Wavefront Sensing of the Human Eye Jim Schwiegerling, PhD & Jochen Straub, PhD

Introduction Want to examine the effects of Conventional Refractive Surgery (LASIK) on the aberrations of the eye. Need to separate the effects of surgery from the effects of healing. Measured wavefront error, visual acuity, contrast sensitivity and pupil size prior to and at multiple time points during a one year time frame following surgery.

Lasik

Design of the study 89 LASIK patients Visit Eyes Pre-op 158 1 week post-op 154 1 month post-op 145 3 months post-op 131 6 months post-op 105 12 months post-op 132 89 LASIK patients Measurements pre-op, 1 week, 1 month, 3 months, 6 months, and 12 months post-op Manifest Refraction Corneal Topography Aberrations Pupil Diameter Contrast Sensitivity

Manifest Refraction No statistically significant change in average manifest refraction is seen during the healing period between the 1 week and any following exam.

Scanning Slit Technique uses white light scatter from cornea and crystalline lens. By refracting through each surface, this technique can measure anterior and posterior corneal shape, as well as anterior crystalline lens shape.

Corneal Topography

Shack-Hartmann Wavefront Sensor Plane Wavefronts Eye Lenslet Array Aberrated Wavefronts Eye Lenslet Array Perfect wavefronts give a uniform grid of points, whereas aberrated wavefronts distort the grid pattern.

Example Images No Refractive Surgery Post-LASIK with VISX Star S2 Low Aberrations High Aberrations

Wavefront Reconstruction PSF

Aberrations The change in aberrations due to surgery is statistically significant for pupil diameters of 5.0 mm and larger. The change in aberrations during healing at 6 months is significant.

Pupil Diameter A statistically significant increase in dark-adapted pupil diameter is measured 1 week after surgery. The pupil diameter returns to its pre-op size after 6 months.

Aberrations & Contrast Sensitivity m=98% m=85% m=70% m=55% m=40% m=25% m=10%

Contrast Sensitivity Contrast Sensitivity Pupil Diameter Aberrations 1 week Decreases Increases Increase 6 months Increases to pre-op Decreases to pre-op 12 months Decreases slightly No change

Discussion The posterior surface of the cornea changes shape as a result of the flap incision and laser ablation, most likely due to a weakening of the mechanicla strength of the cornea. Conventional LASIK reduces refractive error (defocus and astigmatism), but introduces higher order aberrations (mainly spherical aberration). Pupil size is enlarged by surgery and slowly returns to normal during the healing phase. Decreases in visual performance cannot be solely attributed to larger pupil size. Contrast sensitivity is decreased by the higher order aberrations.