We read the recent article titled “Collagen Cross-Linking with Riboflavin in a Femtosecond Laser–Created Pocket in Rabbit Corneas: 6-Month Results” and would like to raise our concerns regarding this animal study.

With a procedure similar to that in a study performed on humans by Kanellopoulos, the authors tried to validate the efficacy of collagen cross-linking, by which corneal collagen is irradiated by ultraviolet A (UVA) light with an administration of riboflavin through a pocket created by a femtosecond laser. A significant increase in the central corneal thickness (CCT) of the rabbits was observed by the authors 6 months after this procedure. However, the rabbits used in the study that weighed approximately 2.5 kg may be growing, rather than full-grown, rabbits. As evidenced by a recent study, corneal thickness in growing rabbits significantly increases during growth. Thus, the observed increase in CCT may not have been the result of the procedure, but rather the result of normal growth. This necessitates a control group in which no intervention is performed and serial CCT measurements are obtained. Additionally, because rabbits have difficulty in fixing, possible interrabbit variability in measured CCT should be considered.

Riboflavin was administered between the flap and posterior stroma. No reports have shown at which depth in the corneal flap riboflavin can soak. Because a possible explanation for the continued decrease in corneal thickness from 1 to 3 months after a standard procedure is that the corneal collagen fibrils (especially the more transverse-oriented anterior fibrils) may be compressed, it is inferred that collagen fibrils in the anterior stoma in this procedure may not have been changed significantly, because an increase in CCT was observed in the same period. This is probably because riboflavin cannot permeate the full thickness of the flap. As demonstrated by a previous study, 65% to 70% of UVA irradiation is absorbed in the anterior stroma. Also, because the rabbit eye has no Bowman layer, the corneal flap may contain more anterior corneal stroma. As such, a considerable portion of the UVA light may be absorbed in the flap with minimal or no riboflavin soaking. Moreover, the authors decreased the time for UVA light (10 minutes), which was only one third of that in an epithelial debridement procedure (30 minutes). Thus, a decreased effect of the UVA irradiation may occur when treating around the dissected riboflavin-soaked corneal stroma. A refined study is necessary to evaluate this procedure further.

Of note, the VisuMax femtosecond laser (Carl Zeiss Meditec AG, Jena, Germany) was set in this study to create a pocket with a spot size of 35 μm and a spot separation of 5 μm. We do not think that these settings are viable and thus can successfully deliver the laser for a pocket creation. According to the available literature, the spot size in the VisuMax system is estimated to be approximately 1 μm.