What would be the tear layer for a patient with Refraction: -2.00 -1.00 x 180, Ks: 43.50/44.50@090, and BCR 43.75?

• A. -0.25 D
• B. +0.25 D (use flatter K)
• C. 0.00 D
• D. -0.75 D

Answer: (B)

Think of the tear layer as a very weak lens sitting between the cornea and the GP lens. Its power is small and tends to be positive, helping to offset part of the eye’s refractive error. To estimate it, you compare the manifest refraction in the principal meridians to the corneal powers, using the corneal plane as the reference (here guided by the mean keratometry and back-vertex value of about 43.75 D).

In this patient, the refraction in the 90-degree meridian is about -3.00 D and in the 180-degree meridian about -2.00 D. The cornea provides roughly 44.50 D in the 90-degree meridian and 43.50 D in the 180-degree meridian. When you use the flatter keratometry (43.50 D) to estimate the tear layer power needed to balance the eye’s optics at the corneal plane, the calculation yields a small positive value of about +0.25 D. So the tear layer would contribute +0.25 D of power, which is why the correct choice is +0.25 D (use flatter K). The other options don’t fit because they imply no tear power or a negative/larger tear power, which would not align with the small positive offset produced in this scenario.