Comparison of bitoric with monotoric laser in situ keratomileusis for the correction of myopic astig

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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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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 cylinderCentral
  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 cylinderlaser 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 which produce a
  spherical corneal profile then any residual
  spherical error is treated

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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 evaluationUCVA, BCVA, manifest
  and cycloplejic refraction, slit lamp exam,
  fundus exam, applanation tonometry, pachymetry
  and corneal topography
• Postoperative evaluationUCVA, BCVA, manifest
  and cycloplejic refraction, slit lamp exam,
  corneal topography and total ablation depth

Inclusion criteriaolder than 18congenital
astigmatism (-1.0 till -6.0 ) stable refraction
Exclusion criteriaBCVA 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
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Nomogram used( Modified Gimbel nomogram )
Example-3.0 / - 4.0 X 180 S.E - 5.0PTK
effect - 4 X 35 -1.4Spherical
treatment - 4 (-1.4) -2.6
Astigmatism- 2.0 X 180 / 2.0 X 90Laser
treatment stages 2.0 X 90 - 2.0 X 180 -
2.6PTK 3 microns

• Calculation determined the laser parameters
  were 1) Calculate spherical equivalent
• 2) determine the PTK 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

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Results

• The Mean age 27.46 years /- 6.3 (S.D) range
  21-49 yearsPreoperative 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 lineslRetreatment for a
  significant residual refractive defect ,
  24 eyes (17.1 ) needed after
  Monotoric LASIK 16 eyes
  (14.6 ) needed after Bitoric LASIK

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Bitoric ablation for astigmatism appear to
besafer, more effective , more tissue sparing
andresulted in a decreased frequency of
reablation than the standard treatment
Conclusion
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.
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Thank you