Análisis biomecánico del disco óptico bajo la variación de presión intraocular y rigidez escleral / Biomechanical analysis of the optic disc under variations in intraocular pressure and scleral rigidity

INTRODUCCIÓN: el incremento de la presión intraocular y las alteraciones detectadas en la apariencia de la cabeza del nervio óptico constituyen factores importantes en la determinación de la progresión del daño glaucomatoso. OBJETIVO: analizar el comportamiento del disco óptico bajo el efecto de la variación de la presión intraocular y de la rigidez escleral, a través de un modelo biomecánico. MÉTODOS: se emplea el método de los elementos finitos. Se definieron varios módulos de rigidez para la esclerótica de 3, 6 y 9 MPa y para la lámina cribosa 0,3 y 0,6 MPa. Todos los tejidos modelados fueron asumidos como materiales isotrópicos con comportamiento elástico e incompresible. RESULTADOS: la mayor concentración de tensiones se localizó en las zonas de la esclerótica peripapilar y en las paredes del canal escleral. Los máximos de tensión (97,523 kPa) y desplazamiento (95,64 µm) se obtuvieron cuando la esclerótica y la lámina cribosa fueron menos rígidas y con la mayor presión intraocular. CONCLUSIONES: la biomecánica del disco óptico influye en el desarrollo de la neuropatía óptica glaucomatosa. El mayor desplazamiento se encuentra en la zona central del disco óptico y está asociado a la pérdida de fibras nerviosas de la retina o al incremento de la excavación papilar en el mecanismo de daño glaucomatoso.

INTRODUCTION: Increase in intraocular pressure and alterations in the appearance of the optic nerve head are important factors in determining the progress of glaucomatous damage. OBJECTIVE: Analyze the behavior of the optic disc under the effect of variations in intraocular pressure and scleral rigidity using a biomechanical model. METHODS: The finite element method was used to define several rigidity modules for the sclera at 3.6 and 9 MPa and for the lamina cribosa at 0.3 and 0.6 MPa. All the tissues modeled were assumed to be isotropic materials with elastic, incompressible behavior. RESULTS: The highest concentration of tensions was located in areas of the peripapillary sclera and scleral canal walls. Tension and displacement peaks (97.523 kPa and 95.64 µm, respectively) were obtained when the sclera and the lamina cribosa were less rigid and intraocular pressure was highest. CONCLUSIONS: The biomechanical characteristics of the optic disc influence the development of glaucomatous optic neuropathy. The greatest displacement is found in the central area of the optic disc, and it is associated with the loss of retinal nerve fibers or an increase in papillary excavation in the mechanism of glaucomatous damage.

Asymmetry Analysis of the Retinal Nerve Fiber Layer Thickness in Normal Eyes using Optical Coherence Tomography

PURPOSE: To investigate the asymmetry of the retinal nerve fiber layer thickness (RNFLT) with respect to the horizontal and vertical meridian and between the right and left eye in normal subjects. METHODS: The RNFLT was measured in 121 normal volunteers by optical coherence tomography (OCT). The RNFLT was analyzed by dividing the circle scanning area (diameter 3.4 mm) around the optic disc into 4 quadrants and 12 sectors. RESULTS: There was a significant difference between the RNFLT of the nasal and temporal quadrant in individual eyes. There was a significant difference between the RNFLT of corresponding sectors with respect to the vertical or horizontal meridian in individual eyes. The nasal and temporal RNFLTs were asymmetrical between the right and left eye in the quadrant and sector analysis. The RNFLT of the nasal and temporal quadrant was thicker in the right eye. The nasal and inferior RNFLT measured by OCT had a significant correlation with degree of refractive error. CONCLUSIONS: In normal subjects without significant anisometropia, there was significant asymmetry of the RNFLT for each eye as well as between the right and left eye.