how to do jcc test ?
A combination of a negative and positive cylinder of equal strength (either 0.25D or 0.50D) is mounted at an angle of 90°.The total spherical equivalent of this lens becomes 0 diopters.
Uses:
i. To find out whether the patient needs a cylindrical correction.
ii. To find out the power of astigmatism
iii. To find out the axis of astigmatism.
i. To find out whether the patient needs cylindrical correction:
For Example:
A 28- year -old patient with AR Readings: RE: -1.0 DS / 0.00 DC
LE: - 1.0 DS/ -0.00 DC
Acceptance: RE: -1.0 DS / 0.00 DC 6/6 LE: -1.0 DS/ -0.00 DC 6/6 (but vision is not very sharp)
If a patient’s vision is not sharp, he or she may have a small cylindrical refractive error which was not picked up by the AR. Now, when we place the cylindrical lens to correct small astigmatism.
We will be faced with two difficulties.
First, since we don’t know the axis, we will have to try at different cylindrical axis to assess where the patient’s vision improves.
Second and most importantly, adding a cylindrical lens changes the spherical equivalent.
Therefore, we will have to adjust the spherical power accordingly to maintain the same spherical equivalent.
This makes our job very difficult. The biggest advantage of JCC is that its spherical equivalent is zero.
We can quickly assess astigmatism by keeping the ‘–‘sign at 90, 180, 45,135°.
If JCC is placed at 180° and the patient’s vision becomes sharper, then this patient needs minus cylindrical correction around 180°.
But when we are finally prescribing the glasses, we should keep the principle of spherical equivalent in mind.
So, during subjective testing, try giving -0.25 DC or -0.50 DC at 180° and reduce the spherical power by - 0.25 DS if needed in case of myopes. We can also check for hyperopes in the same way using the ‘+ ‘sign of JCC.
ii. To find out the correct power of the astigmatism
The cross-cylinder is placed with its axis parallel to the axis of the cylinder in the trial frame (first with the same sign, and then the opposite sign). If vision is improving in any position, then the corresponding correction should be made.
For Example:
A 28- year- old patient with Acceptance: RE: -1.00 DS / -1.75 DC @ 120°
Place the minus cylindrical power of the cross-cylinder at 120° .
Ask the patient if he sees better or worse with a cross-cylinder at 120°. If he sees better, then the cylindrical power needs to be increased. If he sees the worse, it means that we do not have to increase the power.
Now, place the plus cylindrical power of cross-cylinder at 120°.
Ask the patient if he sees better or worse. If the patient sees better, then cylindrical power needs to be reduced.
If the cylindrical power given in the trial frame is correct, then the patient will see worse in both the positions (minus or plus cylinder), because he does not require an additional cylindrical power.
i. To find out whether the patient needs cylindrical correction:
For Example:
A 28- year -old patient with AR Readings: RE: -1.0 DS / 0.00 DC
LE: - 1.0 DS/ -0.00 DCiii.
iii . To find out the correct axis of the astigmatism
JCC can be used to fine tune the axis of the cylindrical prescription.
For Example:
A 28- year-old patient with Acceptance: RE: -1.00 DS / -1.75 DC @ 180°
Place JCC handle at 180° parallel to the axis of the cylindrical power with the minus sign above.
Now, flip and check with a minus sign below.
If the axis of this cylinder is correct, then the patient will see the same or worse in both positions.
But if the patient sees better with any one of the positions, then we need to rotate the cylindrical axis in that direction.
Let us say, in this case, patient sees better when the minus sign is at 45°. Now rotate the cylindrical lens from 180° to 10° (that is towards 45°) and repeat the test.
If there is no further improvement on keeping the handle aligned at 10°, then10° is the correct axis.
If there is improvement in any of the positions, then we should slightly rotate in that direction.
For the plus lenses, we will have to follow the same principle and the plus lens will be rotated towards the plus sign of the JCC, then it shows improvement in the vision.