Silicone hydrogels (SiHys) have become the gold standard for soft contact lens materials. SiHy materials are well known to provide greater oxygen permeability (Dk) in comparison to hydrogel lenses. This increased Dk results in reduced hypoxia-related complications such as vascularization and limbal hyperemia (Maldonado-Codina et al, 2004).
When prescribing soft toric lenses, lens profile thickness strongly influences oxygen transmissibility (Dk/L) and is affected by the toric lens stabilization design, the power, and the material of the lens.
Factors Affecting Lens Profile Thickness of Soft Torics
Stabilization Design Rotational stability in soft toric lenses is crucial for optimal visual quality. Soft torics are typically stabilized with some form of prism ballast or dynamic stabilization/double-thin-zone design.
With a prism-ballast design, a base-down prism is incorporated in the inferior portion of the lens, adding thickness to optimize rotational stability. Thin-zone dynamic stabilization designs thin the contact lens in the vertical meridian, leaving thicker portions in the interpalpebral region of the lens. Both stabilization methods result in a lens with varying thickness profiles, which may affect oxygen transmissibility. Eghbali et al (1996) determined the Dk/L at five locations—the center, 3mm above and below the center, and 6mm above and below the center—of a prism-ballasted hydrogel toric lens. They noted that Dk/L was approximately three times greater at the top 6mm location in comparison to the bottom, thicker 6mm location.
CLINICAL PEARLS IN PRESCRIBING SOFT TORICS
- Consider lens power, rotation stabilization design, and material when prescribing soft toric lenses.
- Reduce wear time if prescribing high-plus or high-minus lenses such as +15.00D or –10.00D.
- Regularly assess the cornea for signs of vascularization.
- Strongly discourage patients from extended and continuous wear of lenses if they have a prescription that requires a thicker lens profile.
Power Contact lens power is another factor that contributes to the thickness profile of a lens. With increasing plus and minus powers, the central lens thickness becomes non-uniform and, therefore, the oxygen transmissibility of the contact lens changes across the lens surface. High-minus-power soft contact lenses with thicker midperipheries produce greater corneal edema due to a greater effective lens thickness over the central zone of the lens (Holden et al, 1983). Adding prism-ballast technology to a high-minus-power lens will further decrease the oxygen availability to the cornea.
Material Research studies have evaluated the influence of lens material and stabilization thickness profiles on oxygen transmissibility. Forister et al (2008) reported that, in comparison to hydrogel lenses, SiHy lenses provided significantly greater oxygenation to the anterior corneal surface under open-eye conditions (silicone hydrogel: 100 mmHg, hydrogel: 35 mmHg). Similar results were reported by Hough and colleagues (2015), who examined the changes in zonal distribution of Dk/L for 16 different silicone hydrogel lenses with varying stabilization designs. They found that for lenses ranging between +6.00DS to –3.00DS, the dynamically stabilized lenses had significantly greater Dk/L in the optic zone in comparison to the prism-ballasted lenses.
When Dk/L Is Critical
Consider silicone hydrogels, especially for patients who need torics in high prescriptions that result in a thicker lens profile. This can reduce the risk of corneal complications resulting from reduced oxygen. CLS
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