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1

Raw data from Ocular Response Analyzer applied for differentiation of normal and glaucoma patients

Jóźwik Agnieszka, Asejczyk-Widlicka Magdalena, Boszczyk Agnieszka, Kasprzak Henryk, Krzyżanowska-Berkowska Patrycja

Optica Applicata

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2020

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Vol. 50, nr 1

147--159

EN

Purpose: Presented study describes new parameters calculated from the Ocular Response Analyzer (ORA) raw data. Such an approach can increase the applicability of the ORA in ophthalmic diagnosis. Among many proposed and examined by us parameters from raw data of the air pressure and applanation curves, only a few were chosen and then applied for characterizing a selected group of patients. Methods: The study included healthy subjects in a control group and patients divided into 2 groups: suspect and glaucoma. A series of four ORA measurements were taken from each subject. The raw ORA data were numerically analyzed and new parameters were calculated from the ORA curves for each measurement. Comparative analysis was carried out for the newly proposed parameters (and original parameters from the ORA device). Results: This interesting finding is that the new parameters showed a statistically significant ability to distinguish the glaucoma suspect group from healthy and glaucomatous patients. Moreover comparable or higher repeatability than for IOPg and CH was obtained. Conclusion: Raw data from the ORA enables definition and numerical analysis of new parameters, characterizing every measurement, which can be successfully used for describing an individual eye and differentiating between some specific groups of patients.

2

Zmierzch tonometrii z pachymetrią?

Flasiński P.

Inżynier i Fizyk Medyczny

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2014

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Vol. 3, nr 4

214--216

PL

Prawidłowe ciśnienie gałki ocznej zależy od równowagi pomiędzy wytwarzaniem a odpływem cieczy wodnistej oka. Ciecz ta spełnia w oku podobną rolę jak krew w organizmie. U zdrowego człowieka ciśnienie oczne mieści się w przedziale 10-21 mm Hg, natomiast o nadciśnieniu mówi się, gdy wartość ciśnienia przekracza 21 mm Hg w jednym oku lub obydwu naraz. Mając na uwadze, że wysokie ciśnienie wewnątrzgałkowe jest głównym czynnikiem rozwoju jaskry, stan nadciśnienia ocznego wymaga ścisłej, regularnej kontroli oraz przeprowadzenia wszystkich niezbędnych badań w kierunku diagnostyki jaskry. O jaskrze możemy mówić, gdy dołączają się objawy neuropatii nerwu wzrokowego – uszkodzenie nerwu z ubytkami w polu widzenia. Do badania ciśnienia wewnątrzgałkowego służą tonometry. Tonometry dzielą się ze względu na zasadę pomiaru na impresyjne, aplanacyjne i bezdotykowe (powietrzne).

3

Applanation pressure function in Goldmann tonometry and its correction

Śródka W.

Acta of Bioengineering and Biomechanics

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2013

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Vol. 15, no. 3

97--106

EN

So far applanation tonometry has not worked out any theoretical basis for correcting the result of intraocular pressure measurement carried out on a cornea with noncalibration dimensions by means of the Goldmann tonometer. All the tables of instrument reading corrections for cornea thickness or cornea curvature radius are based exclusively on measurements. This paper represents an attempt at creating a mechanical description of corneal apex deformation in Goldmann applanation tonometry. The functional dependence between intraocular pressure and the pressure exerted on the corneal apex by the tonometer was determined from a biomechanical model. Numerical GAT simulations, in which this function was also interrelated with the cornea’s curvature radius and thickness were run and a constitutive equation for applanation tonometry, i.e. a full analytical description of intraocular pressure as a function of the above variables, was derived on this basis. The correction factors were defined and an algorithm for correcting the measured pressure was formulated. The presented formalism puts the results of experimental tonometry in new light. Analytical correction factors need not to come exclusively from measurements. A geometric interdependence between them and their dependence on pressure have been revealed. The theoretical description of applanation tonometry contained in the constitutive equation consists of a pressure function developed for a cornea with calibration dimensions and a coefficient correcting this calibration function, dependent exclusively on the cornea’s actual thickness and curvature radius. The calibration function is a generalization of the Imbert–Fick law.

4

Evaluating the material parameters of the human cornea in a numerical model

Śródka W.

Acta of Bioengineering and Biomechanics

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2011

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Vol. 13, no. 3

77-85

EN

The values of the biomechanical human eyeball model parameters reported in the literature are still being disputed. The primary motivation behind this work was to predict the material parameters of the cornea through numerical simulations and to assess the applicability of the ubiquitously accepted law of applanation tonometry – the Imbert–Fick equation. Methods: Numerical simulations of a few states of eyeball loading were run to determine the stroma material parameters. In the computations, the elasticity moduli of the material were related to the stress sign, instead of the orientation in space. Results: Stroma elasticity secant modulus E was predicted to be close to 0.3 MPa. The numerically simulated applanation tonometer readings for the cornea with the calibration dimensions were found to be lower by 11 mmHg then IOP = 48 mmHg. Conclusions: This discrepancy is the result of a strictly mechanical phenomenon taking place in the tensioned and simultaneously flattened corneal shell and is not related to the tonometer measuring accuracy. The observed deviation has not been amenable to any GAT corrections, contradicting the Imbert–Fick law. This means a new approach to the calculation of corrections for GAT readings is needed.

5

Model biomedyczny ludzkiej gałki ocznej

Śródka W.

Prace Naukowe Instytutu Konstrukcji i Eksploatacji Maszyn Politechniki Wrocławskiej. Monografie

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2010

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Vol. 91, nr 34

242-242

PL

Przedmiotem badań jest model strukturalny gaiki ocznej oraz możliwość symulowania za jego pomocą funkcji optycznych oka. Analizie poddano także aspekty mechaniczne pomiaru ciśnienia wewnątrzgałkowego (IOP) techniką określaną jako tonometria. Opracowanie jest próbą stworzenia podstaw teoretycznych mechaniki powłok oka w obu wymienionych zakresach. Obliczenia oparte są na trzech podstawowych założeniach: samonastawności optycznej modelu gałki ocznej, równości ciśnień po obu stronach strefy aplanacji rogówki kalibracyjnej dla ciśnienia nominalnego i dla nienormalnej izotropii materiału. Rozwiązania, osiągane metodą elementów skończonych, uwzględniają fizyczną i geometryczną nieliniowość konstrukcji. Przyjęta strategia obliczeń umożliwia badanie stateczności powłoki rogówkowej w tonometrii aplanacyjnej Goldmanna. Wyznaczone w drodze obliczeń numerycznych ciśnienie aplanacji okazuje się nieliniową funkcją IOP, znacznie odbiegającą od przewidywań Goldmanna. Przeprowadzono szczegółową krytykę tej metody oraz zaproponowano nowy opis teoretyczny pomiaru, a także wynikający z niego formalizm umożliwiający korygowanie odczytu ze względu na grubość rogówki i promień jej krzywizny. Zbadane zostały także pokrewne techniki pomiaru ciśnienia wewnątrzgałkowego nazywane tonometria dynamiczną (DCT) i tonometria rezonansową (ART). Wykorzystanie hipotezy samonastawności oka pozwoliło zintegrować elementy składowe gałki ocznej w jeden spójny układ optyczny. Badanie funkcji optycznych modelu wykazało ścisłą relację zachodzącą pomiędzy materiałem rogówki, rąbka i twardówki. Dla zachowania samonastawności modelu, sieczny moduł sprężystości twardówki musi być około pięciu razy większy od modułu rogówki. Określony metodą odwrotną moduł sieczny rogówki zbliżony jest do 0,27 MPa dla ciśnienia nominalnego. Parametry mechaniczne powłok gałki ocznej nie są wzajemnie niezależne, powiązania między nimi są narzucane przez funkcje optyczne. Zależności te ułatwiają identyfikację strukturalną oka. Odkryte za pomocą modelu efekty nieliniowe w tonometrii aplanacyjnej falsyfikują teorię Goldmanna, a także wynikającą z niej procedurę korekcji wyniku pomiaru IOP. Skutki te obejmują również DCT i ART, oparte na postulatach Goldmanna.

EN

Refraction surgery, tonometry and eye optical system theory are the fields of ophthalmology, in which a biomechanical model of the eyeball could play a significant research and utilitarian role. Attempts at creating such a model have been made since the 1970s. Today when highly sophisticated systems using most advanced methods of structural analysis are available, such problems can be relatively easily solved. Unfortunately, pre-information era assumptions and ways of thinking are still underlying the biomechanical model of the eyeball. This clash of outdated ideas and modern computing tools leads to results which do not find practical application - up to this day the effects of cornea surgeries are empirically predicted, similarly IOP reading corrections in applanation tonometry are experimentally determined. The aim of this research was to diagnose the condition of eye biomechanics, to carry out a critical assessment of the binding formal foundations and to attempt to solve selected problems. The invars process was used to identify the material parameters of the cornea, the sclera and the corneal limbus. In this method, the eyeball model is so designed that its functioning is in agreement with the commonly known experimental results. The results available today relate to tonometry, eyeball stiffness and the cornea. Also the original idea of the optical self-adjustment of the eyeball was used. The number of model assumptions was considerably reduced and the latter were well-founded. The assumptions boil down to the three postulates: abnormal anisotropy of the material, optical self-adjustment of the model and Goldman's postulate that the (nominal) pressures on both sides of the calibration cornea are equal. The calculation eyeball model was solved using the finite element method. Its optical system was built according to the rules predicted by the self-adjustment hypothesis. This new approach to the applanation problem has enabled the investigation of corneal shell stability in GAT. As a result, the hitherto unnoticed influence of IOP on the correction value for the pressure measured by the Goldmann tonometer has been revealed. A detailed critique of the method is presented. A new measurement theory is proposed and a formalism making it possible to correct readings disturbed by cornea thickness and curvature radius variation among people is derived from this theory. Also the IOP measuring techniques: DCT and ART were tested. Numerical simulations showed, contrary to the authors of the techniques, that intraocular pressure measurement results are not in agreement with GAT and need to be corrected as well. Thanks to the eye self-adjustment hypothesis the components of the eyeball could be integrated into one coherent optical system. The examination of the model's optical functions revealed the relationship which must exist between the materials of the cornea, the corneal limbus and the sclera: in order to preserve the self-adjustment of the model, the secant modulus of elasticity of the sclera must be about five times larger than the modulus of the cornea, and the latter is close to 0.27 MPa under natural stress. The investigations showed that above the pressure of 16 mmHg the Goldmann tonometer readings are understated and the deviation from the real IOP value increases with pressure, to as much as 10 mmHg. The same is observed for a cornea with calibration dimensions. This contradicts the Imbert-Fick law. The causes of this phenomenon, until now associated with surface tension in the lacrimal fluid, should be linked with corneal shell stability during flattening. On this basis an applanation pressure function in GAT for the cornea of any dimensions has been developed. The correction formulas for CCT and cornea curvature have been found to depend on IOP and to be mutually dependent. Their analytical form has, besides the empirical basis, a theoretical basis now. Contrary to the common belief, the numerical simulations of DCT suggest that the pressure measured by the tonometer clearly depends on CCT -like in GAT.

6

Biomechanical model of human eyeball and its applications

Śródka W.

Optica Applicata

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2009

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Vol. 39, nr 2

401-413

EN

Attempts at the mechanical identification of the human eyeball are often not very effective for two reasons: the material parameters determined by tension tests on corneal and scleral tissue specimens are not sufficiently accurate while numerical models of the eye, integrating material and geometric parameters, are often based on unrealistic assumptions. The examples presented here cover refractive surgery, Goldmann applanation tonometry and the optical self-adjustment of the eye. The discussed problems are illustrated with calculations showing that it is possible to effectively use a biomechanical model of the eye to identify its material parameters. Also the handicaps, the Imbert-Fick law among them (numerical calculations do not corroborate this law), lying at the basis of applanation tonometry are demonstrated. The conclusions can help to create a realistic numerical model of the eyeball.