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

• Measurements pre-op, 1 week, 1 month, 3 months, 6 months, and 12 months post-op

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

• Manifest Refraction

• Corneal Topography

• Aberrations

• Pupil Diameter

• Contrast Sensitivity

Manifest Refraction

Manifest Refraction: Sphere

-7

-6

-5

-4

-3

-2

-1

0

1

-50 0 50 100 150 200 250 300 350 400

Days after surgery

Sph

ere

in d

iopt

ers

Manifest Refraction: Cylinder

-2

-1

0

1

2

3

-50 0 50 100 150 200 250 300 350 400

Days after surgery

Cyl

inde

r in

dio

pter

sNo 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 lightscatter from cornea and crystalline lens.

By refracting through eachsurface, this technique canmeasure anterior and posterior corneal shape, aswell as anterior crystallinelens shape.

Corneal Topography

Radius of Curvature of the Anterior and Posterior Corneal Surface

5

5.5

6

6.5

7

7.5

8

8.5

9

-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13

Months After Surgery

Rad

ius

of C

urva

ture

(m

m)

Anterior

Posterior

Shack-Hartmann Wavefront Sensor

Plane WavefrontsEye

Lenslet Array

Aberrated Wavefronts

Eye

Lenslet Array

Perfect wavefronts give a uniform grid of points, whereas aberratedwavefronts distort the grid pattern.

Example Images

Low Aberrations High Aberrations

No Refractive Surgery Post-LASIK with VISX Star S2

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.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

3.5 4 4.5 5 5.5 6 6.5 7 7.5

Pupil Diameter (mm)

Abe

rrat

ions

(m

icro

ns)

pre-op

1 week post-op

6 months post-op

12 months post-op

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.

4.5

5

5.5

6

6.5

7

7.5

pre-op 1 weekpost-op

1 monthpost-op

3 monthspost-op

6 monthspost-op

12monthspost-op

Pup

il D

iam

eter

(m

m)

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

Decreases to pre-op

12 months Decreases slightly No change Increase

Changes in Contrast Sensitivity

-0.1

0

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13

Months After Surgery

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.