Anatomical considerations

Published on 08/03/2015 by admin

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Last modified 08/03/2015

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CHAPTER 7 Anatomical considerations

Applied anatomy

The structures in the perilenticular area, the lens capsule, nucleus and cortex, zonules, anterior vitreous, and ciliary body all interact to provide an optically clear (in health) structure which has the ability to focus, to minimize chromatic aberrations and produce the sharpest image possible on the retina for differing lighting conditions and points of focus. The ciliary muscle contracts to reduce the tension in the zonules and allow alteration in shape in the lens capsule, and therefore curvature of the lens. There is much discussion about whether it can continue to function throughout life, but it is thought the ciliary muscle/zonule/capsule interaction have relatively normal function for life ¹.

The zonules are composed of the protein elastin associated with microfibrils of 8 to 12 nm in diameter. Microfibrils contain fibrillins as major constituents. They pass from the ciliary muscle to the lens capsule on both the anterior and posterior surfaces.

The capsule consists of the basement membrane of the lens epithelial cells and is composed of type IV collagen and glycosaminoglycans. It has varying thickness over the anterior and posterior surfaces of the lens, which may be important in the accommodation of the natural lens, so-called accommodative arching. The capsule is thickest in the peripheral regions and thinnest anteriorly and posteriorly ranging from 2–50 microns.

The recess formed by the peripheral anterior capsule, zonules, ciliary body, and posterior surface of the iris forms the ciliary sulcus, which can be used to support the haptic of an intraocular lens. The ciliary sulcus has a diameter of 13–15 mm.

The anterior vitreous maintains a close relationship with the lens, is separated from it by Berger’s space, and attached to the mid-periphery of the posterior part of the lens by Weigert’s ligaments. These are all remnants from the embryological development of the lens (Fig. 7.1).

Fig. 7.1 Lens–vitreous relationships.

The area behind the ciliary body is the pars plana, which joins the peripheral retina. By external landmarks it is up to 4.5 mm from the limbus. It is important, as instruments may be passed through this area without precipitating a retinal detachment.

The lens is a metabolically active structure. The lens structure itself consists of fibers composed of beta-crystallin protein which emanate from the lens epithelial cells, which line the anterior and equatorial parts of the capsule and are laid down in layers with increasing compaction and density in the center or nuclear zone as time progresses. This results in the center of the lens becoming stiffer, compared with the cortex, from 40 years onwards; this reversal in the profile of lens stiffness is partly responsible for the onset of presbyopia. The gradient refractive index structure of the lens aids accommodation and increases the refracting power of the lens.

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