Have you ever wondered about the technology behind your contacts? What are they made of? Why do some brands become dry very quickly? Why are some to be worn for a month and others only for a day?
The technology in your contact lenses has rapidly advanced in the last 5-10 years, much like your phone’s technology. The contact lens market grows each year with the introduction of 2-3 new types of lenses of varying materials and designs.
The contact lens brands of 2022 are more breathable, more comfortable, and healthier for your eyes. More options exist for patients with astigmatism, computer users, and those over 40 wearing reading glasses over their contacts. A contact lens exists for nearly every eyeglass prescription and can be customized for your specific vision needs.
Contact lens materials
Soft contact lenses can be categorized by lens material: HEMA, hydrogel, or silicone hydrogel.
HEMA
The first soft contact lenses were made from HEMA polymers with varying amounts of water. The amount of water in the polymer regulated the amount of oxygen that passed through the lens. HEMA contacts allow less oxygen transmission when compared to today’s lenses and often resulted in eye redness, swelling, and unhealthy changes in the health of the eye. If you had a higher prescription or astigmatism, you were more likely to have problems with thicker HEMA lenses.
Hydrogels
Hydrogel contact lenses followed HEMA lenses and were introduced in the 80s. Hydrogels are gel-like, water-containing polymers (plastics). Hydrogel contacts have higher water content and allow more oxygen to the eye. Their use resolved some of the eye health issues caused by HEMA lenses.
Hydrogels still tend to dry out easily, and as that occurs, less oxygen reaches the eye. Hydrogels may be suitable for daily use by some contact lens wearers, but most of us need lenses that hold moisture after hours of wear time and computer work.
Examples of hydrogel contact lens brands include:
Silicone hydrogels
Silicone hydrogel lens materials were first introduced to the market in the late 90s. 64% percent of the contact lens market is now silicone hydrogel lenses, yet there are still 12 million people wearing older technology lenses that should be upgraded. Patients report better end-of-day comfort and less lens dryness with silicone hydrogel lenses. By adding silicone to the lens, it becomes more porous and allows more oxygen to the eye. They tend to feel a little thicker and more slippery to the touch.
Silicone lenses do tend to collect more debris and protein from the tears. Oil-based makeup tend to stick to them more easily. If you experience these problems, please consult with your doctor for solutions.
Examples of silicone hydrogels contact lens brands include:
Contact lens technology
There are 3 important factors to know about the technology of your contacts:
- Oxygen permeability
- Water content
- Optical design
Oxygen permeability
Oxygen permeability is an important feature of a contact lens. It is important that any material on your eye allows oxygen to the eye, or you may have new, unhealthy blood vessel growth on the cornea, a condition called neovascularization.
Neovascularization can be caused by sleeping in contacts, wearing contacts for too many hours, or wearing contacts that allow less oxygen to the eye. Neovascularization can cause long-term complications and can even limit your ability to have Lasik surgery.
The permeability of a contact lens is referred to as Dk/t where Dk is the measure of a material’s permeability and its thickness at its center. In order to avoid swelling of the cornea, research states the Dk/t needs to be 20 at the center of the cornea and 33 at the periphery for daily wear.
If a contact lens is approved for you to sleep in, the Dk/t must be much higher. For example, AIR OPTIX® NIGHT & DAY® AQUA for wearing overnight has a Dk/t of 140; whereas, the ACUVUE 2® brand has only a Dk/t of 33.
Contact lenses made of the older material, HEMA, had a Dk/t of 40 or less. The Dk/t of hydrogel contact lenses can range from the teens to twenties. Silicone hydrogels have the highest Dk/t with a range from 69 to 140.
Contact lens manufacturers usually add this information for their lenses to their company’s website. Silicone hydrogels have virtually eliminated eye complications like cornea swelling and neovascularization, if worn correctly. The optimal Dk/t for best comfort and eye health is around or greater than 100.
Water content
The water content of a lens has historically been important, particularly in older materials like HEMA and hydrogel lenses. A hydrogel contact lens contains 24% to 78% water volume and a HEMA contact 38%. The relationship has not been as firmly established with silicone hydrogels, but in the past 10+ year history of silicone hydrogels, the amount of silicone has decreased over time, and therefore water content increased in an effort to improve lens comfort.
DAILIES TOTAL1® are considered the premium contact lens. Typically, if you leave your contacts exposed to the air, they will dry up like a raisin. DAILIES TOTAL1® is the first water gradient contact lens that can maintain its hydrated structure all day. The lens features a water content increase from 33% to 80% from the core of the lens to the surface.
Optical design
Your vision in your contact lenses can vary by the material or the optical design of the lens. Each contact lens manufacturer has its own design for astigmatism and multifocal contact lenses. If one brand doesn’t work out for you and you are unable to achieve vision clarity, another brand may be an option.
Differences in the flat/steep shape of your cornea, your pupil size, and the design of the lens may be reasons your vision fluctuates throughout the day or may not be good at a particular viewing distance, far vs. near.
Toric contact lenses for astigmatism come in several different designs including prism ballast spherical base, prism ballast toric base, thin zone spherical base and thin zone toric base.
All toric lens designs have a thicker part of the lens in order to keep the lens from rotating on your eye and causing your vision to go from blurry to clear upon blinking. Contact lenses are different from spectacles in that the lens will always move versus the lenses of your spectacles being locked within the frame. Your doctor will select the best design for you based on the shape, measurements of your eyes, and the behavior of the lens on the eye.
There are two types of multifocal contact lens designs. The most common is comprised of concentric circles of lens powers, like a bulls-eye pattern, prescribed for various viewing distances. There are also blended designs, which keep both the near and distance prescriptions close to the pupil. The doctor will select a lens based on the size of your pupil in various lighting conditions, your occupation, and your specific vision needs.
In conclusion, contact lens material, oxygen permeability, water content, and lens optical design can directl impact your vision, long-term comfort, and eye health. The technology of contact lenses changes with quickly with new industry innovations. Ask your eye doctor about the newest contact lens technology.
Your eyes deserve the best.
