Jackson Cross cylinder

• • Once we have calculated BVS we need to determine the axis and amount of astigmatism
  • Jackson cross cylinder is the most common procedure for determining the axis and the power of the cylindrical power
• A typical Jackson cross cylinder lens is a spherocylindrical lens having a spherical power component combined with a cylinder power component of twice the power of the sphere, and of opposite sign
• JCC available in +/- 0.25, +/- 0.50, +/- 0.75, +/- 1.00 powers, which result in astigmatic intervals of twice these amounts
• • Find best vison sphere (circle of least confusion is on the retina)
  • Estimate the amount and axis of astigmatism (use table)
• Use a circular letter (a one row above the visual acuity), Verhoeff rings or collection of dots
• • Adjust sphere for accommodation (use duochrome). Patient sees equal or slightly better green
  • Choose appropriate cross cylinder:
  • best visual acuity = 6/12 or better, select the ± 0.25D crossed cylinder
  • best visual acuity = 6/18 or worse, select the ± 0.50D crossed cylinder
• • Check if astigmatism is present and if so…
  • Determine the axis accurately:
  • Place the estimated negative cylinder power in the trial frame and remember to add half of the cylinder power as positive sphere to the best sphere
• • Determine cylinder axis by presenting the handle parallel to the cylinder axis
  • Rotate the trial cylinder towards the negative axis of the JCC cylinder
• When the patient is unable to detect difference in clarity between both positions, this is the correct cylinder axis
• • Determine the cylinder power:
  • Present one JCC cylinder (positive or negative cyl) so that one of the axis is parallel with the trial cylinder
  • Increase the cylinder power if the sharpest position occurs with the –ve axis. Decrease lens power if the sharpest position occurs when the +ve axis coincides with the trial lens axis
• • Determine the cylinder power:
  • Add +0.25DS to sphere for every -0.50DC increase in cylinder and vice versa
  • When the patient is unable to detect a difference in clarity between 2 positions, the correct cylinder power has been achieved
  • Recheck axis
  • Refine sphere