prescribing for astigmatism
A New Twist On LARS and RALS
BY PETER BERGENSKE, OD, FAAO
Remember the old controversy about whether or not to consider rotation of the lens when calculating the sum of a toric soft contact lens power and the over-refraction? Although correct, the method was complicated enough to cause a lot of eyecare practitioners to throw up their hands and dial up the lab to let the consultant figure it out. The fact is, compensating for lens rotation is important to get the correct net result, but it doesn't need to be that difficult.
The Old Method
In the past, we were taught that the method of compensating for rotation in the crossed cylinder formula was to first apply RALS (right add, left subtract) to the axis of the lens observed on the eye. For instance, if the lens had axis 90 and it rotated 10 degrees clockwise (base moves left), then the axis used in the crossed cylinder calculation would be 80. That is the axis of the cylinder when the lens is on the eye. The next step is to combine this with the over-refraction in the cross cylinder formula using a pre-programmed calculator or other method. Once that resultant sphere, cylinder and axis are determined, applying LARS (left add, right subtract) gives the appropriate axis.
This method is accurate, but because we don't use it all that often, it is difficult to remember and not intuitively obvious. Fortunately, there is an easier method that is just as correct but easier to remember.
The traditional method effectively rotates the trial lens to the same orientation as the over-refraction and then requires that we figure the expected rotation back in. Really all we need to do is to apply LARS to the over-refraction prior to the calculation. This figures as though we rotated the over-refraction to be at the same orientation as the trial lens. Of course, both these methods assume that the new contact lens will rotate the same as the original lens that we over-refracted.
Over-refraction is a useful and powerful tool that is underutilized in contact lens practice. For complex soft toric contact lenses or even routine cases, the over-refraction can correct for errors that occur due to flexure, draping and tear layer effects. Any time the visual acuity achieved with a soft toric contact lens is less than expected, a sphero-cylinder over-refraction is indicated. This is particularly important when significant rotation is noted, as it is when other effects tend to be most exaggerated. The rule of thumb should be that if the over-refraction improves the acuity, use it.
An alternative method of determining the resultant contact lens power that requires no calculation is to create the original lens power and the over-refraction power with spectacle loose trial lenses, then superimpose these (with the over-refraction compensated with LARS) and measure the result on the lensometer. Although it can be a bit awkward to accurately stack four loose lenses in the lensometer, this method is otherwise simple and accurate.
Dr. 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: email@example.com.