COT Contact Lens Principles 4 — Questions and Answers
Question 1: Which condition describes superficial punctate staining in a 3 o'clock and 9 o'clock distribution on the cornea during RGP lens wear?
- Dimple veiling
- Desiccation staining from incomplete blinking (Correct answer)
- Solution toxicity reaction
- Lens-induced edema
Correct answer: Desiccation staining from incomplete blinking
Peripheral desiccation staining at 3 and 9 o'clock positions occurs when the area adjacent to the rigid lens is not adequately covered by the tear film during blinking.
Question 2: A scleral contact lens differs from a corneal RGP lens primarily because it:
- Uses a higher water content hydrogel material
- Vaults entirely over the cornea and rests on the sclera (Correct answer)
- Is worn only during sleep for orthokeratology
- Corrects presbyopia with alternating segments
Correct answer: Vaults entirely over the cornea and rests on the sclera
Scleral lenses have a large diameter (15–25 mm) that vaults over the entire cornea and lands on the scleral conjunctiva, creating a fluid reservoir.
Question 3: What is orthokeratology (ortho-k)?
- Surgical reshaping of the cornea with an excimer laser
- Use of specially designed rigid lenses worn overnight to temporarily flatten the cornea (Correct answer)
- Fitting of a bandage lens after corneal trauma
- Measurement of corneal curvature with a keratometer
Correct answer: Use of specially designed rigid lenses worn overnight to temporarily flatten the cornea
Orthokeratology uses reverse-geometry rigid lenses worn during sleep to temporarily reshape the corneal epithelium, providing clear unaided daytime vision.
Question 4: Which measurement tool is used to evaluate contact lens fit parameters such as base curve, diameter, and thickness?
- Lensometer (vertometer)
- Radiuscope (ophthalmometer) (Correct answer)
- Goldmann tonometer
- Corneal topographer
Correct answer: Radiuscope (ophthalmometer)
A radiuscope (also called an optical spherometer) measures the back surface radius of curvature of rigid contact lenses.
Question 5: Microcystic corneal edema seen at the slit lamp is an early sign of which contact lens problem?
- Allergic reaction to lens solution
- Chronic corneal hypoxia from insufficient oxygen transmissibility (Correct answer)
- Mechanical trauma from lens edge
- Giant papillary conjunctivitis
Correct answer: Chronic corneal hypoxia from insufficient oxygen transmissibility
Microcystic edema (epithelial microcysts) develops when chronic hypoxia disrupts normal epithelial cell metabolism and turnover.
Question 6: Which lens design is most appropriate for a presbyopic patient requiring good distance and near correction in the same eye simultaneously?
- Monovision with a single-vision lens
- Simultaneous-vision multifocal contact lens (Correct answer)
- Alternating-image translating bifocal lens
- High-add reading-only contact lens
Correct answer: Simultaneous-vision multifocal contact lens
Simultaneous-vision multifocal lenses present distance and near zones to the pupil at the same time, allowing the brain to select the appropriate focus.
Question 7: What is the clinical significance of the 'sagittal depth' (sag) of a contact lens?
- It determines the oxygen permeability of the lens material
- It describes how deep the bowl of the lens is relative to the fitting surface (Correct answer)
- It measures the refractive index of the lens polymer
- It indicates the lens surface wettability angle
Correct answer: It describes how deep the bowl of the lens is relative to the fitting surface
Sagittal depth determines how the lens vaults over the cornea; it is influenced by both the base curve and the overall lens diameter.
COT Contact Lens Principles 4
Which condition describes superficial punctate staining in a 3 o'clock and 9 o'clock distribution on the cornea during RGP lens wear?