JCC: subjective correction for astigmatism

what is JCC used for

jackson crossed cylinder is used for the corection for astigmatism that was suggested by retinoscopy

also called crossed cylinder

equipment needed

hand held JCC

phoropter based JCC

what is a crossed cyliner lens 

a plano positive/plano negative cylinder arranged at right angles to each other 

contains 2 cylindrical powers of equal magnitude but opposite signs orineted 90 degrees apart

cross cyl lens

equal and opposite powers along their principal meridians

negative power in vertical meridian shown by dashed line

positive power in horizontal meridian in blue

2 plano cylinders 

hand held cross cyl 

the red markings : the -0.25 is showing us that this is the axis of the -0.25 cylinder

 white markings - axis of the positive cylinder 

does NOT mean that the power along this meridian is -0.25D 

plano +0.25 and plano -0.25

when combined it forms a sphereo cylindrical prescription of

+0.25DS/-0.50DC

( most posiitve for sphere, S+C= CYL so +0.25 + c = -0.25 : -0.50 )

axis of the + and - are 90 degrees from one another, and are set 45 degrees on either sid of the handle which holds the lens

power along the red axis is 0 as plano

astigmatic interval

distance between the 2 focal lines created by the spherocylindrical lens

often the handle etched on it eg 0.50 , this indicates the astigamatic interval so its -+ 0.25 cross cyl

eg +0.25/-0.50 × 90 

this lens has a power of +0.25 along 90 and -0.25 along 180 

+0.50/-1.00 x ..

this is a x-cyl lens too as the powers are euqal and opposite along its 2 principal meridian

equal and opposite

the power along the red meridian is +0.25

power along the white meridian marked +0.25 is -0.25

twirling the JCC (flip)

twirling gives 2 positions  

when asking to compare the circles, ask which of these two lenses make the circles look clearer and rounder

finding out whether the px prefers a minus cylinder of 180 or plus cylinder of 90 

or minus at/plus at 45/135: the dots

targets to be viewed by px during JCC  

look at circles/dots as they contain all the orinetations 

1: concentric cirlces- verhoeffs rings 

there are 2 sizes of VR. use smaller rings if VA with BMS/BVS is good 

2: dot target , again choose smaller dots if VA with BMS/BVS is good 

3: some choose to use letters that are just above the resolution limit eg if currect best VA is 6/7.5, choose circular letter on 9m line/ should be letter at the end/start of the line, not in the middle 

so O and C would be appropriate 

trial frame axis notation

numbered in degrees

progresses anti clockwise when looking at the px

when inputting a cylinder lens can use this to line the axis with cyl

cyl lens should go i the slot closest to the axis notation , closest to the patients eye

dont put any lenses infront of cyl lenses as this obstructs the view of the cyl axis

what is the JCC based around?

based around the manipulation of the size of theinterval of sturm

the position of the circle of the least confusion (CLC) - a circle in between the 2 focal lines, in relation to the retina

need to detemrine the power and axis of cylinrical lens neeed to correct px astigmatism

done either by refining cyl that was found on ret/autorefractor or

by introducing a dnt ehn refininf if no cyl was found on ret

positive or negative cyls when using JCC

+3.00/-1.00 × 180 is the same as +2.00/+1.00 × 90 

tend to refract using negative cyl

can use the JCC with +cyl or -cylas its the same either way 

mainly -ve cyl so method used is -ve cyl using JCC 

so only paying attention to the -ve markings on the JCC (usually red)

how does the JCC work

need to consider the cylinder power and axis eterminayion deparately 

first consider the power of the correcting cyl

what is the interval of sturm

space between the 2 focal lines formed by an astigmatic eye or lens system

what is the CLC

between  the 2 focal lines, there is a plane where the blur is minimal and approx circular 

this plane is the circle of least confusion and is the ideal place for the retina to be partially corrected or uncorrected astigmatic eye

eg: px has mixed, with the rule astigmatism

+0.50/-1.00 × 180

1 focal line in front of retina, one behind retina at right angle : mixed

between the 2 focal lines is the circle of least confusion (green)

along 180 eye is hyperopic by +0.50, so focal line (V) is behind retina

along 90 eye is myopic by -1.00, so focal line (H) is in front of the retina

CLC is on the retina if one focal line in front other behind, whilst having equal and opposite power

same eg: +0.50/-1.00 ×180

no JCC in place here

note position of front and back focal lines

note size of iterval of sturn also position of CLC

normally have to use a sphere to bring CLC onto the retins (BMS) if clc was initailly in front or behind it

so we only begin to use the JCC when we have used a sphere to place the CLC onto retina

in this eg we dont need a sphere as the clc is already on the retina

place the JCC before the trial frame: for +0.50/-1.00 × 180 

axis 90 offering -0.25 on JCC ( jcc is the light blue line)

ask px to look at the circles ask=nd ask if clearer one way or the other 

focal lines get closer if the cyl axis is correct for the eyes astig 

when JCC is introduced, the interval of sturm changes in size 

in this, the IoS is getting larger as the -ve cyl axis is wrong, its 90 when the eye needs 180

JCC now flipped: rotates axis 90 degrees: +0.50/-1.00 × 180 

in position 2, the minus cyl markings are at 180 

position 2 better as the focal lines are closer together

comparing whether the JCC is better with position 1 or position 2 when asking if the circles look rounder with position 1 or position 2 

compare the size of the IoS 

at position 1: IoS is bigger so worse when minus cyl at an axis of 90 

better when axis is 180 minus cyl 

in eg: +0.50/-1.00 ×180 this is what the px will see

end point is reached when the circles appear thr same for position 1 and 2

effect of adding a -ve cyl from the trial case

if a cylinder from the trial case with the correct axis for that eyes astigmatism , the cylinder will only change the location of one of the eyes focal lines

this is because the trial case cyl has power along one principal meridian only

along the axis of the trial case there is no power, hence no power is added to that meridian

before the -ve cyl from trial case is added, the CLC should be on the retina hence one focal line is in front other behind

the effect of addin an apooropriately oreintated minus trial case cylinder is to move the front focal line back, and rear focal line will remain unchanged

when a minus cyl with approrpoiate axis is added…

blue line shows negative plano cylinder in front of the pateints eye 

moves the front focal line back 

focal line in the rear has no power so doesnt affect this so doesnt move 

IoS gets smaller

CLC is behind the retina so positive sphere power needs adjusting so clc falls back onto the retina.

positive sphere power shifts everything to the left  

what do we do when the CLC is pushed backwards - behind the retina 

the negative cyl from the trial case has pushed the CLC backwards so we need to add a +ve sphere to move the CLC back onto the retina when we have introduced the - ve cyl rom trial frame 

need to add half the amount of cyl introduced of positive sphere 

eg if -1.00DC added, then +0.50DS should be added to the trial frame sphere 

sphere compensation for a change in cyl power

if BMS is +1.00DS and then we add -0.25C x 180 , we dont need to modify the sphere = +1.00/-0.25 × 180

but if we add a further -0.25DC then we need to modify the sphere by +0.25 DS when we add a further -0.25 DC; hence +1.25/-0.50 × 180

if we dont modify the sphere then the i ve cyl will move the clc behind the retina

mainly important in presbyopics

if we make cyl power more negative by -0.50 then need to makw the sphere more positve by adding +0.25 to sphere in trial frame

sphere comp when reducing cyl power

so for every -0.50 DC removed, need to reduce the trial frame sphere power by +0.25 DS

eg. +1.00/-1.00 × 180

using JCC, we decide we need to reduce the cyl by -0.25DC

so +1.00/-0.75 × 180 is now in place

but if we reduce cyl a further -0.25 DC then we need to reduce sphere by +0.25 DS

so we end up with +0.75/-0.50 × 180

eg 1.00/-1.00 × 90

if when cyl power rises to -1.50

add +0.25 DS

= -0.75/-1.50 × 90

how is the JCC used for both cyl power and axis

when checking cyl axis the handle of the JCC is held along the cyl axis in the trial frame

so if axis in trail frame is 170, handle is held along axis, so its held at 80

how is JCC used when checking cyl power 

hold the axis markings of the JCC along axis of the cyl in the trial frame 

red markings along axis of cylinder so perpendicular to the extimated cyl axis