If you wear contact lenses, you probably don’t think about the physical structure or mechanical properties of those little plastic devices you put in your eyes. As long as your contact lenses are comfortable to wear and they correct your vision, you’re good to go.
However, it pays to know about these mechanical properties because they can affect your eyes. Here at Lens, we want to help our customers better understand what they’re putting in their eyes, which is why we want to talk about a little-known but critical mechanical property of contact lenses. In this post, we’ll discuss one specific property of silicone hydrogel contact lenses: the modulus.
What is contact lens modulus?
The modulus of a silicone hydrogel contact lens is the measure of the material’s resistance to deformation under tension or strain. The modulus describes the material’s degree of flexibility or elasticity. In simpler terms, it is the stiffness of a silicone hydrogel contact lens.
The higher the modulus, the stiffer the contact lens material.
Why is the modulus of a contact lens important?
It’s crucial to keep in mind that the modulus is a property of the silicone hydrogel material and not the contact lens itself.
Stiffer or higher modulus silicone hydrogel contact lenses are easier for wearers to handle and insert into their eyes. These silicone hydrogel contact lenses are also more durable than their lower modulus counterparts. High modulus silicone hydrogel contact lenses are more resistant to tearing or ripping. It is also easier to determine if a contact lens is inside out if it’s a high modulus silicone hydrogel material.
How does modulus affect contact lens fit?
The easier handling and durability of high modulus silicone hydrogel materials do come with a few trade-offs. A high modulus silicone hydrogel contact lens is stiff, so it tends to not fit well and not completely conform to the shape of the wearer’s cornea. An improper contact lens fit results in edge fluting, which means the edges of the contact lens are somewhat lifted off the surface of the cornea.
Contact lens fluting often leads to several problems for the wearer. For one, fluting tends to cause supreme eye discomfort and even pain. Apart from discomfort, fluting causes lens awareness or the sensation that there is a foreign body in the eye. Contact lens fluting can also result in the edge of the lens sitting on the lower eyelid, which causes additional discomfort.
Contact lens fluting also results in the appearance of air bubbles between the cornea and the lens material. Improper contact lens fit and fluting often lead to ocular complications. The wearing of high modulus contact lenses is associated with eye conditions such as contact lens-associated papillary conjunctivitis (CLAPC) and giant papillary conjunctivitis (GPC).
The problem with an improper contact lens fit and fluting from high modulus contact lenses is that the fit is unlikely to improve with continuous wear. Wearers can’t easily adjust to ill-fitting high modulus silicone hydrogel contact lenses. Their eye care professional (ECP) will need to prescribe them contact lenses made of a material with a lower modulus.
Meanwhile, low-modulus silicone hydrogel contact lens materials are softer and more flexible than their high-modulus counterparts. The flexibility of a low-modulus silicone hydrogel contact lens means it conforms to the shape of the cornea more easily, making it more comfortable to wear than a high-modulus contact lens. Compared to a high-modulus silicone hydrogel contact lens, a low-modulus lens requires little adjustment. A low-modulus silicone hydrogel contact lens also doesn’t interfere with the eyelids during blinking.
However, low-modulus silicone hydrogel contact lenses are not as easy to handle and insert as high-modulus contact lenses. The softness of low-modulus contact lenses also means that they are less durable than high-modulus contact lenses.
If you are wearing silicone hydrogel contact lenses and find them too uncomfortable despite continuous wear, you may be wearing contact lenses with a high modulus. In this case, you need to consult your ECP about switching you over to a low-modulus contact lens for better comfort.