Treatment-induced shifts of ocular reference axes used for measurement centration Michael Bueeler, PhD, Hans Peter Iseli, MD, Mirko Jankov, MD, Michael Mrochen, PhD PURPOSE: To determine the shifts of the main corneal reference points in dependence of the chosen centration axis for the treatment. SETTING: Federal Institute of Technology Zurich, Institute of Biomedical Engineering, Zurich, Switzerland. METHODS: Computer simulations were performed on several variants of the Gullstrand-Emsley sche- matic eye, which was modified by an off-axis fovea. Refractive corrections were simulated by centering Munnerlyn’s formula on each of the 4 corneal reference points determined in the preoperative eye: the optical axis, the line of sight, the visual axis, and the first corneal reflex. Subsequently, the postoper- ative locations of these axes were determined and compared with the preoperative values. RESULTS: The postoperative line of sight was found to depend least on the choice of the preoperative centration axis for both myopic and hyperopic treatments. It undergoes a maximum movement of 0.040 mm when centering a C5 diopter correction on the preoperative line of sight, whereas the cor- neal reflex, which is used for centering most topography systems, can move by more than 0.10 mm. CONCLUSIONS: Centration of the correction on the preoperative line of sight enabled good compa- rability between preoperative and postoperative measurements that use the line of sight as a reference axis. Yet, centration of the treatment on the preoperative line of sight does not ensure comparability between preoperative and postoperative measurements that use the corneal reflex as a reference axis such as most corneal topography systems. Axis shifts might lead to misinterpretation of data such as a wrong diagnosis of a decentered ablation or changes in the Zernike representation. J Cataract Refract Surg 2005; 31:1986–1994 Q 2005 ASCRS and ESCRS Optical measurements and corneal surgical procedures re- quire proper alignment on the cornea because even small decentration from a determined reference axis will intro- duce new types of optical aberrations. 1–4 However, align- ment of a measuring or treatment device is not a simple task because of the lack of direct target points on the trans- parent cornea. A number of ocular axes can be defined to describe the optical properties of the eye. The points of intersection of these axes with the corneal anterior surface can be used for centering ocular surgical procedures as well as measure- ment devices. Unlike most technical optical devices, the human eye is not a centered optical system and does not contain a true optical axis because the cornea and the lens are slightly decentered and tilted relative to each other. This fact further complicates the determination of ocular reference axes in a clinical environment. In all centering methods, the patient is advised to fixate on a target light, usually located on the optical axis of the technical system. Based on this general principle, the system axis can be cen- tered on various reference points on the eye that are deter- mined by the investigator. The following axes are defined in the eye: the optical axis, the line of sight, the visual axis, the pupillary axis, and the line of the coaxially sighted corneal reflex Accepted for publication March 14, 2005. From the Swiss Federal Institute of Technology Zurich (Bueeler, Mrochen), Institute of Biomedical Engineering, and Institute for Refractive and Ophthalmic Surgery (Bueeler, Iseli, Mrochen), Zurich, Switzerland, and Accuvision (Jankov), London, United Kingdom. Supported by the Swiss National Science Foundation, Bern, Switzerland. No author has a financial or proprietary interest in any material or method mentioned. Reprint requests to Michael Mrochen, PhD, IROC Institute for Refractive and Ophthalmic Surgery, Stockerstrasse 37, CH-8002 Zurich, Switzerland. E-mail: michael.mrochen@iroc.ch. Q 2005 ASCRS and ESCRS Published by Elsevier Inc. 0886-3350/05/$-see front matter doi:10.1016/j.jcrs.2005.03.068 1986 J CATARACT REFRACT SURG - VOL 31, OCTOBER 2005