complex bifocals

Question 1

why do we need complex bifocals?

Answer 1

• when the prescription is the same in each eye, when a patient looks away from the optical centre we know they will be experiencing prism
• but because they are experiencing the same amount of prism in the same direction in each eye , it doesn’t create a problem
• however when a patient has anisometropia, when the patient looks away from the optical centre , they will experience a different amount of prism

Question 2

what is anisometropia?

Answer 2

• unequal refractive power between the eyes

Question 3

what does anisometropia result in?

Answer 3

• results in asthenopia and diplopia due to differential prism and aniseikonia

Question 4

what is differential prismatic effect?

Answer 4

• different amounts of prism experienced by each eye when looking away from the OC, tolerance- only 2 prism dioptres vertically, more horizontally

Question 5

how much does a px look below when they are reading?

Answer 5

• they look 8-10mm below to read

Question 6

which patient can get used to differential prismatic effect ?

Answer 6

• patients that had anisometropia from a young age and worn there glasses and have a reasonable VA

Question 7

which patinets can’t get used to differential prismatic effect ?

Answer 7

• when anisometropia happened suddenly due to pathology or not doing a cataract surgery

Question 8

what is an aniseikonia ?

Answer 8

• aniseikonia - a difference in the retinal image size between the eyes resulting from the difference in prescription
• makes it difficult for the brain to fuse two images into one image because the image is falling into different retinal locations within each eye
• difference in image sizes is 1% of every dioptres anisometropia
• we can only tolerate 1%
• 1 dioptre= 1% difference in retinal image size

Question 9

what can anisokonia cause?

Answer 9

• it can cause distortion , diplopia and can alter patient depth perception

Question 10

what can we do to solve anisokonia ?

Answer 10

• we need to equal out the image size

- we can either make the larger image smaller or smaller image larger

Question 11

how to make smaller image larger - to increase spectacle magnification?

Answer 11

-use seikonic/size lenses to increase magnification

Question 12

what can we use to determine the prismatic effect?

Answer 12

• we can determine the prismatic effect at a point using Prentice’s rule
• P=CF

Question 13

when is the near vision point ?

Answer 13

• near vision point: 8-10 mm down and 2mm in

Question 14

what are the bifocal solutions?

Answer 14

• slab-off lenses/bi-prism
• removes unwanted base down prism from the bottom of the back of the lens
• this results in a line across the lens so an E or D seg with the seg top coinciding with the line looks best

Question 15

why is it not a good idea to slab off a varifocal ?

Answer 15

• we can also slab off varifocal but not good from a cosmetic view
• good to add an anti-reflection coating which reduces the appearance of the line

Question 16

what is a slab on ?

Answer 16

by adding prism to the bottom of the lens

Question 17

what is the minimum prism we can remove in traditional slab off lenses?

Answer 17

• minimum of 2 prism dioptres base down unless freeform

Question 18

which lens do we usually slab off?

Answer 18

• slab off the most negative

Question 19

what is another bifocal solution ?

Answer 19

• franklin split
  . DV and NV portions are completely separate lenses
  . the OCs can be placed where required and prism can be surfaced into either portion to eliminate unwanted prismatic effect
  . not cosmetically good

Question 20

what is a prism controlled bifocal?

Answer 20

• this is another bifocal solution
• usually available in glass only , with visible round 30mm bifocal segment.
• up to 6 △ of extra prism is worked in any direction in the segment only
• always worked into back surface of lens so not visible from front

Question 21

how to work out the base direction of what’s been put into the segment in prism controlled bifocal ?

Answer 21

• when looking at a prism control bifocal, if you want to work out the base direction of what’s been put into the segment.
• its the place where there is least differentiation between the edge of the segment and the lens
• the prism has been dropped in and base direction is thickest point of the prism

Question 22

how to use different size round segment as a solution for patients who are struggling with differential prismatic effect/

Answer 22

• the largest segment is dispensed to the most positive eye ( it induces more base down prism in the segment for reading, to eliminate the base up of the distance portion)

Question 23

how to work out difference in segment sizes ?

Answer 23

20 x differential △ / add

Question 24

what is a fresnel prism ?

Answer 24

• another bifocal solution
• normally used a temporary measure
• will reduce the patient’s VA by approx 2 lines
• increases chromatic dispersion
• may be used for prism in any direction and available from
  1 △ to 30 △
• frensel is a thin sheet of plastic which we can stick onto back surface of lens with water backing
• fresenl is made of lot prisms- thicker end of ridges represents the base direction

Question 25

how to decide which solution is best for patient ?

Answer 25

• cosmesis
• cost
• suitable for prescription
• consider vision