Multiparameter Correction Equation for Goldmann Applanation Tonometry

Purpose. To develop a correction factor to improve the accuracy of intraocular pressure (IOP) measurements made by the Goldmann applanation tonometer (GAT), which considers the combined effects of variations in central corneal thickness (CCT), central anterior curvature (R), age, and the IOP level itself. Methods. Nonlinear numerical simulations based on the finite element method were used to represent corneal behavior under the effect of IOP and external tonometric pressure. The simulations considered various biomechanical corneal properties including the cornea's nonuniform thickness, elliptical topography, weak stromal interlamellar cohesion, low epithelial and endothelial stiffness, and hyperelastic and hysteretic material behavior. The simulations were used to model the GAT procedure on corneas to obtain a correction equation based on the values of CCT, R, age, and IOP measured using GAT (IOPG). The efficiency of the equation in reducing the effects of corneal parameters on IOPG measurements was also assessed using an independent clinical database. Results. The individual effects of variations in CCT, R, and age were estimated at 1.66 mm Hg/100 mu of CCT, 0.89 mm Hg/1 mm of R, and 0.12 mm Hg/decade of age. The correction equation reduced the association between clinical IOP measurements and corneal parameters with r(2) reducing from 11.8 to 0.02%. Conclusions. The GAT correction factor can consider the combined effect of variations in corneal thickness, curvature, age, and IOP. The factor could significantly reduce the reliance of IOPG measurements on corneal stiffness parameters. (Optom Vis Sci 2011; 88: E102-E112)