Chapter 21: Coupling Concept in Astigmatism

Coupling Concept, Coupling, Coupling Adjustment, Steepening and flattening of corneal meridia With any discussion of astigmatism comes the term coupling. What is coupling? Corneal coupling is the spherical change associated with astigmatic treatment. So with incisions, any incision you place on the steep meridian is going to have an equal and opposite effect on the meridian 90° away and so the spherical equivalent doesn’t change. This is 100% coupling. Ablations don’t affect the meridian 90° away and so induces a change in spherical equivalent. Here there is 0% coupling. Now you published a paper on coupling in the Journal of Cataract and Refractive Surgery in 2014. What have you introduced that has been different to what we have known in the past? Coupling got much more complicated when laser treatments came along so the formulas that were prevailing in the 1990’s for coupling worked fine for incisions. As soon as ablations came into view we had a change in the spherical equivalent and that didn’t work. You wanted to have a formula for coupling that works for incisions and ablations and we changed two things: 1) in the coupling formula we just inverted it instead making it flattening of opposite meridian over flattening of treatment meridian 2) instead of using the whole of the SIA we only used the effective SIA and the intended treatment meridian By doing that we were able to use the coupling ratio for both incisions and ablations. How can your definitions of coupling be used clinically? This is the important thing. We created a new parameter called Coupling Adjustment. The coupling adjustment is for any 1.0 dioptre of astigmatism corrected how much of that translates into a spherical change. This needs to be analysed separately for both incisions and ablations and applied to future treatments.