Achieving Excellence in Cataract Surgery - Chapter 12
Achieving Excellence in Cataract Surgery - Chapter 12
Achieving Excellence in Cataract Surgery - Chapter 12
12
Intraocular Lens
Materials and Design
Oliver Findl, MD, MBA
Figure 12-2.
Figure 12-3. Fusion of capsule at haptic-optic junction for different haptic designs. (Left) Acrysof multi-
piece with nearly complete fusion. (Middle) Acrysof single-piece with incomplete fusion which may serve
as one entry site (arrows) for regenerating LECs and no sharp edge at junction. (Right) The Tecnis 1-Piece
IOL incorporates a new feature of the ProTec 360-degree barrier edge.
Figure 12-5. Problems with accommodating IOLs. Infolding of haptics due to capsule constriction with
1CU (left); early PCO due to missing barrier along broad haptic-optic junctions for 1CU (middle) and
Crystalens AT-45 (right).
of IOL power. Should buttonholing not be possible, eye of normal dimensions. Accordingly, in a short eye,
about 0.5 diopters should be deducted from the cal- such a shift would cause more refractive change. These
culated power since the IOL will be more anteriorly IOLs have in common a hinge-like junction of haptics
placed in the eye. to optic that should allow the shifting of the optic when
In cases where no capsule support is given, apart the haptics are compressed. Measurements of IOL
from the classical angle-supported anterior chamber shift with current models have shown only very small
IOL, iris-supported IOLs and scleral-sutured IOLs are amounts of IOL movement and to be very variable
the most popular options (see Figure 12-2). In the case among eyes, both when stimulated with a near target
of the iris-supported IOL with lobster-claw haptics that or pilocarpine-induced ciliary muscle contraction.19-21
are clipped onto iris stroma, they can be clipped onto Apart from lacking evidence of their function, these
the iris from the anterior side or from the posterior IOL designs have had significant amounts of PCO with
sideso-called retropupillary fixation. This IOL style most patients needing Nd:YAG capsulotomies within
has a long track record in aphakic eyes and appears to the first 2 years after surgery (Figure 12-5).22
have a low rate of endothelial cell loss, but do require a
6-mm incision since the aphakic style is currently only
available in PMMA. Intraocular Lens
In the case of scleral suturing of a posterior cham- Optic Design
ber IOL, both foldable and rigid IOLs can be used.
However, there have been several reports of long-term Edge Design
knot erosion resulting in decentration or even sublux- During the past decade it has become clear that
ation of these IOLs as well as late endophthalmitis.18 optic edge design plays an important role in the pre-
The trend is away from sutured IOLs back to modern vention of PCO. When the Acrysof lens (Alcon) was
anterior chamber IOLs and iris-fixated IOLs. introduced in the early 1990s, several studies showed
that PCO development was significantly less than with
other IOLs.23-25 This first was attributed to the acrylic
Special Haptics material and to the surface properties of the IOL.26
Accommodating Intraocular Later it could be shown that the sharp-edge design of
Lenses the lens seemed to be the key factor for this effect.27
Currently available accommodating IOLs are sup- The sharp IOL edge was a result of the manufactur-
posed to work according to the optic shift principle. ing process, and its blocking effect on LEC migra-
Ciliary muscle contraction should result in an anterior tion, therefore, rather coincidental. Further studies
shift of the optic, resulting in an overall increase in confirmed that the rectangular shape of the IOL rim
refractive power of the eye. A 0.7-mm shift would be with its sharp edges, in combination with the acrylic
predicted to achieve 1 diopter of accommodation in an material, was in fact the main reason for the reduced
Intraocular Lens Materials and Design 101
Figure 12-6. Blocking of LEC migration at posterior sharp optic edge due to bending of the capsule (left)
compared to round edge IOL (right).
formation of PCO.28 Studies by Nishi revealed that the of the front and back surface are identical. Some
discontinuous capsular bend seems to be a key factor manufacturers have an asymmetric biconvex optic,
for the preventative effect of a sharp-edge optic.27,29 where the back surface curvature is relatively flat and
The capsular bend at the posterior optic edge causes constant throughout most of the power range and
mechanical pressure and/or contact inhibition of LEC the anterior curvature is varied for IOL power. This
growth on the posterior capsule (Figure 12-6). causes a slight shift of the principal optical plane of
As a result of these findings, several new IOLs with the IOL and also implies that the lens should not be
a sharp optic edge design were introduced in the past implanted front to back in the eye, apart from the
years and compared in clinical trials. In a meta-analysis angulation of the haptics being backward as well. In
of the randomized controlled trials comparing round a symmetrically biconvex lens with no angulation,
and sharp-edge IOLs,30 there was a clear beneficial the IOL could be implanted front to back without a
effect of sharp-edge IOLs concerning inhibition of change in optical power.
PCO. This also confirmed that the sole modification
of the posterior optic edge from a round edge to a Optical Zone
sharp edge leads to a significant reduction of PCO by Most IOLs have a full-size effective optical zone
inducing a discontinuous bend at the posterior capsule of 6 mm in the main range of IOL powers. Therefore,
(Figure 12-7).31,32 the higher powered IOLs will have a thicker optic
Unfortunately, sharp optic edges of IOLs may also than the lower powers. This has the advantage of a
have disadvantages. As described previously, in some full optic zone, but can make folding of the IOL or
cases with implantation of lenses with a rectangular injecting with a shooter variable depending on IOL
edge shape combined with a high refractive index, power. Some IOLs keep a constant center thick-
such as found with the Acrysof lens, an increased ness of the optic and vary the effective optical zone,
incidence of persistent edge-glare phenomena was thereby varying the curvature of the optic and, there-
reported.33,34 Sharp-edge IOL designs cause the light fore, optic power. To my knowledge, there was only
rays that are refracted through the peripheral IOL to one manufacturer (Dr. Schmidt) that actually varied
be more intense on the peripheral retina. Round-edge refractive index of the silicone material used for dif-
IOL designs disperse the rays of light over a larger sur- ferent powers, thereby keeping a constant effective
face area of the retina, leading to less glare. However, optical zone and center thickness.
the half-rounded edge profile of some newly developed
IOLs with a round anterior and sharp posterior optic Special Optics
edge seems to avoid this disturbing side effect.35
Aspherical Intraocular Lenses
Optic Geometry This topic is covered extensively in Chapter 13.
In short, these IOLs are either neutral concerning
Biconvexity SA, therefore not adding SA to the eye, or like most
Most IOLs on the market have a symmetrically models currently on the market have a prolate sur-
biconvex optic, meaning that the radius of curvature face inducing negative SA, which should neutralize
102 Chapter 12
Figure 12-7. PCO 1 year (upper) and 3 years (lower) after surgery for round (left) and sharp (right) edge
optic design for a hydrophobic acrylic IOL.
the positive SA of the average cornea. The aim is to steep axis, adding an opposite clear cornea incision
increase contrast sensitivity under mesopic conditions (OCCI) on the same axis, or making limbal relaxing
where the pupil is dilated. The IOLs have little to no incisions (LRIs) on the steep axis. Most surgeons will
effect when the pupil is small. use a 600-micron knife to perform LRIs. LRIs are able
to reduce corneal astigmatism by as much as 3 diop-
Toric Intraocular Lenses ters. This topic is covered at length in Chapter 16. The
With cataract surgery we can attempt to reduce variability of the outcome is mainly due to interpatient
preexisiting corneal astigmatism using incisional tech- differences in scarring of the corneal tissue, corneal
niques, such as placing the corneal incision on the rigidity, and corneal thickness.
Intraocular Lens Materials and Design 103
life.38 However, good refractive outcome and low
residual astigmatism after surgery are key to success.
Therefore, meticulous biometry and power calculation
are needed. Additionally, since the light is divided and
also some light (about 20%) is lost to higher orders of
diffraction, patients have reduced contrast sensitivity.
Small amounts of PCO may cause substantial loss in
visual functions and Nd:YAG capsulotomy may need
to be performed earlier than usual. Additionally, the
blurred nonfocused image will overlay the focused
image and can cause the photic phenomenon of halos
seen around light sources especially at night with a
larger pupil. These can be disturbing to patients and
are the main reason for explantation of mIOLs.
There are two types of mIOLs: diffractive and
Figure 12-8. Toric IOL with marks for alignment. refractive. Diffractive mIOLs (Figure 12-9) use the
entire optical zone for the creation of two foci and are,
therefore, bifocal mIOLs. The focal points are created
An effective and quite predictable method of using constructive and destructive interference of light
neutralizing corneal astigmatism is the use of toric rays. These phase differences are induced by small
IOLs. The steep axis of the eye needs to be marked steps that are about one-half of the incident light wave-
in the sitting position before surgery since the eye length. In refractive mIOLs, several foci are created by
will undergo some cyclotorsion in the supine position. zones of different surface curvatures of the lens. These
The mean cyclotorsion was reported to be 2 degrees, IOL models will differ according to the distribution of
however, can vary between patients and be up to 10 the zones on the optic surface, and the light distribu-
degrees in individual cases.36 Accurate axis placement tion onto the different foci is pupil size dependent.
of the toric IOL is critical to the outcome since 3% of In general, diffractive mIOLs usually have very
the toric correction is lost for every degree off axis. good near vision outcomes, however, intermediate
Toric IOLs have marks on the IOL optic for alignment vision is poor. In contrast, refractive mIOLs usually
(Figure 12-8). Being 10 degrees off the desired axis have good intermediate vision but relatively poor near
results in about one-third of the toric correction lost. vision. In an attempt to get the best of both worlds, a
Being 30 degrees off results in no toric correction and strategy called mix-and-match with implantation of a
a shift of the axis, and errors beyond that result in an refractive mIOL into one eye and a diffractive mIOL
increase in astigmatism of the eye, being more than
into the contralateral eye has been developed. To date
preoperatively and at a completely different axis (axis-
there are little published data available, but this strat-
flip). Since it is crucial that the IOL does not rotate
egy appears promising in some patients.
inside the capsule bag during capsule shrinkage, there
Another strategy to avoid mIOLs and their poten-
are several different special haptic designs that should
ensure stability. Clinical outcomes with modern toric tial drawbacks as mentioned above is monovision
IOLs have been very promising and rotational stability where both eyes receive standard monofocal IOLs.
appears to be within 2 degrees.37 Good planning and The dominant eye receives an IOL power to achieve
precision during surgery seem to be key to the success good distance vision and the contralateral eye is made
with these IOLs. about 1.25 diopters more myopic to allow intermedi-
ate vision. With both eyes open, the patients usually
Multifocal Intraocular Lenses have satisfactory near vision, at least under good light-
Multifocal IOLs (mIOL) are designed to overcome ing conditions.
the postoperative lack of accommodation by dividing Whether using mIOLs or monovision, patient
the incoming light onto two or more focal points. selection and extensive preoperative counseling are
One of these is used for distance vision, the other for key factors for a good outcome. It appears that patient
near or intermediate vision. These IOLs have shown motivation to achieve spectacle independence may be
to reduce the need for spectacle correction in daily the critical deciding factor for success.38
104 Chapter 12
Clinical Performance of an
Intraocular Lens
Biocompatibility
Phacoemulsification and foldable IOL technology
have permitted the use of small incisions, which results
in less trauma caused by cataract surgery. Immediate
postoperative inflammation is mainly attributed to
surgical irritation of the anterior uvea, which causes
changes in the blood-aqueous barrier.39 Long-term
postoperative inflammation is caused by other factors
such as immunological reactions.
The performance of an IOL is determined by sev-
eral factors such as the surgical technique,40 the peri-
operative treatment,41 the IOL biomaterial and design,
and the host reaction to the lens.
The cellular reaction seen on an IOL is an impor-
tant indicator of the IOLs biocompatibility. On the Figure 12-9. Diffractive mIOL with PCO.
one hand, it consists of macrophages in the form of
small, round cells and foreign body giant cell on the
IOL surface. On the other hand, the cells are LECs entiation into myofibroblasts and the synthesis of col-
after the capsule comes into contact with the foreign lagen fibers.43 These cytokines may act in an autocrine
body IOL. Accordingly, the biocompatibility of an and paracrine fashion, influencing the postoperative
IOL can be divided into two partsthe uveal and the proliferation of LECs in the capsular bag. Thus, Nishi
capsular reaction, as described by Amon.42 and coauthors postulated that fibrous proliferation of
Uveal biocompatibility is defined as the reaction LECs with anterior capsule fibrosis is often associated
of the uvea to the IOL. As a result to the surgical trau- with blood-aqueous barrier disruption, clinically vis-
ma and the IOL, monocytes and macrophages migrate ible as flare in the anterior chamber.43
through the uveas vessel walls into the aqueous and
then onto the IOL surface. Monocytes transform
into small, round cells and macrophages transform Posterior Capsule
into epithelioid and foreign body giant cells that are Opacification
responsible for the phagocytosis of debris. These cells PCO (or after cataract) remains a common prob-
constitute the natural immunological process in a for- lem after cataract surgery with implantation of an IOL.
eign body reaction. It resulted from the transition from intracapsular cata-
Capsular biocompatibility is defined as the reac- ract extraction (ICCE) to ECCE, where the posterior
tion of LECs and the capsule to the IOL material and lens capsule is left intact during surgery. Patients with
design. This encompasses LEC ongrowth, anterior PCO suffer from decreased visual acuity, impaired
capsule opacification, and PCO. The LECs residing contrast sensitivity, and glare disability.
on the posterior side of the anterior capsule (Figure Clinically, two different components of PCO can
12-10), the so-called A-cells, can proliferate onto the be differentiated, namely a regeneratory and a fibrotic
IOL optic from the anterior capsular rim (ongrowth) component (Figure 12-12). Regeneratory PCO is much
and lay down collagen which results in whitening of more common; it is caused by residual LECs from the
the capsule as well as contraction of the capsule, which lens equator region, the so-called E-cells, migrating
in turn may cause rhexis contraction or even phimosis, and proliferating into the space between the posterior
decentration of the IOL, or buttonholing of the IOL capsule and the IOL, forming layers of lens material
(Figure 12-11). and Elschnig pearls. Fibrotic PCO is caused by LECs
LECs also express cytokines, such as interleukin-1, from the anterior capsule that undergo transforma-
interleukin-6, and transforming growth factor , that tion to myofibroblasts and gain access to the poste-
are responsible for LEC proliferation and transdiffer- rior capsule, causing whitening and wrinkling of the
Intraocular Lens Materials and Design 105
Figure 12-11. Complications of extensive fibrotic reaction of capsule: rhexis contraction (left), IOL decen-
tration (middle), partial buttonholing with IOL tilt (right). Arrows indicate location where the rhexis has
slipped behind the optic.
How to Achieve a Low Posterior Capsule As a result, round-edge IOLs have practically disap-
peared from the market. However, although drastically
Opacification Rate
reduced, the problem of PCO has not been eliminated.
Meticulous surgical technique is a prerequisite for
The role of IOL optic material remains unclear; while
low PCO rates. A well-centered capsulorrhexis where
hydrogel lenses have been shown to have a high PCO
the rhexis edge overlaps the IOL optic edge around the
incidence, there is still an ongoing debate about which
entire circumference is necessary to ensure a bending
of the hydrophobic materialhydrophobic acrylic or
effect on the posterior capsule to act as a barrier to
siliconeshould be preferred with respect to PCO
invading LECs. Concerning IOL design, the concept
inhibition. Single-piece IOLs with an incomplete sharp
of a sharp posterior optic edge has been proven to be
optic rim have not shown significantly higher PCO
the most effective method to reduce PCO up to now.
Intraocular Lens Materials and Design 107
Figure 12-13. Examples of the dynamic changes of Elschnig pearls within a month in eyes with PCO; birth
and death (upper), questionable fusion of two pearls and then disappearance (lower).
rates than multipiece IOL designs. However, new Refract Surg. 2008;34(4):677-686.
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