A modified surface replacement prism is statistically equivalent to a flat-surfaced prism. The modified surface prism indicated statistically significant correction for CCT requiring further testing outside the ANSI standard limits (0.500 mm < CCT < 0.600 mm) to examine its full potential.

PurposeClinically evaluate intraocular pressure (IOP) measurements taken with a Goldmann applanation tonometer (GAT) prism and a modified surface Goldmann prism examining measurement differences correlated to central corneal thickness (CCT) and corneal hysteresis (CH) values.DesignProspective, open-label, randomised, controlled, multicentre reference device accuracy analysis.MethodsA GAT and a modified surface GAT prism measured IOP on 243 unique eyes. The study design and methodology complied with International Standard Organization (ISO) tonometer evaluation guidelines, except the inclusion of thin (<500 µm) and thick (>600 µm) corneas. All eyes were randomised to IOP measurement by one of five standard Goldmann prisms and five modified prisms. Pressures were measured by six investigators, two times with each prism for a total of 1936 IOP measurements. Analysis included a multiple linear regression including CCT and CH correlation.ResultsThe difference in IOP measurements of the standard and modified Goldmann prisms correlated well to CCT particularly in thin (<500 µm) and thick (>600 µm) corneas (R2=0.404, p=0.007). Corneal hysteresis (CH) also significantly correlated to the difference in prism measurements (R2=0.125, p=0.039). There was no significant overall mean IOP bias between the two prisms (+0.43 mm Hg in modified, p=0.19).DiscussionThe paired IOP measurement difference between GAT and a modified surface Goldmann replacement prism indicated a statistically significant correlation to CCT and CH. A simple modified replacement prism for any Goldmann-type tonometer may significantly improve IOP measurement accuracy by minimising corneal biomechanical errors associated with CCT and CH.Trial registration numberNCT02990169 and NCT02989909.