Tonometry after Intrastromal Corneal Ring Segments for Keratoconus

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Reliable intraocular pressure (IOP) measurement after intrastromal corneal ring segments (ICRS) implantation is a challenge because of altered corneal morphology. In this study, IOP is measured with four tonometers, compared with Goldmann applanation tonometry (GAT) values and the influence of corneal parameters is established.


This study compares IOP measurements made using different tonometers in patients implanted with ICRS and assesses the effects of central corneal thickness (CCT), corneal curvature, and corneal astigmatism on the IOP measurements obtained.


In this cross-sectional study, IOP was measured using three different tonometers in 91 eyes of 91 patients with corneal ectasia implanted at least 6 months previously with ICRS. The tonometers tested were the TonoPen XL, Pascal dynamic contour tonometer (DCT), and iCare Pro rebound tonometer. GAT measurements were used as reference. Agreement among the IOPs provided by the different tonometers and the influence of corneal variables on the IOP measurements obtained were assessed using the Bland–Altman method, intraclass correlation coefficients, and multiple linear regression analysis.


Mean IOP differences were GAT versus TonoPen XL −0.8 ± 3.07 mm Hg, GAT versus DCT −1.0 ± 3.26 mm Hg, and GAT versus iCare Pro 0.8 ± 2.92 mm Hg. Our multiple linear regression analysis identified CCT as a confounding factor affecting all the tonometer readings but DCT-IOP.


In patients fitted with ICRS, IOP measurements made using the iCare Pro and TonoPen XL showed most agreement with GAT. Intraocular pressure measurements made by DCT were unaffected by corneal topographic factors though this procedure slightly overestimated GAT readings.

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