BASIC INVESTIGATION Effect of Limbal Relaxing Incisions During Phacoemulsiﬁcation Surgery Based on Nomogram Review and Numerical Simulation Jose ´ A. Cristo ´ bal, MD,* Marı ´a A. del Buey, MD,* Francisco J. Ascaso, MD,* Elena Lanchares, Eng,†‡ Begon ˜a Calvo, PhD,†‡ and Manuel Doblare ´, PhD†‡ Purpose: To determine the effect of the parameters related to limbal incisions (length, depth, optical zone, and so on) by revision of different nomograms and analyzing the outcomes of numerical simulation of incisions with a biomechanical model of the cornea. Methods: The Cristo ´bal nomogram was developed based on our experience on the performance of relaxing incisions to correct astigmatism. A numerical model of the eye was used to analyze the effect induced by variation of each parameter compiled in nomo- grams. The biomechanical properties remained invariable for all cases. Different incisions were simulated to study the inﬂuence of each parameter, being one varied while the others remain constant, under equal biomechanical conditions, ignoring the human factor. Results: Quantitatively, simulation of incisions of 45, 60, and 90 degrees; optical zone of 10 mm; and depth of 90% of the thickness induced astigmatic changes of 1.2, 1.4, and 1.9 diopters (D). Paired incisions at 7.5-mm optical zone and 90-degree length induced astigmatic changes of 0.7, 2.6, and 4.4 D for 40%, 75%, and 90% depth, respectively. Paired incisions of 90-degree length and 90% depth for 10- and 7.5-mm optical zones induced 1.9 and 4.4 D, respectively. Qualitatively, the results conﬁrm the guidelines compiled in nomograms. Conclusions: The revision of some nomograms for limbal incisions to correct astigmatism, compared with the outcomes of numerical simulation, leads to common guidelines. Numerical simulation supplies theoretical outcomes that the clinician can take into account to decide the values of the parameters for the surgery, in addition to their clinical experience. Key Words: astigmatism, incisional surgery, nomogram, corneal biomechanics, refractive surgery simulation (Cornea 2009;28:1042–1049) T he original meaning of the word nomogram is a graphical calculating device, a 2-dimensional diagram designed to allow the approximate graphical computation of a function. However, regarding ophthalmology praxis, the nomograms are a set of tables of corrected values or graphics, compiling the authors’ outcomes, which are used as a personal ‘‘abaqus’’to plan the surgical technique. Regarding incisional surgery, nomograms are a reference of the outcomes achieved by a set of values assigned to the parameters that characterize the incisions: length, depth, distance to the optical axis, and so on. 1 There are several nomograms in literature, and they manage the effect of the incisions by the modiﬁcation of some of the parameters, the others remaining constant. The Department of Ophthalmology of the Clinical Uni- versity Hospital ‘‘Lozano Blesa’’ in Zaragoza, Spain, developed a nomogram for limbal incisions, the Cristo ´bal nomogram, 2 which is shown in Figure 1. A schematic form of this no- mogram showing separately the inputs and the outputs is shown in Figure 2. This nomogram indicates the position of the phacoemulsiﬁcation incision, the parameters of the limbal incisions to correct astigmatism according to the preoperative astigmatism, and also the incisions in clear cornea (usually named arcuates), which should be added in cases with preop- erative astigmatism greater than 3 diopters (D). Figure 3 shows some parameters involved in incisional surgery. Table 1 compares some nomograms regarding the parameters used by the authors. The 4 nomograms (Cristo ´bal et al, 2 Nichamin, 3,4 Gills modiﬁed, 5,6 and Wang et al 7 ) obviously consider the preoperative astigmatism, in diopters, to be corrected. Other relevant parameters, such as number of incisions and length of the incisions, are also included in the nomograms by the authors. Nevertheless, other parameters are not generally considered, namely, age, depth of the incisions, and type of previous astigmatism: against the rule, with the rule, or oblique astigmatism. Their inﬂuence is estimated in different ways in those nomograms where they are considered. By way of illustration, Cristo ´bal et al 2 considers the age as a factor to increase or decrease the depth of the incision, whereas Nichamin 3 considers the age in periods of 10 years (from 20 to 80 years old) where the length of arc instead of Received for publication June 4, 2008; revision received January 14, 2009; accepted February 7, 2009. From the *Department of Ophthalmology, Clinical University Hospital ÔLozan‘o BlesaÕ Zaragoza, Zaragoza, Spain; †Group of Structural Mechanics and Material Modelling (GEMM), Arago ´n Institute for Engineering Research (I3A), University of Zaragoza, Zaragoza, Spain; and ‡CIBER on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Arago ´n Institute of Health (ICS), Zaragoza, Spain. Supported by the Spanish Ministry of Education and Science, Madrid, Spain. Research project FIS2005-05020-C03-03. Reprints: Elena Lanchares, Eng, Mechanical Engineering Department, Group of Structural Mechanics and Material Modelling, Arago ´n Institute for Engineering Research (I3A), University of Zaragoza, Marı ´a de Luna 3, E-50018 Zaragoza, Spain (e-mail: elanchar@unizar.es). Copyright Ó 2009 by Lippincott Williams & Wilkins 1042 | www.corneajrnl.com Cornea  Volume 28, Number 9, October 2009