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Comparison of bitoric with monotoric laser in situ keratomileusis for the correction of myopic astigmatism with the Nid PowerPoint Presentation
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Comparison of bitoric with monotoric laser in situ keratomileusis for the correction of myopic astigmatism with the Nid - PowerPoint PPT Presentation


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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.

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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

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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 ablationWhile 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 profileis to steepen the flat meridian and to flatten the steep meridian by equal amounts which produce a spherical corneal profile then any residual spherical error is treated

Aim

(1) Evaluate the effectiveness, preditability & safety of Bitoriclaser ablation.(2) Compare with that of Monotoriclaser ablation

Introduction:

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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:UCVA, BCVA, manifest and cycloplejic refraction, slit lamp exam, fundus exam, applanation tonometry, pachymetry and corneal topography

Postoperative evaluation:UCVA, BCVA, manifest and cycloplejic refraction, slit lamp exam, corneal topography and total ablation depth

Inclusion criteria:older than 18congenital astigmatism (-1.0 till -6.0 ) stable refraction

Exclusion criteria:BCVA worse than 20/70pupil bigger than 6 mm in dim lightevidence of developing cataracthistory of uveitiscorneal dystrophy, glaucoma ,

retinal disease or optic nerve pathologyconnective tissue disease

nomogram used modified gimbel nomogram
Nomogram used( Modified Gimbel nomogram )

Calculation determined the laser parameters were 1) Calculate spherical equivalent

2) determine thePTK effect of the total astigmatism treatment ( Total cylinder X 35% ) this produce the hyperopic shift in refraction there for it is added to the sphere

3) apply spherical treatment adjustment

the spherical component of the refractive correction is 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 myopic (minus) components separately

5) Write laser treatment stages a) Hyperopic cylinder with 5.5 - 9 mm zone b) myopic cylinder with 6.5 - 7.5 mm zone c) nomogram adjusted spherical refractive error

6) for smoothing 3microns PTK are placed in 8 mm zone

Example:-3.0 / - 4.0 X 180

S.E = - 5.0PTK effect =- 4 X 35% = -1.4Spherical treatment =- 4 – (-1.4) = -2.6Astigmatism- 2.0 X 180 / +2.0 X 90Laser treatment stages+ 2.0 X 90 - 2.0 X 180 - 2.6PTK 3 microns

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The Mean age27.46 years +/- 6.3 (S.D) range 21-49 yearsPreoperative refractionwas -0.50 to -10.0 D of sphere with :astigmatismof -0.75 to -2.0 D formonotoric ablation profileastigmatismof -2.25 to -6.0 D forBitoric ablation profileThe mean preop.spherical equivalent (SE)was -1.5 +/- 0.7 range (-3.9 to + 0.50 D )Follow upwas 6 months in all patients

Visual Acuity( 6 months after LASIK )Themean 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 profileThemean BCVAbefore LASIK was 0.71 +/- 0.19 after LASIK was 0.83 +/- 0.15 BCVA 20/40 or better was in 345 eyes ( 100%)in Monotoricprofile: 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 Bitoricprofile: 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 lineslRetreatment for a significant residual refractive defect:, 24 eyes (17.1 %) needed after Monotoric LASIK 16 eyes (14.6 %) needed after Bitoric LASIK

Results:

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Conclusion:

Bitoric ablation for astigmatism appear to besafer, more effective , more tissue sparing andresulted 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 ablationin turn this has the effect of treating high astigmatic errors predictably with a more stable result and with less haze and regression.