Optical coherence tomography quantifies gradient refractive index and mechanical stiffness gradient across the human lens

Abstract

As a key element of ocular accommodation, the inherent mechanical stiffness gradient and the gradient refractive index (GRIN) of the crystalline lens determine its deformability and optical functionality. Quantifying the GRIN profile and deformation characteristics in the lens has the potential to improve the diagnosis and follow-up of lenticular disorders and guide refractive interventions in the future. Here, we present a type of optical coherence elastography able to examine the mechanical characteristics of the human crystalline lens and the GRIN distribution in vivo. The concept is demonstrated in a case series of 12 persons through lens displacement and strain measurements in an age-mixed group of human subjects in response to an external (ambient pressure modulation) and an intrinsic (micro-fluctuations of accommodation) mechanical deformation stimulus. Here we show an excellent agreement between the high-resolution strain map retrieved during steady-state micro-fluctuations and earlier reports on lens stiffness in the cortex and nucleus suggesting a 2.0 to 2.3 times stiffer cortex than the nucleus in young lenses and a 1.0 to 7.0 times stiffer nucleus than the cortex in the old lenses. Optical coherence tomography is suitable to quantify the internal stiffness and refractive index distribution of the crystalline lens in vivo and thus might contribute to reveal its inner working mechanism. Our methodology provides new routes for ophthalmic pre-surgical examinations and basic research. The lens of the eye changes in shape to enable objects at different distances from the eye to be seen clearly. Loss of ability to change the eyes’ focus occurs during aging. We have developed a new way to image the eye that assesses how different lens regions change their shape. We evaluated our approach on twelve people of different ages and showed that those who were older had a stiffer lens, particularly in the central part of the lens. Further development and testing of our method could enable it to be used to both improve routine eye assessments as well as enable more research into how the eye works. Kling et al. use optical coherence tomography to quantify the gradient refractive index and the strain distribution within the human crystalline lens in vivo. A substantial decrease of the strain in the lens nucleus is seen above the age of 50 years, i.e. with the onset of presbyopia.