Contact Lens Principles Flashcards

A patient wearing a rigid gas-permeable (RGP) lens shows apical bearing on fluorescein pattern evaluation. Which base curve adjustment is most appropriate?

Answer: Steepen the base curve to reduce apical touch

Apical bearing occurs when the lens base curve is flatter than the cornea, causing the central posterior surface to contact the apex directly. Steepening the base curve vaults the lens over the apex, converting the fit to the desired apical clearance pattern. Flattening would worsen the bearing.

During over-refraction of an RGP contact lens wearer, the spherocylindrical over-refraction is −0.50 −0.75 × 090. This residual astigmatism is most likely attributable to:

Answer: Lenticular (internal) astigmatism not corrected by the tear lens

A well-fitting RGP lens masks corneal astigmatism via the tear lens. Residual astigmatism found during over-refraction of a spherical RGP is therefore internal (lenticular) in origin — it exists independent of the corneal surface. Corneal toricity would have already been neutralized by the fluid tear meniscus beneath the rigid lens.

A silicone hydrogel lens with a Dk/t of 175 is prescribed for extended wear. The patient develops limbal hyperemia with fine vessel ingrowth after 3 months. The MOST likely etiology is:

Answer: Mechanical lid interaction causing superior limbal compression

At Dk/t of 175, oxygen transmission far exceeds the threshold needed to prevent hypoxic neovascularization (typically >87 for extended wear). Superior limbal hyperemia with vessel ingrowth in high-Dk silicone hydrogel wearers is now recognized as a primarily mechanical phenomenon — tight upper lid compression causes limbal trauma and inflammatory signaling, not hypoxia.

When fitting a toric soft lens, the lens consistently rotates 15° counterclockwise (as viewed from the examiner's perspective). To correct for this misrotation, the cylinder axis of the ordered lens should be:

Answer: Rotated 15° counterclockwise from the spectacle axis

Using the LARS rule (Left Add, Right Subtract): if the lens rotates counterclockwise (Left), you Add the degrees of misrotation to the spectacle cylinder axis. For example, if spectacle axis is 180° and the lens rotates 15° counterclockwise, the ordered axis becomes 180° + 15° = 195° (or equivalently 015°). This compensatory shift ensures the cylinder axis aligns correctly on the eye.

Orthokeratology lenses are designed to temporarily flatten the central cornea. Which corneal refractive zone change most directly accounts for the myopia reduction effect?

Answer: Central epithelial thinning with corresponding midperipheral epithelial thickening

Orthokeratology works through epithelial redistribution, not stromal remodeling. The reverse-geometry lens applies bearing force centrally and creates a reservoir zone in the midperiphery. Central epithelial cells migrate and compress outward, thinning the central epithelium and increasing midperipheral thickness. This redistributes the refractive power of the anterior corneal surface without permanent stromal change.

A patient with keratoconus is being fit with a scleral lens. Vault clearance over the cone apex is measured at 600 µm with OCT after lens settling. The ideal apical clearance target at dispensing (before settling) for most scleral lens protocols is approximately:

Answer: 150–250 µm, allowing for ~100 µm of post-wear settling

Scleral lenses settle an average of 100–150 µm over several hours of wear due to tear reservoir thinning and lens compression of conjunctival tissue. Most fitting protocols target an initial central clearance of 200–250 µm so that after settling the residual vault is approximately 100–150 µm — enough to prevent apical touch without excessive fluid reservoir thickness that degrades vision or promotes hypoxia. A 600 µm clearance at dispensing would result in unacceptably high final clearance.