When prescribed carefully, prism (horizontal and / or vertical) can be crucial in managing conditions such as diplopia, binocular vision dysfunction, and strabismus.1 Using a prism for visual correction has many practical applications; however, unintentionally induced prism can lead to diplopia and asthenopia.2
For some patients, prism-induced disparity exceeding 0.50D may strain the visual system, resulting in asthenopia, blurred vision, eye strain, and diplopia.3-5 Most spectacle wearers can tolerate unwanted prism up to ≤ 0.50Δ vertically and ≤ 1.00Δ horizontally (base-in and base-out).3
Consequently, the American National Standards Institute (ANSI) limits acceptable vertical prismatic imbalance in spectacles to 0.33Δ.6
The use of prism in toric contact lens design
While the use of prisms is most commonly associated with eyeglasses and spectacle lenses, it also plays an important role in contact lens design. Many toric soft contact lenses utilize a prism-ballast, peri-ballast, or thin-zone stabilization design (Figure 1).7
These designs typically involve positioning the prism addition inferiorly by thinning the upper part of the lens, which takes advantage of gravity to help orient and stabilize the lens on the eye.7 Prism-ballast lenses have a thin apex and a thick base,7 as base-down prism adds thickness to the inferior part of the lens. Pressure from the upper lid helps orient the lens, and gravity pulls the thicker region downward.8
Modified prism ballast has an interior thickness area that is smaller than that of a traditional prism ballast, resulting in minimal prism in the optic zone.8 Peri-ballast lenses feature a thick inferior ballast,7 which decreases the amount of prism in the optic zone.8
Thin zone designs, also known as double slab-off or dynamic stabilization, maintain stability through the pressure of the upper and lower eyelids. The lens material is thinner at the top and bottom, allowing it to move beneath the eyelids during blinking.7,9 One of the notable designs is the BLINK STABILIZED Design (BSD), which features four stability zones resulting in a prism-free optic zone.7,10
What are the implications of prism on vision, stability, and comfort?
Prism has the potential to affect contact lens wearers in a number of ways, including having negative impacts on visual acuity, lens stability, and overall comfort.7
Vision
Contact lenses designed with prism or peri-ballast features can induce vertical prism within the central 6mm of the optical zone.7 This prism effect remains uniform throughout the entire optical zone, regardless of the direction of gaze.7 Approximately 50% of patients with astigmatism ≥ 0.75D have monocular astigmatism.7
When fitted monocularly, these ballasted designs can induce up to 0.80D vertical prism imbalance in the visual system, leading to reduced visual quality and discomfort, which are significant factors contributing to contact lens dropout among this group of lens wearers.7 This issue is addressed with prism-free optic zones, which reduce vertical prismatic disparity, as seen in Figure 1.7
Figure 1: Thickness profiles for lenses with differing levels of vertical prism.7
Stability
The mechanics involved in each blink can create forces that disrupt the stability of prism systems, leading to visual instability.11 Prism-stabilized lenses rely on gravity to properly orient their power, functioning optimally when the eyes are in a primary gaze and the wearer’s head is upright.11
Fluctuations in vision may occur when a patient's eyes move into secondary gazes in orthogonal directions or when transitioning from an upright to a supine position.12
Contact lenses designed with vertical and horizontal symmetry can be inserted in any orientation and stabilize quickly.13 This design takes advantage of blink mechanics, making them resistant to head tilt and gravity changes, which enhances their stability on the eye's surface.14
Comfort
Adding a prism to the edge of a contact lens increases the thickness in that area,7 and this interaction can cause discomfort and dryness,8 potentially resulting in contact lens dropout, especially for patients who wear unilateral toric lenses or have monocular astigmatism.7
To improve patient comfort and reduce the risk of unwanted lens rotation, it is important to consider contact lenses that minimize interaction with the lower lid.8
Conclusion
Even though prism is commonly used for lens stabilization, its presence can negatively impact vision, comfort, and the stability of the lens.
Therefore, careful consideration should be taken during lens selection to ensure the appropriate lens is selected for each patient to optimize the wearing experience and to minimize any potential negative effects.
