Sagittal height (SAG)

Sagittal height, often abbreviated to SAG, describes the shape and profile of a contact lens. It is the distance between the centre of the inner surface of the lens and an imaginary straight line drawn between the two opposite edges of the lens. You can think of it as the ‘depth’ or ‘curvature’ of the lens.

This measurement is particularly important for rigid (hard) lenses and for specialised soft lenses such as scleral lenses. The sagittal height determines how the lens rests on your eye: a higher SAG value means a steeper, deeper lens that sits further away from the cornea, whilst a lower SAG value results in a flatter lens that sits closer to the eye.

The SAG is expressed in millimetres and is related to the base curve, the diameter of the lens and the shape of your eye. Two lenses with the same base curve may have different sagittal heights if their diameters differ. That is why opticians and contact lens specialists use the SAG value to precisely match a lens to the shape of your eye.

With scleral lenses, which are larger and rest on the white of the eye (sclera) rather than on the cornea, the sagittal height is crucial. The lens must provide sufficient space between the inside of the lens and the cornea, so that a layer of fluid can remain in place to keep the eye comfortable and ensure the lens stays stable in position.

Why sagittal height is important

The sagittal height determines whether a contact lens fits your eye properly. A lens with too high a SAG may sit too steeply and may not move enough, thereby restricting tear exchange beneath the lens. A lens with too low a SAG may sit too flat, causing the lens to move too much or even press against the cornea. Both situations can lead to discomfort, blurred vision or, in the long term, irritation.

Your optician or contact lens specialist will measure the shape of your eye and calculate or select the correct sagittal height based on those measurements. This is often done using a topographer or other measuring equipment that maps the curvature and profile of your cornea and eyeball.

Sagittal height in scleral lenses

With scleral lenses, sagittal height plays an even greater role than with smaller lenses. Scleral lenses are large enough to extend beyond the cornea and rest on the white of the eye. The space between the lens and the cornea is filled with a fluid, usually a saline solution, which moisturises and protects the eye.

The correct SAG ensures that the lens does not press against the cornea and that the fluid layer remains thick enough throughout the wearing period. Too little space can lead to contact with the cornea and discomfort; too much space can cause the lens to sit too loosely or allow air bubbles to become trapped beneath the lens.

Fitting and check-ups

The sagittal height is determined during your lens fitting. Your optician will have you try on trial lenses and assess how the lens moves, how tear exchange is managed and whether there are any pressure points. Sometimes, several trial lenses are needed to find the ideal SAG.

Even after the initial fitting, sagittal height may remain important. The shape of your eye can change over time, for example due to ageing, eye conditions or following eye surgery. Regular check-ups with your optician help to ensure that the SAG and the fit of your lenses are still correct.