LASIK What are the Limits ?

1. LASIKWhat are the Limits ? A.Marinho,MDPhD 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 actionMYOPIA • 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 -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

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: • - 8.00 (S.E.) or +4.00 (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 -8.00 or hyperopias > +4.00, the limits are defined by the following: • Corneal Thickness • Corneal Curvature • Pupil Size

12. Limits of CRSInclusion 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: - 8.81+ 1.84 • Group d ( K > 34 ) • 20 eyes • Myopia: - 7.14+ 1.43

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 LASIKInclusion criteriaExample • Refraction - 7.00 • Case 1 • Pachymetry 580 • K readings 45 D • Pupil size 4.0mm • Case 2 • Pachymetry 490 • K readings 40 D • Pupil size 6.5mm • GOOD • BAD

19. Limits of LASIKInclusion 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