Anatomical Considerations
Anatomy Review

Anatomical & Physiological Considerations

Successful adaptation to any contact lens, either rigid or soft, is largely dependent upon the adequate wetting of both the lens and the cornea, and sufficient oxygen flow to the corneal epithelium. In addition, a stable and adequate tear film is required as well as some means of removing metabolic by products from the cornea. When there is an adequate tear volume, these requirements can be met with the eyelids.

The health of the anterior surface of the eye is largely dependent upon the balanced interaction of several of its components. These include the eyelids, cornea, conjunctiva, sclera, and tear film. A contact lens on the eye can impose additional burdens to this system. If any one of the components are in some way altered, it can interfere with successful contact lens wear.

Tear Film�Three Layers
Lipid Layer: a fatty material produced by the meibomian glands which forms a very thin layer over the entire surface of the tear film. It functions primarily to prevent rapid evaporation which would result in dry areas on the cornea and subsequent discomfort and corneal damage.

The Mucoid Layer: functions to convert the hydrophobic epithelial layer of the cornea to a hydrophyllic surface. It is the innermost layer of the tear film and located immediately against the corneal and conjunctival epithelial cells. BUT or Break Up Time, indicates the amount of time it takes from a blink until the tear begins to break up or becomes discontinuous. BUT�s shorter than 5-10 seconds are considered abnormal. Short BUTs may indicate a deficiency in the mucoid layer.

The Aqueous Layer: is the middle layer of the tear film and consists of 98% water. However, it also contains ions, and other molecules such as sodium and potassium along with a concentration of protein. A cornea is said to be hypotonic when more water flows in than out causing the cornea to swell. This occurs when normal evaporation is not allowed to occur. An isotonic cornea allows an equal amount of water to flow in as well as out allowing the cornea to maintain its normal thickness.

Corneal State of Partial Dehydration:
A normal cornea is in a continual state of partial dehydration which is also known as deturgence. If more water flows out than flows in, the cornea becomes hypertonic resulting in corneal thinning. Hypertonicity can be caused by a higher salt concentration placed on the cornea. Another aspect of the tear layer which you may need to know is its pH, which refers to its relative acidity or alkalinity. The pH of the human tear is approximately equal to 7.4. pH is significant when considering contact lens solutions.