Common to several earlier CXL studies is the assumption that the occurrence of the stromal demarcation line and its depth act as surrogate parameters for the effect and success of CXL [
2,
6,
7]. To conclude that the depth of the stromal DL directly correlates with the outcome of CXL seems reasonable, given that it represents the transition zone between cross-linked and non-cross-linked tissues. Today, topographical stabilization (
Kmax or
K2.5, respectively) counts as success and an increase in Young’s modulus after CXL can only lead to a stabilization of the cone. Eyes treated with A-CXL(9*10) as used in our trial showed a shallower demarcation line than usually seen in eyes treated with the standard Dresden protocol [
6]. However, eyes with an increase in maximum
K values showed variable depths of the DL which only reinforce the absence of a statistical correlation between DL and changes in
K values (Table
1). To our knowledge, statistical correlations between the DL and changes in parameters such as
Kmax or
K2.5 have only been evaluated to date with a small case series [
8]. In the current trial, we did not find a statistical correlation between DL depth and the changes in
K values. Based on the Bunsen-Roscoe law, which describes the reciprocal relationship between both the total amount of irradiation intensity and that of photochemical reaction, an increase of the radiant energy has been proposed to enable a shorter UV irradiation duration without minimizing the effect of CXL [
9]. Following this hypothesis, accelerated cross-linking protocols with higher irradiation intensities and shorter irradiation times have been introduced [
10]. The equivalence of S-CXL(3*30) and accelerated cross-linking protocols has recently been questioned, and one study showed a superior efficacy of the standard Dresden protocol compared to an accelerated protocol. In the said study, a shallower DL was mentioned in eyes treated according to an A-CXL(9*10), and in fact, an increase in
Kmax was found [
6]. However, according to our results, the central stromal depth of the DL does not correlate with commonly used outcome parameters at all. Doubtless, the depth of the DL and the changes in
K values as found in our study are certainly less distinct than results usually achieved with the standard Dresden protocol, but it seems that unknown factors other than the depth of the DL might influence the outcome of CXL. In our opinion, several parameters could influence the depth of the DL. Newer UV-light sources with a flat beam profile (KXL, Avedro) or “donut-shaped” beam profile (UVX-2000, Innocross) have been developed to obtain an optimized distribution of the energy across the corneal surface [
11]. The depth of the DL could be determined by the beam profile of the UV source used for CXL [
12]. The UV-X 1000 has a Gaussian beam profile and higher UV-A intensities are provided in the center of the beam with a gradual decline of the irradiation intensity towards the periphery [
12]. This could lead to a deeper central demarcation line in patients treated with this device, as shown by Brittingham et al., who used different UV sources with identical soaking time (20 min) and riboflavin solutions [
6].
In addition, new riboflavin solutions with HPMC (hydroxylpropyl methylcellulose) instead of dextran found its way into CXL treatment. Earlier studies mentioned a hypothetical explanation for a shallower DL in rapid protocols in a limited diffusion rate due to a decreased irradiation time during the CXL procedure [
13]. However, according to literature, HPMC significantly increases the penetration of topical fluorescein as an ophthalmic carrier. That may provide the evidence to suggest that HPMC also increases the diffusion rate compared to the standard riboflavin [
14]. Furthermore, a recent study showed comparable riboflavin gradients when comparing 30-min application of riboflavin-dextran and 10-min application of riboflavin-HPMC [
15]. Given that Vibex Rapid ensures the same diffusion rate during 10 min as riboflavin solutions with dextran during 30 min [S-CXL(3*30)], the shallower DL as found in our trial must have another cause. By using iso-osmolar riboflavin solutions with HPMC, corneal thinning can be avoided, an effect commonly observed when using riboflavin solutions with 20% dextran [
16‐
18]. The dehydration and hence corneal thinning induced by riboflavin solutions with 20% dextran as usually used in the standard Dresden protocol most likely explains the deeper demarcation line so commonly observed with this protocol [
5]. Still, as shown in our trial, the depth of the DL does not indicate success of the treatment. There may be evidence that shorter irradiation times negatively influence the polymerization reactions due to insufficient oxygen diffusion [
19]. The Bunsen-Roscoe might not apply to the CXL procedure and a loss of efficacy must be anticipated with higher UV-light intensities [
13]. This could be one explanation why our study showed a worse outcome compared to common results with the S-CXL(3*30). It should be mentioned that one major weakness of this study is its retrospective design which by nature cannot eliminate possible confounders like age, gender, preoperative
K values, and comorbidities like atopic diseases. In conclusion, this study showed that the widely proposed hypothesis of a correlation of the demarcation line depth and CXL outcome does most probably not apply, at least when using the herein described protocol. No correlation was found between the depth of the DL and the change in
K values.