by Dr. Melody Huang
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Contact lens designs refer to more than just cosmetic contact lenses. They include optical designs that correct for a wide range of vision, including myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia (age-related near vision changes).
These are the most frequently prescribed contact lens designs, along with the percentage of contact lens patients in the United States who wear that particular lens design [1]:
Other contact lens designs include:
These contacts suit myopic or hyperopic patients with little to no astigmatism power (also called cylinder or “cyl”). Typically, these include patients with less than -0.75 D of astigmatism.
Spherical lenses are available in daily, 2-week, and monthly replacement schedules.
Note that spherical powers are available in 0.25 D steps. In higher powers, the increments become 0.50 D steps. Typically, this includes any powers above +/-6.00 D, although this number varies slightly in some brands. For example, spherical powers are available in -6.00 D and -6.50 D, but generally not -6.25 D.
If you’re looking at a prescription, you may have noticed that the eyeglass and contact lens prescriptions are different. There are a few reasons for this difference. When determining the power of a spherical contact lens, the eye care practitioner must consider the spherical equivalent and vertex conversion.
The spherical equivalent gives the closest estimate of a prescription without including any astigmatism correction. This number is calculated by taking the cylinder power of the eyeglass prescription and dividing it in half. Then, add the resulting number to the spherical power. You don’t need the cylinder axis for any calculations.
For example, let’s say the eyeglass prescription reads +2.00 -0.50 x 180. First, divide -0.50 in half, which equals -0.25. Then, add -0.25 and +2.00. The resulting spherical equivalent is +1.75 D. Be careful when adding minus and plus signs!
Because spherical power only comes in multiples of 0.25, the eye care practitioner has to use their professional judgment if the cylinder is an odd multiple of -0.25, such as -0.75 or -1.25 (since dividing these numbers in half doesn’t yield a multiple of 0.25).
Vertex conversion is a calculation that compensates for the change in prescription when you go from eyeglasses to contact lenses. Depending on how far a lens sits from your eye, your brain perceives it as stronger or weaker, even though the power of the lens remains the same. This is why the vertex distance (distance from the back of the lens to the front surface of the cornea) is important!
Note that you only have to worry about calculating the vertex conversion if you have a power of +/-4.00 D and up.
For eyeglass lenses, most eye care practitioners use a standard vertex distance of 12 millimeters (mm) from the cornea. Since a contact lens sits directly on the eye, we consider the vertex distance to be 0. Fortunately, you don’t need a fancy math equation to calculate the difference in prescription between a 0 and 12 mm vertex distance!
All you need is a vertex conversion chart, which looks like this:
Here’s how to use the chart:
Let’s say the spherical power of the eyeglass prescription is -8.00 D. If you look up 8.00 on a standard vertex chart, you’ll see that the corresponding minus power is -7.25D. This patient needs a contact lens power of -7.25D. Since most contacts aren’t available in 0.25D steps as we discussed in Part 1 of this series, the eye care practitioner will most often choose -7.00 D. The charts available from CooperVision, such as the one above, already account for power availability and recommend the correct power to order (in our example and the chart above, -8.00D is already converted to the available power of -7.00D).
Toric contact lenses correct astigmatism, along with myopia or hyperopia. Many of the spherical contact lens brands listed above also come in toric designs.
Most toric lenses are available in cyl power increments of 0.50 D starting from -0.75 D. For example, -0.75 D, -1.25 D, -1.75 D, and -2.25 D are common cyl powers. Biofinity® Toric XR and Proclear® Toric XR are available in cyl powers above -2.25 D.
With spherical contacts, you can rotate the lens in any direction, and the power remains the same. With toric lenses, the contact must sit in a specific orientation to provide the proper astigmatism correction.
Contact lens manufacturers use a variety of methods to stabilize their toric lenses. Some of the most common toric designs include [2]:
Toric contacts have markings to help the practitioner see if the lens is sitting in the correct orientation after insertion. If the lens rotates clockwise or counterclockwise, the patient may experience blurry vision.
As with spherical lenses, the practitioner must calculate the toric lens power based on the vertex conversion. Fortunately, CooperVision has a handy OptiExpert® app that does all the hard work. Just plug the eyeglass prescription in!
Multifocal contacts are suitable for presbyopic patients or anyone needing distance and near vision correction. Many of the spherical contact lenses listed above also come in multifocal designs. Toric multifocals, which combine toric and multifocal lens designs into one lens, are available for astigmatism patients.
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Multifocal lenses use what is called “simultaneous images,” which means the patient views multiple prescriptions at the same time. There are two types of simultaneous lens designs [3]:
Monovision is not a lens design, but rather a way of fitting contacts that incorporates multiple lens designs. Instead of multifocal contacts where each contact lens corrects for distance and near, monovision involves one lens to correct the distance vision and the other lens to correct near vision. As you can imagine, monovision takes some getting used to as the brain adapts to this type of vision correction.
Monovision is highly customizable, depending on the patient’s needs. Some practitioners use “modified monovision,” which means fitting one eye with a single vision lens (spherical or toric) and the other eye with a multifocal lens.
Now that you’re familiar with different types of contact lens designs, you can help patients understand why some contact lenses work better for them than others. Lens selection makes a big difference!
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The world of ocular health can be complex and intimidating, especially when you're tasked with choosing the right type of corrective lenses. Among the array of options, scleral contact lenses stand out for their unique design and benefits.
Scleral contact lenses are a type of gas permeable contact lens that vaults over the entire corneal surface and rests on the "white" of your eye (the sclera). Due to their size and unique design, these lenses are often more comfortable than traditional contact lenses, and can provide exceptional vision clarity for individuals with certain eye conditions.
The first reason is related to comfort. Due to their larger size and design, scleral lenses spread the weight across a larger area of the eye. This reduces the risk of discomfort and irritation that can be caused by smaller lenses.
Another reason to consider scleral lenses is their ability to provide superior vision correction for certain eye conditions. If you have keratoconus, for example, a condition that causes your cornea to become thin and cone-shaped, regular contact lenses may not fit properly, leading to discomfort and poor vision. Scleral lenses, however, are designed to vault over the irregular cornea, providing a smooth optical surface that can greatly improve visual acuity.
In addition to comfort and vision correction, scleral lenses can also provide a more stable fit. This is particularly beneficial for individuals with active lifestyles or those who participate in sports.
There are different types of scleral contact lenses, each designed to address specific vision needs and eye conditions. The three main types are full scleral lenses, semi-scleral lenses, and mini-scleral lenses.
Full scleral lenses are the largest type, with a diameter of 18-24mm. They offer the most coverage and are typically used in severe cases of dry eyes or irregular corneas.
Semi-scleral lenses, on the other hand, are slightly smaller, with a diameter of 15-18mm. They are also designed to vault over the cornea but rest on the junction between the cornea and the sclera.
Mini-scleral lenses are the smallest type, with a diameter of 13-15mm. They provide a secure fit and are often used for regular to moderate dry eyes or irregular corneas.
Choosing the right scleral contact lenses is not a one-size-fits-all process. Several factors to consider include your specific eye condition, lifestyle, and personal comfort.
Your eye condition plays a crucial role in determining the type of scleral lens that will provide the best vision correction. If you have severe dry eyes or a highly irregular cornea, for example, you may benefit more from full scleral lenses.
Your lifestyle is another important factor to consider. If you lead an active lifestyle or participate in sports, you may need a lens that provides a more secure fit, such as a mini-scleral lens.
Comfort is also a major consideration. The lens should feel comfortable in your eye, without causing any irritation or dryness. A lens fitting session with an optometrist can help ensure the lens fits properly and comfortably.
An eye exam and consultation with an optometrist is integral to the process of choosing scleral contact lenses. During the exam, your optometrist will assess the health of your eyes, measure the size and shape of your cornea, and discuss your vision correction needs and lifestyle.
This comprehensive evaluation enables your optometrist to recommend the type of scleral lens that best fits your individual needs. Additionally, an optometrist can provide you with instructions on how to properly care for and maintain your scleral lenses, to ensure they last as long as possible and continue providing optimal vision correction.
Choosing the right scleral contact lenses can have a profound impact on your quality of life. By understanding the different types of lenses, considering your specific needs, and consulting with an optometrist, you can find the lenses that will provide you with the best possible vision correction.
For more information on scleral contact lenses or to determine which type is best for your visual needs, visit 20/20 EyeVenue at our office in Westminster or Strasburg, Colorado. Call (720) 740- to schedule an appointment today.
Contact us to discuss your requirements of Double Concave Lenses. Our experienced sales team can help you identify the options that best suit your needs.