Comparison of IVCM and AS-OCT in evaluating cross-linking depth after corneal collagen cross-linking

Objective  To evaluate the corneal stromal demarcation line depth after corneal collagen cross-linking (CXL) using in vivo confocal microscopy (IVCM) and anterior segment optical coherence tomography (AS-OCT) under different ultraviolet irradiation energies.

Methods  A cross-sectional study was conducted. Twenty-two progressive keratoconus patients (28 eyes) who underwent CXL, and 26 patients (48 eyes) with refractive errors who underwent femtosecond laser-assisted in situ keratomileusis combined with accelerated corneal cross-linking (FS-LASIK Xtra) at Xi’an No. 1 Hospital between January and December 2024 were included. According to the ultraviolet energy applied during CXL, eyes were divided into a high-energy group (7.2 J/cm ²) and a low-energy group (1.8 J/cm ²). At 3 months postoperatively, the demarcation line depth was assessed using IVCM and AS-OCT. The study adhered to the tenets of the Declaration of Helsinki and was approved by the Ethics Committee of Xi’an NO.1 Hospital (No. 2025-049). Written informed consent was obtained from all patients prior to surgery.

Results  In the low-energy group, the visibility rate of the demarcation line was 95.8% (46/48) with IVCM and 81.3% (39/48) with AS-OCT, with no statistically significant difference ( χ ²=3.273, P=0.065). In the high-energy group, the visibility rate of the demarcation line was 100% (28/28) with IVCM, which was significantly higher than 67.9% (19/28) with AS-OCT ( χ ²=7.111, P=0.004). The demarcation line depth measured by IVCM and AS-OCT in the low-energy group was 160.0 (140.0, 200.0)μm and 188.0 (143.0, 232.0)μm, respectively, with no significant difference ( P>0.05). The demarcation line depth measured by IVCM and AS-OCT in the high-energy group was 370.0 (322.5, 407.5)μm and 336.0 (297.0, 381.0)μm, respectively, with no significant difference ( P>0.05). The demarcation line depths measured by both IVCM and AS-OCT were significantly greater in the high-energy group than in the low-energy group (both P<0.001). Bland-Altman analysis demonstrated good agreement between IVCM and AS-OCT, with a mean difference of 9.43 μm and the 95% limits of agreement ranging from -67.84 to 86.70 μm, and 96.4%(54/56) of the measurement points fell within this range.

Conclusions  IVCM and AS-OCT provide comparable measurements of the demarcation line depth after CXL, with good agreement between the two modalities. However, IVCM demonstrates superior visibility of the demarcation line compared with AS-OCT.