Multivariate model of refractive shift in Descemet-stripping automated endothelial keratoplasty

Abstract

Purpose: To relate in situ graft shape in Descemet-stripping automated endothelial keratoplasty (DSAEK) to surgically induced refractive error.

Setting: Academic eye institute.

Methods: High frequency arc-scanning ultrasound was performed in 7 patients enrolled in a prospective study of microkeratome-assisted endothelial keratoplasty approved by the Investigative Review Board. A region of interest spanning the horizontal meridian was defined for analysis of epithelial, host, graft, and total corneal thicknesses. Graft thickness profiles were fit by quadratic polynomials where the 2nd-order coefficients represent the posterior corneal curvature contributed by the graft. The curvature coefficient and central graft thickness were analyzed as predictors of induced refractive error.

Results: At final follow-up (mean 5.9 months +/- 3.2 [SD]), 3 patients had a hyperopic shift (+2.50 diopters [D] each), 3 had insignificant (< 0.50 D) refractive shifts, and 1 had a myopic shift. In the group with hyperopic shift, a negative lens effect was predicted by positive curvature coefficients, representing grafts that were thinner centrally than peripherally (mean +22.72 microm/mm(2); range +4.95 to +45.17 microm/mm(2)). In the group with minimal refractive shift, coefficients were less positive (mean +7.28 microm/mm(2); range +2.01 to +13.82 microm/mm(2)). The patient with a myopic shift (-1.00 D) had the only negative curvature coefficient (-0.64 microm/mm(2)). In a 2-predictor model of refractive shift, central graft thickness and the curvature coefficient together accounted for 86% of the variance in the refractive response to DSAEK (P = .025).

Conclusion: Nonuniform thickness profiles and variable central graft thicknesses both contribute to refractive shift after DSAEK.

Figure 1

Appearance of host cornea and donor lenticule by (A) slitlamp biomicroscopy and (B) very high frequency arc-scanning US with the Artemis system.

Figure 2

ArtPro software representations of corneal thicknesses obtained by 6-meridian Artemis scans of patient 1 6 months after DSAEK. Unit of color scale at right is μm. At the far periphery of each scan, there is an abrupt increase in thickness in total corneal thickness, epithelial thickness, and donor thickness and a sharp decrease in host thickness, attributed to software artifact in recognizing the lateral termination of the graft. Within the central and paracentral zones, subtle transitions to warmer colors toward the periphery represent increases in thickness. The lower right map shows donor thickness with the 4.0 mm × 2.0 mm region of interest from which horizontal graft profiles were measured.

Figure 3

Correlation between central US pachymetry and total central corneal thickness by Artemis image analysis on same-day examinations.

Figure 4

The DSAEK graft thickness (μm) averaged vertically across a 2.0 mm high rectangular region of interest spanning 4.0 mm of the horizontal meridian in patient 1. The graft is clearly thicker in the periphery. The quadratic regression equation fit to the profile is shown in the figure. The constant (173.7 μm) represents the minimum of the function, and the 1st-order term characterizes the very weak linear portion of the function. (Note the small coefficient magnitude.) The positive 2nd-order polynomial coefficient (+18.03) reflects the parabolic shape of the graft and reflects negative optical power the graft contributes at the posterior corneal surface. This patient had a 2.50 D hyperopic shift.

Figure 5

Correlation of actual DSAEK-induced refractive shift and refractive shift predicted by a 2-predictor model incorporating (1) the 2nd-order coefficient representing graft curvature and (2) central graft thickness (R² = 86%, P = .02).

Figure 6

Schematic illustration of the hyperopic contributions of (A) a nonuniform graft-thickness profile and (B) the absolute thickness of the graft irrespective of uniformity. In both cases, graft morphology results in a smaller posterior radius of curvature after surgery (r_postop < r_preop).