Title: Ocular functional optical zone: analysis based on polychromatic retinal image quality
1Ocular functional optical zone analysis based on
polychromatic retinal image quality
Li Wang, MD, PhD and Douglas D. Koch, MD Cullen
Eye Institute, Baylor College of
Medicine Houston, TX
Dr. Koch consults for AMO-VISX.
2Background
- Traditionally, the functional optical zone (FOZ)
is analyzed based on corneal topography - Better optical quality of the anterior corneal
surface may not optimize retinal image quality of
the eye due to the effect of the internal optics - The purpose of this study was to investigate,
based on wavefront aberrations of the eye, the
size of the ocular FOZ before and after
wavefront-guided PRK for myopic or mixed
astigmatism with the AMO-VISX CustomVue (CV-PRK).
Holladay Janes. JCRS 2002 Boxer Wachler et
al. JCRS 2002 Racine, Wang Koch. AJO 2006
Tabernero et al. IOVS 2007
3Patients and Methods
- Consecutive cases who underwent CV-PRK for myopic
or mixed astigmatism by one surgeon between
1/1/05 to 6/1/06 - Inclusion criteria
- Available follow-up at 6 months post-op
- CustomVue wavefront measurements 6 mm at both
pre-op and 6 months post-op - 72 eyes of 44 subjects included
- Age 36 9 years (19 to 51 years)
- Pre-op CustomVue SE -4.07 1.98 D (-7.86 to
-0.08 D) - Post-op CustomVue SE -0.36 0.50 D (-1.85 to
0.73 D) - Refractive correction 3.71 1.75 D (0.63 to
7.69 D)
4Methods
- Based on CustomVue wavefront measurements, we
calculated ocular wavefront aberrations for
simulated pupils from 26 mm in 0.1 mm intervals
- Assuming full correction of 2nd order
aberrations, optical image quality of
higher-order aberrations (3rd-6th order) was
evaluated - Using ZernikeTool program (AMO-VISX), we
calculated polychromatic modulation transfer
function (MTF) at 15 cyc/deg (20/40 object) with
Stiles-Crawford effect for 2-6 mm pupils - FOZ was defined as the pupil size over which the
MTF at 15 cyc/deg was 0.1. This values was
chosen since only few eyes had MTFlt0.1 before
surgery and more than half of the eyes had
MTFlt0.1 after the surgery
5All eyes preop
All eyes postop
Mean MTF before and after CV-PRK (1 SD bar)
6FOZ (MTFgt0.1) pre-op vs. post-op
Higher percentages of eyes had FOZ 6 mm and 4
mm before CV-PRK than after CV-PRK (Chi-Square
tests, both Plt0.001)
7Pre-op MTF vs. post-op MTFHow many eyes
improved?
Post-op MTF improved for small pupils
(3-5mm) n17 (23.6)
Post-op MTF improved for pupils up to 6mm n5
(6.9)
8Correlation refractive correction (RC) vs.
post-op FOZ
- In eyes with FOZ lt 6 mm, FOZ decreased with
increasing amount of RC (Pearson coefficient r
-0.42, P0.007)
9Limitations
- Results are not correlated with measures of
visual function - Analysis only done up to 6mm zone
- The FOZ was defined by an arbitrary cut off
value, which was not correlated with the visual
acuity. The FOZ defined with different MTF
values would affect the absolute post-op FOZ
size, whereas the trend of the differences in FOZ
induced by the surgery would be consistent.
10Summary
- The percentage of eyes with FOZ 6 mm decreased
following CV-PRK - Compared with pre-op, image quality following
CV-PRK - decreased in 76.4 of eyes
- improved with small pupils (3 to 5 mm) in 23.6
of eyes - improved with pupils up to 6 mm in 6.9 of eyes
- In eyes with FOZ lt 6mm, there was moderate
correlation between FOZ and amount of RC (r
-0.42, P0.007)
11Discussion
- Results differ from study showing enlargement of
corneal FOZ after CustomVue LASIK (and reduction
after standard LASIK) - FOZ in that study was based only on corneal
refractive power, not HOAs - The reduction in our study is less than recently
reported for corneal FOZ following standard
LASIK - 1.70 mm in that study vs. 0.66 mm in our lt 6 mm
subgroup
Racine L, Wang L, Koch DD. AJO 2006
Tabernero J, Klyce SD, Sarver EJ, Artal P. IOVS
2007
12Next steps
- These results may lead to further refinement of
treatment parameters - Further studies investigating optical FOZ with
various procedures are desirable