Evaluation of Zonal
Readings After LASIK
Timmy Kovoor, MD
Orkun Muftuoglu, MD
V.Vinod Mootha, MD
Steven Verity, MD
R. Wayne Bowman, MD
H. Dwight Cavanagh, MD, PhD
James P. McCulley, MD
► Department Of Ophthalmology
University of Texas Southwestern Medical Center
► The authors have no financial interests in any of
the products or topics mentioned.
► Acknowledgements: Supported in part by an
unrestricted research grant from Research to
Prevent Blindness, Inc., New York, New York.
► Laser in situ keratomileusis (LASIK), photorefractive
keratectomy (PRK), and laser-assisted subepithelial
keratectomy (LASEK) correct myopia by decreasing the
anterior corneal surface curvature.
► The conventional keratometric index of refraction (usually
1.3375) used by most topographers and keratometers to
convert the measured radius into diopters is invalid
because the natural ratio between the anterior and
posterior corneal curvatures no longer exists.
► These instruments thus cannot correctly calculate the
corneal power and usually give a measurement that is
higher than the actual value.
► Overestimating corneal power leads to postoperative
hyperopia for eyes that will have cataract surgery.
► Scheimpflug camera imaging (Pentacam, Oculus,
Wetzlar, Germany) evaluates the anterior and
posterior corneal surfaces.
► It measures the true net corneal power, estimated
ketometric readings, posterior corneal radius,
anterior corneal radius, and corneal thickness.
► The computer software uses the correct indices of
refraction to calculate the total corneal power.
► This measurement of the total corneal power is
called the true net power (true net K) which is
different from the corneal vertex power measured
by manual, automated, or simulated keratometry.
To compare pre & post myopic LASIK
keratometric measurements performed with
Scheimpflug camera imaging with the
values obtained using the clinical history
method and simulated keratometry.
► All procedures were performed at The Laser Vision Correction Center,
University of Texas Southwestern Medical Center at Dallas.
► The study was performed with the approval of the University of Texas
Southwestern Medical Center Institutional Review Board and in
accordance with the Declaration of Helsinki guidelines for human
research and the Health Insurance Portability and Accountability Act
► The Intralase femtosecond laser (AMO Inc, Irvine, CA) was used to
create the flap in all eyes that underwent LASIK. Femtosecond laser
flaps were programmed with the following settings: 120 µm thickness,
9.0 mm diameter, with a 60 KHz repetition rate.
► All eyes underwent wavefront-guided LASIK with VISX S4 CustomVue
with iris registration, (VISX Inc., Santa Ana, CA) for the correction of
myopia or myopic astigmatism.
36 eyes of 19 patients were included in the study
The minimum required follow-up was 6 months after LASIK
Each eye was evaluated by videokeratography (TMS,Tomey, Phoenix, AZ) and
Scheimpflug camera imaging (Pentacam, Wetzlar, Germany).
The surgically induce refractive correction and corneal power were calculated
according to the clinical history method.
To avoid miscalculations due to poor videokeratography quality, both
preoperative and postoperative examinations were performed immediately
after blinking and were carefully inspected before being included in the study.
The following values were analyzed and compared with those obtained with
the clinical history method: mean simulated keratometry (K), mean true net
power (ie, corneal power calculated with the Gaussian optics formula using the
anterior and posterior corneal radii and the corneal thickness), and equivalent
K reading (shown in the Holladay report.)
For each eye, only one good-quality Scheimpflug image (determined when the
quality specification provided by the instrument was ‘‘OK’’) was used.
analyses were performed using
SPSS (SPSS Inc. Chicago, IL). A
► One-way analysis of variance (ANOVA) for
repeated measures with Bonferroni multiple
comparisons were used to compare all
corneal power measurements.
► Preliminary analysis showed that all
assumptions required by the ANOVA were
assessed by Kolmogorov-Smirnov test
The mean age of the patients was 41.7 ± 9.5 years.
Pre- and postoperative refraction and curvature
Measurement Pre-op Post-op P*
SE (D) -4.76 ± 2.21 -0.63 ± 1.47 <0.01
S (D) -5.36 ± 2.12 -0.88 ± 1.49 <0.01
C (D) 1.20 ± 0.9 0.57 ± 0.37 <0.01
R ant 7.69 ± 0.29 8.41 ± 0.55 <0.01
R post 6.39 ± 0.29 6.36 ± 0.28 <0.01
Comparison of K values
Measurement Pre-op Post-op P* P†
1.0 mm EKR (D) 43.78 ± 1.70 39.70 ± 2.65 <0.01 0.39
2.0 mm EKR (D) 43.82 ± 1.68 39.78 ± 2.66 <0.01 0.62
3.0 mm EKR (D) 43.92 ± 1.67 39.63 ± 3.62 <0.01 0.67
4.0 mm EKR (D) 44.09 ± 1.67 40.18 ± 2.57 <0.01 <0.05
4.5 mm EKR (D) 44.19 ± 1.68 40.36 ± 2.54 <0.01 <0.01
5.0 mm EKR (D) 44.31 ± 1.69 40.60 ± 2.51 <0.01 <0.01
6.0 mm EKR (D) 44.58 ± 1.73 41.32 ± 2.50 <0.01 <0.01
True Net Power (D) 42.75 ± 1.62 38.57 ± 2.64 <0.01 <0.01
Sim K (TMS) (D) 44.02 ± 1.16 40.11 ± 1.91 <0.01 <0.05
Clinical Historical Method (D) - 39.68 ± 1.86 - -
† Bonferroni Multiple Comparison Test
The mean simulated K given by TMS was higher than the mean clinical
history method value.
This is consistent with the fact that the simulated K is calculated using
the standard keratometric index (1.3375), which is known to
overestimate corneal power after refractive surgery.
Our results agree with those of Savini et al1.
The true net power in our sample was significantly lower than the
value obtained with the clinical history method.
This discrepancy is likely the result of the different refractive indices
used by the 2 methods; the clinical history method is still based on the
conventional value of 1.3375, whereas the Gaussian optics formula
adopts the true refractive indices of air (1.0), the cornea (1.376), and
aqueous humor (1.336).
Savini et al. Corneal power measurements with the Pentacam Scheimpflug camera after myopic excimer laser
surgery. J Cataract Refract Surg. 2008 May;34(5):809-13.
The mean equivalent K readings at 1.0 mm,
2.0 mm, and 3.0 mm were not statistically
significantly different from the values
derived with the clinical history method.
The 3.0 mm reading was the closest to the
benchmark value, however with high