The uses of the instrument shown in the picture below are ?
A. Refinement of the Astigmatic Axis
B. Refinement of cylinder power of Astigmatism
C. Discovery of Astigmatism
D. All of the above
Ans:D. All of the above.
The instrument shown in the picture above represents Jacksons Cross Cylinder (JCC).
Cross cylinder examination (otherwise known as Jackson’s cross cylinder)
- It is an examination used for the final fine-tuning of the axis and strength of astigmatism after its determination through retinoscopy, stellate cycle or automatic refractometry.
- The sphere in the cross-cylinder is double and of opposite power to the cylinder. The two most commonly used in everyday practice are the following.
- +0.25/ -0.50 (or -0.25/ +0.50) +0.50/ -1.0 (or -0.50/ +1.0)
1. Refinement of the Astigmatic Axis
- This is always done first .
- To refine the axis, the cross cylinder is placed (+0.5D) before the eye with its axis at 45 degree to the axis of the cylinder in trial frame (first with -0.5 D cylinder and then +0.5D cylinder or vice versa) and the patient is asked to tell about any change in visual acuity.
- If the patient notices no difference between the two positions, the axis of the correcting cylinder in the trial frame is correct.
2. Refinement of cylinder power of Astigmatism
- To check the power of cylinder, the cross cylinder of ± 0.25D is placed with its axis parallel to the axis of the cylinder in the trial frame.
- In the first position the cylindrical correction is enhanced by 0.25D and in the second it is diminished by the same amount.
- When the visual acuity does not improve in either of the position the power of cylinder in trial frame is correct.
3. Discovery of Astigmatism
- If no cylindrical correction is present initially, the cross cylinder may still be used, placed arbitarily at 90 degree and 180 degree to check for astigmatism.
- If no preference is found with cross cylinder axis at 90 degree and 180 degree the 45 degree and 135 degree should always be checked before assuring that no astigmatism is present.
- The end of JCC test is when both images appear equal . This occurs because with the correct cylinder power in place and the JCC lens in place the astigmatic interval is the same length in each position.The end of JCC test occurs when the resultant cylinder caused by obliquely crossing two cylinders has been neutralized.