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					  Evaluation of Zonal
Equivalent Keratometry
 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
           Financial Disclosure
► Department   Of Ophthalmology
  University of Texas Southwestern Medical Center
  Dallas, TX.
► 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.
                   Introduction
► 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.
                Introduction
► 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.
               Purpose
  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.
                             Methods
►   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
    (HIPAA).
►   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.
                               Methods
   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.
                Methods
            analyses were performed using
► Statistical
  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
                       Results
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
                                 Results
Comparison of K values
Measurement                        Pre-op         Post-op       P*      P†
EKR
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     -       -
*Students t-test
† Bonferroni Multiple Comparison Test
                                       Discussion
   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.
               Discussion
 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
  variability

				
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