The restoration of all functions of the crystalline lens by its replacement with an artificial lens was the desire of Dr. Harold Ridley who implanted the first intraocular lens more than half a century ago. Restoration of accommodation remains a challenge today. We still cannot adequately and stably restore this dynamic function.

Accommodation is the eye's ability to change the crystalline lens' optical power to focus images on the retina of objects at varying distances. We still lack complete knowledge of the involved mechanisms, aging effects and anatomic variability.

Current IOLs that attempt to achieve adequate accommodation are designed to allow ciliary muscle contraction with vitreal pressure pushing the IOL forward to either shift the optic anteriorly or steepen the IOL's anterior curvature.

Most clinical studies of these IOLS have not separated accommodative amplitudes from pseudoaccommodative amplitudes. Pseudoaccommodation is the focusing of images of objects at varying distances on the retina without a focusing action of the eye.

Any pseudophakic patient can benefit from up to 1.5 D of pseudoaccommodative amplitude resulting from increased depth of focus due to small pupils, symmetrical oblique astigmatism, myopic astigmatism, corneal multifocality or spherical aberration. It is interesting to note that in many studies of the C&C AT-45 lens, investigators have emphasised that patients did not experience halos at night, despite the 4.5 mm optic size—suggesting small pupil size.

No IOL with lens accommodation as a mechanism of action has received US marketing approval to date. According to The Gray Sheet, US Food and Drug Administration's Ophthalmic Device Panel just recently made a recommendation that the C&C Vision CrystaLens AT-45, the first IOL to be considered by the FDA, should be approved but with labelling limiting accommodation to 1D, without mention of the proposed mechanism of anterior shift. According to the panel, C&C did not demonstrate the IOL's mechanism or the longterm stability of the lens hinge.

Laser interferometric studies presented by Findl and Menapace this year in Europe and the United States demonstrated no significant anterior shift of the C&C AT-45 lens with pilocarpine-induced ciliary muscle contraction. Anterior movement with the HumanOptics 1CU was less than 1 mm under the same testing conditions. Some patients even demonstrated posterior shifts with ciliary muscle contraction.

Calculations based on Gullstrand's eye model have shown that about a 2.2 mm anterior shift of an IOL is required to achieve a 4 D difference in IOL focal point power, which is equivalent to a 2.9 D accommodative amplitude at the spectacle plane.

Can we obtain improvement of a mechanical approach for IOL design by combining anterior shift from ciliary muscle contraction with IOL anterior curvature change or a multifocal/aspheric optic? Ease of insertion, aberrations induced by deformability of the optic, the influence of aberrations on depth of focus, and mechanical and optical stability and predictability will all be important factors to evaluate.

Most reliable data to date confirm Helmholtz's capsular theory of accommodation with additional roles of extralenticular components. Lens sclerosis with loss of elasticity appears to be the most important factor in loss of accommodative amplitude.

To what extent can we hope to restore accommodation by refilling the lens capsule? Based on calculations by Van der Heijde, Vilpuru and Terwee, a 2.9 D accommodative amplitude accommodative amplitude at spectacle plane requires a 0.3 mm increase in bag thick ness to achieve a 4.0 D artificial lens focal point power difference.

Since the time of Kessler's first description of lens filling attempts in 1964, techniques and technologies facilitating the attainment of this goal continue to evolve.

Following the contributions of Haefliger, Parel, Nishi, and Hettlich to our understanding of capsular refilling in non-human primate models, Koopmans and Terwee demonstrated this year that there is no age-related decline of lens power during equatorial stretching of human lenses refilled with a soft polymer. In contrast, natural lenses which were subjected to the same technique of mechanical stretching to simulate accommodative mechanisms showed decreases in accommodative amplitudes with age in amounts similar to those measured by Duane.

The choice of polymer will depend on the polymer's Young's modulus of elasticity and index of refraction and our ability to measure refraction intra-operatively.

Although we know that ciliary muscle contractile activity is present even in presbyopic eyes, the exact roles of decreases in unaccommodated ciliary muscle diameter with age and variability of force direction are still unknown.

We need to increase our understanding of accommodation dynamics and the role of higher order aberrations. Dynamic aberrometric measurements are revealing changes in spherical aberrations preceding refractive changes during accommodation. Speed of accommodation will depend on multiple factors.

Multiple aberrations result form inhomogeneities in natural lenses. Different aberrations result when the capsular bag is refilled with a homogeneous polymer. Koopman's experiment revealed negative spherical aberration which was reduced by lens stretching, in young natural lenses but positive spherical aberration which was reduced by lens stretching reduced with lens stretching in lenses refilled with a silicone polymer. The contributory roles of interactions between the polymer, capsule and nonlenticular apparatus will need to be studied.

Leakage of polymer material must be controlled. Polymer pre-curing which could reduce leakage, can produce undesirable optical and mechanical inhomogeneities. Expandable polymers may not completely fill the capsule and may be difficult to remove. The desirability of mechanical plugging of capsulotomies needs to be explored.

Most importantly, the clarity and elasticity of the capsule must be maintained. We must successfully prevent proliferation and migration of anterior and posterior lens epithelial cells without damaging other intraocular tissue.

Restoring, or at least adequately simulating optical and biomechanical properties of the crystalline lens after cataract removal remains to be accomplished in regards to accommodation, but exciting progress has been made.