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OV2021 dispensing > high prescription dispensing > Flashcards

high prescription dispensing Flashcards

1
Q

what is a high prescription ?

A
  1. British standards states that BVD has to be part of the prescription when Rx>+/-5.00D (BS 2738-3)
  2. high index lenses often suggested for Rx>+/-4.00D
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2
Q

what do should lenses provide ?

A

. should provide good vision ( FOV, aberrations, Rx)

. should be cosmetically acceptable to the patient

. should be comfortable

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3
Q

what does a higher index mean ?

A

. higher index=worse vision

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4
Q

what is the effect of lens thickness on lens measurements?

A

. if lens is thick , we need to increase the BVD so the lens isn’t touching the patients lashes when they are blinking

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5
Q

what does providing a lens that has good vision , cosmetically acceptable and comfortable depend on ?

A

to satisfy the lenses providing good vision , being cosmetically acceptable and being comfortable depends on

  • form of the lens
  • material of the lens
  • surface treatments of the lens
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6
Q

what does the form of the lens tell us ?

A
  • tells us the curves that we’re using on F1 and F2 to prescribe the prescription and we know that these can influence the overall thickness and vision that the patient gets
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7
Q

what does the material of the lens depend on ?

A
  • the material of the lens we select will influence the thickness, the minimum centre and edge thickness and the weight or specific gravity
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8
Q

what does the surface treatment of the lens affect ?

A
  • the surface treatment of the lens can affect the vision and the durability of the lens
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9
Q

where is vision best ?

A
  • vision is best at the optical centre and along the optical axis
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10
Q

why has best form lenses been developed ?

A
  • we know the eye doesn’t stay at the optical axis where vision is best and we want the vision off-axis to be as close as possible to optical axis
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11
Q

how can the form of the lens influence vision ?

A
  • curves could aim to remove a particular aberration - aspherics
  • lens should be kept as close as possible to existing lens form
  • minus base toric for minus lenses, plus base toric for plus lenses
  • take into account thick lens equation and BVD
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12
Q

how can the form of the lens influence cosmesis ?

A
  • steeper curve- results in a thicker lens overall
  • form of the lens can alter the minimum centre thickness
    ( thicker centre = thicker lens )
  • aspherics ( flatter and thinner)
  • appearance through the lens
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13
Q

how can the form of the lens influence comfort ?

A
  • larger base curve more difficult to glaze and require a curved frame
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14
Q

how does the material of the lens impact vision ?

A
  • v value of lens material

- can influence lens curves and therefore reflection

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15
Q

how does the material of the lens impact cosmesis ?

A
  • reflections

- lens thickness- reduced with higher index lenses - increases reflections and aberration

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16
Q

how does the material of the lens impact comfort ?

A
  • specific gravity
  • thickness- means a heavier lens, difficult to adjust the frame
  • can calculate the overall thickness of lens by using sag equation
  • safety
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17
Q

how do surface treatment of the lens affect vision ?

A
  • vision can be improved with treatment such as antireflection coating ( MAR)
18
Q

how do surface treatment of the lens affect cosmesis ?

A
  • cosmesis can be altered depending on MAR
  • tints/mirror coating
  • by blocking certain wavelength can improve cosmesis
19
Q

what to consider with mid to high myopes ?

A
  • considerations such as material and an appropriate frame choice can make difference to dispense
  • high refractive index - can reduce the thickness
  • plastic frame - overall weight of the lens will be less
  • decentration of a lens within the frame can result in uneven thickness and a greater thickness on at least one side usually temporal side
20
Q

what to consider with very high minus ( -14.00 and above ) ?

A
  • use a reduced aperture lens
  • available as lenticular or blended lenticular ( only a small section of the lens which is used for seeing and contains optimum prescription )
21
Q

what are lenticular or blended lenticular ?

A
  • only a small section of the lens which is used for seeing
  • eliminate edge thickness
  • easier to glaze
  • manufactured by solid design, bonded or hand edged
22
Q

how does a solid lenticular design look like ?

A
  • will have a ridge at the back between the aperture( bit of lens patient is looking through)
    and the carrier ( vision is poor in the carrier )
23
Q

what does the size of the aperture depend on in lenticular ?

A
  • the size of the aperture is dependent on the lens which is chosen
  • it differs between lenses and manufactures
  • flattened full aperture lenses - 30 mm or 34 mm
24
Q

what are the different apertures for high minus lenses ?

A
  • oval aperture- can be created by using a toroidal rather than spherical surfacing tool to flatten the concave surface
    . can look better than round
  • profile aperture- can look better with unusual frame
    . aperture goes round about 3-4mm from frame edge so carrier is closer to frame edge
    . eyes should sit directly within centre of frame
  • round aperture -
25
Q

what are blended lenticular ?

A

blended - aperture curve is blended with the marginal curve with a convex curve, can be using freeform technology in plastic

  • cosmesis - improved
  • vision - blending of the margin is highly astigmatic , causes a reduction of filed of view
  • minus lenticular lenses are available from a range of manufacturers including Norval
  • minus lenticular lens are available in range of materials
  • minus lenticular lens are available in transition and multifocal design
26
Q

what happens in high positive lenses ?

A
  • a prescription in the aperture of the lens is highly positively and there is a margin where this lens transitions into the carrier lens which can contain some prescription or afocal area
27
Q

what to consider with high positive lenses ?

A

. poor field of view leads to a ring scotoma and a jack in the box affect at the margin

. reduced aperture lenses are available

. available as button, blended or polynomial

. available as bifocals and progressives

. should be UV400 coated

. consider frame fit and size

28
Q

what are button lenticular lenses ?

A

. traditional high positive lenticular lenses
. small positive lens stuck on to a carrier lens
- available in sizes where aperture is 30-40mm and can get them in a round bifocal segment

29
Q

what is zonal aspherics blended margins ?

A

. high positive lenticular lens

. better cosmetically but poor vision through area of blending

30
Q

what is polynomial blending ?

A

. high positive lenticular lens

. better cosmetically and better vision - no aberrational astigmatism

  • eliminates roving ring scotoma and jack in box effect
  • less head movement
31
Q

how to alter Rx for BVD ?

A

use step along

F^2/1000

32
Q

what do we need to take into account with near vision use ?

A
  • need to take into account a near vision effectivity error
  • need to calculate at near as light not from infinity
  • need to take into account the different performance from the thick trial lens form to a thick spectacle lens
  • results in lenses being
    under plussed
    . NVEE= (t/n)L1(L1+2F1)
33
Q

which patients have a larger field of view ?

A

myopic px - larger field of view

34
Q

how is field of view calculated ?

A
  • can be calculated by
    . tan θ = y(37-F)/1000
  • where θ is half the field of view
  • effected by Rx- myopes have an increased FOV, hypermetropes smaller
    -effected by radius of curvature - steeper curves decrease the FOV
  • vertex distance - decreased increases the FOV
35
Q

how to calculate spectacle magnification ?

A

SM= (1/1-dF’v)) x (1/(1-t/nF1))

36
Q

how to reduce spectacle magnification ?

A
  • thinner lens and a flatter front curve will reduce spectacle magnification
37
Q

what to consider when fitting high prescription lenses ?

A
  • consider BVD/pantoscopic tilt
  • consider decentration ( reduce decentration )
  • monocular PDs and heights
38
Q

what is the best way to fit a lenticular or high prescription ?

A
  • get the patient to pick the frame first and adjust the frame and set the trial frame up to the chosen frame characteristics
  • adjust frame to match trial frame
39
Q

how do frames affect vision ?

A
  • vision can be affected by frame regards to field of view
40
Q

how do frames affect cosmesis ?

A
  • decentration
  • power rings
  • minimum blank size
41
Q

how do frames affect comfort ?

A
  • weight of frame
  • frame should be symmetrical
  • frame material
  • size of bearing surface

OV2021 dispensing (7 decks)

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