Presentation on theme: "٤٩ Customized Corneal Ablation Customized LASIK & PRK will dominate in next few years Speedy recovery, good quality of vision satisfactory outcome Disadvantage."— Presentation transcript:
٤٩ 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
٥١ 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
Types of customization Two main methods are available in refractive surgery: 1. Topography guided ablation 2. Wavefront guided ablation
According to recent statistics, approximately 55% of North American refractive surgeons have wavefront analyzers in their practice and routinely perform wavefront-guided ablations.
Definition of Wavefront
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 wavefront error 2)It gives a rough estimate
Types of aberrations Low Order Aberration (LOA) : 1 st & 2 nd order aberration High Order Aberration (HOA) : 3 rd to 6 th or 10 th
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 1 st order aberration, RMS2 is due to 2 nd order and etc. RMSh represents the total RMS of HOA (including 3 rd to 6 th 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
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. Customization can be based on corneal topography or wavefront measurements.
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
١٢ 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
10/14/2007 ٧ Zernikes 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-10 th order = Irregular aberrations, important when pupil is wide dilated. XVth Congress of Iranian Society of Ophthalmology October 2005
Wavefront guided customization Goals a) Spherocylinder correction b) To correct the aberration that are induced by conventional laser vision correction and pre-existing aberration.
Requirment for wavefront 1) Efficient eye tracking 2) Good registeration system for eye 3) Small size laser spot 4) Sufficient corneal bed thickness
Challenges to wavefront Tear film abnormalities Miotic pupil : If severe may be difficult to measure and provide information beyond the 3mm optical zone and therefore require pharmacological dilation.
Optical effects of tear film disruption
Wavefront map with cycloplegic or mydriatic 1) Has significant difference in pre-op refractive error compared to subjective refraction
(Ablation profile algorithm) 1) After good aberration maps, Ablation profile algorithm is needed 2) Should be designed to make the post-op higher order aberrations as minimal as possible 3) Not only correcting the pre-op higher order aberration but also preventing the induction of new higher order aberrations.
٣٥ Conversion to Ablation Profile: Ablation profile is fundamentally inverse of wavefront error map Goal: Correction of refractive error and higher-order aberrations Pupil diameter at least 0.5mm larger than scotopic Limbal marking for cyclotorsion detection Wavefront measurement + corneal curvature + biomechanics ablation profile complex
Registration refers to the method by which a customized ablation pattern based on preoperative wavefront analysis from the aberrometer is superimposed in the same position on the eye during the laser treatment. Registration
Centration is established by the surgeon when the patient fixates on the target light. The instrument captures a video image of the eye, and the surgeon defines the pupillary ring (red) and limbus ring (yellow). These will be used by the LADARVision tracker to follow the eye during application of the customized ablation pattern. LADARWave and LADARVision systems:
Torsional alignment refers to the need to compensate for the cyclotorsion of the eye that occurs when the patient lies down. The eye usually rotates less than 5 degrees, but in some patients, postural cyclotorsion can be as much as 10 to 12 degrees. Torsional alignment
Misalignment 1) A small misalignment in in the axis can have significant impact on the results of procedure 2) A 5-7 degrees of cyclotorsion when changing from sitting position to supine position. 50% of the visual benefit of HOA is lost.
Bausch and lomb zyolptix system has 1) Diagnostic 2) Treatment components for wavefront customized corneal ablation
Diagnostic part of zioptix systems (2172 laser) In zy wave aberometer and ORBscan both machine are integrated in one workstation. It has Hartmann-shack sensor that measures up to 5 th order zernike term. It measures sphere from +6.0 D to -12. D and 0 to 5 D of cylender with pupil diameter from 2.5 to 8.6 mm.
Pupil during exam 1) Orbscan must be performed with an undilatd pupil 2) Wavefront meaurements are taken with a dilated pupil.
Types of beam Has a truncated gaussion 3 beam to achieve the benefits of both beam types. 1 23
Laser beam sizes in Tech217z 1) The 2mm beam treat the majority of the refractive errors. 2) The 1mm beam used for more specific ablation pattern on the transition zone
Eye tracker The system has 120 Hz active eye tracker with a passive automatic shut-off system corrects for intraoperative eye movement in every dimention including cyclotortion and pupil shift
FDA investigation for outcomes for wavefront guided lasik in myopia for 3 separate laser platforms 1) LADAR vision 2) Technolus 217 z for low to moderate myopia 3) visx ConventionalWave front guided 72% ………….20/2089% ……..20/20
Environmental Conditions Control of environmental conditions during treatments is important. In previous U.S. FDA Multi-Center Clinical Trials, the room conditions were: Temperature ranged from 68ºF to 72ºF (20ºC to 22.2ºC) Relative humidity ranged from 40% to 45% Treatments performed at >24ºC were associated with less accurate outcomes Stability of temperature and humidity is important
٤٢ Future of Customized Corneal Ablation LASIK vs surface ablation Each microkeratome induces specific flap only aberration (flap size, thickness) Considering flap aberrations in total treatment calculations
٤٣ Future of Customized Corneal Ablation … cont. Surface Ablations: Show promising results with use of immunomodulating agents Better control of cellular and biochemical reactions Introduction of new drugs to better regulate wound healing and refractive outcome Gene therapy for better control of post laser keratocyte activation and wound healing
٤٤ Multifocal Ablation Presbyopia: Customized multifocal ablation Aberrations may be induced when creating multifocality Potential loss of contrast sensitivity and quality of visual function Future results will be improved: * Wavefront mapping, sophisticated eye trackers and corneal registration * Preoperative simulation of postop condition
٤٥ Laser Delivery Refinements Correction of HOA needs smaller spot delivery >5th order requires mm spot size Smaller spot size needs faster and better eye trackers Smaller ablation depth per pulse provides ideal correction profile for higher orders
Courtesy of IBM The Human Hair (125 microns) Etched with an excimer laser
Thermal Non-thermal ExcimerCO 2 YAG Effects of Laser Ablation
٥٤ Conclusion Wavefront measurement devices and consequently wavefront correction procedures are still in process of evolution Achievement of supervision: with advancement in current procedures will not be a dream in near future