Vectorial astigmatic changes after corneal collagen crosslinking in keratoconic corneas previously treated with intracorneal ring segments: a preliminary study

PURPOSE. To analyze by means of a vectorial method the corneal astigmatic changes occurring after crosslinking (CXL) in keratoconic eyes and to determine the relationship between these changes and the final clinical outcome. METHODS. A total of 16 eyes of 12 patients with the diagnosis of mild to moderate keratoconus and that underwent CXL surgery were included. All surgeries were performed using the standard technique with epithelial debridement. Visual, refractive, keratometric, corneal aberrometric, and corneal biomechanical changes were evaluated during a 24-month follow-up. Additionally, corneal astigmatic changes were analyzed using the Alpins vectorial method: targeted induced astigmatism (TIA), surgically induced astigmatism (SIA), difference vector (DV), magnitude of error (ME), flattening effect (FE), and torque (TRQ). RESULTS. No significant changes in refraction, central keratometry, and corneal higher order aberrations were detected during the follow-up (p >= 0.07). Corneal resistance factor increased at 6 months (p=0.05), but it decreased at 24 months (p=0.01). Postoperatively, the magnitude of SIA vector was significantly lower than the TIA (p <= 0.04). Mean magnitude of DV was 3.33 +/- 2.53 D at 24 months postoperatively. Mean ME remained negative during the follow-up (p >= 0.26). Mean magnitude of FE was also significantly lower than TIA at all postoperative visits (p <= 0.02). Mean magnitude of TRQ vector was 1.31 +/- 1.41 D at 24 months. Significant negative correlations were found between corneal astigmatism preoperatively and postoperative ME and DV at all postoperative visits. CONCLUSIONS. Crosslinking is able to induce a corneal astigmatic change, but it is variable, not predictable, and insufficient to provide an effective astigmatic correction.