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Impressum
Optical principal fundamentals of present concepts in the restoration of accommodation
Ludwig K.
Augenklinik, Klinikum Innenstadt, Ludwig-Maximilians-Universität, München
The abilitiy of the human eye to accommodate to a near stimulus results
from two processes, true accommodative changes of the lens shape and natural
pseudo-accommodative effects. In presbyopia, only these pseudoaccommodative
effects remain. Varying considerably from indiviuum to individuum, they
usually amount to values between 0,5 and 2,0 diopters. Today´s concepts
to restore true accommodation in presbyopic eyes, can be subdivided into
two groups: concepts intending to improve the ability of the natural crystalline
lens to change its shape, and other concepts, that either replace the
natural lens by an IOL from highly malleable material or by an IOL with
the ability to shift anteriorly under intended nearaccommodation. The
present concepts to improve the deformability of the crystalline lens
intend to achieve this aim by increasing zonular tension through an expansion
of the sclera above the ciliary muscle. In the case of the scleral expansion
(SE) technique, true accommodation as its effect has been refuted. Therefore,
only increased pseudo-accommodation due to increased multifocality of
the cornea and/or the crystalline lens can be the causes of an improved
near reading ability. If anterior ciliary sclerotomy (ACS) can achieve
true accommodation, it would do so by using the natural components of
the accommodative apparatus. If not, the same pseudoaccommodative mechanisms
as in SE have to be postulated. Concepts, to replace the natural lens
by an IOL from malleable material would, ideally, also respect the natural
optical structure of the eye and make use of the accommodative apparatus
in order to achieve a desired accommodative response. The ability of so-called
accommodative intraocular lenses to undergo a forward shift under near-accommodation
does, in principle, exist for the natural lens and any other IOL as well.
In those two cases, it is, with an effect of less than 0.5 dpt, only of
minor importance for natural accommodation. In order to provide a sufficient
near accommodation, an "accommodative" IOL should be able to
move forward by about 2 mm. Whether a forward movement of this extent
can be achieved in reality, has still to be proven.