'Technical’ question-regression / residual corneal thickness

[croanster]

I understand it’s common to overcorrect to allow for regression especially in patients that are hyperopic. When they calculate the residual thickness, is that what is left immediately after surgery. And if so, with regression that figure should change quite a bit?

For example, my calculated residual thickness was 321um but at my one day post op I had gone from +3.25 to around -3 and now at +0.75 so in reality with all that regression I might have 350um of eye? Just curious how it works.

Thanks!

[LasikExpert]

The change in refractive error is not made by simply thinning the cornea. The change is made by reshaping the curvature of the cornea. Regression is the cornea moving back toward the original curvature of the cornea as a wound response. Short-term regression can be edema (swelling), but this thickens the cornea, not thins the cornea.

A healthy cornea will not become more thin with regression.

[croanster]

Ok thanks - I was thinking that the cornea would actually thicken with regression not thin as it 'grew back' in the places it was zapped.

Thanks.

[LasikExpert]

The epithelium is the outermost layer of cells on the cornea. These are the fastest reproducing cells in the human body. The underlying layers, bowmans, stroma, decements, and endothelium, do not regenerate. Lasik applys laser energy in the stroma, so those cells do not return.

What changes is the internal biomechanical strength of the cornea and the epithelium. With the internal stresses changed by surgery, the cornea will change shape during the healing process. This is a primary cause of regression. The epithelium assumes the changes are unwanted and attempts to "correct" the surgery. From myopic patients under about 6.00 diopters, the biomechanical and epithelial changes are not enough to effect real change in refractive error. Unfortunately for hyperopic correction, both the biomechanical and epithelial changes dramatically effect the refractive error and is why regression after hyperopic Lasik is so common.