We read with interest the article by Soler and associates comparing pupil-centered vs vertex-centered ablations for the correction of hyperopia. From analysis of the ocular aberrations, they found that in eyes showing large pupil decentration (distance from pupil center to corneal vertex more than 0.25 mm), pupil-centered ablation seemed to induce a lower amount of coma and, consequently, a reduced loss of corrected distance visual acuity compared with vertex-centered patients. Interestingly, in the group with lower pupil decentration, they found better aberrometric outcomes when centered on the corneal vertex.

We think that this discrepancy is attributable to the centration technique and the size of the optical zone (OZ) (the authors do not report OZ size). The effective OZ, regardless of the centration method, should cover the entire functional pupil. If we shift the treatment to the corneal vertex, we should add this shift to the treated OZ twice. Otherwise, a crescent moon segment at the pericentral pupil may be at risk of undercorrection (producing a higher coma induction). A possible explanation of the reported outcomes is that the selected OZ was large enough to cover the pupil for small offset values but not for larger ones (eg, pupil size is 6 mm with a pupil offset of 0.125 mm and an treated OZ of 6.5 mm; pupil size + 2 × pupil offset = 6 + 2 × 0.125 = 6.25 mm < treated OZ). The treatment fully covers the pupil and vertex-centered outcomes may be marginally better than pupil-centered outcomes. However, with an offset of 0.375 mm for the same pupil and OZ size (6 + 2 × 0.375 = 6.75 mm > OZ), the pupil is not fully covered by the treatment. In this case, vertex-centered treatments will induce coma and outcomes will be worse than with pupil-centered treatments.

Centration on the corneal vertex is a reproducible technique when using the videokeratoscopic values. The authors have used the coaxial light reflex under the laser microscope and found that it concurred with the preoperative topographic Cartesian coordinates of the corneal vertex. Determining the coaxial light reflex is subjective, and surgeons may have interindividual differences, depending on eye dominance, eye balance, or the microscope’s stereopsis angle.

The problem with using pupil centration is that the pupil center moves. The authors tried to use the pupil center under mesopic conditions using low illumination during the ablation. Obviously, if they did not use an additional reference during the treatment such as the limbus, which does not move, the ablation could shift with changes in pupil position.

We want to remark that the pupil center is an instable target. A morphologic reference such as the corneal vertex is more advisable. It is reproducible; we do not have problems of asymmetry with loss of ablation efficiency. To solve the problems with spatial shift of the center, we need a third reproducible nonmoving reference such as the limbus. If we shift the treatment to the corneal vertex, we should add this shift twice to the treated OZ.