Biomechanical corneal response measurement after manual insertion of intrastromal rings in patients with keratoconus

PURPOSE: To evaluate the changes in corneal hysteresis before and after the insertion of intrastromal rings in keratoconic eyes. SETTING: Fundacion Oftalmologica del Mediterraneo. Cornea and Refractive Surgery Unit, FOM, Valencia, Spain. MATERIAL AND METHODS: We studied 27 eyes with keratoconus: seven of them were stage II, fourteen stage III, and six stage IV, according to the Amsler-Krumeich classification. Two Keraring® rings (Mediphacos) were implanted in four eyes, while only one ring was implanted in the rest of the eyes. The following was analysed: visual acuity with and without correction, keratometry, pachymetry, and the corneal biomechanical parameters determined by the Ocular Response Analyzer Reichert (ORA), among which were: corneal compensated intraocular pressure (IOPcc), intraocular pressure correlated with Goldmann (IOPg), overall corneal resistance factor (CRF), and corneal hysteresis (CH). The determinations were performed before surgery and three months after intrastromal ring implantation. RESULTS: Mean visual acuity improved from 0.41 ± 0.21 preoperatively to 0.58 ± 0.18 three months postoperatively (p< 0.01). The mean keratometric values decreased. At three months of follow-up, the K1 value changed from 48.55 ± 4.58 preoperatively to 45.69 ± 4.42 (p< 0.01). Likewise, the K2 went from 54.07 ± 5.91 to 51.01 ± 5.64 (p< 0.01). The difference between spherical, cylindrical, and axial refraction values, preoperatively and postoperatively was not statistically significant (p>0.01). Changes in central pachymetry of the cornea prior to and post implantation of the rings were not statistically significant either (p>0.01). Comparing the preoperative and postoperative values obtained by the ORA, the OIPcc, the IOPg, the CRF and the CH values were higher, but not statistically significant (p>0.01). CONCLUSIONS: The ORA performs determinations at the centre of the cornea, not at the periphery where the rings are inserted, therefore it does not appear to be able to predict the biomechanical changes in this area. By knowing the differences in IOPcc, IOPg, CRF, and CH with regard to the depth of the insertion of the ring, we can predict the postoperative IOPcc, IOPg, CRF, and CH values.