Topography

Corneal Topography

Corneal topography is the process by which a detailed contour map of the corneal curvature is mapped using a projected image.

The cornea is the clear window at the front of the eye and the structure upon which a contact lens rests. understanding this structure and it's shape in detail enables us to more accurately fit a contact lens to it in much the same way that a good tailor will accurately take your garment measurements to create a well fitted suit.

The maps can even highlight problems with the corneal structure such as the corneal ectasia's of which keratoconnus is the most common condition. Keratoconnus is a weakness of the corneal causing it to bulge into a cone-like shape. Corneal topography is essential not only to help fit a contact lens (which is vital for the best sight in these cases) but also to monitor the progression of the disease so that intervention can be made if necessary. You can read more about keratoconnus and other corneal conditions on our sister website by clicking here.

The maps produced by the topography machine are in a lot of ways similar to a ordinance survey topographic map of a landscape. If you imagine the cornea as a hill the differing colours on the adjacent map represent the change in height (curvature) of the hill. Red colours are steep curves and the colder colours are shallower curves.

Normal curvature plot. Note the contours of the highlighting the different curvature heights of the cornea.

Astigmatism produces this characteristic bow-tie pattern where one meridian, in this case, 12/6 o'clock is very much steeper than it's counterpart, in the 3/9 o'clock meridian.

Keratoconnus and other corneal abnormalities arising from trauma or post-operativley can distort the natural curvature of the eye into very irregular shapes making it very difficult to focus the light coming through the eye resulting in very poor vision.

3D representation. This is a spherical eye, meanig the curvature is normal but still could be long or short sighted.

3D representation of astigmatism, where the curvature takes on a "rugby ball" like shape. The curvature is very much steeper in the 12/6 o'clock meridian than the 3/9 o'clock.

In the case of Keratoconus the cornea steepens, usually centrally, to form a nipple shaped cone. This is very diificult to coreect with glasses meaning that the only options available in these cases are contact lenses or surgery.