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The optical structure of the eye is composed of two systems for different ranges of vision:
Distance Vision – Determined by length of the eye from the anterior surface of the cornea to the anterior surface of the retina. This is called the axial length and is typically about 24.0 mm in the adult eye. If the axial length is too long, the rays of light will focus in front of the retina, which results in myopia or near sightedness (see picture below). If the axial length is too short, the rays of light will focus behind the retina, which results in hyperopia of far sightedness (see picture below). If the length of the eye allows the rays of light to focus correctly on the retina resulting in good distance vision, the eye is said to be emmetropic. Also the relative shape of the cornea, if asymmetrical in any way, can also affect distance vision due to astigmatism.
Near Vision – Operates through a second system called the crystalline lens which is hidden beneath the iris. This lens changes shape, becoming thicker in its central axis during accommodation which increases its focal power, allowing the eye to see at near. The lens is contained within a capsule made of thin but tough elastic tissue. The capsule which surrounds the lens is held in place by three sets of fibers called zonules which connect the capsule to the ciliary processes and the ciliary muscle. The zonules fall in three categories depending upon where they attach to the capsule. The anterior zonules are a series of attachments that are on top of the capsule about 2-3 mm from the outside edge of the capsule or the equator of the lens. The equatorial zonules are at the edge of the capsule or the equator. And the posterior zonules are on the lower side of the capsule also about 2 – 3 mm from the equator. The zonules are of huge importance in accommodation – almost every theory of accommodation revolves around them in some fashion.
Attached below are several environmental electron microscope pictures of the zonules and their attachment points on the lens capsule (from an article published in November 2006 by Andres Bernal, Jean-Marie Parel, and Fabrice Manns entitled “Evidence for posterior zonular fiber attachment on the anterior hyaloid membrane”). The anterior, equatorial, and posterior zonules are labeled AZ, EZ, and PZ respectively. Also shown is the hyaloid membrane labeled HM or AHM, with which the posterior zonules intersect before attaching to the lens capsule. The hyaloid membrane forms a diaphragm that separates the vitreous in the back of the eye (which starts out as a gel in a young person, but eventually liquefies with age), from the aqueous in the anterior chamber, which is in constant production from the ciliary processes, and then drained through the trabecular meshwork to channels outside of the eye. Also shown is the ciliary body labeled CB to which all of the zonules attach.
The following is a schematic from the same article showing the results of Bernal’s research, especially the intersection of the posterior zonule and the Hyaloid membrane. This schematic will be used later to show the importance of potential positions for the Refocus implants.
Interestingly, while the globe or exterior of the eye reaches its final size and shape by age 19 or 20, the crystalline lens continues to grow on its anterior and posterior surfaces, and perhaps in equatorial diameter, by 20 microns per year much like bark on a tree. An MRI image of the lens in a young eye versus the lens in an old eye is shown below. The image on the right shows the lens of a 26 year old. The image on the left shows the lens of a 49 year old.
Also of particular interest, is the fact that the distance between the equator or outer edge of the lens and the ciliary body (the circumlental space) grows smaller with age. The smaller space between the equator and the ciliary body in the image of the lens in the 49 year old patient above is very obvious. Ron Schachar is one of the first to note this difference, and largely based his theory of accommodation and presbyopia on this concept. Dr. Schachar strongly believed that the diminishing circumlental space resulted from outward growth in the equator of the lens. Other researchers, including Robert Augusteyn from the University of NSW, Sydney, Australia, also agree with Schachar about the linear growth of the lens equator with age.