The steepest meridian is the 45.50 D reading at axis 90. Keratometry is the measurement of the curvature of the front surface of the cornea. It reports the two principal meridians: the one with the most curvature and the one with the least. Each value is written in diopters (D) with its own axis in degrees. The higher dioptric value is the steeper meridian, and its axis travels with it. So 45.50 is the steeper of these two values.
Curvature and power move together. A more steeply curved surface bends light more, so it reads as more power in diopters. The flatter meridian here is 43.00 D at axis 180. The two meridians sit 90 degrees apart in regular astigmatism. Astigmatism means the eye does not bend light equally in all meridians. Keratometry gives an objective, measured value for the corneal part of that astigmatism. Reading the pair correctly matters in daily work. It guides the fit of contact lenses and is an essential measurement before refractive surgery. Keep the number and its axis together, because a value without its axis cannot be used. A rigid lens is chosen from these curves, so a swapped axis sends the wrong lens to the patient.
Read the bigger number as the steep meridian and keep its axis attached. Say it out loud as “steep 45.50 at 90, flat 43.00 at 180” so nothing gets swapped. Then check the written record against what you said before the patient leaves the chair. Recording and reading keratometry values is the heart of the COA keratometry content area.
Incorrect Answers
- B. 43.00 D at axis 180 is the flattest meridian, which is the correct pairing but the wrong one of the two.
- C. This takes the steep power but attaches the flat meridian's axis. Mismatching a value and an axis can ruin a lens order.
- D. This takes the flat power and attaches the steep axis, so both halves of the answer are wrong.
Testing Pearls
- Higher diopter value = steeper cornea; lower value = flatter cornea.
- Keratometry measures the front surface curvature, in diopters, with an axis for each meridian.
- In regular astigmatism the principal meridians are 90 degrees apart.
- Keratometry guides contact lens fitting and is required before refractive surgery.
- Readings are recorded as power and axis pairs, such as 43.00 x 180 / 45.50 x 90.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 5, Optics and Refractive States of the Eye.
