Correlations Between Corneal Biomechanical Properties Measured With the Ocular Response Analyzer and ICare Rebound Tonometry Jorge Manuel Martins Jorge, PhD,* Jose M. Gonza ´lez-Me´ijome, PhD,* Antonio Queiro ´s, MD,* Paulo Fernandes, MD,* and Manuel A. Parafita, PhDw Purpose: To investigate the biomechanical properties of the normal cornea, and correlate them with central and peripheral corneal thickness and age. Methods: Seventy-six right eyes of volunteers were measured with Ocular Response Analyzer (ORA), ICare rebound tono- metry and an ultrasound pachymeter at corneal center and at 4 mm from corneal center in the nasal and temporal directions. Results: ICare readings were significantly correlated with central and peripheral corneal thickness and corneal biomechanical properties. Corneal resistance factor was the biomechanical parameter with the higher correlation with ICare intraocular pressure (IOP) values. ICare tonometry at center and Goldmann equivalent IOP obtained with ORA were significantly higher for thicker than thinner corneas (P <0.05). IOP compensated for corneal properties with the ORA was lower than the remaining IOP values measured in the study. Higher correlation was found between Goldmann equivalent IOP with ORA and ICare IOP values. Conclusions: IOP values obtained with the rebound tonometer are higher in thicker corneas and are positively correlated with biomechanical corneal parameters, namely corneal resistance factor. Although corneal thickness plays a significant role in rebound tonometry, elastic and viscous properties of the cornea seem to play a significant role in the interaction of the tonometer probe with the ocular surface. However, the mechanism behind this process is presently unknown. Key Words: corneal biomechanics, Ocular Response Analyzer, corneal thickness, rebound tonometry, intraocular pressure (J Glaucoma 2008;17:442–448) I ntraocular pressure (IOP) measurements with conven- tional devices have demonstrated to be sensitive to corneal thickness variations. 1–3 In fact, patients with reduced corneal thickness are at a higher risk of suffering from normal or low tension glaucoma and glaucoma damage. 4,5 This is of major relevance in eyes that have undergone refractive surgery. 6 However, the biomecha- nical properties of the cornea can also vary significantly without changes in corneal thickness and this could also affect IOP reliability. In fact, all tonometers that measure through the application of a stress to the corneal tissue are subject to the effects of corneal resistance. This corneal resistance is actually an ‘‘effective property’’ type of parameter. That is, rather than being a specific, intrinsic mechanical parameter like Young’s modulus, corneal resistance is a composite parameter that incorpo- rates the material and geometric properties of the tissue, including the time and spatial dependence of the under- lying corneal material properties. 7 Different instruments have been recently developed for the direct measure of the biomechanical properties of the cornea. A review of those techniques can be found in the literature. 8 However, most recent approaches are based on the noninvasive (air-puff) or invasive (direct contact of a tip) application of a controlled mechanical stress. 8,9 The Reichert ORA (Reichert Inc, Depew, NY) determines corneal biomechanical properties using an applied force-displacement relationship by an air-puff similar to that used in traditional noncontact tonometry. 9 Parameters used to characterize the biomechanical properties of the cornea include corneal hysteresis (CH) and corneal resistance factor (CRF). CH depends on the energy absorbed by the cornea when its tissue is submitted to stress and relaxation, thus inducing a delay in the corneal response to those forces. 9 However, it is necessary to consider that CH and CRF are composite measures, which characterize the structural response of the eye to the measurement device rather than intrinsic material properties of the cornea, such as Young’s modulus. This means that what we call ‘‘biomechanical properties’’ do not represent a specific property of the corneal tissue, but the response of the entire corneal structure to the ORA’s measuring principle. As a result, these properties are only measurable by the ORA—a different device could potentially provide some other Copyright r 2008 by Lippincott Williams & Wilkins Received for publication February 2, 2007; accepted October 20, 2007. From the *Department of Physics (Optometry), School of Sciences, University of Minho, Braga, Portugal; and wDepartment of Surgery (Ophthalmology), School of Optics and Optometry, University of Santiago de Compostela, Santiago de Compostela, Spain. Disclosure: None of the authors has a commercial or financial interest in the instruments or materials used in the study. Reprints: Jorge Manuel Martins Jorge, PhD, Department of Physics (Optometry), Campus de Gualtar, University of Minho, 4710-057 Braga, Portugal (e-mail: jorge@fisica.uminho.pt). ORIGINAL STUDY 442 J Glaucoma  Volume 17, Number 6, September 2008