Transcript LASIK What are the Limits

LASIK
What are the Limits ?
A.Marinho,MD PhD
Departamento de Cirurgia Refractiva
Hospital Arrabida - Universidade do Porto
Corneal Refractive Surgery

LASIK and Surface Ablation are the
most widely practiced and “popular”
forms of refractive surgery

However, even if perfectly
performed,but in a defective indication
can lead to catastrophic situations
Mechanism of action
MYOPIA
Ablates
central cornea
 Changes the
corneal shape
(flattening )
 Changes
dramatically
the corneal
contours (new
edges )

Limits
The normal cornea

Central thickness


mean 520 (480 to 600)
Curvature
 mean
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43D (39 to 46)
Aspheric shape with no edges
General limits of corneal
refractive surgery
We cannot change the shape and
thickness of cornea indefinetely
 The early ideas of corrections up to 30.00 proved wrong
 Biomechanics of the cornea shows that
trying to correct high ametropias lead
almost always to important regression
and reduced quality of vision

REGRESSION


Progressive loss of
effect without
increase in axial
lenght and no
topographic
abnormalities
Not to be confused
with ectasia
Why Regression ?

Epithelial
hyperplasia

Corneal remodeling
Quality of Vision

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
Excellent quality of
vision can only be
achieved up to 5.00
Some decrease up
to -8.00
Poor in high myopia
General limits of corneal
refractive surgery
Most papers dealing with high
ametropia surgery have 6 months to 1
year follow-up……
 So,in an ideal cornea ,we should not
attempt to correct more than:
 - 8.00 (S.E.) or +4.00 (SE)

VERY IMPORTANT
Wave front ablations
 Tissue Saving Nomograms
 Aspheric Ablations


Improve qualty of vision, BUT DO NOT
CHANGE the BASICS of corneal
behaviour
Limits of LASIK

In myopias lower than -8.00 or
hyperopias > +4.00, the limits are
defined by the following:
Corneal Thickness
 Corneal Curvature
 Pupil Size

Limits of CRS
Inclusion criteria

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Total corneal thickness at least 500
micra(Lasik) (480 micra if using FS flaps)
Stromal bed after ablation must be 250
Final corneal curvature must be between 33
and 47D
Full corrected optical zone must be at least
equal to pupil diameter in mesopic conditions
(ray tracing methods)
The cornea is too thin....

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Group a ( < 470  )
was significantly more
undercorrected
and
regressed more than
other groups
In all other groups
results were similar
what suggests that
preop pachimetry is
not so important in the
refractive outcome in
LASIK
0
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
1
2
3
4
5
6
< 470
470 - 490
490 - 510
510 -530
530 -550
> 550
The cornea is too flat...

Group c ( K < 34 )
– 10 eyes
– Myopia: - 8.81 + 1.84

Group d ( K > 34 )
– 20 eyes
– Myopia: - 7.14 + 1.43
L
o
g
C
S
2,65
2,5
2,35
2,2
2,05
1,9
1,75
1,6
1,45
1,3
1,15
1
D
C
Pupil Size
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Pupil size is important and sometimes
overlooked
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Lasik full corrected optical zone must be
at least equal to the mesopic pupil
The cornea is irregular....

Assymetric bow-tie

Assymetric K
readings
Increased posterior
float (Orbscan)
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KERATOCONUS
Limits of LASIK
Inclusion criteria
Example
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Refraction - 7.00
– Case 1
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

Pachymetry 580
K readings 45 D
Pupil size 4.0mm

GOOD

BAD
– Case 2



Pachymetry 490
K readings 40 D
Pupil size 6.5mm
Limits of LASIK
Inclusion criteria

Failure to follow these guidelines lead to
irreversible complications
corneal ectasia
 loss of lines of BCVA ( typical of high myopia
and mainly in hyperopia )
 severe glare and halos

Corneal Ectasia
LASIK IN HIGH AMETROPIA
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Deep ablation – Thin cornea– Unstable
cornea---Regression (ectasia)
Small optical zone---glare and halos-low
mesopic vision
Flattened (or steepned)cornea---distorted
vision—loss of lines of BCVA—low mesopic
vision
Lasik in high ametropia---an unfulllfilled
promise
Phakic/Pseudophakic IOLs
High Myopia
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If the inclusion
criteria are
respected and the
surgery is perfect…
Accuracy, Stability
Long term (15
years) follow-up with
NO complications
MY CHOICE
Conclusions
Limits (Absolute and Particular) of Lasik
must be respected
 Selection of patients is the key for
success
 Classicaly Lasik : 80% MK 20% Laser
 Today: 50% Selection 50% Surgery
