Currently, soft toric multifocal lens fitting sets are not commonly available, so lenses are generally ordered based on calculations. As described in my Jan. 2018 column, astigmatic correction must consistently align with the spectacle astigmatic error and should be ensured prior to employing any multifocal troubleshooting strategies.
A common frustration is to order an initial pair of soft toric multifocals, but when the lenses are fabricated and the patient comes into the office for dispensing, you find that one or both of the lenses rotate on the eye, leading to poor vision. This necessitates that new lenses be ordered, which slows the fitting process and may erode patient confidence.
To expedite the fitting process, consider ordering three diagnostic lenses for each eye when placing your initial order: one on the spectacle minus cylinder axis and one a given number of degrees to the left and right of the spectacle axis. At the initial dispensing, apply the on-axis lens first. If it rotates, you have a lens with an adjusted axis to use, regardless of which way the lens rotates.
Choosing Diagnostic Lens Axes
As discussed in the January column, it is reasonable to use 0.75D of residual astigmatism as the minimum value that is unacceptable for adequate visual performance. How much residual astigmatism a toric contact lens will induce depends on both the cylinder power of the lens and the degree of axis mislocation. Table 1 shows how far off axis toric lenses of various cylinder powers must be to induce 0.75D of residual astigmatism.
|Toric Lens Power (D)||Degrees of Lens Rotation|
For example, if a soft toric multifocal lens with 2.25D cylinder is to be ordered, 10º of axis mislocation will induce 0.75D of residual astigmatism. So, order a lens on axis, one 10º more, and one 10º less (e.g., if the spectacle axis is 020, order lenses at x 020, x 030, and x 010). This approach, with the trial lens that is closest to optimum alignment in place during the return visit, will keep the maximum residual cylinder well below 0.75D. As another example, if working with a lens that has a cylinder power of 4.25D, order on axis and 5º on either side.
When working with a lens with 0.75D of cylinder power, the lens has to rotate 30º off axis to induce meaningful (0.75D) residual astigmatism. You can either order lenses on axis and ±030º, or simply on axis if you feel the lens design is unlikely to rotate to such a large degree.
Employing this approach can help expedite the fitting of any toric lens, but it can be particularly helpful with the more optically complex soft toric multifocal designs. CLS
For references, please visit www.clspectrum.com/references and click on document #268.