Goldmann Tonometry Correction Factors Based on Numerical Simulation

Abstract

Purpose: : To develop a correction factor to improve the accuracy of intraocular pressure (IOP) measurements made by the Goldmann Applanation Tonometer (GAT). For the first time, the correction factor is able to consider simultaneously the effects of variations in central corneal thickness (CCT), curvature (R) and age.

Methods: : Numerical simulation based on nonlinear FE analysis was used to create representative models of the GAT procedure. The analysis considered corneal hyperelasticity, non-uniform thickness and aspherity. The simulation was used in a multi-dimensional study covering eyes with CCT between 0.42 and 0.72 mm, R = 7.2-8.4 mm, IOP = 10-30 mmHg, and age = 50-95 years. In each case, the IOP measurement made by GAT (IOPG) was estimated. The database of input and output parameters was analysed to extract a correction factor equation of the form:Correction factor = IOPG/IOP = A_CCT . A_R . A_Age . A_IOPGwhere the A terms were dependent on the values of the four corneal parameters. The results of the numerical study were compared against earlier correction factors capable of handling variations in either CCT or R. The correction factor was also applied to clinical data obtained at Moorfields Hospital for 532 eyes to assess its effect on the associations between IOPG and the corneal parameters (CCT, R, age).

Results: : Analysis of the multi-dimensional database of input and output parameters resulted in the following values of the correction factor parameters;A_CCT = 2 (CCT - 0.52)² + 1.4 (CCT - 0.52) + 0.5A_R = 1 - 0.1 (R - 7.8)A_Age = 0.0007 (age - 50)² - 0.004 (age - 50) + 2A_IOPG = (IOPG + 39)^-0.5The values of the correction factor matched closely those derived by Orssengo and Pye and by Liu and Roberts, which considered only the variation in either CCT or R. Further, applying the correction factor to the Moorfields clinical dataset resulted in considerable reductions (>70%) in the degrees of association between IOPG and corneal parameters.

Conclusions: : The GAT correction factor developed in this study was the first to consider simultaneously variations in corneal thickness, curvature and age. The results matched closely earlier single-parameter correction factors, and could significantly reduce the reliance of IOPG on corneal dimensions and stiffness. This study demonstrates the potential benefit of numerical simulation in modelling ocular procedures.#

Keywords: intraocular pressure