Checking with the Jackson Cross Cylinder (JCC) is a subjective refraction technique used to refine the cylinder axis and power during an eye exam. Here's a breakdown of how it's done:
1. Cylinder Axis Refinement:
a. Presentation: After a subjective refraction, place the JCC in the trial frame with the handle aligned with the patient's cylinder axis.
b. Questioning: Ask the patient: "When you look at the chart, does it look clearer or darker when the red marks are aligned with the cylinder axis or when the white marks are aligned with the cylinder axis?" (or use similarly colored marks based on the JCC in use).
c. Observation and Adjustment:
- If the patient prefers the position with the red marks aligned with the cylinder axis, rotate the cylinder axis in the direction of the red marks (typically 5-15 degrees, depending on the JCC power).
- If the patient prefers the position with the white marks aligned with the cylinder axis, rotate the cylinder axis in the direction of the white marks.
- Continue this process, making smaller adjustments, until the patient reports no preference between the two JCC positions. This indicates the cylinder axis is properly refined.
2. Cylinder Power Refinement:
a. Presentation: Align the JCC with its minus cylinder axis coinciding with the trial lens cylinder axis.
b. Questioning: Ask the patient: "When you look at the chart, does it look clearer or darker when the red marks are aligned with the cylinder axis or when the red marks are perpendicular to the cylinder axis?"
c. Observation and Adjustment:
- If the patient prefers the position with the red marks aligned with the cylinder axis, increase the cylinder power (make it more minus).
- If the patient prefers the position with the red marks perpendicular to the cylinder axis, decrease the cylinder power (make it less minus).
- Adjust the sphere power by half the change made to the cylinder power, keeping the spherical equivalent constant.
- Continue this process, making smaller adjustments, until the patient reports no preference between the two JCC positions. This indicates the cylinder power is properly refined.
Important Considerations:
- Patient Cooperation: The accuracy of JCC testing relies heavily on the patient's subjective responses. Clear communication and understanding are essential.
- Small Increments: Make small adjustments to the axis and power to avoid over-correction.
- Vertex Distance: Accurate vertex distance is essential for lensometry.
- Trial Frames/Phoropters: Use well-adjusted trial frames or phoropters.
Example from Reference Text:
The provided text snippet suggests asking the patient which "circles look sharper, red or green." Based on their response, the optometrist adjusts the prescription. For instance, if the patient states the "green" circles look sharper, this implies a specific axis or power preference, leading to adjustments in the prescription (in this case, removing "-1.50").
