How to select first base curve of trial RGP lens for keratoconus?

We get a patient with keratoconus and while starting to do the trial fitting, after completing all other examination, we sometimes are perplexed on choosing the first trial lens. There is varied opinion on trial lens selection criteria. So, the base curve or back optic zone radius of curvature of the RGP should be so chosen on the basis of average keratometric value (steepest k plus flattest k divided by 2) of anterior cornea.

Generally accepted idea: 5-6 criteria

1. If average k reading is below 5 mm à Choose the RGP that has BOZR (base curve) 0.3 mm flatter than average 
2. If average K reading is between 5-6 mm à Choose the first RGP lens that has same base curve as average k
3. If average k reading is flatter than 6 mm à Choose the first trial lens that has base curve 0.2mm steeper.

Key words: Keratoconus, RGP fitting, BOZR, Base Curve, Trial Lens, Contact lens trial, RGP criteria, trial lens selection