Small Incision Cataract Surgery (Manual Phaco) Kamaljeet Singh
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Anatomy of the Lens1

BN Chaudhary
Kamaljeet Singh
 
The adult human lens is an asymmetrical spheroid, which does not possess nerves, vessels or con-nective tissue. It is located behind the iris and pupil in the anterior compartment of the eye (Fig. 1.1).
  • The diameter of the lens is 9–10 mm and thickness 4–5 mm, which varies greatly as the eye accommodates for near and distant vision
  • The lens has anterior and posterior surfaces and the border where the two meet is known as the equator
  • The anterior surface is less convex than the posterior, radius of curvature being about 9 mm, while that of posterior surface is 5.5 mm
  • The posterior surface lies in a fossa lined by the hyaloid membrane in front of the vitreous. It is separated from the vitreous by a slight space filled with primitive vitreous
  • The equator of the lens forms a circle lying 0.5 mm within the cilliary processes. The equator is not smooth but shows a number of dentations, which correspond to the attachment of zonular fibres. These dentations disappear when zonules are loose during accommodation.
 
Microscopic Structure of the Lens
The lens consists of:
  1. Capsule
  2. The anterior epithelium
  3. The cement substance of amorphous material
  4. The lens fibres.
Capsule The capsule forms a transparent structure-less highly elastic envelope, which encapsulates the lens material. The anterior capsule is much thicker than posterior. The anterior and posterior capsules are thicker at the equator than at the poles, where the suspensary ligaments are attached. The thickest region up to 23μ is located close to the equator on both the anterior and posterior surfaces. The posterior pole is the thinnest region (4μ) while at the equator (17μm) and anterior pole (9-14μ) is of intermediate thickness.
zoom view
Fig. 1.1: Anatomy of the adult human lens
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The anterior epithelium This is a single layer of cubical cells beneath the anterior capsule (There is no corresponding posterior epithelium). This layer is responsible for all the metabolic and mitotic activity of the lens. This layer produces the lens fibres.
The cubical cells of the anterior epithelium gradually become columnar and elongate towards the equator and are eventually converted into lens fibres.
As these cells elongate into lens fibres, the part, which is in contact with the capsule becomes the posterior part of lens fibres, while the opposite end grows into the anterior portion of the lens fibre.
The cement substance of amorphous material The various elements forming the lens are bound together by an amorphous substance. The cement substance glues the various fibres to each other.
It is found at following sites:
  1. Beneath the capsule both in front and behind.
  2. A thin layer deep to the anterior epithelium.
  3. The central strand.
The central strand occupies the axis of the lens from anterior to posterior pole. Extending towards the equator from this axial collection the amorphous material is collected in the form of Y. The anterior Y is vertical and posterior is inverted (λ). The lens fibres get inserted into these.
The lens fibres Each lens fibre is a long, prismatic sixsided band. Lens fibre is a collection of albuminoid material enclosed in a pseudo-membrane. The membrane is called pseudo because it is composed of the same material as its contents but is denser.
During embryonic development the first lens fibres arise from posterior epithelium, which run from the back to the front of the vesicle. The later fibres are derived from the equatorial portion of the anterior epithelium. The newer fibres are laid external to the deep older fibres and this give the lens a laminated structure.
New lens fibres are laid on throughout life and as the central portion, which corresponds to the keratin layer of the skin cannot be shed, the lens keeps on growing. However, the growth is not proportional to the number of fibres, because the deeper older fibres get shrunken. The lens at the age of 65 years is one-third larger than at the age of 25 years. Hence, we can anticipate bigger nucleus and may need bigger incision while performing surgery in older persons.
The consistency of the lens varies and superficial cortex is softer than central part of nucleus. The nucleus increases in size with age and this becomes flatter with age. However, the refractive power of lens is retained by an increase in the refractive index of the nucleus.
The colour of the lens also changes with age. In the infant and young, it is quite colourless. After about 35 years the central portion develops yellow tinge and gradually becomes darker and more extensive with age. In the older people the lens has amber colour.
Sometimes the lens appears gray in old people when seen by indirect illumination and can be mistaken as cataract by the beginners.
Ciliary Zonule The ciliary zonules consist of fibres arising from the ciliary body to the lens. It holds the lens in position and enables the ciliary muscles to act on it. The zonular fibres are attached at the equator and the anterior and posterior capsule near the equator.
The zonular fibres can be classified in two groups: Main and Auxillary fibres.
  1. Main fibres consists of following fibres:
    1. Orbiculoposterior capsular They originate from the ora serrata and are inserted into the posterior capsule.
    2. Orbiculoanterior capsular They are the thickest and strongest of the zonular fibres. They originate from the pars plana of ciliary body and inserted into the anterior capsule of the lens.
    3. The cilio-posterior capsular fibres They are the most numerous fibres. They arise from the valleys and sides of the ciliary processes. They are directed posteriorly and cross the anteriorly directed fibres and are inserted into the posterior capsule.
    4. The cilio-equatorial fibres They are present only in youthful eyes, originate from ciliary valleys and inserted to the equator of the lens. With age these fibres disappear.
  2. The auxillary fibres Some of these fibres strengthen the main fibres and help to anchor the individual portions of the zonule, while others hold the ciliary body together. These are very fine and run from without inwards and forward.
It is noteworthy that in old age a large number of zonular fibres disappear but some fibres also get thickened.
 
Surgical Anatomy of the Lens
For the purpose of cataract surgery lens can be anatomically divided into:
  1. Capsular bag with sub-capsular epithelium.
  2. Superficial cortex, i.e. soft lens matter that can be aspirated.
  3. Immediate epinucleus with semi-soft lens matter that can be expressed out.
  4. Deep nucleus or a hard core that can be expressed fractured, fragmented or phacoemulsified.
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zoom view
Fig. 1.2: Surgical anatomy of limbus
The capsular bag encapsulates the lens substance. It is highly elastic and hence a big nucleus can be expressed out from a comparatively small capsulotomy or capsulorhexis.
The capsular bag provides support to the IOL within the bag allowing for good haptic placement. The anterior epithelium consisting of cubical cells beneath the anterior capsule is responsible for all the metabolic and mitotic activity of the lens. The cells migrate and elongate towards equator and produce lens fibres. After extra-capsular cataract extraction the remaining lens epithelium especially those in the equator region undergo metaplasia and migrate towards posterior capsule and lead to posterior capsule opacification.
The zonules are inserted into the capsule in a continuous fashion at the equator, anteriorly 2–2.5 mm into the capsule and 1–1.25 mm into the posterior capsule. Hence only a 5–6 mm of zonular free zone of capsule is left for capsulorhexis or capsulotomy.
The lens nucleus has a configuration with a well-defined hard inner nucleus surrounded by semi-soft epinucleus and soft cortical matter. During hydrodelineation the nucleus is separated from epinucleus and this reduces the size of the overall nucleus which can be expressed out from a smaller incision. The epinucleus also forms a cushion beneath the nucleus during phacoemulsification.
 
Surgical Anatomy of the Limbus
Limbus is an important structure from surgical point of view as all the surgery for cataract and glaucoma is performed at the limbus. The external landmarks of the surgical limbus are (Fig. 1.2):
  1. The anterior limbal border It is identified by the insertion of conjunctiva and Tenon's capsule into the cornea, which creates a prominent ridge. This ridge overlies the termination of Bowman's membrane.
  2. The midlimbal line When the conjunctiva is separated from the limbus a bluish transluscent zone 1 to 1.2 mm wide is seen posterior to the anterior limbal border. Posterior to this bluish zone is the white sclera. The line formed at the junction of bluish zone and white sclera is called midlimbal line and it overlies the Schwalbe's line (which is the termination of Descemet's membrane).
  3. Posterior limbal border It lies 1 mm behind the midlimbal line and can be seen only with the use of sclerotic scatter illumination. Posterior limbal border lies approximately over the scleral spur.
The width of the blue limbal zone varies in different quadrants. Maximum width is in the superior quadrant about 1mm. In the temporal and nasal quadrant it is 0.4 mm and in the inferior quadrant 0.8 mm wide. The width of the white limbal zone remains constant throughout.
The midlimbal line is a very important landmark, which overlies the Schwalbe's line and we can remember this by the phrase, “where the white meets the blue Schwalbe's line waits for you.”
But the difficulty is that this landmark is frequently indistinct. Anterior limbal line can be easily distinguished in a limbus based conjunctival flap, but in a fornix-based flap it is frequently irregular and is not a helpful landmark.
FURTHER READING
  1. Peyman Gholam (Ed): Principles of Ophthalmology Jaypee Brothers;  489–91, 532–33, 1987.
  1. Wolf's Anatomy of the Eye 5: 138–42, 1961.
  1. Yanoff M Duker JS(Ed): Ophthalmology Mosby International Ltd: 4–(1.1–1.4), 1999.