A patient with irregular astigmatism after corneal scarring achieves 6/60 with spectacles but improves to 6/9 with a rigid gas-permeable (RGP) contact lens. The mechanism by which RGP corrects irregular astigmatism (spectacles cannot) is:

• A RGP provides higher oxygen transmissibility correcting stromal oedema
• B RGP exerts mechanical pressure to flatten the corneal surface uniformly
• C The tear film between the posterior surface of the RGP and the irregular cornea fills in the surface irregularities, creating a regular refracting surface ✓
• D RGP has a toric back surface that matches the irregular corneal curvature

Explanation

The tear lens (liquid lens) fills the space between the posterior surface of the RGP contact lens and the irregular anterior corneal surface. Since the tear film has the same refractive index as the aqueous (~1.336), and the anterior surface of the tear lens exactly mirrors the corneal irregularity while the posterior surface of the RGP presents a smooth optical surface, the combination creates a regular, smooth refracting system. This is why RGP lenses dramatically improve VA in keratoconus and corneal scarring where spectacles cannot correct the higher-order aberrations.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.