Base down prism ballast approach
Each soft contact lens manufacturer determines the amount and location of prism in their soft toric contact lens designs. Specialeyes has chosen a base down prism ballast approach for their custom soft toric and multifocal toric contact lenses. Each SpecialEyes lens comes standard with 1.0 diopter of base down prism. Reference the shaded gray area within the image below for depiction of prism.
What sets SpecialEyes’ design apart from many of the other custom soft toric contact lenses on the market today is the company’s ability to vary or customize the amount of prism in their lenses. SpecialEyes can vary the amount of base down prism in 0.10 diopter increments up to 4.0 diopters. It is even possible to add prism to a SpecialEyes custom soft spherical contact lens as well.
You may be asking yourself, why would I ever need to alter the amount of prism in a sphere, toric or multifocal toric contact lens? Great question! There are two reasons that stand out. At SpecialEyes we sometimes run across fitting situations where the contact lens fit from a centering and movement on blink perspective appears optimal via slit lamp examination but the lens tends to fluctuate in rotation thus creating instability and fluctuating vision. In this type of situation the base curve and diameter may have already been altered to try and improve fit, yet the unstable rotation still exists. It is in this exact scenario where an increase in the amount of base down prism will help to stabilize the toric or multifocal toric contact lens.
If you find a similar fitting scenario as the one above and feel an increase in base down prism is necessary, start by increasing the prism in 0.25 diopter steps. A general rule to follow is that the higher amount of fluctuating rotation, the greater amount of additional prism will be needed to correct the unstable lens.
Another scenario where adding additional prism to soft toric contact lenses may prove beneficial is for treating a binocular dysfunction such as vertical heterophoria which is a vertical misalignment of the eyes causing vertigo, double vision, headaches, fatigue, etc.
So, if you experience unstable rotation issues while fitting custom soft toric contact lenses or have a patient with a binocular dysfunction consider adding additional prism to the contact lens if warranted.
The SpecialEyes Team