Presbyopia, accommodation, and mature catenary

Abstract

Coleman and Fish described a mechanical model to support the catenary theory of accommodation and lens change in the human eye. A key element of the theory is that in accommodation the equatorial lens diameter decreases relative to the equatorial lens diameter in the unaccommodated state and that it is this decrease in the lens diameter that leads to the “rounding up,” or steepening of the central lens radius of curvature. Their Figure 6B shows data points which seem to support these findings. However, Figure 4, which is a series of photographs of their lens models, does not support the data reported in Figure 6B. In the 1560-ml model and the 1740-ml model, the equatorial diameter is larger in the accommodated state. In the 1980-ml model, the diameter is larger in the unaccommodated state. In the 2100-ml model, the diameters in the two states seem to be equal within the limit of my ability to measure the photographs with calipers.It is extremely difficult in vivo to compare the equatorial lens diameter in the unaccommodated vs. accommodated state because there are often changes in the position and perspective of the eye under study. Accommodative models are less vulnerable to this problem. The actual photographs (raw data) of Coleman and Fish do not seem to support their conclusions. Coleman and Fish described a mechanical model to support the catenary theory of accommodation and lens change in the human eye. A key element of the theory is that in accommodation the equatorial lens diameter decreases relative to the equatorial lens diameter in the unaccommodated state and that it is this decrease in the lens diameter that leads to the “rounding up,” or steepening of the central lens radius of curvature. Their Figure 6B shows data points which seem to support these findings. However, Figure 4, which is a series of photographs of their lens models, does not support the data reported in Figure 6B. In the 1560-ml model and the 1740-ml model, the equatorial diameter is larger in the accommodated state. In the 1980-ml model, the diameter is larger in the unaccommodated state. In the 2100-ml model, the diameters in the two states seem to be equal within the limit of my ability to measure the photographs with calipers. It is extremely difficult in vivo to compare the equatorial lens diameter in the unaccommodated vs. accommodated state because there are often changes in the position and perspective of the eye under study. Accommodative models are less vulnerable to this problem. The actual photographs (raw data) of Coleman and Fish do not seem to support their conclusions.