Three-Year Corneal Biomechanical Outcomes Assessed by CORVIS ST Dynamic Scheimpflug Imaging After Epithelium-Off Conventional Versus Accelerated Cross-Linking in Progressive Keratoconus.

PURPOSE: To compare long-term corneal biomechanical and tomographic responses using dynamic Scheimpflug analysis, three years after epithelium-off (epi-off) conventional corneal collagen cross-linking (C-CXL; Dresden protocol) versus epi-off accelerated cross-linking (A-CXL) in patients with progressive keratoconus.

METHODS

Prospective comparative study. 81 patients were assessed; treatment was performed in 40, including 23 bilateral keratoconus treatments and 17 unilateral treatments, resulting in 63 eyes included in the final 36-month analysis cohort. Eyes underwent epi-off CXL with either conventional protocol (3 mW/cm2 for 30 min; 5.4 J/cm2; n=34) or accelerated protocol (9 mW/cm2 for 10 min; 5.4 J/cm2; n=29). Assessments at baseline, 6, 12, 24, and 36 months included Kmax, uncorrected visual acuity (UCVA), corrected distance visual acuity (CDVA), pachymetry, and Corvis ST dynamic corneal response parameters: deformation amplitude (DA), applanation times (A1T, A2T), peak distance (PD), highest concavity (HC) radius, stiffness parameter at first applanation (SP-A1), and BAD-D. Primary endpoint: between-group differences at 36 months and longitudinal change in key biomechanical parameters-secondary endpoints: Kmax, UCVA, and CDVA at each follow-up. Welch's t-test compared groups at each time point; repeated-measures modeling evaluated time, group, and time × group interaction.

RESULTS

Both protocols demonstrated significant longitudinal biomechanical remodeling and clinical stabilization over 36 months (time effect p < 0.001 for DA, SP-A1, A1T, A2T, PD, and HC radius). At 36 months, between-group differences were not statistically significant for Kmax, UCVA, CDVA, pachymetry, DA, A1T, A2T, PD, HC radius, SP-A1, or BAD-D (all p > 0.05). Longitudinal modeling demonstrated a significant time × group interaction for SP-A1 (p=0.042), indicating a modest protocol-dependent divergence in stiffness trajectory over time, while DA, A1T/A2T, PD, and HC radius showed parallel improvements without significant interaction. Pachymetry showed early thinning (6 months) followed by remodeling and stability by 36 months. No sight-threatening complications occurred.

DISCUSSIONS: Both epi-off conventional and accelerated CXL produced sustained biomechanical remodeling and clinical stabilization over 36 months. Although final between-group differences were not statistically significant for Kmax, visual acuity, pachymetry, deformation amplitude, applanation times, peak distance, HC radius, SP-A1, or BAD-D, repeated-measures analysis identified a significant time × group interaction for SP-A1, suggesting a modest protocol-dependent divergence in the trajectory of corneal stiffening. These findings indicate that matching total fluence does not necessarily produce identical biomechanical time courses and that irradiation duration may still influence the pattern of postoperative strengthening.

CONCLUSIONS

Three years after treatment, both epi-off C-CXL and epi-off A-CXL stabilized progressive keratoconus and produced sustained biomechanical remodeling detectable by dynamic Scheimpflug imaging. Final 36-month outcomes were comparable between protocols in this cohort; however, the stiffness trajectory (SP-A1) differed modestly over time, supporting the concept that irradiation duration may influence the time-course of biomechanical strengthening even when total fluence is matched.