Dr. Mita Joshi, J11818
Twirling The Cross
Refraction is an art and mastering it requires patience and passion. It is not simply prescribing glasses but fine tuning your refraction to match the patient’s needs and expectations,
Stokes introduced the concept of using cylinders to refine refraction in 1849.He used two cylinders with equal and opposite powers to assess cylinder requirement. The cylinders could be rotated to achieve a range of power from zero (when the axes are parallel) to maximum (when axes are perpendicular)
Jackson modified the Stokes’s lens and introduced the Crossed Cylinder or Cross Cylinder in 1887.
The Jackson Cross Cylinder consists of a combination of Two Cylinders with equal and opposite Dioptric powers, The dioptric power ranges between 0.25-1.0 D. The resultant combination can also be expressed as a spherocylindrical combination of a cylinder of a specific power with a sphere of twice the power and opposite sign
+ 0.12 D cyl × 90°-0.12 D cyl × 180°
+ 0.25 D cyl × 90°-0.25 D cyl × 180°
+ 0.50 D cyl × 90°-0.50 D cyl × 180°
+ 1.00 D cyl × 90°-1.00 D cyl × 180°
The Axes are marked and the respective cylinders denoted with red for minus and white for plus cylinder.But the markings may be different for different manufacturers and must be verified.
The terms used in relation with a cross cylinder include ‘Twirl “ or ‘Flip’ is used to denote movement of the cross cylinder in such a way that the side of the cross cylinder facing the examiner faces the patient at the end of flipping it.
The Cross cylinder is used to
- Refine Axis of Cylinder
- Refine Power of Cylinder
- Determine presence of cylinder power in spherical correction
| Refining Axis of Cylinder
Once the spherical and cylindrical correction are in place, ask the patient to focus on 6/12 or 6/9 line on Snellen’s Chart. Keep the cross cylinder in front of the trial frame with the handle parallel to the axis of cylinder in trial frame. In this position, the axes of the cross cylinder straddle the axis of the correcting cylinder at 45° each. The patient is asked to compare the images seen before and after the placement of the cross cylinder. The cross cylinder is now flipped and the patient is now asked to compare the images between the two positions of the cross cylinder .One of the two positions would have a sharper or darker or more legible image. The cylinder in the trial frame is rotated toward the axis on the cross cylinder having the same sign as that of the cylinder in trial frame. Suppose there is a minus cylinder in the trial frame at 160 degree axis. Cross cylinder with handle at 160 degrees is flipped and patient observes an improvement in a particular position. Move the cylinder in trial frame by 5 or 10 degrees towards the minus cylinder axis in the cross cylinder. Again make the handle parallel to new axis position in trial frame and twirl the cross cylinder. Ask patient for image improvement and rotate axis accordingly. There will be a position of the lens in trial frame at which flipping or twirling the cross cylinder will result in equally blurred images. Thus the axis of trial frame cylinder is refined Determining the power The axis once determined, the power is refined. The cross cylinder is placed in front of the trial frame with the cylinder parallel to the trial frame axis. Flipping the cross cylinder will add or subtract from the trial frame cylinder. Ask the patient to tell which image is clearer and add or subtract from the trial frame cylinder and repeat the process till the image is blurred on both positions of the cross cylinder. Checking presence of Cylinders After spherical correction is finalized , put the cross cylinder at 90 and 180 degrees and ask for image clarity.if blurred equally,keep at 45 and 135 degrees and recheck. Thus the cross cylinder forms an important part of refractive armamentarium
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