bifocals Flashcards
what is presbyopia?
- term given to a normal reduction in accommodation with age
- normal physiological condition
- starts at about 40 years
why does presbyopia happen?
- 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
what are the glasses options for presbyopia?
- 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
what are other options that have been developed if you don’t want single vision lenses?
1- bifocals
2- trifocals
3-progressive lenses
what are bifocals?
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
what are trifocals?
trifocals - focuses at distance, intermediate and near
- modified form of bifocals which allows to focus at three distances
what are progressive lenses?
- 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
when are bifocals dispensed?
- 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
what is the near portion or segment in a bifocal lens?
- 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
what are the advantages of bifocals ?
- 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
what are the disadvantages of bifocals?
- 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 -
how are bifocals described ?
- 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
what are the different bifocal shapes that we can order ?
D seg
R seg
C seg
E seg
what is a D seg?
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
what is a R seg ?
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
what is a C seg?
C segment looks like a D segment , but has a curved top to it
what is a E seg?
- 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
what are the features of D segs?
- 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
what are R seg features ?
cosmetic - good - segment not as noticeable
vision - seg exerts base DN prism - good for positive lens
availability - excellent - available in large and small sizes
what are C seg features ?
- 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
what are executive lenses features?
- cosmetic - poor - straight line across - most noticeable
- vision - good optics, no jump - one of the best
- availability - harder to get hold of
what are the measurements we need to take to order bifocal lenses?
- 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
what do we need to take for horizontal measurements?
- we need to measure the patient’s binocular distance vision PD
- we need to measure patient’s near binocular PD
why we do we need to measure the patient’s binocular distance vision PD?
- we do this because when patient is looking through the distance portion of the bifocal, they will be looking through their distance prescription
what does the combination of having a distance and near PD allow us to calculate?
allows us to measure the geometrical inset of the segment
what is the geometrical inset?
- the distance between the distance OC and the midpoint of the segment diameter ( based on the near OC)
how to calculate how much inset is required for a patient?
- you need to take away your near PD from your distance PD and divide by 2
- ( distance - near ) /2
how to measure near PD?
- make sure text is held where the patient will read
- replace with accommodative target
- measure
why is the segment not directly underneath what we’re looking?
- 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
what does the vertical position of the bifocal segment tell us ?
- vertical position of the bifocal segment tells the lab how high up the bifocals should be set
how are the heights of the bifocal measured?
- 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
how to record segment top position ?
- 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
how to record segment height?
- 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
what does an appropriate segment height mean?
- 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
what happens if height of segment is too high ?
- if too high - segment interrupts distance vision
-
what happens if height of segment is too low?
- if too low - struggle to get near vision
where do we want to place segment ?
- it is best to place the segment at the lower limbus of the pupil
when will we want to place segment higher or lower than the lower limbus?
- 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
why might a measurement be taken incorrectly ?
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
