Contact Lens Practice Pearls
A Closer Look at Soft Toric Contact Lens Calculators
By John Mark Jackson, OD, MS, FAAO
I was recently asked for my opinions on over-refraction (OR) calculators for soft toric lenses. Which one is the best to use? Let's take a look at some of the currently available calculators.
Why Use Calculators?
A calculator is typically used to adjust the prescription of soft torics due to lens rotation. Adjusting the axis using LARS (left add/right subtract) is common, but relies on an accurate observation of lens rotation. The higher the astigmatic correction, the more precise the measurement has to be, and a good OR and a calculator can often give you a better resultant lens.
I chose to test four commonly used calculators to see how they compare.
But first, the clinical data. I'm using a fairly easy scenario to illustrate things. I chose a spectacle prescription of –1.00 –2.00 x 180 and a contact lens with the same power. I will pretend the lens rotates 15 degrees to my right. This means that the patient looks through axis 015 after the lens rotates. LARS, then, would tell us to change the lens axis to 165 (right-subtract, so 180-15=165).
With the original lens rotating 15 degrees, I should get an over-refraction of +0.50 –1.00 x 143 (based on math; I discussed this in the June 2007 Prescribing for Astigmatism column). Let's see how the calculators handled this.
Testing the Calculators
Ophthalmicalc is an online application from the University of Melbourne (you can easily find it by Google searching “ophthalmicalc”). This calculator asks for OD or OS, the spectacle prescription, the lens power, and the OR; it tells you the expected rotation and the lens power you should try next. Ophthalmicalc got it right, telling me the lens rotated 15 degrees to the right and to order –1.00 –2.00 x 165. It describes rotation as nasal/temporal, which some may find easier to visualize.
ToriTrack is available from CooperVision (you can easily find it by Google searching “Coopervision ToriTrack”). ToriTrack asks for the spectacle prescription, lens power, and the OR. It gave me the correct lens to try next, but didn't tell how much rotation I should expect to see.
EyeDock.com is a subscription website with a great deal of information for eyecare providers. They also have an iPhone app with a nice interface. The calculator asks for the spectacle prescription, the lens power, and the observed lens rotation. EyeDock gave me the correct lens to try, but you must accurately measure the rotation. It works well, but I would prefer it to calculate without my input of the rotation.
OptiCalc is an iPhone app with several contact lens tools. The OR calculator asks for spectacle prescription, lens power, and OR. It also chose the correct lens to try next, but adjusted the axis to 160. Presumably this is because most lenses come in 10-degree axis steps, but I would rather make that decision myself since high cylinder powers sometimes come in smaller steps.
All are Good Choices
The bottom line here is that they all work well. My favorite is Ophthalmicalc because it gives me the most information. I find it useful to know the amount of rotation I should expect. If the mark is not where it says it should be, it probably means the lens parameters are not labeled correctly.
One final note: the calculators are only as good as the data used. If the OR itself is suspect, you can't rely on the answer. Also, because lenses may be within ANSI standards but off a little from the label, that can impact the precision of the results as well. Use your clinical judgment in addition to these tools to provide optimal patient care. CLS
|Dr. Jackson is an associate professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at firstname.lastname@example.org.|