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Anatomy of lens
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The center of the anterior surface is called
anterior pole and it is situated 3mm away
from the posterior (endothelial) surface of
cornea. The center of the posterior surface is
called posterior pole. The distance between
these poles is measured as lens thickness. The
thickness of lens is 3mm at birth, whichincreases to 6mm in older age.
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The marginal circumferences of the lens,
where anterior and posterior surface meet,
are known as equator. Equatorial or lens
diameter is generally measured in nasal to
temporal dimension. The equatorial diameter
of lens is 6.5 mm at birth, which reaches to 9-
10 mm in adult life..
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The equator of the lens is encircled by the
cilliary processes of the cilliary body and held
in position by zonules (see later) laterally. The
distance between lens equator and cilliary
processes is 0.5mm. Lens equator has a
serrated or ridged appearance which is caused
by the zonular fibers and this serration orridges disappears during accommodation due
to relaxation of the zonular fibers
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Refractive power:
The diopteric power ofhuman eye is approximately 58diopters. The refractive power of crystalline lens is
about 15 diopters. Though lens has less refractivepower than cornea, it has the ability to change itsshape with the help of cilliary muscle, by which it canchange its diopteric power, allowing the distant andnear vision. However this property changes with age.
Lens has a refractive index of 1.39 (1.36 in peripheryand 1.40 centrally - a property which is termed asgrading refractive index)
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The eye has the capacity to adjust its focusfrom distance to near objects because of theability of the lens to change shape, a
phenomenon known as accommodation. Theinherent elastic property of the lens allows itto become more or less spherical dependingon the amount of tension exerted by the
zonular fibers on the lens capsule. Zonulartension is controlled by the action of theparasympathetically innervated ciliary muscle
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When cilliary muscles contracts, relaxation ofzonular tension occurs. The lens then assumesa more spherical shape, resulting in increased
dioptric power whichhelps to bring nearerobjects into focus.
Ciliary muscle relaxation causes the zonulartension to increase. As a result, lens flattens,whichhelps in bringing more distant objectsinto view.
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PresbyopiaThis condition may develop around the age of 40-50 yrs. With increasing age, with the continuingappositional growth of lens fibers and due to agerelated changes in lens capsule, the elasticity ofthe lens diminishes. The contractility of the ciliarymuscle also diminishes due to the structuralchanges in the muscle. As a result lens fails to
change its shape sufficiently duringaccommodation.
g
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Structure of lens:Lens is histologically composed of three structures-lens capsule,lens epithelium and lens fibers.
Lens Capsule:
Lens capsule is a transparent covering that surround the entire lens.Histologically it is a basement membrane, secreted by lensepithelium and lens fibers .The capsule is produced anteriorly bythe lens epithelium and posteriorly by the elongating fiber cells. It iscomposed of type IV collagen fibers and sulphatedglycosaminoglycans. Though it has no elastic tissue, it is highly
elastic in nature because of lamellar or fibrillar arrangement offibers. This property of the lens gradually decreases with age.
Lens capsule is thickest near equator and thinnest at posterior pole.Thickness of anterior lens capsule increases with age, whereasthickness of posterior capsule remains constant or changes slightly.
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This extreme thinness of the posterior capsule
makes it more vulnerable for posterior
capsular tear or rent during cataract surgery.
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CapsulorhexisDiameter of the adult lens is approximately 10mm and anarea of diameter 6mm is the zonule free area in anteriorcapsule. In cataract surgery, a circular opening is made
(capsulorhexis) with in this area. To overcome the elasticstrength of the capsule, while performing continuouscurvilinear capsulorhexis, two types of forces are applied-tearing by stretching (force is applied perpendicular to thedesired direction of tear which is uncontrolled) and tearingby shearing (force is applied perpendicular to the capsular
plane and it is more controlled).As the capsule in children ishighly elastic than in adults, it becomes very difficult toperform continuous curvilinear capsulorhexis in suchpatients
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Near the equator.the cells of the anterior lens
epithelium,elongate and becomes columnar(E
cells).As they elongate their apical parts lies
deeper to other cells which are placed more
anteriorly.Thus these elongated epithelial cells
trasformed to lens fibres at the equator.
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After cataract or Posterior capsular opacification
After cataract surgery, residual epithelial cells may cause posterior capsuleopacification. E cells migrate posteriorly along the posterior capsule andoften forms large balloon like bladder cells, known as Wedl cells. These areclinically termed as Elschnig pearls. Each pearl represents the failed
attempt of a epithelial cell to differentiate into a new lens fiber. E cells arealso responsible for a dumb bell dough-nut-shaped opacification, knownas Soemmerings ring.Remnant cells on the anterior capsule after cataract surgery differentiateinto spindle-shaped, fibroblast-like cells, which are known asmyofibroblasts. They express smooth muscle actin filament, expressedcommonly in smooth muscle cells and become highly contractile. These
cells proliferate and migrate to the posterior capsule and form a layer bysecreting extracellular ground substances and a basement membrane likematerial. Cellular contraction by this highly contractile cells leads tothe formation of folds and wrinkles in the posterior capsule
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