MORE ABOUT DOUBLE-K: WHEN YOU DON´T HAVE PREOPERATIVE DATA
What about if we do not know the pre-laser K readings?. Should I still use double-K?.
The answer is YES. How to do it…, there are two ways:
1- Use an standard K reading. In 5980 eyes seeking for refractive surgery in Clinica Baviera (Spain), the average K reading was 43,60. Other studies have reported slightly steeper K readings (Khabazkhoob M, Hashemi H, Yazdani K, Mehravaran S, Yekta A, Fotouhi A. Keratometry measurements, corneal astigmatism and irregularity in a normal population: the Teharan Eye Study. Ophthalmic Physiol Opt 2010;30(6):800-805).
I would use 43,60 as my first choice
2- Calculate an approximate preoperative reading if you know K post and refraction corrected. You have to take into account that K readings after refractive surgery tend to over-estimate the power of K after myopic corrections and under-estimate K after hyperopic corrections. In myopic correction I would deduct 0,15 diopters each diopter corrected (for example 38 diopters K reading and-6 diopters corrected, I would use 38 – (6 * 0,15) = 37,10, as the supposed real post-op K and add the 6 diopters corrected. So, estimated pre-op K reading = 43,10).
It is not an accurate system at all because it has a few wrong assumptions, but it could be useful for double-K calculation.