Arthur Cummings MD, FRCS Wellington Eye Clinic, Dublin How to avoid the induction of aberrations in corneal laser refractive surgery Arthur Cummings MD, FRCS Wellington Eye Clinic, Dublin

Lower Order Aberrations LOA’s remain the key factor in overall patient satisfaction A patient with -0.75D of myopia and 0.05µ of HOA is more symptomatic than a Plano patient with 0.4µ of HOA

Rule No. 1 Use outcomes analysis to refine nomograms and get the refractive results on target IBRA by Zubisoft DataGraph Med SurgiVision Refractive Surgery Consultant

Higher Order Aberrations The most common aberrations that can be induced are coma and spherical aberration Coma is induced by means of decentred ablations Spherical aberrations are induced by small optical zones, very large corrections on old technology lasers

Coma: C7 and C8 Mainly caused by poor alignment under the eye tracker Mistaken belief that all modern eye trackers can compensate for everything Alignment under the laser: check with cross-hairs No crossing of legs when lying down on the laser bed Occluding 2nd eye and fixating with the eye being treated Ensuring that the eye is perpendicular to the laser beam

Laser beam Well Centred Poorly Centred or Decentred

Green Flashing Light is in middle of 4 Yellow Lights Correct fixation Green Flashing Light is in middle of 4 Yellow Lights

Incorrect fixation Green Flashing Light is NOT in middle of 4 Yellow Lights

Incorrect fixation Green Flashing Light is NOT in middle of 4 Yellow Lights

Centration of Ablation Pupil centre? Corneal Apex? Somewhere in between (along chord length)?

Additional Sources of Error Wavefront Congress February 2015 Santa Barbara, California, USA Charles Campbell Consultant to AMO / VISX

Refractive error introduced during surgical treatment Misplacement of the treatment Rotation error Centration error Change in ablation rate

Comments on rotation error If the magnitude of the correction is small large rotational errors have little effect in degrading vision For aberrations expressed in terms of Zernike coefficients, terms with larger meridional indices, m, are affected more by rotational error In most cases common astigmatism has by far the largest magnitude and so it is usually the only aberration really affected by rotation error

Cause and prevention of rotation error Rotational misalignment can occur in the placement of the head beneath the laser system The eye often rotates from its normal – head vertical – orientation when the head becomes horizontal during surgery (cyclo-rotation) Iris registration has proved to be an effective way optically sensing the rotational orientation of the eye during surgery and providing a correction to the treatment routine

Difference between rotation and centration errors For decentration the full corrective effect is still applied whereas for rotation the desired refraction effect is changed. For decentration errors the induced errors are always of a lower order than the correction whereas for rotation the induced error is of the same order as the correction.

A more important effect of treatment misplacement (VISX) The effect of the transition zone of the ablation when it enters the pupil of the eye

Cause & Effect of a transition zone lenslet Caused by high radial curvature in transition zone Creates a secondary, displaced image The power of lenslet is proportional to the change in corneal curvature in the optical zone, i.e. to the magnitude of correction Visibility of the secondary image is proportional to ratio of the area of the lenslet to the rest of the pupil area and the contrast of object and background

Minimizing decentration (VISX) Plan the treatment to have the optical zone centered on the mesopic pupil center It not advisable to center the ablation on the corneal apex because the corneal apex is usually displaced from the mesopic pupil center Use iris registration or centration on the pupil center as viewed through the operation microscope The pupil restricts during treatment which usually decenters it from the mesopic pupil. So centering on the treatment pupil will usually cause some ablation decentration. But this effect is very small in general ~ 1/16 mm to 1/8 mm

Change in ablation rate during treatment Tissue dehydration or an excessive fluid film on the stromal tissue will cause the ablation rate to change Dehydration – causing extra ablation – is most likely due to air flow over the treatment area during the procedure

Manufacturer's Efforts Sissimos Lemonis Dr. Sam Arba_Mosquera Dirk Muehlhoff Charlie Campbell Dr Gerhard Youssefi WaveLight Schwind Zeiss VISX Technolas With grateful thanks and acknowledgment to these gentlemen for their time and input

WaveLight

WaveFront Optimized Ablation Profile Designed by WaveLight Designed to change the lower orders (sphere & cylinder) without changing the higher orders (aberrations) Based on data in the literature on how much spherical aberration and higher order astigmatism is induced by the standard (Munnerlyn) formulae

The Philosophy WAVEFRONT OPTIMIZED PROFILE A circular laser beam is being projected onto the cornea Normal ablation Reflection Circular in the centre Elliptical in the periphery. Ablative power is lost in several ways. Light is being reflected. The ellipse covers a larger area, resulting in a lower energy density. The lower ablation rate causes a spherical aberration resulting in poor night vision. Reduced Ablation, lower fluence 24

Wavefront-Optimised There is a numerical compensation (increasing the number of laser spots) as the ablation moves peripherally to compensate for the loss in ablation efficiency

Manufacturer's Efforts WaveLight Schwind Zeiss VISX Technolas

Schwind

Schwind 1) Compensation of Spherical Aberration and focus shift based on K-readings Arba Mosquera S, de Ortueta D. Analysis of optimized profiles for ‘aberration-free’ refractive surgery. Ophthalmic Physiol Opt;. 2009; 29: 535-548 Rationale: Avoid hyperopic shift in deep ablations due to tissue removal (i.e. shortened axial length)

Schwind 2) Different aspherical compensation as a function of the OZ Arba Mosquera S, de Ortueta D. Analysis of optimized profiles for ‘aberration-free’ refractive surgery. Ophthalmic Physiol Opt;. 2009; 29: 535-548 Rationale: Avoiding induction of spherical aberration (but also HO Astigmatism) Disclosure: The compensation is discrete and stepwise in 0.5mm OZ steps, so that e.g. 6.24 and 6.26mm OZ receive different compensations, but 6.26mm and 6.74mm receive the same compensation

Schwind 3) Asymmetric offset to compensate for the visual axis Arba Mosquera S, Ewering T.  New asymmetric centration strategy combining pupil and corneal vertex information for ablation procedures in refractive surgery: theoretical background.  J Refract Surg. 2012 Aug;28(8):567-75 Rationale: Avoiding induction of coma aberration from defocus correction (but also trefoil aberration from Astigmatism correction).  Saving tissue since the visual axis (if determined) can be selected for the ablation, but the ablation remains truncated (and tilt-compensated) concentric to the pupil boundaries Disclosure: Determination of the true visual axis is an open quest yet

Schwind 4) Dehydration model to compensate for treatment duration 5) Corneal and Flap thickness compensation de Ortueta D, von Rüden D, Magnago T, Arba Mosquera S.   Influence of stromal refractive index and hydration on corneal laser refractive surgery.  J Cataract Refract Surg. 2014 Jun;40(6):897-904 Rationale: Cutting a planar flap means steepening the stroma by an amount equal to the flap thickness Disclosure: Flaps may be far from planar, and may deviate from what was planned originally

Schwind 6) Different ablation spots for LASIK/FemtoLASIK compared to TransPRK (epithelium) and to PRK/LASEK  Rationale: The cornea is a layered structure with different cellular properties at different layers, leading to different ablation properties at different layers

Schwind 7) Automatic dynamic transition zone calculation   Schwind 7) Automatic dynamic transition zone calculation Rationale: ensuring the same treatment always receives identical TZ Disclosure: Limits the exploration capabilities of the surgeon

Schwind 8) Adjustment to ablation depth for age   Schwind 8) Adjustment to ablation depth for age  Luger MH, Ewering T, Arba-Mosquera S. Influence of patient age on high myopic correction in corneal laser refractive surgery. J Cataract Refract Surg. 2013 Feb;39(2):204-10 Rationale: Age affects not only the accommodative amplitude of the patients, but also the water content in the cornea, leading to (slightly) different ablation rates at different ages Disclosure: Age itself is not the actual driving force for that, so we are using correlation but not a root-cause-effect, other elements may contribute as confounding factors

Manufacturer's Efforts WaveLight Schwind Zeiss VISX Technolas

Zeiss

Zeiss For ZEISS, our profile is a result of a stepwise development process. To come up with our current AAA profile design, a big number of cases treated with the previous profile have been analysed for topographic changes pre- to post-op and the learnings have been incorporated into a so-called energy correction function which accounts for the loss of ablation efficiency of a particular laser pulse in the periphery of the cornea. That energy correction functions accounts for the effect of oblique incidence and the associated projection and reflection losses.

Manufacturer's Efforts WaveLight Schwind Zeiss VISX Technolas

AMO VISX

What can be done to decrease this effect? Move the transition zone outward Estimate the effect and include an opposite aberration into the treatment

Laser Vision Correction ablation targets created using Wavefront deflection information from a wavefront eye refraction  with no need to reconstruct the wavefront Anterior corneal surface information from Keratometer measurements or Full corneal topography CustomVue treatments are created with this system today How is this done? Why is this desirable?

The gradient of the ablation target is then THE TASK: find the ablation target  the amount of corneal tissue to remove The ablation target, At, is the difference between the pre-op anterior corneal shape, Spreop, and the post-op anterior corneal shape, Spostop The gradient of the ablation target is then

Method to find the ablation target Directly calculate the gradient of the pre-op anterior corneal surface from its measured surface shape Find the gradient of the desired post-op anterior corneal surface using the measured wavefront gradient, the desired wavefront gradient, and known pre-op anterior corneal surface gradient Reconstruct the ablation target from its gradient information

Information needed Measured exiting wavefront gradient values (ray direction information) Measured anterior corneal shape  full topography or keratometry measurements with a surface reconstruction Desired final refractive error

The basic philosophy underlying the method Before and after laser vision correction, rays from the Hartmann-Shack retinal source follow the same paths until they reach the anterior corneal surface Measurement of the exiting rays and the corneal surface allow the ray directions just before final refraction to be determined Using this ray direction information, the anterior corneal surface shape is changed until the rays, following final refraction, take the desired paths

Using the basic philosophy to find the desired anterior corneal gradient Uncorrected surface normal Uncorrected exiting ray Corrected surface normal Corrected ray Entering ray

Ablation target creation: a three-step process Project the measured wavefront gradient values onto the specified corneal ablation grid Calculate the ablation target gradient values from the pre-operative corneal surface gradients, the wavefront gradient values, and the desired wavefront gradient values using ray tracing formulas Reconstruct the ablation target from its gradient values

FIRST STEP: Project the measured rays onto the corneal surface Hartmann-Shack lenslet image plane in the exit pupil of the eye Rays exiting the uncorrected eye cornea Measurement location Refraction location Visible iris Rays just before refraction at the anterior corneal surface

The measured rays are interpolated to find rays that strike the corneal surface at the chosen ablation grid locations p is the pitch of the Hartmann-Shack lenslet array g is the chosen ablation grid spacing Rays measured by the Hartmann- Shack array, spaced apart by pitch p in the lenslet array plane Rays that would strike the corneal surface spaced apart by the chosen ablation grid pitch g on the corneal surface

Second step: Use ray-tracing formulas to find the new surface gradient values Snell’s law in vector form Since the entering rays are the same before and after ablation Dividing the component equations gives the x- and y-gradient values

Third step: The ablation target is reconstructed from the difference in the corneal surface gradients before and after ablation Fourier gradient reconstruction allows rapid, accurate reconstruction due to the fact that the gradient data have been prepared on a square grid

Advantages of the method Wavefront and corneal surface information are fully integrated Full corneal topography provides More accurate corneal height information for the ray interpolation step More accurate local corneal gradient information for the reconstruction step More accurate laser ablation compensation to be made for tipped surface effects Full use made of wavefront information without the need to reconstruct the wavefront No need to decide on the wavefront reconstruction method used

Manufacturer's Efforts WaveLight Schwind Zeiss VISX Technolas

Technolas

B&L Aspheric Ablation Profile We were actively rolling this (ablation profile) into the field in 2006. On the basic design of the aspheric algorithm nothing has changed since then and we are so happy with the outcomes that we use the same algorithm even in our most recently launched TENEO excimer laser which is our new platform.

Moving on to Customized Ablations.. There are treatments available to correct higher order aberrations That is a different topic to today’s presentation Wavefront-guided Topography-guided Asphericity-guided Decision-tree follows

Asphericity-controlled Refraction, Visual Acuity Corneal Topography Visual Quality Topography irregular ? Mesopic visual symptoms ? BCVA < 6/6 ? Evaluation of HOA Wavefront Maps Aberrometry Induction of C12 ? WF Maps Valid ? WaveFront Optimized Asphericity-controlled WaveFront Guided Topography Guided

Benefits of Custom-Q Custom Q treatments take Wavefront Optimization one step further Wavefront-Optimization based on pre-op K’s and mean asphericity (K and Q) Loss of LASER-pulse efficiency (caused by the angle of incidence) are customized to the pre-operative corneal curvature and asphericity

Ablation design for Custom Q and all custom-guided procedures Q pre-op = -0.9 Less deflection = good LASER-pulse efficiency Q pre-op = -0.28 More deflection = less LASER-pulse efficiency

Summary There are surgical skills and disciplines to acquire and adhere to that can reduce the level of induced aberrations Manufacturers have improved the ablation profiles the eye-trackers and taken other issues into account to provide surgeons with the highest level of sophistication and safety that we have ever enjoyed

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