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DISPENSING > bifocals > Flashcards

bifocals Flashcards

1
Q

what is presbyopia?

A
  • term given to a normal reduction in accommodation with age
  • normal physiological condition
  • starts at about 40 years
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2
Q

why does presbyopia happen?

A
  • it happens due to the thickening of lens with age , and as lens continues to lay down layers this means that the lens becomes stiffer and harder to change shape
  • when the lens becomes stiffer there is a reduction in the ability to change shape - which means patient’s loose the ability to see closer objects and therefore require some form of correction to help them see it near
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3
Q

what are the glasses options for presbyopia?

A
  • single vision lenses
  • single vision lenses allow a patient to see the prescription through the whole lens but only allowed to see one distance at a time

. full lens dedicated to prescription
. two separate pair
. not very useful for people who multitask
. when you have reading prescription on you won’t be able to see anything in the distance

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4
Q

what are other options that have been developed if you don’t want single vision lenses?

A

1- bifocals
2- trifocals
3-progressive lenses

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5
Q

what are bifocals?

A

bifocals - focuses at distance and near

  • use distance and near prescription
  • much more convenient for patient’s allowing them to watch tv and read at same time
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6
Q

what are trifocals?

A

trifocals - focuses at distance, intermediate and near

- modified form of bifocals which allows to focus at three distances

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7
Q

what are progressive lenses?

A
  • progressive lenses
  • these lenses contain prescriptions catering from distance all the way to near vision
  • but the size of area that you have to look through for each prescription varies on the type of lens given
  • they have some form of peripheral distortion - so the patient has to look through specific areas of lens to get clear vision

e.g.
. varifocals
. enhanced readers/occupational lenses

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8
Q

when are bifocals dispensed?

A
  • bifocals are dispensed when there are problems with accommodation
  • usually due to presbyopia
  • can also be due to other problems with accommodation such as in children or if there is an issue with the lens
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9
Q

what is the near portion or segment in a bifocal lens?

A
  • near portion ( segment) contain the near vision prescription - which is a combination of the distance prescription and the addition that has been provided to help the patient see up close
  • the addition is always a positive value - generally same for left and right eye
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10
Q

what are the advantages of bifocals ?

A
  • bifocals allow clear vision at any 2 distances
  • wide reading area- can range in size - so can order a wider or smaller segment
  • more convenient than 2 separate pairs
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11
Q

what are the disadvantages of bifocals?

A
  • visible line between distance vision and reading area, so indicative of patient’s age
  • smaller seg diameters have limited field of view compared with single vision
  • patients over 55yrs will start to require intermediate correction on top of distance and near
    e.g. computer screen
    may need to recommend trifocal or progressive lens
  • prismatic effect- induced by segment
  • condition called jump -
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12
Q

how are bifocals described ?

A
  • bifocals are described by their segment shape and size

-e.g. D28
D= shape
28 = size - means at its widest point the segment will measure 28mm across

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13
Q

what are the different bifocal shapes that we can order ?

A

D seg

R seg

C seg

E seg

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14
Q

what is a D seg?

A

D seg - has a straight top to the segment - so looks like capital letter D on its side
- the value of the number written next to the D refers to the width of the shape at its widest point

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15
Q

what is a R seg ?

A

R seg- stands for round segment

  • the dividing line of this segment is single circle arc
  • sometimes the whole of the segment will fit into a lens, in which case it looks like a full circle
  • depending on how high up with in the lens we place the segment , you may only see part of circle
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16
Q

what is a C seg?

A

C segment looks like a D segment , but has a curved top to it

17
Q

what is a E seg?

A
  • E segment stands for executive segment
  • the segment dividing line crosses the whole lens
  • for an executive segment you will only write an E seg- you wouldn’t write a size next to the E as the size will go through the whole lens
18
Q

what are the features of D segs?

A
  • very commonly prescribed lens
  • from a cosmetic view point - its reasonable - straight line is noticeable though
  • vision - seg exerts base up prism - this is good for minus lenses - this is because as you look down in a minus lens, you naturally experience some base down prism, so the segment exerting some base up can be a good thing as it reduces the amount of prism the patient experiences
  • availability - excellent - available in large and small sizes
19
Q

what are R seg features ?

A

cosmetic - good - segment not as noticeable

vision - seg exerts base DN prism - good for positive lens

availability - excellent - available in large and small sizes

20
Q

what are C seg features ?

A
  • cosmetic - better than D segs - curved top not as noticeable
  • vision - seg exerts base UP prism - good for minus lenses
  • availability - reasonable selection of sized
21
Q

what are executive lenses features?

A
  • cosmetic - poor - straight line across - most noticeable
  • vision - good optics, no jump - one of the best
  • availability - harder to get hold of
22
Q

what are the measurements we need to take to order bifocal lenses?

A
  • need to measure the patient so that the bifocal segment sits in a comfortable place for reading

. need to record

  • distance binocular PD
  • near binocular PD
  • heights
23
Q

what do we need to take for horizontal measurements?

A
  • we need to measure the patient’s binocular distance vision PD
  • we need to measure patient’s near binocular PD
24
Q

why we do we need to measure the patient’s binocular distance vision PD?

A
  • we do this because when patient is looking through the distance portion of the bifocal, they will be looking through their distance prescription
25
Q

what does the combination of having a distance and near PD allow us to calculate?

A

allows us to measure the geometrical inset of the segment

26
Q

what is the geometrical inset?

A
  • the distance between the distance OC and the midpoint of the segment diameter ( based on the near OC)
27
Q

how to calculate how much inset is required for a patient?

A
  • you need to take away your near PD from your distance PD and divide by 2
  • ( distance - near ) /2
28
Q

how to measure near PD?

A
  • make sure text is held where the patient will read
  • replace with accommodative target
  • measure
29
Q

why is the segment not directly underneath what we’re looking?

A
  • this means that segment isn’t directly underneath where they’re looking- this is because when we are looking at something at near, our eyes converge , so by moving segment inwards, the patient has the ability to access the segment easily and to use as much of it as possible
30
Q

what does the vertical position of the bifocal segment tell us ?

A
  • vertical position of the bifocal segment tells the lab how high up the bifocals should be set
31
Q

how are the heights of the bifocal measured?

A
  • can be measured from HCL or the bottom rim
  • glasses need to adjusted so that they are sitting well before you start
  • measure onto the lens that the patient is going to wear
  • take into account previous wear
32
Q

how to record segment top position ?

A
  • how far segment top is from HCL
    e.g. 4 below HCL
    tells us that the top of segment needs to be placed 4mm below the HCL of frame
  • usually require this measurement
33
Q

how to record segment height?

A
  • this measurement is referring to where the top of the bifocal will sit in reference to the lowest tangent of lens
    e. g. 24 mm from lowest tangent
  • lowest tangent can mean different things to different people
  • can mean the point at which the lens meets the frame
  • can mean the bottom of the frame it self so including the depth of frame going around lens
  • can measured directly below where the distance optical centre would be
  • can be measured from lowest point of frame
  • due to this confusion you will be required to use segment top position
34
Q

what does an appropriate segment height mean?

A
  • means that the patient will easily be able to access the segment itself - but its position will not affect the when they’re looking straight ahead
35
Q

what happens if height of segment is too high ?

A
  • if too high - segment interrupts distance vision

-

36
Q

what happens if height of segment is too low?

A
  • if too low - struggle to get near vision
37
Q

where do we want to place segment ?

A
  • it is best to place the segment at the lower limbus of the pupil
38
Q

when will we want to place segment higher or lower than the lower limbus?

A
  • if the patient is used to wearing a segment on the low side than we can take into account when measuring the next pair
  • might want to take segment higher if the patient does a lot of near vision work and is generally slight down
  • might want to take segment slightly lower if the patient is rarely going to use near vision portion and is more concerned with the distance
39
Q

why might a measurement be taken incorrectly ?

A

1- when frame is sitting low on patient’s face - this means that the patient is going to have very limited distance vision through this lens and bifocal is going to be very high up
- when patient wears the frame at right position - you will find that the height measurement which you previously took would be too high and you will need to remeasure

2- when patient’s eyelid is drooping - make sure you are measuring at the point where the iris meets the sclera

DISPENSING (19 decks)

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