8/6/2019 lens veh

1/32

8/6/2019 lens veh

2/32

Anatomy of lens

8/6/2019 lens veh

3/32

8/6/2019 lens veh

4/32

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.

8/6/2019 lens veh

5/32

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..

8/6/2019 lens veh

6/32

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

8/6/2019 lens veh

7/32

8/6/2019 lens veh

8/32

8/6/2019 lens veh

9/32

8/6/2019 lens veh

10/32

8/6/2019 lens veh

11/32

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)

8/6/2019 lens veh

12/32

8/6/2019 lens veh

13/32

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

8/6/2019 lens veh

14/32

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.

8/6/2019 lens veh

15/32

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

8/6/2019 lens veh

16/32

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.

8/6/2019 lens veh

17/32

This extreme thinness of the posterior capsule

makes it more vulnerable for posterior

capsular tear or rent during cataract surgery.

8/6/2019 lens veh

18/32

8/6/2019 lens veh

19/32

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

8/6/2019 lens veh

20/32

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.

8/6/2019 lens veh

21/32

8/6/2019 lens veh

22/32

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

8/6/2019 lens veh

23/32

8/6/2019 lens veh

24/32

8/6/2019 lens veh

25/32

8/6/2019 lens veh

26/32

8/6/2019 lens veh

27/32

8/6/2019 lens veh

28/32

8/6/2019 lens veh

29/32

8/6/2019 lens veh

30/32

8/6/2019 lens veh

31/32

8/6/2019 lens veh

32/32