Non-Invasive Ocular Rigidity Measurement: A Differential Tonometry Approach.

Efstathios T Detorakis,Emmanuela Tsaglioti,George Kymionis

doi:10.14712/18059694.2015.99

Efstathios T Detorakis, Emmanuela Tsaglioti + Show 1 more

Open Access

https://doi.org/10.14712/18059694.2015.99

Copy DOI

Abstract

Taking into account the fact that Goldmann applanation tonometry (GAT) geometrically deforms the corneal apex and displaces volume from the anterior segment whereas Dynamic Contour Tonometry (DCT) does not, we aimed at developing an algorithm for the calculation of ocular rigidity (OR) based on the differences in pressure and volume between deformed and non-deformed status according to the general Friedenwald principle of differential tonometry. To avoid deviations of GAT IOP from true IOP in eyes with corneas different from the "calibration cornea" we applied the previously described Orssengo-Pye algorithm to calculate an error coefficient "C/B". To test the feasibility of the proposed model, we calculated the OR coefficient (r) in 17 cataract surgery candidates (9 males and 8 females). The calculated r according to our model (mean ± SD, range) was 0.0174 ± 0.010 (0.0123-0.022) mmHg/μL. A negative statistically significant correlation between axial length and r was detected whereas correlations between r and other biometric parameters examined were statistically not significant. The proposed method may prove a valid non-invasive tool for the measurement method of OR, which could help in introducing OR in the decision-making of the routine clinical practice.

Highlights

Ocular Rigidity (OR) is an important property of the ocular tissues associated with their resistance to mechanical deformation [1]
The latter displays a dynamic change in its elastic properties due to constant changes in the amount of blood contained in uveal vessels in response to a variety of physiological factors, such as the cardiac cycle, respiratory movements or intraocular pressure (IOP) changes [6]
To correct for deviations of Goldmann applanation tonometry (GAT) IOP from true IOP in eyes with corneas different from the “calibration cornea” we applied the Orssengo-Pye algorithm [17]: Taking into account that the basic definition of OR relates with the association between pressure and volume changes in the eyeball, we explored the possibility to take advantage of the “delta”, i.e. the difference (ΔIOP) between applanation (GAT) and non-applanation (DCT) tonometry, in association with the volume displaced during the applanation phase of GAT

Summary

Introduction

Ocular Rigidity (OR) is an important property of the ocular tissues associated with their resistance to mechanical deformation [1]. In the case of the eyeball, which is filled with incompressible content (aqueous humor and gel-like vitreous body), rigidity is affected by the elastic properties of the sclera and cornea and by other factors, such as the vascular uveal layer [5]. In the case of the cornea, bio-mechanical behaviour includes a visco-elastic or anisotropic element, implying that that the rate at which a load is applied changes the measured value for cornea’s Young’s modulus [7,8,9] The latter describes the resistance of corneal tissue to mechanical deformation and corresponds to the relation between tensile strain and tensile stress of corneal tissue [9]. Reported corneal Young’s modulus values range from 0.159 MPa to 57 Mpa (mean 0.29 ± 0.06 Mpa) [7,8,9], reflecting the complexity of ex-vivo corneal bio-mechanical behaviour

Results

Discussion

Conclusion