Changes in ocular dimensions and refraction with accommodation.

The purpose of this study was to measure the changes in ocular dimensions with accommodation, with particular reference to the radius of curvature of the posterior surface of the crystalline lens. The increase in power of the eye with accommodation is considered to arise primarily from a decrease in the radius of curvature of the anterior surface of the lens, with the role of the posterior surface somewhat unclear. We measured the axial dimensions (A-Scan ultrasonography), corneal radius of curvature (keratometry), refractive error (auto-refractor) and radii of curvature of the lenticular surfaces (video phakometry) for 11 subjects, mean age 21.2 +/- 2.6 years, for five levels of ocular accommodation up to 8.00 D. At maximum accommodation the mean changes were a decrease in anterior chamber depth of 0.24 mm, an increase in lens thickness of 0.28 mm, a decrease in radius of curvature of the anterior surface of the lens of 4.95 mm and 1.34 mm for the posterior surface. The corresponding increase in power of the lenticular surfaces for an equivalent refractive index of 1.422 for the lens was 5.53 D and 3.10 D for the anterior and posterior surfaces respectively. No significant changes were recorded in axial length or vitreous chamber depth. We conclude that when crystalline lens power is calculated on the basis of an equivalent refractive index, changes in the posterior surface of the lens contribute around one third of the increase in lens power associated with 8.00 D of ocular accommodation.