contact lens case reports
Computer-assisted Toric Soft Lens Fitting, Part 2
BY PATRICK J. CAROLINE, FCLSA, FAAO, & MARK P. ANDRÉ, FCLSA, FAAO
The ultimate goal of any toric soft lens is to position the lens cylinder power directly over the cylinder power of the eye. When one orders the lens cylinder axis at the same axis as the spectacle Rx, and the rotation
Rx and the rotation marker positions exactly at 6 o'clock, appropriate alignment results.
However, if the rotation marker positions on either side of 6 o'clock, the cylindrical power of the lens no longer aligns with the cylinder of the eye, causing cross cylinder effect.
Traditionally, the LARS pneumonic or sphero-cylinder overrefraction and adjustment for lens rotation has compensated for such lens malalignment. Accurate estimation of lens rotation is often subjective and can result in calculation errors, especially in higher cylinder powers. Additionally, beneath a toric soft lens a number of unforeseen variables may exist, including vertexing errors and masking of corneal cylinder as well as lacrimal lens and lens draping effects. A simple sphero-cylinder over-refraction and accurate cross-cylinder computation is the best way to uncover the influence of these variables.
The ToriTrack Calculator
Recently CooperVision introduced ToriTrack, a new technique for calculating cross-cylin der effects. The ToriTrack does not require data related to on-eye lens rotation, but calculates lens malalignment based on three optical variables: manifest refraction, lens power on the eye and sphero-cylinder over-refraction.
Patient NV had been experiencing blurry vision with toric soft lenses. Her manifest refraction was: OD 8.50 1.50 x 70, 20/20, OS 7.75 1.50 x 105 20/20 and her SCLs were: OD 8.00 1.25 x 70 20/25, OS 7.00 1.25 x 105, 20/30. The lens rotation markers positioned at the desired 6 o'clock position OU. The sphero-cylinder over refractions were OD +1.00 1.00 x 30, OS +1.50 2.00 x 155 (Figure 1) 20/20 OU. We entered the data into the ToriTrack, revealing an axis shift of 25 degrees OD and 59 degrees OS with new resultant powers of OD 7.50 1.00 x 45 and OS 6.50 1.00 x 164. Final VAs were clear and stable at 20/20 OU (Figure 2).
|Figure 1. Sphero-cylinder over-refraction OS.||Figure 2. Resultant calculated lens parameters OU.|
This case illustrates how all too often the resultant lens powers make little or no sense when compared to the baseline manifest refraction.
Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University. Mark André is director of contact lens services at the Oregon Health Sciences University.