Isolated human lens interferometric surface radius of curvatures: Implications for the mechanism of accommodation.

Accurate central surface radius of curvature (RoC) measurements of isolated human lenses are essential for understanding the zonular forces required to modify human lens shape to focus at near; i.e., accommodate. The human lens can be described as an encapsulated oblate spheroid, with its minor axis aligned with its optical axis. The lens is suspended by zonular fibers that originate from the epithelium of the ciliary body and insert into the equatorial region of the lens capsule. According to Helmholtz's theory of accommodation when the eye views a distant object (the unaccommodated state), the ciliary muscle is fully relaxed and the zonules are under maximal tension. This tension flattens both the central and peripheral lens surfaces resulting in minimal central optical power (COP). During near focus (accommodation), contraction of the ciliary muscle reduces zonular tension, allowing the elastic capsule to restore the lens to a more rounded shape. This increases the curvature of the lens surfaces, central thickness, and COP. Consequently, isolated lenses without zonular tension from young donors (20-30 years old) would be expected to exhibit maximum COP. However, the companion independent profilometric equation fitting study found that, within central optical zones ≤ 3 mm, 10 fresh isolated lenses from donors in this age range actually had minimal COP. The present study utilizes a white light scanning interferometer (WLSI) with a 10x objective that was validated by measuring RoCs of glass and porcine lenses. Fourteen transparent human lenses were obtained from both eyes of seven donors aged 20-30 years of whom 2 were female and 5 were male. One lens of each donor was placed in preservative media and the contralateral lens in culture media within 11:26 ± 5:15 (range: 4:47-21:54) of the donor's death. Two of the lenses stored in the culture media had torn capsules and were excluded from the study. Central thickness and WLSI surface vertex RoCs of 12 lenses were measured within 16:27 ± 5:22 (range: 10:11-25:33) of the donor's death. Mean central thickness, anterior and posterior vertex RoCs and COP were 3.54 ± 0.07 mm, 10.2 ± 0.9 mm, 6.8 ± 1.0 mm, and 20.7 ± 2.1 diopters, respectively. These results confirm the companion study that isolated human lenses have low COP consistent with the unaccommodated state of lenses in vivo. Therefore, relaxation of all the zonules does not increase COP and cannot be the basis for the mechanism of accommodation. These results have implications for the development and treatment of myopia, presbyopia, glaucoma, cortical cataracts and design of accommodative intraocular lenses.