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Soft Toric Rotation: How Much is Too Much?
BY THOMAS G. QUINN, OD, MS, FAAO
I placed soft toric lenses on my patient's eyes. After settling, she returned to the exam room reporting clear vision. Indeed, the patient's acuity measured 20/20. I then lined her up behind the slit lamp, only to find a lens rotating 20 degrees off axis. How can this be? What do I do now?
The Flinch Level
Let's approach these questions by asking: how much astigmatic blur can most patients easily tolerate? One way of answering this is to ask yourself at what level of astigmatism you would fit a patient with a toric rather than a spherical lens. There is evidence to support the notion that we should consider toric lenses at an astigmatic level of 0.75DC. In other words, up to about 0.50DC is "okay" in most cases.
Analyzing the Prescription
Accepting 0.50DC of astigmatic blur as our "flinch level," let's look at my patient's spectacle prescription: –3.50 –0.75 × 170. I had applied a diagnostic toric soft lens of this same power to the eye. How much crossed cylinder effect would I expect to be induced by a –0.75D cylinder lens rotated off axis by 20 degrees?
It has been calculated that a toric lens rotating 30 degrees will induce crossed cylinder effect equal to the toric power in the lens. Or, every 10 degrees off axis induces one-third of the toric lens power. In this case, 0.75DC divided by 3 is 0.25DC of crossed cylinder effect per 10 degrees of rotation.
In my patient's case the 20 degrees of off-axis rotation induced only 0.50D of crossed cylinder effect. She was clearly able to tolerate this without difficulty.
Sphere Power Considerations Had my patient's spherical error been closer to emmetropia, 0.50DC of astigmatic blur would have been a more significant part of the total refractive error, and therefore may have been more visually problematic. An axis adjustment would be strongly indicated in such a case.
Cylinder Power Considerations Table 1 shows how much crossed cylinder effect can be expected with various degrees of axis misalignment with four common toric lens powers. Obviously, the more toric power in the lens, the more critical it is to have proper rotational performance. Looking at it another way, the lower the toric power in the lens, the less critical it is to have it perfectly align with the eye's astigmatic error. In general terms, each 0.50D decrease in toric lens cylinder power allows for about five more degrees of axis mislocation before it becomes visually significant for most patients.
To Change or Not to Change?
Should my patient's lens axis be changed? Though such action would be reasonable, I elected not to change the lens since the patient was happy and comfortably reading the 20/20 line. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #160.
Dr. Quinn is in group practice in Athens, Ohio. He is a diplomate of the Cornea and Contact Lens Section of the American Academy of Optometry, an advisor to the GP Lens Institute and an area manager for Vision Source. He has served as an advisor or consultant to Coopervision, CIBA Vision and Vistakon and has received research funding from AMO, B&L, CIBA Vision, Coopervision and Vistakon.