EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/99908
December 2012 EW RESIDENTS 45 EyeWorld journal club Review of ���Contribution of posterior corneal astigmatism to total corneal astigmatism��� by Ken Downes, M.D., Edie Deniro, M.D., Geoff Wilkes, M.D., Alan Lowinger, M.D., Patrick Coady, M.D., Michael Clamp, M.D., Ako Takakura, M.D., Maya Ling, M.D., Adrian Dokey, M.D., Sally Lin, M.D., and Kevin Denny, M.D., service chief, Cataract and Anterior Segment Surgery, California Pacific Medical Center, San Francisco Kevin Denny, M.D., service chief, Cataract and Anterior Segment Surgery, California Pacific Medical Center (CPMC) This month's JCRS includes the paper that Doug Koch delivered for his Kelman Innovator Lecture at the 2012 ASCRS���ASOA Symposium & Congress. I asked the CPMC residents to review it for this month's journal club. ���David F. Chang, M.D., EyeWorld chief medical editor O ver the years, cataract surgery has evolved to fall squarely within the realm of refractive surgery. Ophthalmologists are continuously looking for ways to further improve the predictability of post-cataract surgery refractive outcomes when choosing intraocular lenses and planning their surgeries. Correcting pre-op astigmatism with toric intraocular lenses and limbal relaxing incisions is now commonplace in ophthalmology, and understanding how to best correct astigmatism is important in the march toward more accurate and predictable surgical outcomes. In their recent article, Douglas Koch, M.D., and his colleagues at the Cullen Eye Institute, Houston, address posterior corneal astigmatism, which may become an important Contribution of posterior corneal astigmatism to total corneal astigmatism Douglas D. Koch, M.D., Shazia F. Ali, B.S., Mitchell P. Weikert, M.D., Mariko Shirayama, M.D., Richard Jenkins, M.D., Li Wang, M.D. J Cataract Refract Surg (December) 2012; 38:2080-2087 Purpose: To determine the contribution of posterior corneal astigmatism to total corneal astigmatism and the error in estimating total corneal astigmatism from anterior corneal measurements only using a dual Scheimp���ug analyzer. Setting: Cullen Eye Institute, Baylor College of Medicine, Houston. Design: Case series. Methods: Total corneal astigmatism was calculated using ray tracing, corneal astigmatism from simulated keratometry, anterior corneal astigmatism, and posterior corneal astigmatism, and the changes with age were analyzed. Vector analysis was used to assess the error produced by estimating total corneal astigmatism from anterior corneal measurements only. Results: The study analyzed 715 corneas of 435 consecutive patients. The mean magnitude of posterior corneal astigmatism was ���0.30 diopter (D). The steep corneal meridian was aligned vertically (60 to 120 degrees) in 51.9% of eyes for the anterior surface and in 86.6% for the posterior surface. With increasing age, the steep anterior corneal meridian tended to change from vertical to horizontal, while the steep posterior corneal meridian did not change. The magnitudes of anterior and posterior corneal astigmatism were correlated when the steeper anterior meridian was aligned vertically but not when it was aligned horizontally. Anterior corneal measurements underestimated total corneal astigmatism by 0.22 @ 180 and exceeded 0.50 D in 5% of eyes. Conclusions: Ignoring posterior corneal astigmatism may yield incorrect estimation of total corneal astigmatism. Selecting toric intraocular lenses based on anterior corneal measurements could lead to overcorrection in eyes that have with-the-rule astigmatism and undercorrection in eyes that have against-the-rule astigmatism. Financial disclosure: The authors received research support from Ziemer Group (Port, Switzerland). In addition, Dr. Koch has ���nancial interests with Alcon (Fort Worth, Texas), Abbott Medical Optics (Santa Ana, Calif.), Calhoun Vision (Pasadena, Calif.), NuLens (Herzliya Pituach, Israel), and OptiMedica Corp. (Sunnyvale, Calif.). Top, left to right: Ken Downes, Edie Deniro, Geoff Wilkes, Alan Lowinger, and Patrick Coady. Bottom, left to right: Michael Clamp, Ako Takakura, Maya Ling, and Adrian Dokey Source: Geoff Wilkes, M.D. player when planning to correct pre-op astigmatism. In this large, single-center, retrospective case series, the authors analyze measurements gathered from a Galilei Dual Scheimpflug Analyzer (Ziemer Ophthalmic Systems, Port, Switzerland) from 715 corneas of 435 consecutive patients presenting to their clinic for cataract and refractive surgery assessment from January 2008 to March 2011. The study���s intended purpose was to determine both the contribution of posterior corneal astigmatism to overall corneal astigmatism and the error introduced when estimating total corneal astigmatism from anterior corneal measurements alone. They included an age-stratified group of patients all with ���good-quality Scheimpflug analyzer scans,��� an absence of previous ocular trauma, ocular surgery, corneal or ocular disease, and a lack of contact lens wear for 2 weeks prior to measurements. Results included reporting magnitude of posterior corneal astigmatism, correlations of anterior and posterior astigmatism, age-related changes in anterior and posterior astigmatism, and the error from estimating corneal astigmatism from anterior corneal astigmatism alone. Regarding the study���s methods, we felt further explanation of why an eye might not have a ���good quality��� scan may prove useful for most ophthalmologists who presumably do not have access to a Scheimpflug analyzer in their own offices. Did these eyes have any common characteristics that will allow an ophthalmologist to predict poor quality Scheimpflug analysis? Can anything be gathered from the data of the poor quality scans? Are there any trends in posterior corneal astigmatism in these eyes despite the poor scans? When considering the magnitude of posterior corneal astigmatism found in this study, it is encouraging to note that the authors��� findings are quite complimentary to several previous studies, including the largest by Ho et al.,1 which used an alternate Scheimpflug analyzer (Pentacam, Oculus, Lynnwood, Wash.). While the authors do note that a weakness of this study is their inability to validate the accuracy of posterior corneal measurements, the similar findings between studies certainly seem to lend support to each other���s findings. When discussing the magnitude of posterior corneal astigmatism, the authors report it exceeding 0.50 D in 9% of eyes in their study. We think it might be interesting to know which type of eyes these outliers represent. Do these 9% of eyes have anything in common? Would sub-analysis yield any clues to better determine which patients might continued on page 46