Goldmann Tonometer Prism with an Optimized Error Correcting Applanation Surface

Abstract

PurposeWe evaluate solutions for an applanating surface modification to the Goldmann tonometer prism, which substantially negates the errors due to patient variability in biomechanics.MethodsA modified Goldmann or correcting applanation tonometry surface (CATS) prism is presented which was optimized to minimize the intraocular pressure (IOP) error due to corneal thickness, stiffness, curvature, and tear film. Mathematical modeling with finite element analysis (FEA) and manometric IOP referenced cadaver eyes were used to optimize and validate the design.ResultsMathematical modeling of the optimized CATS prism indicates an approximate 50% reduction in each of the corneal biomechanical and tear film errors. Manometric IOP referenced pressure in cadaveric eyes demonstrates substantial equivalence to GAT in nominal eyes with the CATS prism as predicted by modeling theory.ConclusionA CATS modified Goldmann prism is theoretically able to significantly improve the accuracy of IOP measurement without changing Goldmann measurement technique or interpretation. Clinical validation is needed but the analysis indicates a reduction in CCT error alone to less than ±2 mm Hg using the CATS prism in 100% of a standard population compared to only 54% less than ±2 mm Hg error with the present Goldmann prism.Translational RelevanceThis article presents an easily adopted novel approach and critical design parameters to improve the accuracy of a Goldmann applanating tonometer.

Goldmann tonometer error correcting prism: clinical evaluation

“…Low correlation coefficients as seen with the corneal curvature and corneal rigidity correlations are common in clinical IOP studies due to the multiple variables in measurement error 19–21. The assumption is that the biomechanical error relationships are linear when in fact there is evidence that they may be nonlinear which may add to a lower correlation 18. Correlation coefficients >0.40 seen with the CCT, IOP, and multiple regression analysis statistically indicate that the CATS prism is significantly less sensitive to corneal biomechanical errors than GAT IOP measurement.…”

“…Also the direct comparison of IOP measurements between the two prisms averaged over all IOP’s indicated a 1:1 correlation further supporting the lack of difference between the two prisms. The previously published study included cadaver eyes which also indicated negligible difference between the two prisms when comparing to intracameral transducer measured IOP 18. Although the average bias is negligible, ~50% of the population has corneal biomechanical variability which adds significant error to the GAT IOP measurement 6–8…”

“…The CATS tonometer prism, illustrated in Figures 1 and 2, mathematically reduces the GAT IOP measurement error due to all recognized variations in corneal biomechanics by ~50% 18. All physical dimensions, materials, and properties of the CATS prism are identical to the GAT prism with the exception of the curved applanating surface of the prism.…”

“…Mathematical modeling with finite element analysis and manometric IOP referenced cadaver eyes were used to optimize and validate the design. The optimized CATS prism was designed to measure the same pressure as a GAT prism in corneas with measured average corneal properties in corneal thickness, curvature, rigidity, and tear film 18. However, ~50% of the patient population do not have average corneas 6,8,14,15…”

Intraocular Pressure Measurement Accuracy and Repeatability of a Modified Goldmann Prism: Multicenter Randomized Clinical Trial

“…The CATS prism has demonstrated improved IOP accuracy mathematically and by direct GAT comparison, as well as by comparison to transducer intracameral pressure. 12,13,14,15,16 …”