ABO Contact Lens Fitting 4 — Questions and Answers
Question 1: Residual astigmatism with an RGP lens is BEST corrected by:
- Increasing lens diameter
- Using a back-surface toric RGP lens (Correct answer)
- Decreasing center thickness
- Steepening the base curve
Correct answer: Using a back-surface toric RGP lens
Residual (lenticular) astigmatism uncorrected by the tear lens requires a toric RGP design, typically back-surface toric.
Question 2: The Holladay-Prager formula and similar tools are used in contact lens practice primarily for:
- Calculating vertex distance power conversion (Correct answer)
- Determining scleral lens landing zone width
- Predicting lens rotation in toric fits
- Measuring tear film breakup time
Correct answer: Calculating vertex distance power conversion
Vertex distance power conversion formulas (like Holladay's) convert spectacle Rx to contact lens power, especially important above ±4.00 D.
Question 3: A new soft lens wearer reports blurry vision that clears momentarily after blinking. The MOST likely cause is:
- Incorrect lens power
- Lens dehydration or poor wettability (Correct answer)
- Giant papillary conjunctivitis
- Corneal edema
Correct answer: Lens dehydration or poor wettability
Post-blink clearing of blur suggests the tear film temporarily restores optical quality, pointing to lens dehydration or poor wettability.
Question 4: Which of the following BEST describes 'limbal-to-limbal' fitting for a soft contact lens?
- Lens diameter equals the horizontal visible iris diameter (HVID)
- Lens diameter extends 1–2 mm beyond the HVID onto the sclera (Correct answer)
- Lens diameter is 0.5 mm smaller than HVID
- Lens covers only the pupillary zone
Correct answer: Lens diameter extends 1–2 mm beyond the HVID onto the sclera
Standard soft lenses are fit so their diameter extends approximately 1–2 mm beyond the limbus onto the sclera.
Question 5: A contact lens patient shows 3 and 9 o'clock corneal staining with an RGP lens. This is MOST likely caused by:
- Excessive lens movement
- Inadequate edge lift and poor tear meniscus at the lens periphery (Correct answer)
- Solution toxicity
- Lens power error
Correct answer: Inadequate edge lift and poor tear meniscus at the lens periphery
Desiccation at the 3 and 9 o'clock positions occurs when the nasal and temporal tear meniscus is disrupted by a poorly designed lens edge.
Question 6: Extended wear (EW) contact lenses primarily require higher Dk/t values because:
- The patient cannot insert or remove lenses daily
- Closed-eye oxygen tension is much lower, requiring higher transmissibility to prevent hypoxia (Correct answer)
- Higher Dk lenses are more resistant to deposits
- EW lenses must correct higher refractive errors
Correct answer: Closed-eye oxygen tension is much lower, requiring higher transmissibility to prevent hypoxia
During sleep, the palpebral conjunctiva reduces corneal oxygen supply to ~8%, so lenses with very high Dk/t (≥87 for EW) are needed to prevent hypoxia.
Question 7: Which lens material property is most important in reducing lipid deposit formation on contact lenses?
- High water content
- Low water content and high silicone content
- Ionic surface charge (FDA Group IV materials) (Correct answer)
- High refractive index
Correct answer: Ionic surface charge (FDA Group IV materials)
FDA Group IV lenses (high water, ionic) attract the most protein and lipid deposits due to their ionic surface charge.
ABO Contact Lens Fitting 4
Residual astigmatism with an RGP lens is BEST corrected by: