Jackson Cross Cylinder

• What is it?

  • used to determine axis and power of astigmatism
  • used as a more accurate alternative to block&fan - 2.5 compared to 5 degs of accuracy
  • cannot be used if > 1.00 DC difference compared to result found in ret
  • Target = concentric circles (Verhoeff circles) or anything circular

  *size must be better than patient's best sphere VA for that eye
• Components of JCC:

  • Handle = at 45 deg to each axis

  *no. of handle corresponds to astigmatic interval b/w the 2 powers (-0.25DC and +0.25DC = 0.5DC)

  • Cylindrical lens = marked by axis at 90 deg to each other (where there is no power)

  *available in 0.25,0.5,0.75,1.00 powers - use higher powers for lower vision patients

  *use JCC that is less than or equal to the power of the estimated cylinder

  *one colour = +ve cylinder , other colour = -ve cylinder

  • Axis - HANDLE PARALLEL TO TRIAL FRAME AXIS (one colour only)
  • Power - AXIS MARKINGS ON JCC PARALLEL TO TRIAL FRAME AXIS (both colours)
• Best Sphere VA vs Estimated Astigmatic Error:

  • 6/5 = 0.00 DC
  • 6/6 = -0.50 DC
  • 6/9 = -1.00 DC
  • 6/12 = -1.50 DC
  • 6/18 = -2.00 DC
  • 6/24 = -2.50 to -3.00 DC
  • 6/36 = -3.25 to -4.00 DC
  • For every -0.50DC added in trial frame cylinder....

  *Add +0.25 DS to the sphere

  • For every +0.50DC added in trial frame cylinder...

  *Add -0.25 DS to the sphere
• Advantages over Fan&Block:

  • accurate to 2.5 deg - bracketing technique
  • unaffected by head tilt
  • performed for both distance and near vision
  • more accurate for lower power cylinders
  • circular targets are everywhere - domiciliary

  Disadvantages:

  • Alternating choice (where fan&block is simultaneous choice) - patient has to remember and cannot see options at once

  *older patients may require more time to respond

  • Fan and block more accurate for higher power cylinders
• Selection of Appropriate X-Cyl:

  • 0.25 - if refined V/A (after retinoscopy) is better than 6/12
  • 0.50 - if refined V/A (after retinscopy) is worse than 6/12

  *If V/A has improved after refining axis, go back to smaller JCC - the higher the cyl, the more specific the axis has to be
• Why do we leave duo chrome on equal/green before JCC?

  • if clearer on red, circle of least confusion in front of retina (not on)
  • Position 1 - posterior focal line will go on retina but increases astigmatic interval

  *patient will report it to be clearer but not circular

  • Position 2 - reduces astigmatic interval but both planes still in front of retina, leaving patient myopic

  *patient will report it to be circular but not clearer

  Better on green for younger people...

  • px can accommodate the circle of least confusion to move from behind retina to on the retina
  • not presbyopes though