The Sclera and Induced Abnormalities in Myopia
The sclera is a tough, translucent fibrous coat that provides a set shape and volume of the eye and functions as a protective casing its fragile internal contents. Axial myopia, the typical form seen in highly myopic patients, is associated with stereotyp
- PDF / 1,561,255 Bytes
- 16 Pages / 595.28 x 790.87 pts Page_size
- 11 Downloads / 232 Views
The Sclera and Induced Abnormalities in Myopia Richard F. Spaide
The main constructural element of the eye is the sclera, a tough, translucent fibrous coat that provides a set shape and volume of the eye and functions as a protective casing for its fragile internal contents. The sclera is a composite of an interwoven network of collagen fibers (mostly type I) embedded in a hypocellular ground substance matrix. The sclera has some similarities to a pneumatic tire. The collagen fibers are analogous to the plies in a tire; they are relatively less distensible fibers embedded in a matrix that is more distensible. A pneumatic tire is inflated by air, while the sclera is inflated by the intraocular pressure. The mechanical engineering advantages of this arrangement include strength, sufficient rigidity without brittleness, and little need for intrinsic blood supply or cellular turnover. Because the eye has structural rigidity, its length and shape are not altered with eye movement. On the other hand the eye is easily deformable without suffering internal or external damage during ordinary use in life. Blood vessels and nerves of various sizes penetrate the sclera, and design features of the scleral openings help mitigate against loss of the intraocular contents. The muscles of the iris and ciliary body attach to the sclera as do the extraocular muscles used for movement of the globe. The sclera, which accounts for more than 90 % of the surface area of the eye [1], merges anteriorly with the optically clear specialization, the cornea. Many changes occur in the sclera of high myopes, and these changes and the abnormalities they may induce are the focus of this chapter. In most high myopes the eye undergoes normal development in utero and early childhood to be followed later by progressive scleral thinning and ocular expansion. With knowledge of the base anatomy and the induced alterations caused by the expansion, the subsequent
R.F. Spaide, MD Vitreous, Retina, Macula Consultants of New York, 460 Park Ave, 5th Floor, New York, NY 10021, USA e-mail: [email protected]
abnormalities associated with myopia are easier to understand. As such this chapter starts with a review of the embryology and development of the sclera, its anatomy, and mechanical properties. Features of what happens in myopization will then be presented, followed by how these forces cause specific clinically recognizable alterations in the sclera and associated structures. One specific and important induced alteration, the staphyloma, is sufficiently complex as to require treatment in its own accompanying chapter entitled, creatively enough, “Staphyloma.”
8.1
Embryology and Development of the Sclera
The evagination of the optic vesicle starts in the fourth week and invaginates to form the optic cup in the fifth week. A thickening of the overlying ectoderm called the lens placode develops around this time and will eventually invaginate to form the primordial lens. The sclera is derived from the neural crest and, to a much lesser extent, the mesoderm,
Data Loading...
