Resultant vertical prism in toric soft contact lenses

- All toric soft contact lens (TSCL) designs evaluated had vertical prism in the optic zone except ACUVUE OASYS® for ASTIGMATISM, which had virtually none.
- TSCL designs utilising prism-ballast and peri-ballast for stabilisation have vertical prism in the central optic zone.
- In monocular astigmats fitted with a TSCL, vertical prism imbalance could create or exacerbate disturbances in binocular vision function.

PurposeRotational stability of toric soft contact lenses (TSCLs) is achieved using a range of designs. Designs utilising prism or peripheral ballast may result in residual prism in the optic zone. This study quantifies the vertical prism in the central 6 mm present in TSCLs with various stabilisation methods.MethodVertical prism was computed using published refractive index and vertical thickness changes in the central optic zone on a full lens thickness map. Thickness maps were measured using scanning transmission microscopy. Designs tested were reusable, silicone hydrogel and hydrogel TSCLs: SofLens® Toric, PureVision®2 for Astigmatism, PureVision® Toric, Biofinity® Toric, Avaira® Toric, clariti® toric, AIR OPTIX® for ASTIGMATISM and ACUVUE OASYS® for ASTIGMATISM; with eight parameter combinations for each lens (−6.00 DS to +3.00 DS, −1.25 DC, 90° and 180° axes).ResultsAll TSCL designs evaluated had vertical prism in the optic zone except one which had virtually none (0.01 Δ). Mean prism ranged from 0.52 Δ to 1.15 Δ, with three designs having prism that varied with sphere power. Vertical prism in ACUVUE OASYS® for ASTIGMATISM was significantly lower than all other TSCLs tested.ConclusionsTSCL designs utilising prism-ballast and peri-ballast for stabilisation have vertical prism in the central optic zone. In monocular astigmats fitted with a TSCL or those wearing a mix of toric designs, vertical prism imbalance could create or exacerbate disturbances in binocular vision function. Practitioners should be aware of this potential effect when selecting which TSCL designs to prescribe, particularly for monocular astigmats with pre-existing binocular vision anomalies, and when managing complaints of asthenopia in monocular astigmats.