Optical and Transition Zone Volume Components in Hyperopic Corneal Ablation and Hyperopic KLEx Designs

To develop an analytic framework that decomposes stromal tissue volume in hyperopic corneal refractive surgery into optical zone (OZ) and transition zone (TZ) components, and to quantify the influence of TZ geometry and minimum central thickness in hyperopic keratorefractive lenticule extraction (KLEx).

Corneal profiles were modeled as conic sections; hyperopic curvature change was related to dioptric correction by a paraxial expression linking R₁ and R₂. The OZ volume was the volume of revolution of the sagittal difference between baseline and target surfaces. The TZ was parameterized by an analytic blending function defined over [a,b] (with a = S/2 and b = (S+T)/2) and exponent m, yielding a closed-form V_TZ. Hyperopic KLEx designs were represented by a central thickness offset (CTO, p) added within the OZ.

For baseline geometry (S = 6 mm, T = 1.5 mm, m = 2), V_TZ/V_OZ was 0.60 and remained essentially constant across D = +1.00 to +6.00 D. Across six representative combinations of OZ diameter and TZ width evaluated at D = +1.00 D, V_TZ/V_OZ ranged from 0.17 to 1.56, corresponding to 14% to 61% of total modeled volume. A 30-µm CTO increased total volume by 1,092 nL, 79% at +1.00 D, and 38% at +6.00 D.

Across the representative diameter combinations evaluated, the TZ accounted for 14% to 61% of total modeled volume. For fixed geometry, the TZ-to-OZ volume ratio changed little with correction magnitude. A non-zero minimum central thickness added a fixed-volume increment whose relative contribution was greatest at low corrections.