Approaches to cataract surgery vary in patients with previous refractive surgery. John A. Hovanesian, MD, FACS, interviews Douglas D. Koch, MD, about performing cataract surgery on patients with prior refractive surgery.
An excerpt from the article:
Hovanesian: Why is it so difficult to measure the cornea of post-refractive surgery patients?
Dr. Koch: There are two reasons that it is difficult to obtain good corneal measurements in patients who have had prior refractive surgery. We must remember that the cornea has two curvatures, a front and a back, and when we use a standard keratometer or topographer, we are only measuring the front, but we are giving an overall value for corneal power. That overall value is based on some assumption about what the back corneal power is. If you subject a patient to PRK or LASIK, you change the front corneal power but not the back, and so you alter the relationship. These devices use a different index of refraction to compensate for the back corneal power, but if you change the relationship between front and back, that index of refraction is no longer valid.
Unfortunately, because different levels of correction change the front in different magnitudes relative to the back, there is no new single value for refractive index you can just plug into those machines to get your true corneal power. So what we are doing is measuring the front and having to make a sophisticated guess about the back, and that guess is often not as accurate as we would like it to be.
Read the entire interview here: http://www.osnsupersite.com/view.aspx?rid=27382