LASIKWhat are the Limits ?

A.Marinho,MD PhD Departamento de Cirurgia RefractivaDepartamento de Cirurgia Refractiva

Hospital Arrabida - Universidade do PortoHospital 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 actionMYOPIA

Ablates central Ablates central corneacornea

Changes the Changes the corneal shape corneal shape (flattening(flattening ))

Changes Changes dramatically the dramatically the corneal corneal contours (contours (new new edgesedges ) )

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

00.250.5

0.751

1.251.5

1.752

0 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

11,151,3

1,451,6

1,751,9

2,052,2

2,352,5

2,65

D C

Log CS

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