ISSN 2415-3060 (print), ISSN 2522-4972 (online)
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JMBS 2020, 5(1): 135–142
https://doi.org/10.26693/jmbs05.01.135
Clinical Medicine

Results of Modeling the Stress–Strain State of the Eye Cornea with Extraocular Muscles Pathology

Kochina M. L. 1, Kovtun N. M. 2
Abstract

A significant share of eye diseases in children is occupied by the pathology of oculomotor apparatus accompanied by strabismus and visual binocular disturbances. Strabismus, in addition to a cosmetic defect, is accompanied by serious disorders of the child's binocular and monocular functions. According to various authors, 0.5–3.5% of children suffer from apparent strabismus. In the structure of children's eye pathology in Ukraine, strabismus takes the second place after refraction anomalies. Its specific gravity is 25.6% of cases. The determination of the structural and functional state of extraocular muscles still presents certain difficulties, since most of the existing methods are subjective and require personal contact of the doctor with the patient, which is difficult in the case of young children or people with mental disabilities. On the other hand, the same deviations of the eye can occur with various structural and functional disorders of the extraocular muscles, or without them, when there are other reasons associated with refractive errors, sensory disturbances, etc. For the differential diagnosis of the pathology of extraocular muscles and other causes of strabismus, as well as the choice of tactics for surgical treatment, objective methods are needed. Their results should not depend on contact with the patient or the qualifications of the doctor. Among such methods the most promising is the polarization–optical method. The use of polarized light to study the eye cornea allows visualizing the distribution of internal stresses arising in it due to the mechanical effect of direct extraocular muscles and intraocular pressure. When illuminating the cornea of the living eye with polarized light, an interference pattern similar in shape to a rhombus is observed on it. Its angles are based on the vertical and horizontal diameters of the eye cornea. It is proved that the symmetry of this interference rhombus depends on the balance degree of the direct extraocular muscles action, which ensures the eye rotation and fixes it in the right position. In case of the extraocular muscles action imbalance the shape of the interference rhombus changes. This can occur while increasing or decreasing the effort of one or more extraocular muscles. For clinical use of the described changes in the shape of the interference pattern, it is necessary to detail their features with various combinations of structural and functional pathology of extraocular muscles. For this, models of the stress–strain state of the eye were used. The purpose of the work was to determine the features of interference patterns observed on the eye cornea in polarized light in the pathology of extraocular muscles using models. Results and discussion. To construct the calculated interference patterns, an improved model of the stress–strain state of the eye was used. The improvement of this model, constructed using the theory of thin shells, consisted in taking into account the real place of attachment of extraocular muscles (in accordance with the anatomical ideas), as well as various possible options for their structural and functional pathology. The difference between the proposed approach and the known one also consists in determining the values of the geometric parameters of the calculated isochroms constructed for various variants of structural and functional changes in extraocular muscles. Using an improved model of the stress–strain state of the eye, a calculation was made and isochroms were constructed for cases of displacement the effort application of each of the four rectus muscles along the line of action (approximation of the point of application of effort to the limb and its removal from it in the range from 1 mm to 4 mm from normal provisions). As previous studies have shown, informative indicators for assessing the state of extraocular muscles are segments of the diagonals of the interference rhombus and the angles between them. Conclusion. As a result of the analysis of the shape of the calculated isochroms using informative parameters, we found out that the approximation of the place of application of muscle force to the limb from the normal position, as well as its removal, caused not only a change in the ratio of segments of the corresponding diagonal, but also a shift of the isochromic angles from the corresponding meridians. Tears of the upper corner of the isochrome were observed when the attachment point of the superior rectus muscle was removed from the normal position towards the equator of the eye. The obtained results indicated that determining the cause of strabismus using interference patterns should include not only their shape analysis, but also the results of other studies of the strabismus patient.

Keywords: extraocular muscles, strabismus, interference patterns, stress–strain models of the eye

Full text: PDF (Ukr) 768K

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