Astigmatism is a common vision condition caused by an irregularly shaped cornea or lens, making the eye shaped more like a football than a perfect sphere. To correct this, a specific lens called a toric lens is required, which has different refractive powers across its surface. Colored contact lenses are purely cosmetic devices designed to alter or enhance the eye’s natural shade. Combining these functions into a colored toric lens is difficult due to a complex intersection of physics, manufacturing precision, and market forces, leading major manufacturers to largely discontinue the product.
The Technical Challenge of Toric Lenses
Toric lenses require their corrective power to remain in a precise rotational position on the eye. A toric lens must align its specific axes of correction with the corresponding axes of the eye’s astigmatism, as even a slight rotation can result in a significant loss of visual clarity. To ensure the lens stays oriented, manufacturers engineer stabilization mechanisms directly into the lens structure.
A common method is prism ballast, which involves adding a zone of increased thickness, typically at the bottom of the lens. This thicker area interacts with the lower eyelid during a blink and uses gravity to pull the lens back into its correct vertical alignment.
Another design, known as dynamic stabilization, creates thin zones at the top and bottom edges of the lens, allowing the eyelids to engage and center the lens during the blinking process. The success of a toric lens relies entirely on this delicate balance of thickness profiles and material properties to maintain consistent, stable vision.
Manufacturing Hurdles Combining Color and Correction
The process of adding cosmetic color directly conflicts with the delicate engineering required for toric lens stability. Color is applied by embedding pigments within the lens material, often through a sandwiching technique to prevent the pigment from touching the eye. This pigment application introduces a foreign layer and alters the lens’s physical structure.
Pigment layers can disrupt the exact thickness profile required for stabilization zones, potentially interfering with the lens-to-eyelid interaction. This disruption causes the lens to rotate or oscillate, leading to fluctuating vision.
Additionally, the thicker zones created for stabilization already reduce oxygen transmissibility to the cornea. Adding a layer of opaque pigment further impedes the flow of oxygen, compromising eye health and comfort. Combining these complex features often results in a product that fails to meet stringent safety standards for both rotational stability and oxygen permeability.
Market Economics and Profitability
The decision to discontinue colored toric lenses is largely a financial one driven by a highly specialized and low-volume market. Correcting astigmatism requires three distinct parameters: sphere power, cylinder power, and axis. This results in an exponentially greater number of unique prescriptions compared to standard spherical lenses, which only require sphere power.
Each unique combination of these three parameters represents a Stock Keeping Unit (SKU) that a manufacturer must produce and warehouse. For a niche product like a colored toric lens, only a small fraction of customers will require any specific SKU, leading to extremely high inventory costs and a low return on investment. Producing a complex toric design in multiple colors for low-volume sales makes the product commercially unsustainable for major companies.
Current Alternatives for Astigmatism Patients
Despite the lack of mass-market colored toric lenses, patients with astigmatism still have viable options to achieve a change in eye color. For individuals with mild astigmatism (typically less than -0.75 diopters), an eye care professional may fit them with a standard spherical colored contact lens. In these cases, the tear film under the lens can mask the astigmatism, providing acceptable vision.
Another option is to seek out custom-made colored toric lenses from specialized optical laboratories. These lenses are manufactured to an individual’s precise specifications, including color, power, and stabilization design. While significantly more expensive and requiring several weeks to produce, this option offers the highest chance of success for those with higher astigmatism.
Advancements in digital printing technology are making colored lenses more precise, and a few small manufacturers have introduced limited-range colored toric products to the market, signaling a potential change in availability.