Theories of Presbyopia (1997-1999)
Dr Harvey Burd, Dr Stuart Judge and Mr Mark Flavell
The conventional understanding of accommodation, described by Helmholtz
in 1909, is that the shape of the lens is controlled by the ciliary muscle.
When the eye views a distant object, the ciliary muscle is relaxed and
radial forces are applied to the lens equator by the zonule. In this
configuration, the optical power of the lens is at a minimum. To view
close objects, the ciliary body contracts and the force in the zonules
reduces. This causes an increase in polar curvature of the lens surfaces
and a consequential increase in optical power of the lens.
However, the Helmholtz view of the accommodation process is not universally accepted. In particular, Schacher et al. 1993 suggest, on the basis of a mathematical model of the accommodation process, that the conventional Helmholtz mechanism of accommodation is incorrect.
We conducted a detailed study of the Schachar et al. 1993 analysis and concluded that the model was misleading because it neglected certain non-linear terms. A corrected numerical model, including these additional terms, was developed. This new model exhibited classical Helmholtzian behaviour (see Burd, Judge and Flavell 1999)
Burd, H.J., Judge, S.J. and Flavell, M.J. (1999) Mechanics of Accommodation of the Human Eye. Vision Research 39, 1591-1595