Reply : Q-factor customized ablations (2006)

Thank you for the refreshing comments about our publication on Q-factor optimized ablation profile. Positive critics are the motor for scientific progress; however, it is good practice and style to declare financial interest as an author, which the authors of the letter have failed to do. At least one author (T.M.) is employed by the Schwind Co. Ortueta et al. state that “[u]p to now, there is no proof that the asphericity alone plays a major role in the visual process” and cite, to our surprise, the article by Somani et al.1 In the abstract of this paper, we find the following statement regarding simulations of optimal optics: “Simulations showed that the (optimal) corneal Q-value can vary from more to less prolate depending upon the shape of the internal surface.” Yes, there may not be one optimal Q-value that fits all eyes, but we know that a target Q-factor should be clearly negative (prolate), which is not achieved today. If this is not proof enough, let us compare the results of U.S. Food and Drug Administration trials on ablation profiles. The simple wavefront-optimized profile is equivalent to or even outscores most wavefront-guided profiles relative to increase or loss of best spectacle-corrected visual acuity by compensating for the induced spherical aberration, which is equivalent to the induction of a prolate cornea.2 Indeed, there is proof available to motivate the refractive surgery community to go forward and aim for postoperatively prolate corneas (probably including the correction of other aberrations. We agree that the one-fits-all Q-factor is problematic for various reasons. We have discussed this topic in our response to the letter from Kezirian3 (see preceding letter and reply). In brief, this was just a starting point; as the next step, we are looking for a nomogram for Q-factor adjustment to preserve the preoperative Q-value. Ortueta et al. believe that biomechanical responses of the cornea cannot be compensated for by the Q-factor optimized treatment. We absolutely disagree. In fact, without knowledge of the preoperative biomechanical parameters of an individual cornea to be treated, we cannot address the individual biomechanical response, but we can compensate for the average biomechanical response, which obviously always goes in the direction of an oblate cornea. Roberts interpreted our undercorrection of the Q-factor as an underestimation of the biomechanically driven induction of oblateness (Cynthia Roberts, PhD, personal communication, June 2006) and she voted for a biomechanically optimized ablation pattern by adding a better Q-factor adjustment. The authors are wrong when they state there was no significant difference in the Q-factor between the wavefront-guided and the custom-Q group. For corrections of −5.0 diopters and less, and this is the group of interest, there was a statistically significantly smaller shift toward oblateness of the cornea in the custom-Q group. In essence, we are happy having initiated a discussion that draws our focus back to the cornea and its shape. We introduced the wavefront-guided profile some years ago and performed the first treatments in 1999. Since then, we have learned that the wavefront-guided treatment has advantages and disadvantages. To believe that wavefront-guided treatment is the best for everyone is a gross oversimplification and in most cases a significant overkill regarding preoperative analysis. Keratorefractive surgery takes place in the cornea, and it is our job to produce a postoperative cornea that has a “more or less prolate shape,” guaranteeing optimal optics. As mentioned in our paper, the wavefront-guided ablation profile is a brilliant alternative but the results of the FDA trial demonstrate that only a small fraction of patients truly benefits from this approach.