An D or more.2 For these higher levels

An intraocular lens is a lens implanted in the eye, usually
as a treatment for cataracts or myopia. Intraocular lenses have been used to
correct errors such as near-sightedness, far-sightedness, and astigmatic eyes
since 1999, with more than 6 million lenses being implanted annually1.
First used in 1949, vast improvements have been made in their design
since, which has led to cataract surgery becoming a precise refractive surgical
technique, about a thirty minute procedure.
We will first look at the different types of IOLs available at present, these
include Toric, Multi-focal, Accommodating, and Aspherical Intraocular lenses.
Toric IOLs are used to correct for astigmatism, and are dependent on the
accuracy of placement within the eye. Multi-focal IOLs create multiple focal
points in the eye, to correct both distance and near vision at once.
Accommodating lenses are used to restore the accommodative response, by using
the optic shift principle. Lastly, aspherical lenses are used to improve the
optical quality of the image created by the IOL. Similar to toric lenses,
aspherical lenses are dependent on their centration and tilt.

Corneal astigmatism, which is where the curvature of the
cornea is uneven, leading to improper refraction of light rays, is present in
22.2% of patients of cataract surgery to a level of 1.5 D or more.2
For these higher levels of astigmatism, toric IOLs are used as they are not
dependent on the healing response of the cornea, and they have shown to be an
effective method of correction. They are however, dependent on rotational
stability within the eye and axis misalignment could result in reduction of
astigmatism correction, or if the rotation is past 30 degrees, the post
operative astigmatism will be higher than the level of corneal astigmatism. To
assist in correct placement, toric IOLs have two markings present on the optic,
to line up with the axis of astigmatism. When in the eye the IOL is rotated
until the markings align with the corneal astigmatic axis, by reference markers
placed on the eye prior to surgery. Different designs of IOLs show different
levels of rotational stability once in the eye, with friction between the lens
and the crystalline bag the main factor in keeping the lens stable. Lens
material affects friction, with silicon lenses showing less friction than
acrylic and PMMA lenses. Lens size in relation to capsular bag size is also
important; if the IOL is too small, there is not enough contact, hence not
enough friction for stability, and if too big, the IOL can stretch and distort
the bag.  Small side struts on the IOL,
known as haptics are also used for stability. These can be open or closed loop,
with both having their own advantages and disadvantages.

Multifocal IOLs are mainly used for surgical correction of
presbyopia, the hardening of the lens which occurs naturally with age and
results in light focusing behind the retina. Multifocal IOLs are used as their
use is independent of ciliary body function. The lenses are currently the most
reliable for recovering both near and distant vision. Multifocal lenses create
focal points in the eye, corresponding to the working distances. There are
different types of multifocal lenses, with two main types: diffractive and
refractive design. Diffractive lenses can then be categorised into fully
diffractive or partially diffractive, and refractive lenses can be divided into
either concentric or sectorial. Fully diffractive multifocal IOLs have many
concentric rings on their surface. Each ring creates a diffraction pattern, and
by changing the width of the rings, the diffraction pattern can be used to
create two focal points within the eye. Asymmetrical diffractive IOLs split the
light unequally and can be used to transmit a higher amount of light to either
near or distance.
Partially diffractive IOLs have a diffraction pattern over a specific area of
the lens. An example of this type of lens is appodised diffractive multifocal
IOLs, where only the central 3.6mm of the lens has the diffraction pattern
present. The central part of the lens distributes the light between distance
and near vision equally, then moving towards the outer part of the lens, the successive
rings distribute more light towards the distance. This lens is pupil dependent,
as when the size of the pupil increases more light is distributed to the
distance.
Concentric refractive multifocal IOLs have concentric zones that differ in
curvature, which creates varying refractive powers. The different zones of
refractive power create multiple focal points within the eye. These lenses are
also pupil dependent, as the refractive zones are relatively large. Refractive multifocal
IOLs do not precisely split the light into two focal points, but into a small
spread of light near the focal point area. This leads to a broader range of
near visual acuity instead of the precise optical quality seen with a single
focal point.3
Sectorial refractive multifocal IOLs, also known as rotationally asymmetrical,
are the newest type of multifocal lens. The current technique of implantation
is to position the lens towards the inferior portion of the eye. This lens
gives two precise focal points, independent of the pupil. As the lens in
non-concentric, it does not produce the conventional halo effect, glare is
instead restricted to an area matching the position of the segment.

Accommodating IOLs are used to restore the ability of the eye
to adjust its focusing power at all distances. As the eye ages, the lens inside
the eye becomes less flexible, resulting in the inability to focus near vision.
This is a sign of presbyopia in people over 40. As the ciliary muscles are
still active, accommodating IOLs were designed to shift forwards with ciliary
muscle contraction. This is known as the optic shift principle. There are two
types of accommodating IOLs, single and dual optic. Single optic accommodating
lenses are designed to move anteriorly with accommodative effort, increasing
the effective lens power. Different single optic lenses rely on ciliary muscle
contraction producing either an increase in vitreal fluid pressure, which would
push the lens forward, or releasing the zonule tension, allowing the capsular
bag of the lens to push against the haptics. Dual Optic lenses are designed to
fill the capsular bag.they have a high powered positive anterior optic and
negative posterior optic. The haptic attaches to both optics and keeps the two
lenses separated. While the ciliary muscles are relaxed the optics are close to
each other, and under accommodation effort, the capsular bag compresses the
haptics, moving the anterior lens forward. The anterior lens of a dual optic
system is more powerful than that of a single optic system, meaning dual optic
lenses have higher accommodating power.

Aspheric IOLs are monofocal IOLs, used to correct spherical aberration
in the eye. A lens with spherical aberration will focus rays that pass through
the peripheries of the lens closer to the lens than the rays that pass through
the centre. Other types of IOL may also increase the positive spherical
aberration of the eye, which naturally increases with age. Using an aspheric IOL
adds negative spherical aberration to compensate for the positive spherical
aberration of the eye. This would be an aberration control aspherical IOL, which
is depenent on the lens being centrally positioned in relation to the visual
axis. If this centralisation is off by more than 0.5mm, the reduction of spherical
aberration is lost, and if the is also tilt introduced, the effects of
spherical aberration can be compounded.4
Aberration neutral aspherical IOL are aberration free, meaning they are
intended not to introduce any aberration to the eye, but will not correct the
spherical aberration of the cornea either. Centration of the lens is less
important with aberration neutral lenses. In order to obtain the best results
after implantation, corneal spherical aberration would ideally be measured
accuratley prior to the surgery, and the lens selected accordingly.

Power calculation of the IOL is important as it ensures that
the lens fits the needs and requirements of each patient. There are four
factors involved in the power calculation; the eyes axial length (AL), the corneal
power (K), the postoperative IOL position within the eye (estimated lens
position, ELP), and the anterior chamber constant

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