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Introducing and measuring cornea and sclera deformability parameters on the basis of Schiøtz tonometry: mathematical modeling and clinical evaluation in Primary Open Angle Glaucoma (POAG)
Author(s) -
Iomdina E.N.,
Lyubimov G.,
Moiseeva I.,
Stein A.,
Kiseleva O.,
Archakov A.
Publication year - 2016
Publication title -
acta ophthalmologica
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.534
H-Index - 87
eISSN - 1755-3768
pISSN - 1755-375X
DOI - 10.1111/j.1755-3768.2016.0488
Subject(s) - sclera , cornea , ophthalmology , glaucoma , medicine , open angle glaucoma , intraocular pressure
Purpose To obtain additional information on the biomechanical status of the eyeball on the basis of static loading by the Schiøtz tonometer, to be applied to POAG .Methods Earlier, we proposed a mechanically correct theory of Schiøtz tonometry, which represents the eyeball as a shell characterized by two elastic constants separately responsible for the cornea and the sclera properties, and used the theory here. We examined 41 patients (43 eyes) aged 55–72 (mean age 64 ± 4.0 years) with non‐operated POAG, including 20 eyes with early and 23 eyes with advanced stage of POAG. 15 patients with no eye pathology of the same age group served as control. The examination included differential Schiøtz tonometry using a GlauTest 60 tonograph. Results Applicability limits are estimated for the standard relations used in processing the clinical data. Based on differential tonometry, we introduced a coefficient γ , equal to the ratio of the IOP difference to the plunger weight difference, and looked at how γ depends on the cornea and sclera stiffness and the true IOP. The γ coefficient, which mainly characterizes the scleral rigidity, was determined at 1.65 ± 0.25 mm Hg/g for the early stage of POAG and 1.88 ± 0.13 mm Hg/g for the advanced POAG stage, with a control group average of 1.47 ± 0.10 mm Hg/g (p < 0.05). On average, γ was 27% greater for the POAG patients, whereas the mean Friedenwald's rigidity coefficient only differed from the norm by 7%. Conclusions Changes in the mechanical parameter proposed testify to changes in the elastic characteristics of the eyeball in POAG. The coefficient γ reflects individual abnormalities of the biomechanical status of the sclera for the glaucomatous eye more adequately than Friedenwald's rigidity coefficient. Practical recommendations are proposed. The study was supported by RFBR (project No. 14‐01‐00475).