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M. Neuburger, D. Boehringer, T. Reinhard, J. F. Jordan; Dependency of the Corneal Hysteresis on the Intraocular Pressure - An in vitro Model. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5553.
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Goldmann Applanation Tonometry (GAT) is still the gold standard for measuring intraocular pressure (IOP). Nevertheless, several influencing factors such as central corneal thickness (CCT) have been discussed. The Ocular Response Analyzer (ORA) is a new device for measuring IOP. It is designed to take into account the biomechanical properties of the cornea during the measurement. It therefore calculates the so-called corneal hysteresis (CH) and a corneal resistance factor (CRF). We aimed at investigating whether the CH really describes an independent biomechanical property of the cornea or whether this parameter itself depends on IOP.
We determined the CH of 9 donor corneas at different IOP levels. The corneas were therefore clamped into an artificial anterior chamber developed by our group. IOP in this artificial anterior chamber was adjusted manometrically to 10, 20, 30, 40 and 50mmHg. At the beginning and at the end of each measurement, we determined the CCT by ultrasonic pachymetry. We also determined IOP by the iCare, TonoPen and Perkins tonometer. CH and CRF were obtained using the ORA.
We found a statistically highly significant dependency of the CH from the adjusted IOP: CH decreased with increasing IOP. Mean value at 10mmH was 14, at 20mmHg 16, at 30mmHg 13, at 40mmHg 5 and at 50mmHg 2. CRF showed no dependency on IOP and fluctuated between 14 and 19. CCT had no influence neither on CH nor on CRF. Most accurate IOP readings were obtained by the TonoPen, followed by the iCare, the Perkins tonometer and the ORA.
Our results suggest that the CH as measured by the ORA is not an independent biomechanical property of the cornea. Either CH itself is dependent on the IOP, or the ORA is incapable to measure CH accurately at higher IOP levels. This is an important new finding for the clinical interpretation of the IOP measurement obtained by the ORA. It also underlines the impact of the biomechanical properties of the cornea during IOP measurement.
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