Advanced Techniques
Astigmatism

Because it is necessary to inhibit lens rotation, toric lenses will typically come with reference markings so the fitter can determine how the lens is oriented on the cornea. If one were to superimpose the face of a clock over the cornea, these markings, which may be circles or lines, are located at the six-o-clock or three and nine-o-clock meridians. Each hour on the clock represents 30�. So if a lens becomes oriented at five-o-clock that means it has rotated 30� to the right, while seven-o-clock is 30� to the left. The expression LARS stands for left add, right subtract. If the lens is rotated to the fitter�s left, the appropriate number of degrees is added to the prescribed axis. If it is rotated to the right the appropriate number of degrees is subtracted. Lens rotation can be measured using a slit lamp equipped with a protractor. The use of trial lenses is especially important when fitting toric contact lenses.

 

Engraged Markings on Soft Toric Lenses

 

LARS: Left add, right subtract