Quantification of Gaze-Dependent Corrective Errors With Fresnel Prisms: Implications for Strabismus Management.

PURPOSE: Fresnel prisms are widely used to manage strabismus. Although prism power is typically selected based on the deviation in the primary gaze position, actual deflection varies with the gaze direction because the angle of incidence changes. This study quantified gaze-dependent corrective errors in the secondary and tertiary positions.

METHODS

Fresnel prisms (10Δ-40Δ) were mounted on a two-axis rotation stage. Deflection was measured by projecting a laser spot onto a tangent screen 100 cm away while rotating the prisms horizontally and/or vertically (±30°). Two configurations were tested: eyeward prism serration (EPS) and outward prism serration (OPS). Geometric modeling was used to predict the deflection of a prism attached to a spectacle lens, and corrective errors across the gaze field were visualized as contour maps using Kriging interpolation.

RESULTS

Corrective errors varied with prism power, gaze position, and application method (EPS vs. OPS; factorial ANOVA, P < 0.05). Prisms > 20Δ exhibited substantial, spatially complex errors. In the EPS configuration, horizontal gaze shifts toward the apex caused rapidly increasing overcorrection, whereas shifts toward the base led to undercorrection. In the OPS configuration, this gradient was reversed on the horizontal axis (P < 0.0001). Vertical gaze shifts not only amplified the horizontal corrective effects but also created unintended vertical components (perpendicular to the principal section).

CONCLUSIONS

Fresnel prisms introduce nonlinear, gaze-dependent corrective errors despite the full correction of the strabismic deviation in the primary position. Recognition of these optical properties may help optimize therapeutic outcomes and enhance diagnostic accuracy.