Contact Lens Principles Flashcards

A patient with keratoconus is being fit with a rigid gas-permeable (RGP) lens. The fluorescein pattern shows central pooling with mid-peripheral bearing and edge lift. Which fitting relationship does this describe?

Answer: Apical clearance fit

Apical clearance (steep fit) is characterized by central fluorescein pooling (dye accumulates under the central lens where it vaults the cone), mid-peripheral bearing (darkened annular zone of lens-cornea contact), and peripheral edge lift. This is the classic steep-fitting pattern. Apical touch shows central bearing (dark center), and alignment shows a uniform green pattern across the central zone.

Which parameter defines the distance from the back surface of a contact lens to the front surface of the cornea at the lens periphery, and is critical for ensuring adequate tear exchange?

Answer: Axial edge lift (AEL)

Axial edge lift (AEL) is measured parallel to the optical axis and quantifies how far the peripheral back surface of the lens separates from a theoretical extension of the corneal surface. Adequate AEL ensures the tear pump mechanism functions and prevents debris accumulation. Radial edge lift (REL) is measured along the lens radius and is geometrically distinct from AEL. Edge clearance refers to physical space at the lens margin, while peripheral curve width is a lens design parameter rather than a fitting measurement.

A soft toric contact lens is consistently rotating 15° nasally after each blink. Using the LARS rule to compensate for this rotation during lens ordering, how should the axis be adjusted for a patient whose spectacle refraction is -2.00 -1.50 × 090?

Answer: Order axis 105

The LARS rule: if the lens rotates Left, Add the degrees of rotation to the axis; if it rotates Right, Subtract. Nasal rotation in the right eye corresponds to clockwise rotation when viewed from the front, which is rightward rotation. Therefore, we subtract: 090 − 15 = 075... Wait — nasal rotation in the right eye moves the lens nasally, which is temporal-to-nasal. From the examiner's view facing the patient, the right eye's nasal side is to the left, so nasal rotation is a leftward (counterclockwise) rotation. LARS: Left → Add. 090 + 15 = 105. Order axis 105.

The Dk/t value of a contact lens material represents oxygen transmissibility. If Lens A has a Dk of 100 barrers and a center thickness of 0.20 mm, and Lens B has a Dk of 150 barrers and a center thickness of 0.35 mm, which lens delivers more oxygen to the cornea?

Answer: Lens A, with a Dk/t of approximately 500

Dk/t = Dk ÷ thickness (in cm). Lens A: 100 ÷ 0.020 cm = 5,000 (×10⁻⁹). Lens B: 150 ÷ 0.035 cm = 4,286 (×10⁻⁹). Lens A has higher Dk/t despite lower Dk, because its thinner center thickness compensates. A higher Dk material does not automatically mean more oxygen transmission — thickness is equally critical. Water content affects Dk in hydrogel lenses but is already reflected in the Dk value.

Contact lens-induced superior epithelial arcuate lesions (SEALs) are most commonly associated with which combination of factors?

Answer: High-Dk silicone hydrogel lenses with tight fit and steep base curve

SEALs are crescent-shaped epithelial staining lesions at approximately the 10–2 o'clock positions, paradoxically associated with high-Dk silicone hydrogel lenses — not hypoxic lenses. The leading hypothesis is mechanical: silicone hydrogel materials are stiffer (higher modulus), and when combined with a steep/tight fit, the rigid lens edge impinges on the superior epithelium beneath the lid during blinking. Low-Dk lenses cause hypoxic complications (neovascularization, edema) rather than SEALs. Excessive movement and flat fits cause inferior mechanical trauma, not superior arcuate lesions.

When using videokeratography to fit a scleral lens, a practitioner observes a limbus-to-limbus corneal diameter (HVID) of 11.4 mm and significant irregular astigmatism post-penetrating keratoplasty. The initial trial lens lands with mid-peripheral bearing visible on OCT. Which adjustment is most appropriate?

Answer: Increase the landing zone curvature (flatten the haptic) to reduce bearing

In scleral lens fitting, mid-peripheral bearing on OCT indicates that the landing (haptic) zone is too steep relative to the scleral contour, creating excessive compression at the landing zone. The correction is to flatten the haptic/landing zone curves, which opens up the angle and reduces localized bearing. Changing the base curve primarily affects central vault — not peripheral landing. Reducing diameter would change where the lens lands but not address the bearing geometry. A limbal zone adjustment addresses clearance over the limbus, not mid-peripheral haptic bearing.