prescribing for astigmatism
Soft Lens Wizard in Your
BY PETER D. BERGENSKE, OD, FAAO
Experienced fitters of toric soft lenses often find that simple compensation for rotation does not provide a satisfactory visual result. When the
patient's vision does not meet expectations even though the lens power and orientation seem appropriate, over-refraction of the current lenses can often provide the solution for solving the uncorrected refractive error. The over-refraction information is then used in cross-cylinder calculation to determine a final lens prescription. In this case, all the fitter needs to know is the power of the lens, its rotational orientation and the over-refraction to compute the final lens power. This method works well, yet it relies on accurate measurement and appropriate compensation for rotation.
The ToriTrack software available from
CooperVision, rather than relying on rotation, anchors its calculations around the spectacle correction. This is a disadvantage potentially in cases of irregular astigmatism, but it has several advantages, not the least of which is that the fitter can virtually ignore the rotation. Actually, the rotation is ignored for purposes of the calculation, but it is still important to assess and note rotation in the event that a new lens rotates significantly different from the lens used in calculation.
ToriTrack is available via an online calculator at CooperVision's web site
(www.coopervision.com), but a particularly appealing feature of ToriTrack is that it is available for free by download, or one can request a copy of the software on CD. Moreover, the software will run not only on a PC, (sorry, no Mac), but is perhaps most useful in its format for Palm OS. This portability keeps the software at hand for ready use in the busy clinic.
To use ToriTrack , start by entering the patient's spectacle prescription, which forms the baseline reference for the rest of the calculations (Figure 1). Then enter the prescription of the trial contact lens and the over-refraction, and ToriTrack goes to work. The spectacle prescription is automatically vertexed to the plane of the cornea. The degree of mislocation is deduced from this set of information and automatically compensated in the cross cylinder-calculation. A resultant lens prescription is then displayed.
|Figure 1. Enter the spectacle Rx into the ToriTrack software.
|| Figure 2. The software calculates the contact lens and parameters.
A contact lens selection feature is built into the ToriTrack software that combines information on corneal curvature and horizontal visible iris diameter to determine a "best fit" contact lens available in the needed prescription (Figure 2). For obvious reason, this is aimed at Cooper-
Vision products, so this aspect is not useful for other brands, yet the over-refraction software can be applied with any toric soft contact lens design.
Bergenske, a Past Chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for over 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. Email him at:
Contact Lens Spectrum, Issue: October 2002