1. LASIK What are the Limits ?
A.Marinho,MD PhD
Departamento de Cirurgia Refractiva
Hospital Arrabida - Universidade do Porto

2. 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

3. Mechanism of action MYOPIA
Ablates central cornea
Changes the corneal shape (flattening )
Changes dramatically the corneal contours (new edges )

4. Limits The normal cornea
Central thickness
mean 520 (480 to 600)
Curvature
mean 43D (39 to 46)
Aspheric shape with no edges

5. General limits of corneal refractive surgery
We cannot change the shape and thickness of cornea indefinetely
The early ideas of corrections up to proved wrong
Biomechanics of the cornea shows that trying to correct high ametropias lead almost always to important regression and reduced quality of vision

6. REGRESSION
Progressive loss of effect without increase in axial lenght and no topographic abnormalities
Not to be confused with ectasia

7. Why Regression ?
Epithelial hyperplasia
Corneal remodeling

8. Quality of Vision
Excellent quality of vision can only be achieved up to -5.00
Some decrease up to -8.00
Poor in high myopia

9. 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:
(S.E.) or (SE)

10. VERY IMPORTANT Wave front ablations Tissue Saving Nomograms
Aspheric Ablations
Improve qualty of vision, BUT DO NOT CHANGE the BASICS of corneal behaviour

11. Limits of LASIK
In myopias lower than or hyperopias > +4.00, the limits are defined by the following:
Corneal Thickness
Corneal Curvature
Pupil Size

12. Limits of CRS Inclusion criteria
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)

13. The cornea is too thin....
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

14. The cornea is too flat... Group c ( K < 34 ) 10 eyes
Myopia:
Group d ( K > 34 )
20 eyes
Myopia:

15. Pupil Size Pupil size is important and sometimes overlooked
Lasik full corrected optical zone must be at least equal to the mesopic pupil

16. The cornea is irregular....
Assymetric bow-tie
Assymetric K readings
Increased posterior float (Orbscan)
KERATOCONUS

18. Limits of LASIK Inclusion criteria Example
Refraction
Case 1
Pachymetry 580
K readings 45 D
Pupil size mm
Case 2
Pachymetry 490
K readings 40 D
Pupil size mm
GOOD
BAD

19. 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

20. Corneal Ectasia

21. LASIK IN HIGH AMETROPIA
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

22. Phakic/Pseudophakic IOLs High Myopia
If the inclusion criteria are respected and the surgery is perfect…
Accuracy, Stability
Long term (15 years) follow-up with NO complications
MY CHOICE

23. 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