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H.E. Kanngiesser, M. Nee, C. Kniestedt, C. Inversini, R.L. Stamper; Simulation of Dynamic Contour Tonometry Compared to an In Vitro Study Revealing Minimal Influence of Corneal Radius and Astigmatism . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2641.
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Purpose: The Dynamic Contour Tonometry (DCT) is the first direct and non-invasive procedure for intra-ocular pressure (IOP) measurement. To predict the influence of corneal topography on tonometric readings the behavior of cornea and tonometer was simulated. To investigate experimentally these influences tonometric measurements on human cadaver eyes were performed using DCT, Goldmann Applanation Tonometry (GAT) and Pneumotonometry (PTG) simultaneously. Methods: To perform the simulation explicit mathematical methods served for calculation of the geometry and the capillary forces. Finite element methods furnished models for the cornea and the pressure sensor, and served for calculation of the force distribution between tonometer and cornea. In the in vitro study sixteen freshly enucleated human eyes were dehydrated using Dextran until relatively stable Central Corneal Thickness was achieved. Corneal Radius (CR) and Astigmatism (AS) were measured for each eye using Orbscan. The eyes were then connected to an intracameral manometric system varying intraocular pressure from 10 to 35mmHg. DCT, GAT and PTG readings were compared with direct manometry. Results: The simulation revealed inaccuracies in the theory introduced by Goldmann et al. (1957) concerning the capillary force and the tension in the cornea that misled former approaches of direct non-invasive tonometry. Simulation predicted a weaker influence of corneal topography on DCT than on GAT. In the in vitro study DCT measurements on human cadaver eyes differed on average 0.38±0.44mmHg from the corresponding manometric readings, whereas in GAT and PTG these differences were substantially higher, with -3.36±1.22mmHg and -3.68±2.01mmHg, respectively. No significant relationship was found between CR and the DCT (R2= 0.0011; P=0.76) and GAT readings (R2=0.031; P=0.10). The PTG readings, however, were significantly correlated to CR (R2=0.080; P<0.01). Again no significant relationship was found between AS and DCT readings (R2=0.014; P=0.28), whereas GAT (R2=0.35; P<0.001) and PTG (R2 =0.13; P<0.001) were significantly correlated, with slopes of -1.63mmHg/mm and -1.29mmHg/mm, respectively. Conclusions: In the in vitro model, corneal astigmatism appears to influence both Goldmann applanation and Pneumotonometry measurements. Pneumotonometry may also be influenced by corneal radius. In contrast, Dynamic Contour Tonometry appears to be relatively independent of both corneal radius and astigmatism.
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