Fundamental principles and anatomical considerations

The change in optical power of the eye in order to produce a sharp retinal image of objects at different distances is called accommodation. The accommodative apparatus of the eye consists of the ciliary muscle, the zonular fibers which form the suspension apparatus of the crystalline lens, and the crystalline lens itself. During accommodation (far–near focusing) the tension of the zonular fibers is reduced by the contraction of the ciliary muscle ring and under the influence of the elastic lens capsule the shape of the lens changes to its mechanical resting state12. This increases the surface curvature and the anteroposterior thickness of the lens and reduces its equatorial diameter. These changes contribute to an increase in optical power of the eye.

Pseudoaccommodation

Accommodation (i.e. real change in optical power) must be distinguished from the ability to see clearly at near. Even in the presbyopic or pseudophakic eye, depth of focus (e.g. with pupil constriction) bifocality or multifocality (astigmatism, multifocal IOL) can provide a variable degree of functional near vision. Such effects are known as ‘pseudoaccommodation’.

Clinical features and pathophysiology of presbyopia

The decrease of accommodative amplitude begins at an age of 10–12 years and ends with a nearly complete loss of accommodative ability by the age of 50–55. Presbyopia has been attributed to a loss of elasticity of the ciliary muscle and the choroid as well as geometric changes and hardening of the lens, especially the lens nucleus, with advancing age. Although the underlying causes are still a matter of scientific discussion, it is basically accepted that the hardening of the lens substance represents a substantial limiting factor for the accommodation in the presbyopic eye4. Experimental results suggest that other important structures such as ciliary muscle and lens capsule still remain at least partially functional even in the presbyopic eye.

Goals and indications of surgery

Natural accommodation has both accommodative and pseudoaccommodative components. Treatment can be performed by replacing the lens with a multifocal or accommodative implant in cataract surgery. If the crystalline lens is still clear, however, it may be preferable to treat presbyopia in a way that allows for preserving the lens. This has been attempted, e.g. by scleral expansion or by generating multifocality in the cornea with special excimer laser profiles. All procedures used so far are not able to restore accommodation comparable to the young eye. Generally, refractive surgical procedures for the restoration of accommodation can be distinguished in pseudoacommodative and accommodative.

Pseudoaccommodative procedures