optics of CLs Flashcards
what is subjective refraction?
. spectacle prescription in front of the ocular surface ( spec RX)
. BVD=12mm
what is spectacle RX?
. spectacle prescription in front of the ocular surface ( spec RX )
why do we need to convert spectacle RX into ocular RX?
. when we give px cls, we will get rid of vertex distance and we will push the prescription all the way onto the cornea
. therefore we need to convert spectacle RX into ocular RX, which takes into account the disappearance of the 12 mm vertex distance
how do we convert spec RX to ocular RX?
by using the back vertex distance table
why is there a difference between spec RX and ocular Rx?
the difference is due to the back vertex distance
why does the back vertex distance table start at 4?
. anything below a prescription of 4 will have the same spec Rx and ocular Rx
what to do when px is astigmatic ?
need to correct spec Rx to ocular Rx in both meridians
what rules to follow when prescribing cls?
. usually prescribe the least negative ( round down )
. round sph down
and round the cyl up
. you can round both sph and cyl down and make sure you do an over refraction on top of the cls
where is refractive astigmatism located ?
. located in the cornea or the crystalline lens
. refractive astigmatism=corneal + lenticular
how do we get refractive astigmatism ?
. from spectacle Rx
where do we get corneal astigmatism from ?
. from keratometry or topography
how do we measure the curvature of the cornea ?
. keratomerty - measures the curvature along the two principle meridians of the cornea over three to four mm
. topography
what is topography ?
. topography is used to identify the full shape of the cornea by measuring thousands of points along a nine mm diameter
. gives more information than keratomerty
. shows the power of the cornea along the principal meridians
why is a topographer more useful than a keratometer ?
. in keratometry we only get information about the principal meridian centrally, where as in a topographer we get information about the principle meridian both centrally and in the periphery
how is the radius of the cornea calculated?
. the radius of the cornea is calculated in mm
. if you put a circle along the curvature that follows the curvature of the cornea you can measure the radius by going from the centre of the circle all the way to the side
. the distance between the centre of the circle to the side is measured- vertical measurement
. there is a difference between the 2 principle meridians
. when the radius is bigger that means that the circle is bigger and that means that the curvature is flatter
how do we calculate corneal astigmatism ?
. rule of thumb: 0.05 difference = 0.25D corneal astigmatism
. 0.05= smallest available unit radius
. 025D= smallest unit power
how do we calculate lenticular astigmatism ?
lenticular astigmatism = refractive astigmatism - corneal astigmatism
what is WTR ( with the rule astigmatism )?
. the vertical meridian is steeper, so the radius is smaller
what is ATR ( against the rule astigmatism )?
. the vertical meridian is flatter, so the radius is bigger
what does it mean that if all the astigmatism the patient needs is located in the cornea?
. it means that the patient is an excellent candidate for rigid corneal lens
. this is because a rigid corneal lens doesn’t change shape when we put it on the patient’s eye
what is the problem with a soft toric lens ?
. lenses don’t stabilise very much
. every time you blink the cl will move
. rotation in contact lens when you blink- due to the axis changing
-you get poorer vision than in specs
how does a rigid lens correct astigmatism ?
. the tears will fill the gap between the cornea and contact lens
. RGP CL neutralises almost 90% of corneal astigmatism - so dont have to give cyl- so can just give them the sphere -
. doesn’t neutralise the lenticular astigmatism
-if patient wants toric lens- then you would have to prescribe the whole prescription for e.g -2.00/-2.00x180 whereas just RGP needs -2.00 .
what happens to corneal power when you spherical rigid corneal lens ?
. instead of calculating the air/tear interface, we now calculate the astigmatism in the tear/cornea interface
. when spherical RGP lens is placed on the cornea, tears replace air ( and cornea replaces tears )
. (n) for air is replaced by (n) for cornea
. corneal astigmatism is reduced when a spherical RGP is placed on the cornea
what does the choice for cl depend on ?
. the choice for cls depends on the amount of astigmatism there is
when can you order a spherical RGP?
if corneal astigmatism is anything between 0 and 2.50D
you can order spherical RGP lens
-IF TOO Much difference in both principal meridians then the lens will start rocking on the eye- not good- fit good in one merdian but not in the other- lots of tears which will need to fit gap- distance too big- uncomfortable - px will blink lens out
what to order if corneal astigmatism is more than 2.50DC?
. a toric lens must be ordered
.back surface toric RGP will be ordered if corneal astigmatism is more than 2.50D
what is a back surface toric RGP?
. you will order the curvature of the lens to the same curvature as you found in keratomerty ot topography
. back surface of lens is ordered to the two principle meridians that you have measured
-corrects the back of CL - cornea and tears side
what happens when you have remaining lenticular astigmatism ?
. you order a front surface toric lens for any remaining lenticular astigmatism
. lenticular astigmatism is corrected on the outside of lens- side of the air and tears
what is induced astigmatism ?
. happens when you are having a back surface toric because there is a difference in the refractive index of the lens and tears which can cause induced astigmatism
what is a bi toric lens?
. when you order back and front surface toric
what to order with SCL?
for any astigmatism you need to order a toric CL
when are SCL not manufactured ?
. SCL are not manufactured unless they have a CYL of 0.75D or more
what happens if CYL is less than 0.75D?
. you order a mean spherical equivalent , so you add half of the cyl to the sphere
what is problems with SCL?
. need to be stabilised to avoid rotation , they move
what to do if corneal and lenticular astigmatism cancel each other out ?
. spherical SCL is best solution
. if you are putting a RGP lens , the lenticular astigmatism will be uncorrected
- so if you read the keratometry readings of a px with no astigmatism (no cyl) and they have keratometry reading with a difference therefore if you prescribe them RGP lenses the LA will remain uncorrected therefore better to give Spherical SCL
what do large oblique cyl do when changing to CLs?
. the patient might experience distortion if they have spectacle rx with astigmatism
. large oblique cylinders may produce some spatial distortion when changing to CLs
. when you wear a spectacle Rx with oblique cyl and you look at an object, the retinal image is slightly tilted , however when you wear a spectacle for a while your perception is that the object is straight
. when you fit them with cls, the retinal image will be straight and the perception is of object is tilted
what is corrected retinal image ?
when a spectacle or cls are worn this is referred to as corrected retinal image
how do cls affect visual acuity ?
. myopes get more magnification with cls compared to spectacles , improved VA
. hyperopes get less magnification with cls compared to spectacle, reduced VA
. this is because in cls we decrease the BVD, myopes receive more magnification
what is the prismatic effect when wearing cls?
. myopes use more convergence when wearing cls compared to wearing glasses
. when myopes wear spectacles they receive base in at near
. when myopes wear cls, cls move with the eye, so when eyes turns in, the base in is lost and no decentration
. myopic eye needs to accommodate and coverage more with cls
. the opposite for hyperopes, they might like reading with glasses
what is the effect of accommodation when wearing cls?
. myopic eye needs to use more accommodative efforts when wearing cls
. in hyperopes : delay presbyopia when wearing CLS
what is anisometropia ?
. there is a significant difference in the spherical refractive errors between2 eyes
what is the effect of relative spectacle magnification when wearing cls?
- refractive anisometropia: good for cl correction
- this is because you get similar retinal images - axial anisometropia: this means one eye is longer than other
- larger retinal image, cls not good choice
what is the effect of field of view when wearing cls?
. larger FoV with CLs . loss of spectacle frame . less blind spot . especially appreciated by high positive Rx . more natural view with CLs
why is there a big advantage of spectacle wear over contact lenses when it includes prismatic prescription ?
- in spectacles you have option to divide prism between eyes, you can go vertical or horizontal
. you can use fresnel prisms for high values - in contact lenses
- normally only base down prism fitting
- maximum is base down
- vertical prism cannot be divided
- horizontal and base up prisms only in scleral lenses
