The ability to incorporate prism into scleral GP contact lenses is a helpful tool for practitioners to correct residual astigmatism. For many years, prism ballasting has been the gold standard for stabilizing a lens that has front-surface toricity. This is crucial to provide optimal vision and to minimize rotational forces that occur with blinking.

Most GP contact lens manufacturers use 0.75 to 1.50 base-down prism diopters to provide rotational stability, which is typically minimal enough that patients can compensate for this induced vertical deviation. However, it is now more common to achieve stabilization with scleral GPs by taking advantage of patients’ scleral toricity and utilizing toric peripheral curves on the back surface of the lens.

The beauty of both corneal and scleral GP lenses is their ability to neutralize corneal astigmatism by creating a tear lens. However, it is important to consider the uniformity of the thicker tear layer under scleral contact lenses (Ramdass, 2016). Any scleral lens that decenters will generate a prismatic tear layer, with thinner clearance superiorly and an excessive amount of clearance inferiorly (Michaud, 2018). This will induce higher-order aberrations (HOAs), and the best way to compensate optically for HOAs is with cylindrical lenses. This may explain why some patients have residual astigmatism with scleral GP lenses and not with corneal GP lenses.

Correcting Small Amounts of Tropia

Until recently, it has been uncommon to correct a tropia with prism in GP contact lenses because the prismatic effect tends to be partially neutralized by the tear layer. Thus, the magnitude of incorporated prism would have to be significantly larger compared to the required amount to achieve the desired effect, often causing the lens profile to become extremely thick. Nonetheless, a few contact lens manufacturers offer the option of incorporating prism as a means of optical correction for patients who have ocular misalignment.

An impression-based custom scleral lens offers the option to correct optical prism in scleral lenses. This lens is capable of providing up to 8D of prism (4D in each eye) either vertically or horizontally. Optical prism is achieved by altering the thickness of the anterior surface of the lens within the optic zone, effectively creating a tilted surface and inducing prism correction. The peripheral portion of the scleral lens remains uniform in thickness and is stabilized by the toricity of the sclera. Additionally, the thickness of the tear reservoir remains stable throughout the optic zone (Sindt and Slater, 2018).

Expanded Use of Prism

For many years, practitioners have prescribed spectacles to be worn over scleral lenses for patients who have residual astigmatism or ocular misalignment. However, new technological advances in scleral lens designs can utilize prism to visually correct small magnitudes of tropia. With these advances, patients may appreciate significant improvement in visual quality and eliminate, or reduce, the need for overlay spectacle correction with lens wear. CLS

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