Several laboratory and clinical studies
22–26 have been done with the aim to demonstrate the equivalence between conventional and accelerated UV-A irradiation protocols. The hypothesis of equivalence has been made based on the Bunson-Roscoe law of reciprocity.
27,28 On the other hand, this hypothesis has been shown not to be true in biological tissues.
1 Accordingly, Schwarzschild
29 proposed a modification of the reciprocity law, which later became known as Schwarzschild's law. In this study, we found significant differences in riboflavin consumption between tissues that have been irradiated with the conventional and accelerated UV-A protocol. The conventional irradiation of the stroma achieved on average 20% more consumption of riboflavin than the accelerated one. It should be the object of further investigation whether this difference may lead to a different amount of additional cross-linking bonds between stromal proteins and therefore to different biomechanical strengthening of the corneal stroma in patients. So far, to our knowledge, no previous experiment provided evidence of such difference,
22,23,25,30 except for a biomechanical study on porcine eyes by Hammer et al.
31 These authors found 15% lower stiffening effect with increasing UV-A irradiance from 3 mW/cm
2 to 9 mW/cm
2 and decreased time from 30 to 10 minutes, respectively. Several factors, including differences in tissue specimens (humans, nonhumans), soaking protocols, UV-A devices, biomechanical testing, etc., make comparison between different studies challenging. Further work is needed to determine the exact relationship between different UV-A irradiation protocols, riboflavin consumption, and biomechanical strengthening of the human corneal stroma. This knowledge would enhance our understanding of the arrangement and interactions between stromal components, which have been shown to be highly correlated to corneal depth. In previous work,
32 the depth-dependent change of stromal mechanical elasticity from anteriorly high to posteriorly low has been correlated with a gradual change of structural collagen features within the stroma. Since the riboflavin consumption was homogeneous throughout stromal depth, we may expect the stromal stiffening gradient not to be greatly changed after corneal cross-linking; however, there will be an overall increase of the stromal mechanical elasticity, as found in previous inflation testing of human cadaver eyes.
33