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Jun Liu, Junhua Tang, Xueliang Pan, Paul A. Weber; Effect of Corneal Modulus and Thickness on Tonometric Measurements of IOP - Experimental Studies in Enucleated Canine Eyes. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6268.
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Numerical models have predicted that corneal stiffness could significantly affect the accuracy of tonometric measurement of intraocular pressure (IOP). The purpose of this study was to experimentally examine the effect of corneal stiffness and central corneal thickness (CCT) on Goldmann Applanation Tonometry (GAT) and Tonopen measurements of IOP in a canine eye model.
Thirteen enucleated canine globes were obtained within 2 hours postmortem. CCT was measured by using an ultrasound pachymeter (DGH-550BPL, DGH Technology). IOP was manometrically controlled by using a saline column adjusted to 15 mmHg or 30 mmHg and confirmed by a pressure sensor that was connected to the anterior chamber. IOP was measured by using a GAT (AT900, Haag Streit, Switzerland) and a Tonopen (Tono-pen XL, Reichert, Inc.). Both devices were calibrated prior to experiments and re-checked at the completion of the experiments. Each pressure measurement was repeated three times and the average was used for further analysis. Corneas were then dissected into 3.5mm by 10mm strips. The stress-strain relationship of the corneal strips was measured by using a Rheometrics System Analyzer (RSA III, TA Instruments, New Castle, DE).
The mean CCT was 589.2±55.2 µm in the measured canine eyes. At a true IOP of 15 mmHg and 30 mmHg, the GAT readings were 5.3±1.8 mmHg and 17.1±1.7 mmHg, respectively. The corresponding Tonopen readings were 12.4±1.6 mmHg and 22.1±2.4 mmHg. The mean Young’s modulus (secant modulus at 1% strain) of the dissected canine corneal strips was 1.55±0.34 MPa. The correlation between corneal stiffness (i.e., Young’s modulus) and GAT or Tonopen error was 0.33 or 0.37 at 15 mmHg, and 0.59 or -0.30 at 30 mmHg. The correlation between CCT and GAT or Tonopen error was 0.01 or 0.09 at 15 mmHg, and 0.09 or 0.06 at 30 mmHg.
Both GAT and Tonopen underestimated IOP in canine eyes. This is likely due to these devices being calibrated for human eyes with properties and dimensions different from those of canine eyes. Tonopen appeared to be more accurate than GAT for IOP measurements in canine eyes. Our results also provide preliminary experimental evidence for a correlation between corneal stiffness and tonometric errors that is likely stronger than the correlation between CCT and tonometric errors. Furthermore, the influence of the corneal properties on tonometric measurement errors appeared to be different for normal and high true IOP levels. Future studies should verify these findings in human eyes with a larger sample size.
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