A rigid gas-permeable contact lens with base curve 7.40 mm is fitted on a cornea with keratometry 7.50 mm @ 180 and 7.90 mm @ 90. The lens fit is likely to be:
- A Too steep (flat cornea, steep lens)
- B Too flat (steep cornea, flat lens) ✓
- C Aligned fit (lens and cornea match)
- D Cannot be determined from this data
Explanation
The base curve of 7.40 mm is steeper (more curved) than the flattest K of 7.50 mm. However, the steepest K is 7.90 mm, meaning the lens base curve is actually flatter than the average corneal curvature and much flatter than the steep meridian. For a flat-fitting relationship, the lens base curve should be flatter than the corneal K. Here the lens is steeper than the flat K but much flatter than the steep K, creating an unstable, flat-fitting relationship. The key concept: lens base curve 7.40 mm < corneal curvature on average means lens is relatively flat compared to corneal shape.
Reference: Contact Lenses, 6th Edition (Phillips, Speedwell), 6th ed.
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Written and medically reviewed by the StethoPrep medical team.