Limbus

The limbus is the circular transition zone between the cornea and the white of the eye(sclera). This zone is approximately 1 to 2 millimetres wide and runs like a ring around the cornea. On the outside of your eye, you can see the limbus as the edge where the transparent part of your eye meets the white part.

The limbus is not a sharp boundary, but a gradual transition where the tissue of the cornea slowly changes into the firmer tissue of the sclera. This area contains important structures, including blood vessels that supply the cornea with oxygen, and stem cells that can repair damaged corneal tissue.

For contact lens wearers, the limbus is relevant because the edge of the lens often rests on or near this area. Soft contact lenses usually cover the limbus partially or completely, whilst rigid lenses often remain just within the limbus edge. The oxygen supply and comfort of the lens depend in part on how the lens relates to this area.

Function of the limbus

The limbus plays several key roles in corneal health. The limbus contains blood vessels that supply oxygen and nutrients to the edge of the cornea. The cornea itself has no blood vessels, so this supply from the limbus is vital for its outer layer.

In addition, the limbus contains limbal stem cells. These cells can produce new corneal cells and play a role in repairing minor damage to the corneal surface. In a healthy limbus, the cornea remains clear and is able to recover from minor irritation.

The limbus and contact lenses

The position and design of a contact lens determine how the lens relates to the limbus. Soft lenses are usually larger than the cornea and cover part of the limbus. This can affect the oxygen supply to the limbal zone, particularly with lenses that have lower oxygen permeability.

Rigid lenses are smaller and often sit just inside the limbus, on the cornea itself. With a good fit, the limbus remains unobstructed and the supply of oxygen from the limbal zone can continue unhindered. A lens that is too tight or does not move enough can exert pressure on the limbal zone and cause irritation.

Limbal redness

Sometimes the limbus area can become red or irritated. This can be caused by prolonged lens wear, a poorly fitting lens, or an insufficient supply of oxygen. Allergic reactions or irritation caused by lens solution can also lead to limbal redness.

If you have red or irritated eyes around the limbus area, remove your lenses and have them assessed by your optician or optometrist. Persistent redness may indicate a problem with the fit, the material or the care of your lenses.