Methods for characterising patient-specific corneal biomechanics

• Autores: Miguel Ángel Ariza Gracia
• Directores de la Tesis: Begoña Calvo Calzada (dir. tes.), José Félix Rodríguez Matas (dir. tes.)
• Lectura: En la Universidad de Zaragoza ( España ) en 2017
• Idioma: español
• Tribunal Calificador de la Tesis: Anna Pandolfi (presid.), Miguel Angel Martínez Barca (secret.), María José Rupérez Moreno (voc.)
• Programa de doctorado: Programa de Doctorado en Ingeniería Biomédica por la Universidad Politécnica de Catalunya y la Universidad de Zaragoza
• Materias:
  • Física
    • Óptica
      • Óptica geométrica
  • Ciencias de la vida
    • Biofísica
      • Biomecánica
  • Ciencias médicas
    • Ciencias clínicas
      • Oftalmología
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• Resumen

  • SUMMARY This thesis addresses the problem of generating a patient-specific numerical model of the human cornea, using its patient- specific geometry and mechanical properties. We propose several methods to reconstruct the patient-specific geometry of the cornea in an automatic fashion, as well as different protocols to determine patient-specific material properties of the corneal tissue under different loading conditions. Ocular healthcare has become rapidly important during the last decade, supported by the increment of safety in surgical procedures, and the reduction of associated costs. However, over or un- der corrections after refractive surgeries are still causing certain degree of visual impairment, patient dissatisfaction, and an increment of costs due to secondary surgeries. Furthermore, concern about the underlying biomechanical sources of ectatic diseases, such as Keratoconus, has encouraged the search for a better definition of the mechanical properties of the ocular tissues. In this vein, non-contact tonometers (NCTs), or air-puff devices, have become a reference in Ophthal- mology to perform intrasurgical assessment, or to provide an insight into the mechanical properties of the corneal tissue in pathological and healthy eyes.

    Throughout the course of this dissertation, our contribution to the field of Corneal Biomechanics is presented in different milestones. First, we carry out a theoretical in silico study to better understand the physical grounds of NCTs, the role of different ocular features (intraocular pressure, material stiffness, and geometry), and what these clinical tests really characterize. Second, we develop a novel automatic methodology to reconstruct patient-specific corneal geometries where data is available (provided by commercial topographers), allowing to simulate a general air-puff test. Third, we propose different mathematical techniques to predict patient-specific material properties of the cornea based on clinical biomarkers (maximum displacement in a NCT, intraocular pressure, and corneal geometry), showing the capabilities of numerical methods in helping to assess in clinics. Fourth, we propose a novel numerical-experimental protocol so as to determine the mechanical properties of the corneal tissue using inflation and bending tests. In this way, we try to avoid an ill-posed material optimization by minimizing both stress states simultaneously. Fifth, using the information provided by our patient- specific material protocols, we simulate patient-specific Astigmatic Keratotomy surgeries in animal models (New Zealand Rabbit). To ensure that equivalent optical metrics are used when comparing experimental and numerical data, we have developed, and validated using commercial software, an in-house ray tracing algorithm. Thus, a consistent validation and optimization procedures are carried out. Sixth, as NCTs involve the coupling between fluids (air and humors) and a structure (eyeball), fluid-structure interaction (FSI) simulations are applied to improve NCTs’ simulations, including more realistic load transfers and boundary conditions. Additionally, we study whether FSI simulations are mandatory or under what hypothesis they can be accurately substituted by pure mechanical dynamic simulations.

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