Refractive Lens Surgery

siloxane, the refractive index can be increased
and at the same time the specific
gravity increases well over 1. Such materials
are used in high refractive index foldable intraocular
lenses (IOLs). In order to counteract
excessive increases in specific gravity, a
third comonomer can be introduced [35].
Silicone polymers by themselves are liquids.
They can be crosslinked into gels. The
stiffness of such gels depends on the length of
polymer chains between crosslinks.
It appears that the gels used in the published
literature have been produced by
crosslinking of vinyl-ended polysiloxane
with hydrosilyl-type crosslinkers, facilitated
by a platinum catalyst. This is a commonplace
route to obtain silicone gels at low temperatures
within a reasonable amount of time. Using
traditional nomenclature, a part A containing
polymer and catalyst, and a part B
containing polymer and crosslinker are formulated.
When the two parts are mixed, the
crosslinking reaction commences. A lens replacement
material should have a Young’s
modulus of about 1 kPa. A typical foldable
IOL has a modulus about 1,000 times higher,
i.e., similar to a presbyopic crystalline lens.
How this low modulus is achieved is mostly
considered proprietary knowledge.
Alternatively, curing can be initiated by
light – photoinitiation. With such a system
there is no need to mix components, but the
formulation must be protected against light
until the right moment. After injection into
the bag, crosslinking is started by exposure to
light. The initiation requires light of sufficient
energy. Ultraviolet is harmful and therefore
blue light is preferable. Photoinitiation of silicone
curing is known in ophthalmology in
conjunction with the light-adjustable lens
from Calhoun [36]. Photocuring silicones for
lens refilling have been revealed recently by
Garamszegi and coworkers [37] and are being
investigated by Parel’s group [9].
Chapter 18 Injectable Polymer 181
18.4.2 Hydrogels
Hydrogels are another class of potential candidates
for a lens replacement material. In
contrast to silicones, these polymers contain
water. The desired refractive index requires a
rather high percentage of polymer. Too much
polymer can make the hydrogel too viscous
for injection. Therefore polymers with high
intrinsic refractive index must be sought.
With hydrogels it is crucial to control the
polymer/aqueous interaction. If a polymer
that is water soluble is injected into the bag,
the hydrogel will expand upon crosslinking.
This makes the degree of filling difficult to
control and the capsule can even burst. If the
polymer is not water soluble, it cannot form
an injectable hydrogel. To be useful the polymer
must be just on the limit – swell but not
dissolve in water. Hydrogels are intuitively attractive,
as they are felt to be close to natural
materials. In fact, the proteins of the crystalline
lens are technically hydrogels.
Kessler [1] tried Damar gum and Agarwal
[4] gelatin, in both cases without success.
De Groot and coworkers studied a number
[38, 39] of hydrogel systems with the aim of
using them as accommodating lens replacements.
Poly(ethylene glycol) diacrylate was
used to crosslink a copolymer of Nvinylpyrrolidone
and vinyl alcohol by photopolymerization,
using a phosphine oxide
initiator. Lenses were formed in pig cadaver
eyes. The lenses formed had the transparency
of a 25-year-old human lens.A novel hydrogel
based on poly(1-hydroxy-1,3-propandiyl)
showed promise in forming a material with
low modulus. In a different approach, small
particles were crosslinked to form a loosely
crosslinked gel [40]. The particles provided
refractive index and the loose gel low modulus.
The idea of crosslinking particles has
been pursued by Pusch [41].
Murthy and Ravi [42] used poly(ethylene
glycol)-based hydrogels as mechanical
probes to study accommodation. Lenses were
formed in porcine cadaver eyes. The softest