"My Size is 8.5"
Occasionally I'll get a patient call or tell me something like this: "hey doc my LAST Rx (that had since become blurry) was a "8.5" base curve. These NEW ONES that YOU Rx'd are "8.7". Well these new ones move around a lot and I'm going to need you to give me the NEW, clearer (usually they say "stronger", but I digress) Rx in "my size" which is 8.5." No. It doesn't work like this. "Your size" is not 8.5. 8.5 is kind of a standard/common base curve for a 14.0mm (or smaller) diameter soft spherical lens. Now you need TORICS (astigmatism). Toric lenses are usually larger than 14.0 to add in the prism ballast (weight or "carrier") to stabilize them. Well when you INCREASE the diameter you must also increase (flatten) the base curve. Those go together. You can't just increase the diameter and keep that "8.5" base curve that you think is "your size" because if that were made it would likely "squeeze" you too hard and be very, very uncomfortable (assuming same material and design, for those of you fellow eye docs following this). Just like "8.5" is kind of a standard base curve for soft, disposable, 14.0mm diameter spherical silicone hydrogel contacts, "8.7" is kind of the standard base curve for torics. I often say that fitting soft contact lenses is like fitting tee shirts: you're a "medium" or a "large" or an "XL". You're not a "38 regular" tee shirt. They're not that specific. THE REAL problem in the case above is the SAME problem EVERY new toric wearer has: torics move. Their old spherical contacts moved, too. They just didn't know it. It's not that 8.5 is "your size". Your size is "medium", which is what most adult nearsighted patients are. Don't get too caught up on what you think your "base curve" should be.