Abstract
Custom Spectral Domain Optical Coherence Tomography (SD-OCT) provided with automatic quantification and distortion correction algorithms was used to measure anterior and posterior crystalline lens surface elevation in accommodating eyes and to evaluate relationships between anterior segment surfaces. Nine young eyes were measured at different accommodative demands. Anterior and posterior lens radii of curvature decreased at a rate of 0.78 ± 0.18 and 0.13 ± 0.07 mm/D, anterior chamber depth decreased at 0.04 ± 0.01 mm/D and lens thickness increased at 0.04 ± 0.01 mm/D with accommodation. Three-dimensional surface elevations were estimated by subtracting best fitting spheres. In the relaxed state, the spherical term accounted for most of the surface irregularity in the anterior lens (47%) and astigmatism (70%) in the posterior lens. However, in accommodated lenses astigmatism was the predominant surface irregularity (90%) in the anterior lens. The RMS of high-order irregularities of the posterior lens surface was statistically significantly higher than that of the anterior lens surface (x2.02, p<0.0001). There was significant negative correlation in vertical coma (Z3 (-1)) and oblique trefoil (Z3 (-3)) between lens surfaces. The astigmatic angle showed high degree of alignment between corneal surfaces, moderate between corneal and anterior lens surface (~27 deg), but differed by ~80 deg between the anterior and posterior lens surfaces (including relative anterior/posterior lens astigmatic angle shifts (10-20 deg).
Highlights
The optical components of the eye are the cornea and the crystalline lens; they must be transparent and have appropriate shape and refractive indices for providing an optimal retinal image
Anterior and posterior lens radii of curvature decreased at a rate of 0.78 ± 0.18 and 0.13 ± 0.07 mm/D, anterior chamber depth decreased at 0.04 ± 0.01 mm/D and lens thickness increased at 0.04 ± 0.01 mm/D with accommodation
As in previous studies reporting wave aberrations, we found that the spherical term changed toward negative values with accommodation in the anterior lens surface but this tendency is not observed in the posterior lens surface
Summary
The optical components of the eye are the cornea and the crystalline lens; they must be transparent and have appropriate shape and refractive indices for providing an optimal retinal image. The cornea accounts for most of the optical refractive power, while the crystalline lens provides approximately one third of the total static refractive power of the eye and is the responsible for the focusing ability in young eyes (process known as accommodation) [1,2,3,4,5,6,7,8,9]. The ciliary muscle contracts, relaxing the tension on the zonular fibers and changing the crystalline lens geometry, primarily increasing the curvature of its surfaces. These changes overall contribute to additional ~10 diopters (D) of refraction in the young adult eye; by age 45 most of the accommodation amplitude is lost (process known as presbyopia) [10,11]. There are numerous reports in the literature reporting lens radii of curvature, measured generally with adapted commercial or custom-developed instruments: Purkinje imaging [12,13], Scheimpflug camera [14,15,16], Magnetic Resonance Imaging (MRI) [17] and, recently, Optical Coherence Tomography (OCT) [18,19,20,21]
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