What would be the residual astigmatism from the predicted contact lens power for a patient with Refraction: -2.00 -1.00 x 180, Ks: 43.50/44.50@090, BCR: 43.00?

• A. PL
• B. Plano
• C. 0.00 D
• D. +0.50 D

Answer: (A)

The key idea here is how residual astigmatism is determined when predicting a spherical GP lens power. You compare the corneal astigmatism seen on kerato‑dioptric measures with the manifest refractive cylinder.

From keratometry, the cornea shows 1.00 D of astigmatism, with the flatter meridian at 090 and the steeper at 180. The manifest refraction has 1.00 D of cylinder at 180. Those two cylinders are equal in magnitude but oriented 90 degrees apart. When you choose a spherical GP lens power, the spherical component is set to correct the myopia, and with the base curve chosen close to the cornea (BCR around the flattest meridian), the tear lens and the lens curvature tend to neutralize the corneal astigmatism in both meridians. With this arrangement, the residual cylinder after applying the predicted lens power ends up being zero.

So, the predicted lens power leaves no remaining astigmatic error, i.e., the residual astigmatism is plano. This is why the best answer is plano (no cyl). If there were a mismatch in magnitudes or axis alignment, a residual astigmatism would remain, potentially requiring a toric GP lens.