Lens stabilisation

Lens stabilisation ensures that a contact lens does not rotate freely on your eye, but remains in a fixed position. This is particularly important for toric lenses used to correct astigmatism and for multifocal lenses, as the correction must be positioned exactly in the right place to ensure clear vision.

With a standard spherical lens, the direction of rotation doesn’t matter, as the power is the same throughout. However, a toric lens has different powers in different directions, and a multifocal lens has zones for distance and near vision. If such a lens rotates, the correction shifts and your vision becomes blurred, double or distorted.

How does lens stabilisation work?

Manufacturers use various designs to stabilise a lens:

• Prism ballast: the bottom of the lens is slightly thicker and heavier, so that gravity pulls the lens downwards and keeps it in position.
• Double thin zones: thin areas above and below the lens, where the eyelids hold the lens in place and centre it when you blink.
• Truncated lower edge: the lens has a straight, truncated edge at the bottom which rests against the lower eyelid.
• Dynamic stabilisation: a combination of thickened and thinned areas across the lens, which work in conjunction with eyelid movement to ensure the lens quickly returns to the correct position.

Each brand and type of lens may use its own stabilisation method. Most modern toric lenses are designed to return to the correct position within a few seconds of being inserted or when you blink.

Why is the correct position important?

With astigmatism, the curvature of your cornea runs in a specific direction, known as the axis. A toric lens compensates for this by having a different power along precisely that axis. If the lens rotates by a few degrees, the correction no longer aligns with the axis of your astigmatism, and your vision becomes less sharp. The same applies to multifocal lenses: the reading zone must be in front of your pupil, otherwise you will be looking through the wrong part of the lens.

How can you tell if the stabilisation is working properly?

If your lens is properly stabilised, your vision will be clear immediately after insertion, or within a few blinks. If you notice that your vision becomes blurred or improves every time you blink, or if letters sometimes appear double, the lens may be rotating. This could be because the stabilisation does not fit your eye shape properly, or because the lens does not fit you optimally.

The influence of the tear film and eyelid tension

The stabilisation works in conjunction with your tear film and the tension of your eyelids. If you have dry eyes or, conversely, excessive tearing, the lens may move differently. The shape and tension of your eyelids also play a role: tight eyelids hold the lens in place more effectively, whilst loose eyelids provide less grip.

Fitting and check-ups

When fitting toric or multifocal lenses, your optician or optometrist will always check that the lens is stable. Sometimes, small markings on the lens are used to see by how many degrees the lens is rotating. If the stabilisation isn’t working properly, a different type or brand of lens with a different stabilisation method may be a better fit.

Lens stabilisation and activity

During sport or sudden head movements, a lens may shift slightly for a short time, but a well-stabilised lens will quickly return to its correct position. If you notice that your lens often shifts whilst you’re moving, discuss this with your optician. Sometimes a slightly tighter fit or a different stabilisation design can help.