Corneal Thickness–Correcting Goldmann Applanation Prism Surface
Purpose
Examine the design requirements and present solutions for a simple modification to the Goldmann applanation tonometer which greatly negates the errors induced by a varying corneal stiffness and thickness.
Methods
Design criteria, solution methods, and prototype testing are presented for a simple modified Goldmann applanation tonometer designed to minimize the contribution of corneal stiffness to intraocular pressure measurement. The Design concentrates on modifications to the shape of the applanating surface of the Goldmann applanation prism. Both mechanical and mathematical model design optimization and prototype demonstration of reduced corneal stiffness sensitivity are demonstrated.
Results
A modified Goldmann applanation tonometer prism is able to reduce the corneal thickness sensitivity of patients having corneal thicknesses between 480 and 630 microns to +/- 2 mmHg. This compares to +/-5 mmHg with the standard prism over the same range. The greater accuracy of the modified surface indicates 92% of patients will be measured within 2 mmHg without corneal thickness correction and 77% of patients to within 1 mmHg without correction.
Conclusion
A modified Goldmann Applanation prism is able to greatly improve the accuracy of Intraocular pressure measurement. The modification cost is a fraction of what is currently available and used to correct intraocular pressure for both corneal thickness and corneal stiffness. Additional optimization may be able to further improve accuracy and negate the use of corneal pachymetry for all but advanced glaucoma patients.