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000234_Mohamed_Awadalla_Poster_ASCRS_2008

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					Comparison of bitoric with monotoric laser in situ
keratomileusis for the correction of myopic
astigmatism with the Nidek EC-5000 Laser.


                                    By

                     Mohamed Abdul-Rahman Awadalla,FRCS
                             Magrabi Eye Hospital
                                   Egypt


                                                     1
         Aim
(1) Evaluate the effectiveness, preditability & safety of Bitoric laser ablation.
(2) Compare with that of Monotoric laser ablation


   Introduction:
Nidek EC 5000 is a LASIK machine using the scanning slit technique So :
When the Excimer laser uses the negative cylinder:
Central ablation along the steepest meridian will flatten the steepest meridian but also
will induce some flattening in the flattest meridian ((Coupling effect)) which will induce
a positive sphere which has to be compensated by spherical hyperopic ablation
While when the Excimer laser uses the positive cylinder:
laser will steepen the flattest meridian with no significant effect on steepest meridian
because ablation is not performed in the central area

The princible of the Bitoric ablation profile
is to steepen the flat meridian and to flatten the steep meridian by equal amounts
                                                                                   2
which produce a spherical corneal profile then any residual spherical error is treated
     Methods
Retrospective study included a comparative analysis
of 230 eyes of 135 patients with myopic astigmatism who underwent LASIK
using the Nidek EC 5000 excimer laser and the Moria M2 microkeratome.
    With the Bitoric nomogram ( 105 eyes of 65 patients)
    and the monotoric nomogram ( 125 eyes of 70 patients)

Preoperative evaluation:                      Inclusion criteria:
UCVA, BCVA,                                   older than 18
manifest and cycloplejic refraction,          congenital astigmatism (-1.0 till -6.0 )
slit lamp exam, fundus exam,                  stable refraction
applanation tonometry,
pachymetry and corneal topography             Exclusion criteria:
                                              BCVA worse than 20/70
                                              pupil bigger than 6 mm in dim light
Postoperative evaluation:                     evidence of developing cataract
UCVA, BCVA,                                   history of uveitis
manifest and cycloplejic refraction,          corneal dystrophy, glaucoma ,
 slit lamp exam,                              retinal disease or optic nerve pathology
corneal topography and total ablation depth                                      3
                                              connective tissue disease
   Nomogram used
   ( Modified Gimbel nomogram )                                    Example:
                                                                   -3.0 / - 4.0 X 180
Calculation determined the laser parameters were
1) Calculate spherical equivalent                                  S.E = - 5.0
2) determine the PTK effect of the total astigmatism treatment     PTK effect =
    ( Total cylinder X 35% ) this produce the hyperopic shift         - 4 X 35% = -1.4
in refraction there for it is added to the sphere
3) apply spherical treatment adjustment                            Spherical treatment =
    the spherical component of the refractive correction is          - 4 – (-1.4) = -2.6
determined by:
    a) the spherical equivalent
    b) PTK effect (hyperopic shift) of the cylindrical treatment
4) divide the astigmatism by 2 and write hyperopic (plus) and      Astigmatism
        myopic (minus) components separately                       - 2.0 X 180 / +2.0 X 90
5) Write laser treatment stages                                    Laser treatment stages
      a) Hyperopic cylinder with 5.5 - 9 mm zone                   + 2.0 X 90
      b) myopic cylinder with 6.5 - 7.5 mm zone                    - 2.0 X 180
      c) nomogram adjusted spherical refractive error              - 2.6
6) for smoothing 3microns PTK are placed in 8 mm zone (6           PTK 3 microns

                                                                                        4
     Results:
The Mean age 27.46 years +/- 6.3 (S.D) range 21-49 years
Preoperative refraction was -0.50 to -10.0 D of sphere with :
              astigmatism of -0.75 to -2.0 D for monotoric ablation profile
              astigmatism of -2.25 to -6.0 D for Bitoric ablation profile
The mean preop.spherical equivalent (SE) was -1.5 +/- 0.7 range (-3.9 to + 0.50 D )
Follow up was 6 months in all patients
Visual Acuity (       6 months after LASIK )
     The mean UCVA was 0.7 +/- 0.23 (range 0.3-1.0)
               was 20/40 or better in 120 eyes ( 88.3%) & 20/20 in 48 eyes (35.6%) in Monotoric profile
             was 20/40 or better in 101 eyes ( 92.6%) & 20/20 in 21 eyes (19.9%) in Bitoric profile
     The   mean BCVA before LASIK was 0.71 +/- 0.19
                         after LASIK was 0.83 +/- 0.15
                        BCVA 20/40 or better was in 345 eyes ( 100%)
               in Monotoric profile: 7 eyes (5.1%) lost 1 Snellen line of BCVA,
              13 eyes (10%) gained 1 line,2 eyes (1.5%) gained 2 lines,0 eyes (0%) gained 3 lines
              In Bitoric profile:     4 eyes (3.6%) lost 1 Snellen line of BCVA,
              25 eyes (22.9%) gained 1 line,7 eyes (6.5%) gained 2 lines,2 eyes (2%) gained 3 lines
l

Retreatment for a significant residual refractive defect:
,
            24 eyes (17.1 %) needed after Monotoric LASIK
            16 eyes (14.6 %) needed after Bitoric LASIK                                               5
 Conclusion:
               Bitoric ablation for astigmatism appear to be
             safer, more effective , more tissue sparing and
             resulted in a decreased frequency of reablation
             than the standard treatment
Why?
* Optically leads to a nearly spherical cornea as it ablates a cylindrical profile in the steeper meridian to
flatten it and ablates midperipherally in the flat meridian to steepen it (unlike ablation in a single
meridian which results in loss of physiological surface profile)

* Reduces the effective optical zone and the edge profile by treating half the cylinder in the steep
meridian and the other half in the flat meridian which creates a smooth transition between the treated and
untreated cornea

* Needs less tissue removal for the same refractive defect by balancing the negative and the positive
ablation

in turn this has the effect of treating high astigmatic errors predictably with a more stable result and
with less haze and regression.                                                                        6
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