Contact Lenses & Fitting Flashcards

When evaluating a rigid gas permeable (RGP) lens fit on a normal cornea using sodium fluorescein, the ideal fitting relationship shows:

Answer: Alignment fit with minimal apical touch and moderate peripheral edge lift

An ideal RGP fit on a normal cornea demonstrates alignment with minimal apical touch and adequate peripheral clearance to allow tear exchange and prevent peripheral seal-off.

Scleral contact lenses vault completely over the cornea and limbus, resting on which tissue?

Answer: The bulbar conjunctiva overlying the sclera

Scleral lenses bear entirely on the bulbar conjunctiva overlying the sclera, completely vaulting the cornea and limbus with a fluid reservoir beneath.

Orthokeratology contact lenses are designed to achieve their primary refractive effect through which mechanism?

Answer: Temporarily reshaping the corneal epithelium via reverse geometry lenses worn during sleep

Orthokeratology uses reverse geometry RGP lenses worn overnight to temporarily redistribute and reshape the corneal epithelium, flattening the center and steepening the midperiphery to reduce myopia.

Prism ballast stabilization used in soft toric contact lenses works by:

Answer: Adding base-down prism that weights the inferior lens, using gravity and lid forces to resist rotation

Prism ballast incorporates a wedge of base-down prism that makes the inferior portion of the lens thicker and heavier, using gravity and upper lid pressure to maintain rotational orientation.

In a patient with keratoconus, rigid gas permeable contact lenses improve visual acuity primarily because they:

Answer: Create a smooth anterior refracting surface independent of the irregular cornea using a tear lens

RGP lenses mask irregular corneal astigmatism in keratoconus by creating a new smooth refracting surface; the tear lens between the flat posterior RGP surface and the irregular cornea fills in the irregularity.

Which multifocal contact lens design presents both distance and near focal points to the retina simultaneously regardless of gaze direction?

Answer: Simultaneous vision concentric ring design

Simultaneous vision designs use concentric alternating zones of distance and near power so both focal points are always projected onto the retina, relying on neural adaptation to select the appropriate image.

Based on the Holden–Mertz criteria, what minimum Dk/t (×10⁻⁹) is required for a daily wear soft contact lens to prevent clinically significant corneal edema?

Answer: 24

Holden and Mertz established that a minimum Dk/t of 24 ×10⁻⁹ is required for daily wear to prevent clinically significant corneal swelling, while extended wear requires 87 ×10⁻⁹.