Eyeworld Supplements

center and flatter toward the corneal periphery, and this is the ideal shape in order to have no spherical aberration within the system as a whole. For exam- ple, the normal cornea of someone who has not un- dergone previous surgery may be 43.00 D or 44.00 D in the center, while farther out in the periphery, it may flatten down to 42.00 D or 41.00 D. Applying a laser treatment just to the central cornea will flatten the center relative to the periphery, creating an oblate cornea. This will induce aberrations on a wavefront map and visual symptoms for the patient. Choosing a target Q that is prolate helps the laser to achieve the final desirable corneal shape. For pri- mary eyes, I try to preserve whatever Q value the patient presents with, i.e., the delta-Q is zero. Most primary eyes are quite prolate and the goal is to pre- serve that prolate shape. So, for those patients, the Q value does not change. It is important to remem- ber that the patient's primary concern is his or her final visual acuity, and this depends largely on the refractive outcome, so our primary duty is to try to address the refractive component at the time of surgery. Rating the map With practice, it becomes easier to identify topo- graphic maps as high or low quality. One nice feature is that the Topolyzer provides an automated rating of the map by placing a green tick next to a map you may want to use, a yellow tick next to a map you need more time to think about, and a red tick next to a map that is of poor quality. Then, it only takes a minute to load all the maps into the ALLEGRETTO WAVE excimer laser. It may take an extra few minutes to decide which maps to use and to amend or tweak the refraction if necessary, but those few extra minutes are time well spent to give patients the vision they deserve. T he ultimate goal of topography- guided custom ablation treatment (T-CAT) diagnostics is to capture a reproducible topographical image of the corneal irregularity that can then be used to drive a topography-guided procedure. When I am considering the use of T-CAT, I find that it is helpful to identify the patient's primary complaint. This information combined with a sketch that I ask the patient to draw to illustrate his or her visual symptoms (while best corrected) helps to inform my decision about whether T-CAT is the appropriate treatment. In the FDA clinical trial of T-CAT technology, in- vestigators used a minimum of 4 topography images of each eye captured with the ALLEGRETTO Topolyzer (Alcon, Fort Worth, Texas) to create a treatment plan. Some important clinical pearls to keep in mind in an effort to get a good scan include selecting patients with good correlation between topographic (corneal) astigmatism and refractive astigmatism; using the same target Q value as the preoperative value; using the "tilt-off " tissue-sparing default setting; making sure that the patient is properly aligned under the laser; and comparing ablation profiles of T-CAT vs. wavefront (when available). Methodical map analysis When analyzing the topographic map quality, it is important to review the entire treatment zone. The surgeon should examine the rings for continuity using the measurement function in the overview screen, and make sure the mires are as crisp and clear as possible. Then, the surgeon should look for discontinuity and dry spots (Figure 1); compare and analyze higher order aberrations; ensure that there is a good correlation between refractive astigmatism and topographic astigmatism; ensure that the detection of the concentric rings and the detection of the pupil are accurate; and ensure that the image is centered. Once I am satisfied that the patient's topo- graphical maps are of high quality, I upload that information to the excimer laser to generate the ablation profile. The next step is to choose a target Q value for the treatment. The Q value is the rate of change of corneal curvature; a perfect circle has a Q value of zero. All the spots, left, right, central or peripheral to this particular spot, will fall away at the same rate. A normal prolate cornea is steeper in the Figure 1. Ring pattern: Look for discontinuity, dry spots, etc. within the ring pattern. Interpreting topographic information with T-CAT diagnostics by Arthur B. Cummings, MB ChB, FCS(SA), MMed(Ophth), FRCS(Edin) Dr. Cummings is a consultant ophthalmologist at the Wellington Eye Clinic and Beacon Hospital in Dublin, Ireland. He is a consultant for Alcon. Dr. Cummings can be contacted at abc@wellingtoneyeclinic.com. For important product information about the WaveLight Excimer Laser System, please see page 6. 4 A monumental stride forward in refractive outcomes

• Cover