To compare the characteristics of corneal stromal demarcation line after different surgical methods of riboflavin/ultraviolet A corneal collagen cross-linking (CXL) in early keratoconus, and analyze the influence of the demarcation line on the cross-linking effect.

A non-randomized controlled clinical study was conducted.Sixty-nine eyes of 69 patients treated with riboflavin/ultraviolet A CXL in the Eye Hospital of Shandong First Medical University from May 2019 to February 2021 were included.According to the cross-linking methods, the patients were divided into epithelium-on treatment group (21 eyes) and epithelium-off treatment group (48 eyes). There were 25 eyes in 5.4 J energy group and 44 eyes in 7.2 J energy group.The morphology and changes of corneal stromal cross-linking reaction (corneal stromal demarcation line) were observed at 2 weeks, 1, 3 and 4 months after operation.Changes in the thinnest corneal thickness (TCT), uncorrected visual acuity (UCVA, LogMAR), best corrected visual acuity (BCVA, LogMAR) and corneal maximum curvature (Kmax) were recorded.This study adhered to the Declaration of Helsinki.The study protocol was approved by the Ethics Committee of Eye Hospital of Shandong First Medical University (No.2019.05). Written informed consent was obtained from each subject.

Of the 69 eyes after operation, 44 eyes (63.77%) had demarcation lines, and 25 eyes (36.23%) had no demarcation lines.The occurrence rate of demarcation lines in the epithelium-on treatment group was 79.17%(38/48), which was significantly higher than 28.57%(6/21) in the epithelium-off treatment group (χ²=16.186, P<0.01). The occurrence rate of demarcation line in 5.4 J energy group was 72.00%(18/25), and the 7.2 J energy group was 56.80%(25/44), with no significant difference (χ²=1.565, P=0.302). Slit lamp microscopy and anterior segment-optical coherence tomography showed that the demarcation line appeared at 1-2 weeks after operation, gradually converged and strengthened after 1 month, turned diffuse, blurred and faded by degrees after 2-3 months, and basically disappeared after 4 months.The depth of the demarcation line reached 141-423 μm, with an average depth of (263.44±84.22)μm.Scanning laser confocal microscopy showed that corneal stromal cells were activated and light reflection was enhanced after CXL.Collagen fibers extended vertically and horizontally, crisscrossed, and were in a reticular arrangement.The TCT decreased from preoperative (458.69±38.28)μm to (443.86±36.54)μm at 4 months after operation, showing a statistically significant difference (t=6.705, P<0.001). There was no significant difference in the TCT reduction between groups with and without demarcation lines (t=1.684, P=0.100). At 4 months postoperatively, the UCVA of all eyes increased from preoperative 0.74±0.37 to 0.69±0.38, and the difference was statistically significant (t=2.109, P=0.039). There was no significant difference in BCVA between before and after operation (t=1.006, P=0.319). There was no significant difference in change of UCVA and BCVA between groups with and without demarcation lines (t=0.065, P=0.949; t=0.346, P=0.730). There was no significant difference in Kmax in all patients between before and after operation (t=0.050, P=0.950). There was no significant difference in the Kmax change between groups with and without demarcation lines (t=-0.739, P=0.464). The change in TCT in the epithelium-off treatment group was significantly greater than that in the epithelium-on treatment group (t=2.815, P=0.008). There was no significant difference in UCVA, BCVA and Kmax changes between epithelium-on and epithelium-off treatment groups (all at P>0.05). There was no obvious corneal scarring, infectious keratitis, corneal endothelial decompensation or other complications.

The demarcation line after CXL may be a sign of the depth of cross-linking reaction, which is more prone to occur after the epithelium-off operation method.Both the epithelium-on and epithelium-off operation methods have similar therapeutic effects.Demarcation line after different cross-linking methods has no significant influence on the cross-linking effect in keratoconus.

Keratoconus; Riboflavin/ultraviolet A corneal collagen cross-linking; Demarcation lines; Scanning laser confocal microscope; Collagen fiber