Conductive keratoplasty and the coupling phenomenon

Purpose. To determine whether changes in corneal astigmatism with astigmatic conductive keratoplasty (CK) treatment obey Gaussian coupling (i.e., the steepening of the flat axis associated with the flattening of the steep axis) and to measure the coupling ratio and the coupling constant to determine the effect of astigmatic CK treatment on spherical equivalent. Methods. Retrospective review of 33 eyes in 24 patients who had undergone CK for hyperopia and who were treated intraoperatively for induced astigmatism. Induced astigmatism was determined by comparison of keratometric readings before and after CK. Results. The coupling ratio was calculated according to two equations: clinical coupling ratio (1.61 ± 0.81 diopters) and coupling ratio of the surgically induced refractive change (1.57 ± 1.16 diopters). Values of the coupling ratio between 0.72 and 1.88 indicate coupling. Conclusions. The cornea does not behave according to Gauss's law of elastic domes. The targeted flat axis and steep axis 90° away are affected by CK in opposite but not equal amounts. Therefore, when correcting surgically induced astigmatism with CK, the overall spherical equivalent of the patient will change because the coupling ratio is not equal to 1.