Chapter 6: Astigmatic Lenses Flashcards
What is astigmatism?
All meridians in the lens do not have the same curvature
What are the 2 types of astigmatic lenses?
- Cylindrical
- Toric (sphero-cylindrical)
What is a cylindrical lens?
- Have one plane srface and other forms part of a cylinder
- in one meridian, lens has no vergence power (the axis).
- in the meridian at right angles to the axis, the cylinder acts as a spherical lens
What is the total effect of a cylindrical lens and what is this called?
Formation of a line image of a point object - called the focal line, parallel to the axis of the cylinder
What is a Maddox rod formed of?
Series of powerful convex cylindrical lenses mounted **side by side **in a trial lens
What is the Maddox rod used for?
Diagnosis of extraocular muscle imbalance
How does Maddox rod testing work?
- patient views distant white point source of light through the Maddox rod
- light in meridian parallel to the axis of each cylinder passes through undeviated therefore a row of foci 90 degrees to the axis of the Maddox rod formed - appears as a line
- light incident on MR in meridian 90 degrees to its axis is converged by each cylinder a real line focus between the rod and the eye
- this focus is too close for distinct image to be formed on retina so is scattered over wide area, doesn’t confused composite line image
Where is the source of light in relation to the patient when using a Maddox rod?
it must be far enough away for its rays to be parallel on reaching the patient (at least 6m); Maddox rod is close to eye in the trial frame
What is important to remember about the image seen by the patient from a Maddox rod?
the line seen lies at 90 degrees to the axis of the rod (not the real line image of the rod)
the Maddox rod glass is tinted red so the composite line image is also red
How is the Maddox rod used to test muscle balance?
- rod is palced close in front of R eye 9in trial frame), distant white spotlight viewed by both eyes
- R eye sees a red line at 90 degrees to axis of MR, L eye sees white spotlight
- two eyes see dissimilar images so any muscle imabalance will become manifested
How is horizontal muscle imbalance tested for with the MR?
- rod must be horizontal to give a vertical line (and vice versa)
- the position of the red line moves in the opposite direction from the deviation of the eye
- e.g. for the R eye, if the vertical line is to the left of the white dot, indicates exophoria
What is a toric lens?
cylindrical surface that is curved in both vertical and horizontal meridians (not to same extent)
meridia of max + min curvature = principal meridia; 90o to each other
What is the base curve of a toric lens?
the prinicpal meridian of minmum curvature (and therefore minimum power)
What is another name for toric lenses?
sphero-cylindrical lenses
Why don’t toric (sphero-cylindrical) lenses produce a single defined image?
principal meridians form separate line foci at right angles to each other - Sturm’s conoid
What is the the interval of Sturm?
the distance between two line foci formed by the 2 prinicpal meridians of toric lenses
What is the circle of least confusion (akak circle of least diffusion)?
plane where two pencils of light formed by toric lens intersect
What type of images are formed at all planes other than the circle of least confusion between Fh & Fv?
blur circle images
How is a toric (sphero-cylindrical) lens notated and what does this represent?
- can be thought of as a spherical lens with cylindrical lens superimposed
- writted as a fraction; spherical power is numerator, cylindrical power is denominator
- e.g. power of +2D in one principal meridian and +4D in oher principal meridian can be regarded as a +2D sphere with a +2D cylinder superimposed
- this is written +2.0 DS / +2.0 DC
Why might it be useful to calculated the power of the spherical lens of closes overall effect to a given toric lens (spherical equivalent)?
reveals whether the eye is essentially hypermetropic, emmetropic or myopic
this consideration is important in choice of IOL power for individual patient
How do you calculate the spherical equivalent power from a toric lens prescription e.g. +2.00 DS/ -2.00 DC?
Algebraic addition of the spherical power and half the cylindrical power:
2+ (-1) = +1.00 DS
How does the focal point of a toric lens relate to that of a spherical equivalent lens?
focal point of the spherical equivalent would coincide with the circle of least confusion of the Sturm’s conoid
What is a cross cylinder?
- sphero-cylindrical (toric) lens in which the power of the cylinder is twice the power of the sphere and of the opposite sign
- net result is same as superimposing two cylindrical lenses of equal power but opposite sign with axes at right angles
What is another name for the cross-cylinder?
Jackson cross-cylinder
What are 2 uses of the Jackson cross-cylinder?
- toric lens used to check orientation of the trial cylinder by superimposing it with its axis lying oblique to the axis of the trial cylinder
- can also check **power ** of the cylinder by superimposed further cylinders of varying power and sign in the same axis as the trial cylinder
How is the Jackson cross-cylinder set up?
- the lens is mounted on a handle which is placed at 45 degrees to the axes of the cylinders (see image)
- the axes marked on the lens are the axes of no power of the individual cylinders
- the power of each cylinder therefore lies at **90 degrees to the marked axis **and coincides with the marked axis (of no power) of the other cylinder (of opposite sign)
How do the axes marked on the Jackson cross-cylinder and the axes of refractive power relate to each other?
those marked on the cylinder are in opposite positions from the axes of refractive power
How are cross-cylinders named?
by the power of the cylinder (marked on the handle)
e.g. in image would be a 1.00 dioptre cross cylinder (-0.5 DS/+1.0 DC)
What are the 2 powers in which cross cylinders are available?
+0.5 and +1.0 D
What is the function of the 1.00 D cross-cylinder? 2 things
- used to check the axis of the trial cylinder
- and the power in patients with poor visual acuity
What is the function of the 0.5 D corss-cylinder?
used to check the power of the trial cylinder where the patient has good vision
What test type must a patient look at during use of the Jackson cross cylinder and why?
test type 2 lines above the smallest they can see
because cross-cylinder blurs the vision and larger letters are used to make discrimination between positions of the cross-cylinder easier for the pt
How is the cross-cylinder used to check the axis?
- cross-cylinder held before eye with handle in line with the axis of the trial cylinder; then turned over, and patient asked which position gives a better result
- cross cylinder is held in preferred position and axis of trial cylinder rotated slightly towards axis of same sign on the cross-cylinder
- process repeated until trial cylinder is in correct axis for the eye, at which time rotation of cross cylinder will offer equally unacceptable visual alterations
How is the cross-cylinder used to check power of the trial cylinder?
cross-cylinder is held first in one axis then the other overlying the trial cylinder - this increases then decresing the power of the trial cylinder
How can a cross cylinder be used to confirm the absence of a cylinder?
- cross-cylinder is offered as an addition to the trial sphere in four different orientations, with its + axis at 90°, 180°, 45°, and 135°.
- If the patient prefers one of these options to the sphere alone, a cylindrical correction is necessary.
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What is lenticular astigmatism?
astigmatism caused by the lens (rather than cornea); keratometry readings may not show significant difference but power cross demonstrates difference in different meridia
How do you calculate lenticular astigmatism?
difference between overall manifest refraction which shows overral amount of astigmatism and keratometry readings (showing any corneal astigmatism)
How are cross-cylinders named?
by the power of the cylinder (marked on the handle)
e.g. in image would be a 1.00 dioptre cross cylinder (-0.5 DS/+1.0 DC)
