Reader and Industry Forum
A New Method for Determining Toric Lens Rotation
by Ane Murueta - Goyena Larrañaga
A recent survey indicates that 45 percent of the population has 0.75DC or greater of astigmatism (Morgan et al, 2012). Although soft toric contact lens prescribing has gained in popularity in the last few years, astigmatism of 0.75D or less remains largely uncorrected (Efron et al, 2011). Several factors may contribute to the under prescribing of soft toric contact lenses, but certainly one of the most common is the belief of many practitioners that it takes too much chair time (Jackson, 2012).
The aim of this article is to explain an alternative method for determining soft toric contact lens rotation. This method is faster to perform compared to over-refraction and provides more accurate results compared to LARS (left add, right subtract). Its name is NIPO—a mnemonic acronym that stands for negative in, positive out.
The NIPO Method
Following are the four steps of the NIPO method:
1. Fit a soft toric contact lens and allow it to settle for at least 10 minutes.
2. Set a negative cylinder on the back side of the trial frame and a positive cylinder on the external side (Figure 1).
a. Use a negative cylinder that is equal to the refractive astigmatism (power and axis). The negative cylinder must neutralize the refractive astigmatism. Note that occasionally the refractive astigmatism differs from the lens cylinder axis and/or from the spectacle’s cylinder power.
b. Make sure the positive cylinder’s power is the same as the negative cylinder’s power, but in a random axis.
Figure 1. NIPO method: negative In, positive out.
3. Turn the positive cylinder until the patient reports that best visual acuity (VA) is achieved.
4. If necessary, over-refract with spherical lenses.
Imagine that a patient has the following actual refraction: OD −2.00 −3.25 × 095 and OS −1.50DS. To allow binocular tolerance, the spectacle prescription has less cylindrical power in the right eye: OD −2.00 −1.75 × 095 and OS −1.50DS.
However, this power difference can be tolerated in contact lenses. Note that stock contact lens axes are in 10-degree steps, and the requested axis is not available. The lens powers would be OD −2.00 −3.25 × 090 and OS −1.50DS.
The negative cylinder power should be −3.25 × 095, which is the same as the refractive—real— astigmatism. This power doesn’t coincide with the spectacle power, nor does the axis coincide with that of the contact lens.
How the NIPO Method Works
The negative cylinder in the trial frame corrects the refractive astigmatism. Therefore, the patient will have a hyperopic astigmatism, which is induced by the lens cylinder (Figure 2). The power of this astigmatism is known (it is the lens cylinder power); however, we will determine the axis that neutralizes the astigmatism using a positive cylinder, i.e., we will turn the positive cylinder until it aligns with the astigmatic axis—when best VA is achieved (Figure 3).
The difference between the negative cylinder and the positive cylinder give us the degrees of lens rotation and position:
• If the positive cylinder axis is greater than the negative cylinder axis, subtract the difference from the patient’s spectacle axis.
• If the positive cylinder axis is less than the negative cylinder axis, add the difference to the patient’s spectacle axis.
Figure 2. Soft toric lens is rotated. The negative cylinder neutralizes the refractive astigmatism. Hyperopic astigmatism results because of the lens' negative cylinder at an unknown axis.
Figure 3. The positive cylinder neutralizes the astigmatism induced by the contact lens. Both are oriented on the same axis.
Why Should You Use NIPO?
I’ve found that NIPO is faster to perform compared to over-refraction, which means less chair time and more efficiency.
The LARS method is a great technique for estimating the direction of lens rotation, but not the quantity. To calculate the amount of rotation, we need a reticule in our slit lamps, which may not be available in a given office. But every practitioner has a trial frame and loose cylinders!
NIPO also provides valuable information that other methods cannot. Remember that in the NIPO method, the positive cylinder neutralizes the contact lens cylinder when both are on the same axis. This can be helpful for situations in which we are not sure about the actual contact lens axis because of manufacturer error, contact lens swapping between eyes, etc.
This is an easygoing and straightforward tool when fitting soft toric contact lenses. Keep NIPO in mind! CLS
For references, please visit www.clspectrum.com/references.asp and click on document #211.
Ms. Murueta-Goyena has a master in optometry degree by Centro Boston of Optometry, Madrid, and she attended a summer program at The New England College of Optometry. She practices at Gran Via optic centre in Bilbao, Spain. Her main interest is in contact lens practice.