Why a Lens for Astigmatism Must Never Spin on Your Eye

An ordinary lens for short-sightedness can turn on the eye without any harm.

A round lens can spin a full circle and its power stays exactly the same.

A lens for astigmatism is different. It has to be right not only in power but in direction. So it has to settle into place on the eye and stay there.

Astigmatism is an eye that needs power in one direction only

The American Academy of Ophthalmology describes astigmatism as a cornea or lens whose normal curvature is not perfect. A normal eye is evenly rounded like a basketball. An astigmatic eye is curved more in one direction, like an American football.

Light then fails to gather at a single point, and the focus differs by direction. Which direction is more out of focus decides which direction the correction has to go.

That is why a prescription carries three numbers.

• SPH: the power for short- or long-sightedness
• CYL: the size of the power that corrects astigmatism
• AXIS: the direction in which that correction must sit, expressed between 0 and 180 degrees

Glasses are fixed in a frame, so the AXIS stays where it belongs. A contact lens floats on the eye and moves every time you blink. A correction that needs a direction has to be carried on something that moves.

How to read each number on a prescription is covered in Six Codes Hidden on One Sheet of Paper.

A small turn takes away part of the correction

What happens when the lens rotates?

A clinical article in Contact Lens Spectrum explains it this way. For every degree a toric lens is off its axis, roughly 3 to 3.5 percent of the cylinder correction is left uncorrected. A 10-degree rotation therefore removes about 30 to 35 percent of the astigmatic correction.

The same article adds that with a low cylinder power a misalignment of around 10 degrees can sometimes be tolerated, while the higher the power, the more even a small deviation is felt.

So a toric lens is not something you can judge by power alone. Staying in place on the eye is a condition of the correction.

Three designs that keep a lens from turning

Manufacturers shape the lens so it engages with the eyelids and settles into position. Three approaches appear most often in clinical literature.

1. Prism ballast

The lower part of the lens is made thicker. When you blink, the upper eyelid slides over the thin upper part and pushes the thicker lower part down. Clinical writing compares this to the "watermelon seed principle": press a watermelon seed with your finger and it slips away, and in the same way the pressure of the eyelid pushes the lens into the same place.

2. Thin zone (double slab-off)

Material is removed from the top and bottom of the lens to make it thin, and the thicker middle part, which sits in the space between the eyelids, does the work.

3. Eyelid-stabilized

The lens is symmetrical left to right, with four stabilization zones whose thickness changes gradually so that they work together with both upper and lower eyelids. There is no prism in the optical part of the lens. It is described as a design intended to find its position quickly after it is placed on the eye.

In all three, the key is to match the distribution of thickness to the movement of the eyelids. There is no conclusion that one approach is better for everyone. A study comparing prism ballast with the eyelid-stabilized design also reports that the two were similar in rotation and in symptoms.

Even so, a lens can still turn

Better design does not remove rotation entirely. The clinical literature points to the interaction between lens and eyelid as the main cause of toric rotation. The shape of the eyelid opening, the direction the lids move when you blink, head tilt and the direction of the astigmatism all play a part.

That is why a practitioner places the lens on the eye, lets it settle, and looks at which way and by how many degrees it has turned. If the lens turns left, that amount is added to the prescribed axis; if it turns right, it is subtracted. In clinical practice this is called "LARS" (Left Add, Right Subtract).

It is also why the AXIS printed on the box can differ from the AXIS on your glasses prescription without being an error. It may be a number that already allows for the angle at which the lens settles on the eye. Why this difference arises is also covered in Why Are Your Glasses and Contact-Lens Prescriptions Different If Your Eyes Are the Same?.

Things worth knowing

• The same power can behave differently when you change brand. Stabilization design differs from product to product, so when you change a toric lens it is safer to consult a professional. Matching numbers do not guarantee the lens will sit in the same place. This connects with Why Do Contacts With the Same Base Curve and Diameter Fit Differently?.
• Lens position can change with posture. If your vision blurs when you tilt your head or change position, remember the situation and mention it at your check-up.
• Do not rub your eye to turn the lens into place. Trying to rotate the lens with a finger can injure the eye.

A lens for astigmatism is chosen for power and direction together

The two main kinds of contact lens for astigmatism are toric soft lenses and RGP (rigid gas permeable) lenses.

But the performance of such a lens is not set by a single power number. How the lens engages with the eyelids and how stable it is on the eye work together. That is why anyone with astigmatism should always check CYL and AXIS when they receive a prescription.

Lenses for astigmatism can be seen in the Toric Lenses collection. Choose according to your prescribed power, and ask a professional if you have questions.

References

• American Academy of Ophthalmology, What Is Astigmatism?
• Contact Lens Spectrum, Don't Forget to LARS
• Contact Lens Spectrum, Problem Solving Soft Toric Contact Lenses
• Contact Lens Spectrum, A Difference in Designs

This article is general information. The type of lens for astigmatism and how to wear it depend on the individual eye, so follow the examination and prescription of a professional.