Purpose: To analyze corneal astigmatic changes after implantation of intrastromal corneal ring segments (ICRS) in keratoconic eyes using the Alpins vectorial method. Setting: Vissum Corporation, Alicante, Spain. Methods: Keraring ICRS were implanted in eyes with a diagnosis of keratoconus. One of 3 surgeons performed the ICRS implantations using femtosecond technology and following the same protocol. Visual, refractive, keratometric, and corneal aberrometric changes were evaluated during a 12-month follow-up. Corneal astigmatic changes were also analyzed using the following Alpins vectorial components: targeted induced astigmatism (TIA), surgically induced astigmatism (SIA), difference vector, magnitude of error, flattening effect, and torque. Results: Postoperatively, there was significant visual improvement (P = .03), significant central flattening (P = .03), and a significant reduction in manifest astigmatism (P<.01). The magnitude of SIA vector was significantly lower than the TIA postoperatively (P≥.02). The mean magnitude of the difference vector 3 months postoperatively was +2.96 diopters (D) ± 1.68 (SD). The mean magnitude of error remained negative and unchanged (P≥.10). The mean magnitude of the flattening effect was significantly lower than the TIA at all postoperative visits (P≤.01). The mean magnitude of torque vector was 1.21 ± 0.98 D at 3 months. Significant negative correlations were found between preoperative corneal astigmatism and the magnitude of error and difference vector at all postoperative visits. Conclusion: Although ICRS implantation significantly reduced the magnitude of corneal astigmatism, there was a trend toward undercorrection and the meridian of correction was not appropriate in all cases, showing the need for nomogram adjustments. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. © 2010 ASCRS and ESCRS.