Customized Corneal Ablation Customized LASIK & PRK will dominate in next few years Speedy recovery, good quality of vision satisfactory outcome Disadvantage of conventional refractive surgery in some patients: * Increase in HOA * Reduction in visual quality ٤٩
Wavefront Customized Visual Correction Ocular wavefront sensing: Will be increasingly employed Will become routine in vision assessment Wavefront customization is employed to optimize any refractive surgery procedure ٥١
Wavefront Customized Visual Correction (cont) Future wavefront customized refractive procedures Implantation of optimized IOL’s e.g Technis aspheric lens Customized IOL’s preinsersion, customized phakic IOL’s
Types of customization Two main methods are available in refractive surgery: • Topographyguided ablation • Wavefrontguided ablation
According to recent statistics, approximately 55% of North American refractive surgeons have wavefront analyzers in their practice and routinely perform wavefront-guided ablations.
Wavefront-Guided (Customized) Excimer Laser Refractive Surgery • Definition: Wavefront-guided custom ablation is used to correct higher-order aberrations, in addition to spherocylindrical correction.
Incidence in General population 10-15%have significant higher-order aberration
Hard contact lenses Corrects higher order aberrations resulting from the cornea.
How to measure the higher order optical aberration? • By using a Wavefront analysis system called aberrometry
Pupil and Wavefront • Larger diameter leads to a larger Wavefront error
Evaluation of vision quality Compromised more at dim light during night and represented by: a) Double vision b) Ghosting c) Glare d) Halos e) Starbursts d) Reduced contrast sensitivity
Quantification of magnitude of the aberration 1)Usually by RMS wavefronterror 2)It gives a rough estimate
Types of aberrations • Low Order Aberration (LOA) : 1st & 2nd order aberration • High Order Aberration (HOA) : 3rd to 6th or 10th
How to evaluate the quantity of optical system • Root Mean Square ( RMS ) The RMS is SD of height (depth) of wavefront relative to the reference at all the point in the wavefront. RMS is very useful measure of optical quality. • Peak-To-Valley (PV) Distance from the highest to lowest point on the deformed wavefront relative to the reference wavefront.
RMS1 is qualitative presentation of 1st order aberration, RMS2 is due to 2nd order and etc. • RMSh represents the total RMS of HOA (including 3rd to 6th OA) • RMSg represents the total RMS of HOA and LOA • Total RMS increases with aging • LASIK increases RMS especially in subjects with larger pupils • RMSh> 0.3 and > 20% of total RMS is clinically significant and indicative for customized ablation
Wavefront measuring devices • Hartmann-shack style devices are the most common used
Hartmann-shack style 1)A narrow beam of light is projected on to the retina and the light reflected from the fovea passes through the lens and the cornea and exists the eye
Customization can be basedon corneal topography or wavefront measurements. • Corneal topography guided ablation has been attempted on patients with regular and irregular astigmatism, decentered ablations, and central islands. The irregular astigmatism group is more challenging and may benefit most from corneal topography guided ablation as the systems become more refined.
Hartmann-shack aberrometer • A low intensity laser beam is directed to retinal surface • Light rays from this laser spot are reflected back to the front of the eye
Laser spot on retinal surface light rays are reflected back to the front of the eye Lens arrey focuses these light rays Photodetector (charged coupled device,CCD) Dots of light
Real eye with Aberrations Considering refractive lens at corneal surface Excimer etching by customized ablation Converting wavefront to flat wavefront Perfect focus on fovea Ideal eye ١٢
Zernike’s Polynomials Zero order (no order)=axial symmetry, flat wavefront First order = linear aberration, tilting around a horizontal (x) or vertical axis (y) Second order = focus shift, spherical defocus or astigmatism Third order = corresponding to coma, triangular astigmatism Fourth order = spherical aberration, complex patterns Fifth-10th order = Irregular aberrations, important when pupil is wide dilated. 10/14/2007 ٧ XVth Congress of Iranian Society of Ophthalmology October 2005