There have long been concerns that the cornea should not end up too flat after laser vision correction eye surgery for correcting nearsightedness (myopia). This new study suggests this may not be the case.
During LASIK or PRK laser eye surgery, the excimer laser reshapes the cornea. When someone is nearsighted, their cornea is too steep. When laser vision correction is performed on nearsighted patients, the laser precisely reshapes the cornea to make it flatter so that it focuses the light more perfectly.
For years, surgeons have felt that the cornea could only be flattened to a certain level and, below that leve lof corneal flatness, visual quality would suffer. Current FDA guidelines for LASIK include a limit for how flat the cornea should be made when treating nearsightedness. Depending on how flat a patient’s cornea is before surgery, this may mean there is a limit to how much nearsightedness could be corrected before that limit of corneal flatness is reached. Some patients have even been told they are not good candidates for laser eye surgery, whether LASIK or PRK, due to their cornea being too flat pre-operatively. From a practical standpoint, this issue of maximal flatness typically only limits the amount of myopia laser vision correction in patients with very high degrees of nearsightedness.
A recent study from Israel, however, scientifically looked at eyes with a post-operative degree of flattening from PRK laser eye surgery that had previously been considered as making the cornea “too flat”. Interestingly, the researchers in this study did not find any detrimental effects from making corneas this flat. In this study, the charts of 5410 eyes that had undergone PRK laser eye surgery to correct nearsightedness from 2006 to 2010 were retrospectively reviewed. In the study, the researchers looked at those eyes whose corneas ended up being “too flat” by conventional definition. The steepness or flatness of the cornea is measured by a unit called the “K” value. K values in normal, unoperated human eyes range typically from 41 to 47 with higher numbers being steeper. In this study, eyes that ended up with K values below 35 (i.e. very flat) were analyzed. In the study, these very flat corneas with K values less than 35 after PRK laser eye surgery did not show any loss of corrected visual acuity (the best vision with corrective lenses) compared to their pre-operative best corrected visual acuity. The authors concluded there was no concern, at least based on this study, with using the laser in PRK surgery to make corneas flatter than had previously been thought prudent.
In my own practice, I have a conservative approach and, despite this study, I continue to follow FDA guidelines for a limit to how flat I make the cornea after surgery. However, the study is interesting and if further studies align with these results, we may learn that we safely can provide treatment to those patients who, in the past, we have cautioned against undergoing LASIK or PRK due to concerns about the flatness of their corneas.
See Also
Visual Acuity Outcomes in Eyes with Flat Corneas after PRK
A study showing that flat corneas after PRK did not create visual problems after laser eye surgery.
Patients with Potentially Flat K’s for LASIK
An article from an American Society of Cataract and Refractive Surgery journal about flat corneas after LASIK.
Steep, Flat, and Irregular Corneas in LASIK
Review of Ophthalmology Journal about LASIK if flat, steep, and irregular corneas.
Flat Corneas
Online discussion about LASIK in flat corneas.
The Effect of Preoperative Keratometry on Visual Outcomes After Moderate Myopic LASIK
A study of the effect of preoperative corneal steepness as measured by keratometry values on visual outcomes after myopic LASIK.
FDA Labeling for VISX CustomVue
FDA guidelines for VISX CustomVue Wavefront LASIK for minimal K values (corneal flatness) after wavefront LASIK.