EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/276058
E W FEATURE 112 between that incision and the dis- tance to the limbus." When this is done manually, practitioners tend to put the incision just inside the lim- bus. "I think a lot of the refinement yet to come in femtosecond LRIs is to hone down the white-to-white a nd to know where the incision is in relationship to the physical surgical limbus," Dr. Alpins said. Another obstacle in attaining best correction of corneal astigma- tism with femtosecond lasers is determining the true power of such a stigmatism. Dr. Alpins said that simulated keratometry, used to help ascertain corneal astigmatism, relies on only one ring of topography readings—the seventh ring, located roughly over the 3 mm zone. "The thing about using only one ring when the topographers have up to 32 rings is that it's a bit of a lucky s hot," Dr. Alpins said, adding that in a fat cornea the ring will sit over the 4 mm zone and in a steep one over the 2 mm zone. "It doesn't measure the same point in the cornea every time if you use only one ring," he said. To try to obtain better results, Dr. Alpins has been using the param- eter corneal topographic astigma- tism (CorT). For this, he considered all of the rings and all of the data captured on corneal topography and took the vectorial average of them all. "What we showed was the CorT value of the anterior cornea was so much more accurate than the SimK, manual keratometry, paraxial curvature matching and corneal wavefront," Dr. Alpins said. "It was the best of them all." Dr. Alpins is now using three machines to ascertain total corneal power—the Sirius Corneal Topographer (CSO, Florence, Italy), Pentacam (Oculus, Arlington, Wash.), and Ziemer (Port, Switzer- land). Together these offer a CorT number that is more accurate not just for the anterior cornea but for the whole cornea, Dr. Alpins said, adding that this will give practition- ers the number needed for LRI and toric IOL placement. Douglas D. Koch, MD, profes- sor and the Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute, Baylor College of Medicine, contends there's a vari- ability introduced into femtosecond measurements by two things that don't currently get considered. "One of them was pointed out by Dr. Holladay in terms of toric lenses, and that is the fact that there is less effective toricity as the anterior chamber deepens and as the IOL power diminishes," Dr. Koch said. With astigmatism measurement in general, there is a lot of variability in anterior corneal measurement. "You do measurement with three or four devices and it's astonishing the amount of variability that occurs." The second issue is the posterior cornea, which introduces an ele- ment of astigmatism that can be 0.5 D or more. "We are struggling to get devices that accurately predict it on a patient-by-patient basis," he said. Once that occurs, Dr. Koch thinks, there will be a distinct, incremental improvement in outcomes with the femtosecond lasers. Dr. Holladay pointed out that tilt and decentration can be factors because the human eye is not along the optical axis. The fovea, which has the finest vision, is not at the posterior pole, it's five degrees temporal, allowing people to avoid having to look through all the wires of the optic nerve. As a result we also end up with a lens that's tilted about five degrees. "Both that decen- tration and tilt result in about 0.5 D of against-the-rule astigmatism," Dr. Holladay said, adding that this also includes the against-the-rule astig- matism from the posterior cornea. "That has always been there, but February 2011 Femtosecond phaco techniques March 2014 TORIC/LRI MARKERS S9-2033 Pre-Operative Alignment Marker For marking patient on gurney or at slit lamp prior to procedure • Three fine marking points at 3, 6 and 9 for accurate reference • Properly angled handle allows an unimposing approach to patient S9-2060 Mendez Style Degree Gauge Intra-operative ring for aligning to steep axis and defining LRI points • Large 11.7mm internal diameter provides for maximum visualization • Beveled surface reduces reflections form microscope lighting • Crisp laser etched markings every 5˚ for greater marking accuracy S9-2065 Axis Marker Works inside the Degree Gauge to mark prime meridian (steep axis) for LRIs and Toric alignment. • Fits inside Degree Gauge perfectly • Crisp edges create a fine demarcation • Length of marks accommodates both limbal and corneal marking S9-2050 LRI Marker Provides crisp marks at commonly used LRI Positions • Fits inside Degree Gauge perfectly • Creates symmetrical, opposing marks for 30˚, 45˚ and 60˚ incisions • Fine arc connects degree endpoints for easy, acurate incisions 2500 Sandersville Rd., Lexington, KY 40511 USA Phone: 800-354-7848, 859-259-4924 F ax: 859-259-4926 E-Mail: stephensinst@aol.com www.stephensinst.com Operating continued from page 110 Corneal topographic astigmatism (CorT) better matches the manifest refractive cylinder (R) than simulated keratometry (Sim K), according to Dr. Alpins. Source: Noel Alpins, MD continued on page 114 108-125 Feature_EW March 2014-DL2_Layout 1 3/6/14 3:59 PM Page 112