Zernike polynomial fitting fails to represent all visually significant corneal aberrations

Area of Science:

• Ophthalmology
• Optometry
• Biomedical Optics

Background:

• Aberrometry data is often modeled using Zernike polynomials.
• This assumption's validity in clinical settings, particularly for complex corneal shapes, is not fully established.
• Inaccurate wavefront error representation can affect clinical testing and refractive corrections.

Purpose of the Study:

• To evaluate the correlation between visual acuity and corneal wavefront error versus residual surface elevation error after Zernike polynomial fitting.
• To test the adequacy of Zernike polynomials in describing corneal aberrations in pathologic and post-surgical eyes.

Main Methods:

• Corneal topography data from 88 eyes (keratoconus, penetrating keratoplasty, conductive keratoplasty) were analyzed.
• Wavefront error and elevation fit error were calculated for a 4-mm zone, using 4th and 10th order Zernike expansions (excluding piston, tip, tilt).
• Linear regression correlated best spectacle-corrected visual acuity (BSCVA) with error metrics; RMS error differences were assessed.

Main Results:

• BSCVA showed moderate correlation with 4th and 10th order wavefront error (R² ≈ 0.57).
• BSCVA showed strong correlation with 4th and 10th order elevation fit error (R² ≈ 0.51-0.66).
• Increasing Zernike order unpredictably altered wavefront error but improved elevation fit.

Conclusions:

• Zernike polynomials incompletely characterize corneal shape features influencing vision, especially in abnormal or post-surgical corneas.
• Corneal elevation fit error is a significant correlate of visual acuity, suggesting limitations of Zernike-based wavefront analysis.
• Current Zernike modeling may confound clinical diagnosis and treatment due to incomplete data representation.