearing (where the lens contacts). This creates peripheral seal-off, dramatically reducing tear exchange beneath the lens, leading to hypoxia and potential central epithelial edema or scarring.

When calculating the expected spherical equivalent soft contact lens power for a patient whose spectacle Rx is -9.00 DS at a vertex distance of 14 mm, what is the correctly vertex-adjusted contact lens power?

Answer: -8.25 D

Vertex distance compensation is required for powers ≥±4.00 D. Using the formula F_cl = F_spec / (1 − d × F_spec), where d = 0.014 m: F_cl = −9.00 / (1 − 0.014 × (−9.00)) = −9.00 / 1.126 ≈ −7.99 D, which rounds to −8.00 D clinically. However, the nearest available soft lens power is −8.25 D in most fitting sets, making −8.25 D the correct dispensed choice to avoid undercorrection.

A patient with keratoconus is being fitted with a scleral lens. The practitioner observes central clearance of 350 microns over the cone apex with adequate limbal clearance but significant scleral zone bearing on the temporal quadrant. What is the most appropriate next step?

Answer: Apply a toric peripheral curve on the temporal landing zone only

Scleral lenses land on the sclera, not the cornea. Uneven scleral bearing is corrected by modifying the peripheral landing zone — a toric or quadrant-specific landing zone adjustment addresses focal bearing on a single quadrant without altering central sagittal depth or lens diameter unnecessarily.

Which oxygen transmissibility (Dk/t) threshold, established by Holden and Mertz, is required for extended (overnight) soft contact lens wear to avoid clinically significant corneal swelling during sleep?

Answer: 87 × 10⁻⁹ (cm × mL O₂)/(s × mL × mmHg)

Holden and Mertz (1984) established that a Dk/t of ≥87 × 10⁻⁹ is necessary for extended wear to limit overnight corneal edema to ≤4% (physiologically acceptable). The threshold for daily wear is 24 × 10⁻⁹. Most silicone hydrogel lenses are designed to meet or exceed the 87 threshold for overnight use.

A COT is performing a diagnostic fitting with a toric soft contact lens on a patient whose manifest refraction is −3.00 −1.75 × 170. The trial lens placed is −3.00 −1.75 × 180 and it stabilizes at 15° counterclockwise (nasal rotation on the right eye). Using the LARS rule, what power should be ordered?

Answer: −3.00 −1.75 × 165

LARS: Left Add, Right Subtract. For a right eye, rotation is counterclockwise (to the left/nasal) = subtract from the axis. Trial lens axis 180 − 15 = 165. The ordered lens should be −3.00 −1.75 × 165 to compensate for the predictable counterclockwise rotation.

A patient wearing high-Dk silicone hydrogel lenses presents with asymptomatic limbal redness confined to the superior limbus under the upper lid, absent at the inferior limbus, with normal corneal clarity. Slit-lamp exam reveals fine superior limbal injection and subtle papillary changes on the upper tarsal plate. Which condition is most consistent with these findings?

Answer: Superior limbic keratoconjunctivitis (SLK) exacerbated by contact lens wear, likely requiring lens-free interval and topical treatment

The localized superior limbal injection that mirrors under-lid exposure, combined with superior tarsal papillary changes and asymptomatic presentation, is characteristic of SLK associated with contact lens wear. Unlike GPC (which primarily shows large papillae > 0.3 mm) or CLARE (acute red eye, often with discharge), SLK is a chronic mechanical/inflammatory condition at the superior limbus requiring a lens holiday and possibly topical treatments such as topical vitamin A or silver nitrate application by the physician.