HomeArticlesJackson Cross Cylinder [ Sub – ENG ] – Refraction part 2
Jackson Cross Cylinder [ Sub – ENG ] – Refraction part 2
October 13, 2019
Hi to everybody and welcome to OptoTubeChannel, the first Italian YouTubeChannel dedicated to Optometry Today let’s talk about Jackson Cross Cylinder test, which is usefull to evaluate astigmatism For example when we perform subjective refraction and see that his/her visual acuity doesn’t improve by adding lenses, and if we ask him/her about his/her perception, he/she says that is distorted, that is the case of astigmatism. So let’s use Jackson Cross Cylinder to evaluate and compensate astigmatism. JCC is a bicylindric lens which power is +/- 0.25 or +/- 0.50 and it is placed into a tool which can make rotate it and switch side. So the first step is to check cylindric lens axis which compensates astigmatism. To do it let’s see to the patient a clock chart, which is a figure very similar to a clock with dashed lines into, placed through different space directions. At this point let’s ask to the patient if there’s a line or a lines group which are seen in a different way. So the patient could answers that there is a line group seen better, so in that case put the cylinder axis through this direction. But the most frequent answer is that patient say that there’s a lines group or a direction seen worst and in that case put cylinder axis through the orthogonal direction compared to worst lines. If we use astigmatic clock we can use the ” 30 rule ” which says to multiply for 30 the smaller number of the lines seen worst. For example if are seen worst the lines on 39 direction, let’s multiply 3 for 30=90 degrees which is the orthogonal direction to the worst lines direction. If we want to use others chart, not always this rule is true for construct reasons. So at this point let’s set the found axis into our phoropter or trial frame and let’s start from power evaluation of the cylindrical lens. We can make this by JCC placing it ahead patient eye. So let’s set the minus axis parallel to the preset axis ( which axis we found by astigmatic clock ). At this point let’s use 0.25 minus lens, asking to the patient if he/she has a better perception in this case or when we switch JCC side (so its axis using the plus ) or if the two perception ar similar. If they are similar we can stop here, or we could make more precise axis and come back to visual acuity evaluation. If the two perceptions aren’t similar, we have to increase cylinder power ( or in any case modify cylinder power ) until all lines will be seen in the same way. Let’s consider that, each -0.50 D of minus cylinder , we have to add +0.25 D of plus spherical. Conversley if we use plus cylinder on the trial frame. So if the patient says to see in tha same way every direction of the astigmatic clock, we can refine cylinder axis. So let’s rotate of 45 degrees the JCC, one of its axes because they rotate mutually in the same moment. So let’s rotate again of 10 degrees toward the minus axis, asking to the patient if he/she prefears this side ( axis ) or the other ( so changing JCC axis=plus axis ) or if this configurations / perceptions are similar. If they are similare, we could come back to visual acuity evaluation; if they aren’t, we have to continue to search the most precise axis. Obviously higher is the lens power, preciser will have to be the axis! With lower cylindrical lens for not precise axis, there aren’t many induced effect by wrong axis set. When we find the wright axis which let the patient see all lines in the same way, we can remove JCC remove astigmatic clock too, and come back to the chart we used to evaluate visual acuity So in this case we evaluated and compensated astigmatism during subjective refraction by JCC a very usefull test so! So this is all about JCC test, if you liked leave a like and subscribe channel! Let’s see to the next opto-video! OPTO-STYLE!!!