Quantification of the surgically induced refractive effect of intrastromal corneal ring segments in keratoconus with standardized incision site and segment size

PURPOSE: To determine the surgically induced refractive and keratometric effects of intrastromal corneal ring segments (ICRS). SETTING: St. Paul's Eye Unit, Royal Liverpool University Hospital, Liverpool, United Kingdom. DESIGN: Case series. METHODS: Keratoconic patients had implantation of 2 vertically aligned 0.3 mm Intacs ICRS with the incision site at 90 degrees. Principal outcome measures were changes in visual acuity and refractive and keratometric effects (analyzed using methods of Kaye and Harris) 4 months postoperatively. Subgroup analysis was based on the Krumeich classification. RESULTS: Twenty-nine eyes (29 patients) were included. There was a significant difference between the mean preoperative (-5.16 + 1.93 x 162.05) and postoperative (-2.64 + 1.07 x 137.25) refractive errors (P=.01), with a mean refractive surgical effect of +1.35 +1.48 x 88.71 (SD + 2.76 + 4.78 x 149.62), and between the mean preoperative (51.32/2.28 @ 122.33) and postoperative (48.23/2.36 @ 92.10) keratometric powers (P=.001), with a mean keratometric surgical effect of -3.87/2.54 @ 61.77 (SD 1.37/4.20 @ 136). The irregularity index in the 3.0 mm zone decreased from 11.56 diopters (D) to 8.78 D. Refractive and keratometric surgical effects showed little difference within stages I through III and were greater in stage IV. CONCLUSIONS: Placement of two 0.3 mm ICRS caused overall flattening of the cornea (2.00 D to 3.00 D), with more flattening orthogonal to the incision site at 90 degrees. Variability of this effect, principally in the steep and flat meridians, limited the predictability of ICRS. Corneal behavior may differ with other incision sites.