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Contact Lenses > CL Designs > Flashcards

CL Designs Flashcards

1
Q

Define a spherical contact lens

A

A lens with 1 base curve (fitting curve) on the back and the same power in every meridian

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2
Q

Define a toric contact lens

A

A lens with different powers at two meridians 90º apart. They can have 1 or 2 fitting curves, depending on the design. These lenses will have scribe marks to indicate rotation

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3
Q

What are the three types of contact lens stabilization? Are these techniques used on soft lenses, GP lenses or both?

A
  1. Prism Ballast - soft or GP lenses
  2. Dynamic/Blink Stabilization - soft only
  3. Truncation - soft or GP lenses (soft is very rare)
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4
Q

What would happen to a lens without stabilization

A

The lens would rotate 15-60º which causes discomfort

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5
Q

Explain how a prism ballast creates stabilization. What is a disadvantage to a prism ballast?

A

There is a thickened portion at the bottom on the lens so that the your eye blinks the “thicker” portion will alway be towards the bottom. Plus gravity helps the weighted portion of the lens stay down

Similar to a watermelon seed: if you squeeze a watermelon seed it will always shoot out towards the widest part of the seed

When you lay down the lens will rotate 90º.

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6
Q

Explain how Blink Stabilization works

A

The front surface of the lens has small “speed bumps”. The eye sits within the groove and keeps the lens from rotating

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7
Q

Explain how truncation works

A

The bottom of a round lens is cut off so there is a flat side that rests on the lower lid.

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8
Q

Define a Multifocal Lens. Why are these not popular?

A

A contact lens designed for presbyopia. Dry eye is more common as you age, so patients often do not enjoy wearing CLs

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9
Q

Define Cosmetic Contact Lenses

A

(Arguably all CLs are cosmetic. )

Soft lenses used to change eye colour (opaque and enhancers - for dark and light eyes)

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10
Q

Why are GP lenses not used for cosmetics (changing eye colour)?

A

GP lenses are smaller than the HVID so they are ineffective at changing the eye colour

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11
Q

Define Therapeutic/Bandage Lenses

A

An SCL used for protection - like a bandaid after PRK. It can be used to deliver medications, hide a disfigurement (Coloboma or Aniridia), Myopia control, X-chrome (R/G colour deficiency)

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12
Q

Define a Scleral Lens. Material?

What is another name for this?

A

A lens that covers the cornea an some/most of the sclera. They used to be made of Glass or PMMA, but now they are made of SCL and GP materials

AKA a haptic lens

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13
Q

What is the wear time for Scleral Lenses? What are they used for?

A

Typically 4 hours. Typically used for movie sets, Keratoconus and bandage lenses or severe dry eye

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14
Q

What is the purpose of a lenticular GP?

A

Used to minimize the weight of a high plus Rx. It looks like a fried egg (2+ radii on the front surface)

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15
Q

What is the name of the central area fo a Lenticular Lens? What about the peripheral area?

A
  1. The optic cap

2. The carrier

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16
Q

Define Lenticularization. When is it used?

A

An edge design put on the front surface of a GP or hard lens. typically used on lenses that are more than +/- 2.00 D

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17
Q

What is the typical lenticularization for high plus lenses? Why?

A

High plus lenses are heavy ad want to slide down into the lower fornix.

Typically have a myoflange around the edge of the lens which gives the lid something to grab onto to keep the lens higher

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18
Q

What is the typical lenticularization for high minus lenses? Why?

A

High minus lenses have thick edges which can be uncomfortable.

Typically get a hyperflange (thin edge) so the lid can blink over easier and it is more comfortable. Referred to as a CN bevel

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19
Q

Describe Fenestration. What is it used for?

A

Drilling 1 or more 0.4 mm holes in the periphery of a GP lens. It was used for PMMA lenses to allow more oxygen through the material.

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20
Q

What is a downside to fenestration on a lens?

A
  • It weakens the structural integrity of the lens (easier to chip and warp)
  • The holes can become plugged with protein and can lead to infection
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21
Q

What is PMMA? Who would benefit most from a PMMA lens?

A

Polymethylmethacrylate - A form of Acrylic

  • This is an old material of hard lens, but it is NOT OXYGEN PERMEABLE (dK = 0), but the diameter is small so the edge of the cornea breathes better
  • Lens material with great optics (n=1.49) and holds its shape well
  • Parkinson’s patients as they are less likely to scratch the lens while cleaning
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22
Q

What are the Generations of GP lens materials?

A
  1. CAB (Cellulose Acetate Butyrate)
  2. Sylicone Acrylate
  3. Fluorocarbonate Silicone Acrylate
  4. Fluorocarbons
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23
Q

Define CAB Lenses

A

Cellulose Acetate Butyrate. The first generation of GP lens. It is very scratch resistant.

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24
Q

What are Sylicone Acrylate Lenses.

What are some advantages over CAB lenses?

What are two examples of lenses with dK values?

A

The second generation of GP lenses.

Higher dK values so they are more breathable than CAB lenses

Boston 2 - 12
Boston 4 - 26

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25
Q

What are Fluorocarbonate Silicone Acrylate Lenses.

What is the approximate dK value?

Give an example

A

The third generation of GL lens material.

dK ≃ 70

Boston Equalens

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26
Q

What is Fluorocarbons.

How does this compare to Fluorocarbonate Silicone Acrylate

What is an example

A

The fourth generation of GP lens material

They took out the silicone so it is less breathable than the 3rd generation material.

Quantum 2 - dK is 92, but the range of dK for this material is 30-92

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27
Q

What is the relationship between Silicone and breathability?

A

Silicone is a harder material but it adds oxygen permeability

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28
Q

What is Fluorosilicone Acrylate?

A

The most modern GP lens material. It has the best dK and breathability.

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29
Q

Define dk Value.

A

A value that quantifies the breathability of a contact lens.

30
Q

What is the standard lens for measuring dK value?

A

A -3.00 lens

31
Q

What are the overnight standard dK values for soft and GP lenses? What do they need?

What is this referred to?

Why are they different?

A

GPs dK ≥ 60
SCL dK ≥ 100
BUT THEY NEED FDA APPROVAL REGARDLESS OF DK

Overnight is referred to as continuous wear

These differences have to do with the diameter of the lens, as GP lenses are smaller and more oxygen can get to the periphery of the cornea

32
Q

What is Dr. Fatt’s Principle?

A

There are two pays to double the oxygen transmissibility of a HYDROGEL lens

  1. Reduce the centre thickness by Half
  2. Increase K by 20%
33
Q

What lenses does Dr. Fatt’s principle not work?

A

For Silicone Hydrogel lenses (Group 5)

34
Q

If a patient has dry eyes, what type of water contact do you want for the,?

A

LOW water content (low K)

35
Q

What is the difference between transmissibility and permeability?

A

Permeability is the breathability of a lens material (the d value)
Transmissibility is the amount of oxygen that is actually getting through the lens (Affected by Dr. Fatts’ Principle)

36
Q

What are the generations of Soft Contact Lenses?

A
  1. Hydrogel - (Hydroxyethylmethacrylate (HEMA))
  2. Glycerol Methacrylate (GMA)
  3. Silicone Hydrogel (SiHy)
37
Q

Describe HEMA lenses

A
  • They are a soft CL material that is “semi scleral” (covers the cornea and some of the sclera. They are known for their initial comfort (a hydrophilic material)
38
Q

What is the diameter of fit for SCL and GP lenses?

A

SCL: HVID + 2 ≃ 14 (on average)
GP: HVID - 2 ≃ 9 (on average)

39
Q

Describe GMA lenses

A

Another soft CL material, but not used often because it is more expensive). It has a smaller pore structure than HEMA so there is a lesser chance of proteins getting stuck on Helens surface

40
Q

Describe Silicone Hydrogel Lenses

A

A hybrid of Hydrogel plus Silicone.

  • It has the highest dK values so it breathes well
  • ** Does not rely on centre thickness or Water content (so a +6.00 has the same dK as -3.00)
  • Silicone is hydrophobic so the lens does not dry out easily and can be worn longer
  • Does not attract proteins, so it stays cleaner longer
  • DOES attract lipids
41
Q

What classifies low vs high water lenses? What are their benefits?

A

High water: >50% - Dries out, but has higher dK/wear time

Low water: < 50% - No dry out

42
Q

What classifies Non-ionic vs Ionic lenses? What are their benefits?

A

Non-Ionic - Neutral surface so no static charge (does not attract proteins)

Ionic - Surface is charged (attracts proteins) - bad for allergies, flu, central cycle, systemic diseases

43
Q

Describe FDA Group 1 lenses

A

Low Water, Non-Ionic

  • Good for dry eyes (does not dry out)
  • Low dK ∴ shorter wearing time (8-10 hours)
  • Does not attract proteins
44
Q

Describe FDA Group 2 lenses

A

High Water, Non-Ionic

  • Not good for dry eyes (they dry out)
  • Higher dK ∴ longer wear time (12-14 hours)
  • Does not attract proteins
45
Q

Describe FDA Group 3 lenses

A

Low Water, Ionic

  • Good for dry eyes (does not dry out)
  • Low dK ∴ shorter wearing time (8-10 hours)
  • DOES attract proteins
46
Q

Describe FDA Group 4 lenses

A

High Water, Ionic

  • Not good for dry eyes (they dry out)
  • Higher dK ∴ longer wear time (12-14 hours)
  • DOES attract proteins
47
Q

Describe FDA Group 5 lenses

A

Silicone Hydrogel

  • Low water, so they do not dry out easily
  • Highest dK ∴ longer wear time or continuous wear
  • Stiffer lens
  • Does not attract proteins, but DOES attract lipids
  • Dr. Fatt’s rule does not apply
48
Q

Describe Conventional Lenses

A

1 pair of non-disposable lenses that last 1.5 years. They can be daily wear (Low water ∴ 10 hours) or flexiwear (High water ∴ 12-15 hours)

49
Q

What is the benefit to conventional lenses?

A

Economical ($90-$180)

50
Q

What is the disadvantage to conventional lenses?

A

Higher risk of infection. The lenses are being worn and handled for a longer period of time before being disposed of

51
Q

Describe Frequent Replacement Lenses. What are some examples?

A

Lenses designed to be work for a certain longer amount of time.

  • Semi-anually (every 6 months)
  • Quarterly (every 3 months)
52
Q

Describe Disposable Lenses. What are some examples?

A

Lenses designed to be replaced after a short amount of time

  • Monthly
  • Bi-weekly
  • Daily
53
Q

What is the wear time for Dailywear lenses?

A

Typically 10 hours. These lenses are low water (good for dry eyes), low dK (low oxygen transmissibility)

54
Q

What is the wear time for Flexiwear?

A 
Typically 12-15 hours
High water (does not dry out) and high dK (good transmissibility)
55
Q

What are the 3 Manufacturing methods? What types of lenses are each of them used on?

A
  1. Lathe Cut (GP and SCL)
  2. Injection Molding (SCL only)
  3. Spin Casting (SCL only)
56
Q

Describe Lathe Cut Manufacturing

A

The most accurate method of making both SCL and GPs. It is similar to making candlesticks by carving/grooving the curves into the lens

57
Q

What are some advantages and disadvantages to Lathe Cut Manufacturing?

A

Advantages

  • Most accurate method
  • Best for custom parameters (Diameter, BC, Rx, Optic Zone, peripheral curves)

Disadvantages

  • Slow (typically a few days)
  • Expensive
  • Hard to generate large batches
58
Q

Describe Injection Molding

A

A Liquid polymer is poured into a mold and a second mold squeezes towards the first (but never touch) to form both the front and back curves to create SCLs. Then UV/heat is applied to solidify the polymer.

Most common method

59
Q

What are some advantages and disadvantages to Injection Molding?

A

Advantages

  • The molds never touch, so they do not have wear and tear
  • Fast
  • Mass production
  • Gives consistently and accurate results

Disadvantages
- Not as easy create custom results

60
Q

Describe Spin Casting

A

A liquid polymer is placed in a mold that is spinning. Depending on the RPM the liquid polymer is displaced differently to create different Rx types (SCL). Then UV/heat is applied to solidify the polymer.

61
Q

What are some advantages and disadvantages to Spin Casting?

A

Advantages
- Very fast

Disadvantages

  • Prescirpiton Limitations (high cyl, presbyopia)
  • Not as consistent or accurate
62
Q

What is the tear exchange rate for SCLs and GPs?

A

SCL 2%

GP 20%

63
Q

What is the purpose of peripheral curves (GP)?

A

Provide edge lift and tear exchange. Typically 0.3 mm wide

64
Q

What is the purpose of secondary curves (GP)?

A

Provides stability not he cornea ∴ comfort. It works like an anchor to keep the lens on the cornea. Typically 0.3 mm wide)

65
Q

Describe the GP base curve

A

The section of the GP lens that sits on the Cornea, this is also the section that houses the optic zone (on the front)

The size is the is the remaining diameter after you subtract the peripheral and secondary curves

66
Q

Describe the function zones.

What are the three types of blend and what do they mean?

A

The place where the curves meet. They are blended to avoid arcuate abrasions

The blend can be heavy, medium or light. A heavy blend will result in more movement

67
Q

Example: The Diameter of a GP lens is 10.2, what is the size of the optic zone

A

Optic Zone = Total Diameter - Diameter of secondary curves - diameter of peripheral curves
OZ = 10.2 - (2 ・0.3) - (2・0.3)
OZ = 9.0 (you subtract the diameter off BOTH SIDES

68
Q

Name the 5 advantages of SCL over GP (4 main benefits)

A
  • Initial comfort
  • Convenient + spare lenses easily available
  • Less expensive
  • Usually have stock lenses
  • Less skill needed to fit/dispense**
  • Less chair time needed to fit**
  • ***Better for part time wear
  • ***Cosmetic Lenses
  • ***Bandage/Therapy lens
  • ***Good for contact sports (large diameter and don’t fall out easily)
  • Less likely to scratch eye on insertion/removal
  • Less likely to get debris behind lens
  • No corneal Molding
  • Easier to learn
69
Q

Name 5 advantages of GP over SCL

A
  • Sharper optics (no drying out, lenses keep shape)
  • Custom Rx and fit parameters
  • Easier to keep clean (less chance of infection
  • Best option for Dry eye patients (lenses do not dry out)
  • Easier to insert, remove and handle (eventually)
  • Higher tear exchange ∴ more oxygen to the eye (higher dK)
  • Long lifespan (5-7 years) with modifiable parameters (but they cannot change the base curve or make the lens larger)
  • Can remove scratches
  • Can correct/mask corneal astigmatism
  • Durable lens (no ripping or tearing)
  • Great long term comfort because the lens does not contact the corneal tissue
70
Q

What is an X-chrome lens?

A

A red coloured hard or soft lens that transmits light in the red zone from 590 ti 700 nm. It is intended to improve colour discrimination for the colour blind

71
Q

How do you use an X-Chrome lens?

A

Place the lens on the non-dominant eye!

Contact Lenses (8 decks)

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