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Understanding the Limits of LASIK Surgery: An Expert's Insight

Learn about the mechanisms and limits of LASIK and surface ablation surgeries, including corneal reshaping, regression risks, and achieving quality vision. Explore the inclusion criteria, factors affecting outcomes, and potential complications. Gain valuable knowledge from an expert in corneal refractive surgery.

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Understanding the Limits of LASIK Surgery: An Expert's Insight

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  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

  17. 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

  18. 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

  19. Corneal Ectasia

  20. 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

  21. 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

  22. 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

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