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 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 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 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.... 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

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

Pupil Size Pupil size is important and sometimes overlooked 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) KERATOCONUS

Limits of LASIK Inclusion criteria Example 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

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 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 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