Hyaluronic acid (hyaluron, sodium hyaluronate, hyaluronan) is widely spread in the human body. Hyaluronic acid has an important function in connective tissue, skin, intervertebral discs, joints and nerve tissue and is also contained in the tear fluid. Hyaluronic acid is able to bind very large amounts of water and, when dissolved in water, acts as a lubricant.
Hyaluronic acid is built up and constantly catabolized by the body. In the outer layer of the cornea of the eye (epithelium) about half of the hyaluronic acid present there is broken down daily and replaced by newly formed hyaluronic acid. The cornea is not supplied by blood vessels. The hyaluronic acid in the spaces between the cells therefore plays a decisive role in supplying the cornea with nutrients and disposing of metabolic products, as well as in cell differentiation, cell renewal and wound healing.
From a chemical point of view, hyaluronic acid is a chain-like arrangement of sugar molecules. A hyaluronic acid chain can consist of 500 to 100,000 chain links.
The length of the hyaluronic acid chains determines their physical properties and their biological function.
The healthy tear film combines a high viscosity (viscous) when the eye is open with a low viscosity (thin) when the eye is blinking. Thus, on the one hand it remains on the eye surface for a long time, on the other hand it ensures a smooth gliding of the eyelid over the eye surface and prevents blurred vision (streaky vision) after the blink.
Only eye drops containing hyaluronic acid with an extremely long chain length (Hylan A) give the drops flow properties similar to those of the natural tear film, reduce the friction between the eyelid and the eye surface during the blink of the eye the most, remain on the eye surface the longest after the drops have been applied and bind the most water. In addition, very long hyaluronic acid chains prevent inflammation of the eye and the eyelid margins and contribute to the healing of existing inflammation.
Eye drops with medium chain length hyaluronic acid behave neutrally with regard to inflammation, while short hyaluronic acid chains can contribute to an intensification of the inflammation.
Of all hyaluronic acid eye drops, Comfort Shield® eye drops contain the hyaluronic acid with the longest chain length (Hylan A, approx. 3 MDa), while Comfort Tears® eye drops contain medium chain hyaluronic acid (approx. 2 MDa).
In eye drops hyaluronic acid increases the viscosity of the solution). In eye drops with hyaluronic acid of a very long chain length (Hylan A), the molecular chains can become entangled with each other when the eye is open. These eye drops therefore remain on the surface of the eye for up to more than an hour.
When the eyelid is blinking, the long hyaluronic acid molecules arrange themselves in parallel, making these eye drops just as thin as eye drops with hyaluronic acid of shorter chain length.
Only the hyaluronic acid (Hylan A) used in Comfort Shield® offers optimum effectiveness and tolerability.
Further information on the flow behaviour of commercially available hyaluronic acid eye drops and the role of hyaluronic acid in maintaining the physiological balance of the eye is provided in the enclosed technical articles.
Hyaluronic Acid Eye Drops – What you should know about their rheological properties
Dr. Wolfgang Müller-Lierheim:
Tear substitutes – News about hyaluronic acid
Dr. Wolfgang Müller-Lierheim:
