Ocular Biomechanics: Measurement Parameters and Terminology

Abstract

doi:10.3928/1081597X-20110519-01 T raditionally, biomechanical properties of the cornea have been assessed in vitro by measuring stress-strain and Young modulus on corneal strips. In 2005, the Ocular Response Analyzer (ORA; Reichert Technologies, Depew, New York) was launched as the fi rst commercial device claiming to provide in vivo measurements of corneal biomechanics.1 It utilizes a dynamic bi-directional applanation process in which two applanation pressure measurements are recorded: one while the cornea is moving inward, and the other as the cornea returns. The primary output measurements are Goldmann-correlated intraocular pressure (IOPg), and a new parameter called corneal hysteresis (CH), defi ned by Reichert as the difference between the two pressure values of the inward and outward applanations. The ORA provides two additional new parameters: corneal-compensated intraocular pressure (IOPcc) and corneal resistance factor (CRF), which was thought to be an indicator of the overall “resistance” of the cornea. Given the promising nature of the possibility of measuring biomechanics in vivo, many studies covering a wide range of topics have been performed and published using the ORA in the past 5 years. The majority of articles addressed the following topics: 1) IOP, CH, and CRF measurements in patients with glaucoma, 2) changes in CH and CRF after corneal refractive surgery, and 3) changes in CH and CRF in keratoconic patients and the possibility of using CH and CRF for assisting in the detection of early keratoconus. With this initial body of articles published, we can now start to evaluate the clinical utility of the ORA in refractive surgery. This editorial reviews the use of the ORA for measuring IOP, CH, CRF, and future parameters and discusses the commonly used terminology. IOP MEASUREMENTS Corneal-compensated IOP has been shown to be comparable to IOP measured with dynamic contour tonometry,2 and IOPcc has also been shown to be independent of central corneal thickness.3 Therefore, as the proportion of patients electing to have corneal refractive surgical procedures continues to increase, IOPcc will remain a valid and powerful tool to diagnose and monitor patients with glaucoma after corneal surgery.