CONTACT LENSES TRUSTED AROUND THE WORLD
Anatomy and physiology of the eye
The orbit is a bone cavity or a socket in which the eyeball is located. It consists of seven small bones that protect the eye from physical damage.
The tissues surrounding the eye, including eyelashes and eyelids, are called the adjacent organs.
Eyelashes consist of several rows of short hairs located along the edge of the eyelids and perform the following functions:
– holding particles of air dust, eyelashes prevent clogging of the eyes;
– contribute to the rapid closure of the eyelids with the threat of damage.
Eyelashes are characterized by natural growth and loss. New eyelashes grow quickly.
Each eyelid consists of a movable plate of fibrous tissue that covers and opens the front of the eyeball. Outside, the eyelids are covered with skin, and from the inside – with the mucous membrane. Distributing the tear fluid secreted by the lacrimal glands, the eyelids protect the eye from any foreign substances that can damage the cornea. Lacrimal puncta are also located on the eyelids, through which a tear is discharged.
Both eyelids are covered by a sensitive membrane, which is called the conjunctiva. Palpebral conjunctiva lines the inner surface of the eyelids and passes into the bulbar conjunctiva. The bulbar conjunctiva is located on the front of the eyeball (the visible white part of the eye) and is in contact with the surface of the palpebral conjunctiva.
Since the conjunctiva is a continuous shell, contact lenses cannot be behind it, that is, behind the eye. The conjunctiva is in constant direct interaction with the lenses, therefore, for the successful use of contact lenses you need a healthy conjunctiva. With eye irritation, the conjunctival blood vessels dilate and the eye turns red. The conjunctival membrane contains glands that contribute to the formation of a tear film to moisturize the surface of the eye.
A layer of water and nutrients on the cornea is called a tear film. This film is constantly produced and removed from the surface of the eye. The lacrimal glands of the eyelids participate in its formation, the outflow occurs through the lacrimal puncta into the nasolacrimal duct and further into the nose. The tear film provides nutrition to the cornea in the form of oxygen, glucose, salts and minerals. It moisturizes the cornea and eyelids, preventing them from drying out, and also acts as a lubricant, facilitating the sliding of the eyelids along the cornea. Foreign particles dissolve in the tear film and are removed from the surface of the eye. Dangerous microorganisms are exposed to tear antibacterial enzymes (lysozymes).
The tear film consists of three layers:
– external lipid (fat)
– central water
– and the inner mucin layer, which promotes the connection of the film with the cornea
The lipid layer is a fatty film that prevents the tear film from drying out. The water layer is a slightly alkaline solution (pH 7.35), consisting of water (98%), minerals and nutrients, enzymes, (ions), dissolved salts and proteins. This layer, which accounts for up to 90% of the thickness of the tear film, provides the cornea with oxygen and nutrients. The mucin (mucous) layer binds the lacrimal film and the corneal epithelium.
An important factor in adapting to contact lenses is the chemical structure of the tear film, since the lenses lie on it. Typically, the film thickness is 7 μm. The average volume of tear fluid in the eye is 6 μl. 10–20 seconds would be enough for the entire tear film to evaporate, but we involuntarily blink every 5–10 seconds and restore the film.
Typically, patients, who have problems with any layer of the tear film are advised against wearing contact lenses. The condition of the tear film can be assessed in two ways: in the Schirmer test, strips of paper are used to indirectly determine the amount of the water layer of the film, the test for tear breakup time (TBUT) shows the time in which the tear film evaporates from the surface of the cornea (normally – 10-15 seconds).
The cornea is a transparent, avascular, highly sensitive, multilayered, domed membrane that borders the anterior chamber of the eye. Convex in the center, the cornea is flattened at the transition to the sclera, which is called the limbus. The changing curvature of the cornea makes it difficult to measure and adjust contact lenses. Therefore, contact lenses usually have different curvatures: central and peripheral.
