9.1 the jackson cross-cylinder

Question 1

What is the Jcc?

Answer 1

-is a lens comprising of positive and negative cyls with equal magnitude placed with her axis perpendicular to each other
-Both axis should be 45 degrees to handle.
-comes in different powers
-

Question 2

What is the refinement of cylinder axis based on?

Answer 2

the obliquely crossed cylinders

Question 3

What is refinement of cylinder power based on?

Answer 3

increasing and decreasing the power of the cyl without changing the sphere component.
You can achieve this by simply flipping the cross cyl

Question 4

What is the jcc marked with?

Answer 4

• marked with axis values
• The cylinder axis are opposite with powers
• On THE VERTICAL meridian is a minus. cylinder axis
• on the horizontal meridian is a positive cyl axis

Question 5

What does negative power cause on JCC?

Answer 5

Displacement along 90

-

Question 6

What does positive power causes on the JCC?

Answer 6

cause the displacmemtn along 180

Question 7

How is cross cyl hold?

Answer 7

with its handle along the direction of cylinder axis

-cross cyl is flipped or twirled between position 1 and position 2

Question 8

What does the patient look at?

Answer 8

• rings of dots. OR
• Verhoeff rings
• They are asked whether they look more round or sharp in position 1 or position 2

Question 9

What is the effect of twirling the cross cyl at position1 and position 2?

Answer 9

• rocking axis of correcting cyl
• ask patient whether they prefer the axis being slightly flatter or steeper
• As from showing position 1 and then position 2 - the clearer lines move in position slightly and we just ask patient which ones they prefer more.

Question 10

Why does

Answer 10

obliquely crossed cylinders.

Question 11

Why do we use power vector analysis?

Answer 11

when it is obliquely crossed

Question 12

What can we confirm using the theory of obliquely crossed cylinders ?

Answer 12

that the cross cyl acts to increase and decrease the axis value of the correcting cyl put in place

Question 13

How do we do power vector analysis?

Answer 13

Using the online calculator

Question 14

What is the direction the axis shifts in by over laying the cross cyl on the figures which showed the clearest lines on the fan chart?

Answer 14

Axis shifts towards to NEGATIVE JCC axis for negative cyls

- if one position is preferred, you see where the negative axis

Question 15

How do the powers act in the JCC?

Answer 15

-power is acting right angles to the axis

Question 16

When does the fan chart appear much more clearer?

Answer 16

when the astigmatism is less for example from 3D to 1.50D which is now much clearer.
-Then the axis shift is much larger -

Question 17

What happens when there is a greater axis shift?

Answer 17

there is weaker astigmatism

-Which is an advantage

Question 18

How can we calculate the effect of axis shift caused by the cross cyl for different levels of astigmatism ?

Answer 18

• See table in Slide 13
• as you increase power of cross cyl the axis changes increases- which means if you have a good level of VA , and small level of astigmatism , we would want the weaker cross cyl- use higher cross cyl power when dealing with lower levels of vision.
• The level of shift decreases as we go to higher levels of astigmation - which is good because our tolerance to axis errors decreases for higher levels of astigmatism
• If we are a moderate/high astigmatism - will be twitchy on getting the exact right astigmatic axis
• cross cyl will make small changes on astigmaitc axsi - as we get to 3/4/5 D we only have a few degrees - which can be critical and can reduce vision
• If you are a low astigmatic- will not notice much on vision- so creating larger changes with cross cyl - for l

Question 19

What is a good adavantage of the cross cyl?

Answer 19

the larger changes in axis with cross cyl with lower levels fo astigmatism but getting smaller with he higher levels of astigmatism

Question 20

Now how can the cross cyl be used to refine cylinder power?

Answer 20

Once the cylinder axis has been found, the JCC is held with its axes along and perpendicular to the correcting cylinder axis- now not to change the axis but th power

• The cross cyl has 0 BVS
• not going to change spherical component
• BVS is kept on the retina

Question 21

What is the effect fo the JCC when flipping to find powers?

Answer 21

The effect of the JCC is to increase the magnitude of the cyl in one orientation and decrease it in the other
-Now when ask the subject whether the circles are clearer in one direction or the other- asking whether they preferred as slightly increased level of astigmatic correction or slightly decreased level of astigmatic correction

Question 22

How can we work out the resultant?

Answer 22

by combining the trial case lens with the cross cyl powers
- the example shown ins slide 15 -

as -3.00 along 135 and power (+0.50)
-3.00+0.50= -2.50D
The other meridian is 0.00 across 45 but -0.50 in cross cyl - so 0-0.50= -0.50D
so now for the resultant you have to find difference hence put in the subtract sign to find it= -0.50– (+) 2.50 = +2D of astigmatism in this orientation

So now if you flip cross cyl 
-carry out same calculation
-3.00-0.50 = -3.50D X 135
0.00+0.50= +0.50 X 45
\+0.50 x 45
\+0.50-- (+) 3.50 = 4D of astigmatism

Question 23

What powers changes the cyl powers by ?

Answer 23

±0.50 x-cyl changes the cyl power by 1.00DC

±0.25 x-cyl changes the cyl power by 0.50DC

Question 24

Summary

Answer 24

The JCC is the preferred method for refining cyl power and axis but it doesn’t work for all patients
Changing axis is based on the theory of obliquely crossed cylinders
Astigmatic power change can be worked by resolving principal meridian powers