Viscoelastic properties of the iris in different types of glaucoma

Introduction. The iris is involved in maintaining the ocular hydrodynamic homeostasis as one of the elements of the anterior chamber angle of the eye. From there, the iris is one of the parts of its drainage area. The anterior chamber angle’s structure can predispose to an increase in intraocular pressure and thereby provoke the development of primary angle-closure glaucoma, a disease accompanied by clogging of the drainage area of the eye and, accordingly, an increase in intraocular pressure.

Objective. To analyse the viscoelastic properties of the iris in primary angle-closure glaucoma and primary open-angle glaucoma.

Materials and methods. The study material was a fragment of the iris obtained through iridectomy in the course of sinus trabeculectomy, which is considered the treatment for glaucoma surgery. A total of 43 samples (43 patients) were obtained and analysed, with 20 samples obtained from patients with primary angle-closure glaucoma (group 1), and 23 samples from patients with primary open-angle glaucoma (group 2). A standard ophthalmological examination, which included visometry, autorefractometry, tonometry, biomicroscopy, and indirect ophthalmoscopy, was carried out. The special examination included static perimetry, gonioscopy, ultrasound biomicroscopy or optical coherence tomography of the anterior chamber angle, optical coherence tomography of the disk of optic nerve.

Results and discussion. The avascular part of the stroma (designated by us as S) is the most rigid part of the iris, according to the study. It is characterized by the greatest efforts of the viscous dynamic resistance to the indenter (0.4–2.0 gf/мм2 × S), as well as the maximum integral tensile strength (up to 4.9 × 10-2 N). The inner (vascular) part of the stroma has the lowest dynamic viscosity, according to this study.

Conclusion. The samples vary enormously both in the biomechanical characteristics and relative thickness of this layer. It was observed that this biomechanically incompetent structure completely collapses and ceases to exist in partial dehydration of the sample. 

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