Development and validation of a correction equation for Corvis tonometry

Primary objective: This study uses numerical analysis and validation against clinical data to develop a method to correct intraocular pressure (IOP) measurements obtained using the Corvis Tonometer for the effects of central corneal thickness (CCT), and age. Materials and Methods: Finite element analysis was conducted to simulate the effect of tonometric air pressure on the intact eye globe. The analyses considered eyes with wide variations in IOP (10-30mmHg), CCT (445-645 microns), R (7.2-8.4mm), shape factor, P (0.6-1) and age (30-90years). In each case, corneal deformation was predicted and used to estimate the IOP measurement by Corvis (CVS-IOP). Analysis of the results led to an algorithm relating estimates of true IOP as a function of CVS-IOP, CCT and age. All other parameters had negligible effect on CVS-IOP and have therefore been omitted from the algorithm. Predictions of corrected CVS-IOP, as obtained by applying the algorithm to a clinical data-set involving 634 eyes, were assessed for their association with the cornea stiffness parameters; CCT and age. Results: Analysis of CVS-IOP measurements within the 634-large clinical data-set showed strong correlation with CCT (3.06mmHg/100microns, r(2)=0.204) and weaker correlation with age (0.24mmHg/decade, r(2)=0.009). Applying the algorithm to IOP measurements resulted in IOP estimations that became less correlated with both CCT (0.04mmHg/100microns, r(2)=0.005) and age (0.09mmHg/decade, r(2)=0.002). Conclusions: The IOP correction process developed in this study was successful in reducing reliance of IOP measurements on both corneal thickness and age in a healthy European population.