Refractive Lens Surgery

180 S. Norrby
there was fibrin formation in the anterior
chamber in the early postoperative period,
later followed by opacification of the capsule
due to lens epithelial cell proliferation. Recently,
we have managed to control the postoperative
inflammation by steroid therapy
and prevent capsule opacification by means
of a cytotoxic compound. The clear eyes now
allow measurement of refraction in accommodated
and unaccommodated states using a
Hartinger coincidence refractometer.Accommodation
is induced by means of a miotic
agent (pilocarpine or carbachol). Accommodation
of about 3 diopters has been measured
up to 6 months postoperatively [29]. This research
was carried out in part in collaboration
with Dr. Adrian Glasser, Houston, Texas.
18.4 The Materials
The crucial properties for a lens replacement
material are refractive index, modulus (softness),
and, to a lesser extent, density.
The natural lens has a gradient refractive
index. The index is lower at the surface and
increases towards the middle. Gullstrand [30]
calculated that a homogeneous material replacing
the crystalline lens should have an index
of 1.413 for the unaccommodated state,
and 1.424 for 9.7 diopters of accommodation.
In accordance with the Dubbelman eye model
[31], the equivalent refractive index for a
35-year-old person is 1.427 in the unaccommodated
state and 1.433 for 4 diopters of ac-
Fig. 18.7. Young’s modulus of human lens
material at different ages. Data of Weeber
et al. [34] compared to those of Fisher [56].
Printed with permission of Henk Weeber,
who provided the graph
commodation. That the equivalent refractive
index increases with accommodation is due
to the gradient refractive index of the crystalline
lens.A homogeneous replacement will
therefore produce less accommodation for
the same amount of lens curvature change, as
pointed out by Ho et al. [32] .
Fisher [33] found the elastic modulus of
the human lens to be about 1.5 kPa and to increase
slightly with age. More recently, Weeber
et al. [34] measured shear compliance
(the inverse of modulus) of human crystalline
lenses as a function of age. They found
lens compliance to decrease (increase in stiffness)
by a factor of 1,000 over a lifetime. Figure
18.7 shows the data of these two papers in
comparable units.While the results of Weeber
et al. explain better why lens stiffness prevents
accommodation, they are comparable
to those of Fisher for young lenses, which are
the target for a lens replacement material.
The density of an artificial lens material
should be slightly higher than that of water to
avoid flotation, yet not so dense as to cause
inertia forces on the zonules when the head is
shaken.
18.4.1 Silicones
In the early literature most research groups
appear to have used poly(dimethyl siloxane)
– common silicone. It has a refractive index of
1.40 and a specific gravity of 0.98. By copolymerizing
dimethyl siloxane with diphenyl