Soft torics are better than ever, but there are still situations in which GP torics will give better results. In general, GPs provide better optics for astigmats, especially so as the amount of refractive cylinder increases. Let’s review the types of GP torics, potential candidates, and how to get started with each.
Starting the Fit
First, determine whether the eye needs spherical or toric base curves (BCs). With only K values, the K toricity is your guide. If the value is less than 2.00DC, use a spherical BC. If it is greater than 3.00DC, toric BCs are needed. If it is between the two, start with a spherical BC and change depending on the fit quality.
A better method is to use corneal topography. Elevation differences between the major meridians give a better indication than K values. If the elevation difference exceeds 60 to 80 microns at an 8mm chord, use toric BCs (Figure 1).
Spherical Base Curve Lenses For a spherical BC, choose between a sphere GP or a front-surface toric (FST) lens. This is an easy choice. Determine how much residual astigmatism (RA) will be present with a sphere lens by finding the mismatch between the refractive and keratometry cylinders.
If the predicted RA is greater than 0.75DC, consider using an FST. The lab will add toric power to the front surface to correct the RA and prism to inhibit lens rotation. Simply give the lab the patient’s Ks and prescription, or the best-fitting sphere lens and sphero-cylindrical over-refraction (SCOR), and the lab will do the rest.
Modify the lens fit as needed for best alignment, diameter, edge clearance, and lid interaction. Note any lens rotation and SCOR, and the lab will refine the parameters.
Toric Base Curve Lenses With toric BC lenses, the two options are bitoric and back-surface toric (BST). The difference is purely optical. If the refractive cylinder is 1.5 times the K cylinder, that eye needs a BST. For example, if the refraction is +3.00 –6.00 x 180 and the K values are 42.00/46.00 @ 090, there is 4.00DC of corneal toricity and 6.00DC of refractive astigmatism. Because 4.00 x 1.5 = 6.00, this eye gets a BST. All other situations will need a bitoric lens to get the correct lens power.
Fitting the two lens types is very similar and very straightforward. Online calculators are available, such as the GP Lens Institute’s at gpli.info/lens-calculator , to determine the initial parameters, or GP labs can help design the lens.
To troubleshoot, note any problems with lens alignment in either meridian, document any lens rotation, and do a SCOR. As always, your GP lab can help with any parameter changes.
The toric BC options typically have very little induced cross-cylinder effect with lens rotation, and the natural toricity of the lens and cornea together prevents most lens rotation in the first place. The result is spectacular optics for high astigmats.
Always keep GP options in mind when dealing with astigmats. Soft torics aren’t the best choice for everyone, and patients will thank you for providing them with the best options. CLS