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Contact Lens year 1 > unit 5 > Flashcards

unit 5 Flashcards

1
Q

◦ Optical parameters

A

Power
Optical quality
Scattering

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2
Q

Material properties

A

Water content
Modulus
Wettability
Surface friction
Oxygen permeability
Refractive index
Spectral transmission

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3
Q

Lens geometry

A

Diameter
Sagittal depth
Thickness
Radius of curvature
Edge shape

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4
Q

Optical Properties
* Power

A

◦ ii) Lensometer/focimeter
Measure back vertex power (BVP)
Careful prep is required when measuring in air
–Clean, dab dry
State-of-the-art instruments immerse the lens in a saline-filled container

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5
Q
  • Factors that compromise optical quality are:
A

◦ Inhomogeneous material
◦ Surface scratches/deposits
◦ Lathe marks (from manufacture)
◦ Material inclusions
◦ Air bubbles

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6
Q
  • Equipment
A

◦ Slit lamp
On patient’s eye
Holding lens
◦ Contact lens loupe

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7
Q

Material Properties

A

Properties can vary widely between soft lens materials
◦ Most of the properties are linked to water content
◦ Need to be measured individually for each material

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8
Q

Water Content

A
  • Gravimetric method is an accurate technique
  • Mass of water in hydrated contact lens
  • Compare to mass of the hydrated material
    -Water content = (mhydrated - mdry) / mhydrated x 100
    -m= mass
  • Requires specialized lab equipment
    ◦ Sample lenses are dried in an oven and the weight measured using an analytical balance
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9
Q

Modulus

A

Stiffness of material
◦ “Young’s modulus”
* Softer lens is more comfortable
* Stiffer lens easier to handle

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10
Q

Wettability

A
  • Surface wettability is an important factor affecting comfort
  • Conventional hydrogels
    ◦ Wettability increases as amount of water increases
  • Silicone hydrogels
    ◦ Hydrophobic nature
    Need a surface coating
    Wetting agents added to the material or packaging solution
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11
Q

Surface Friction

A
  • Impacts comfort of the lens when blinking
    ◦ Friction between the contact lens, cornea, and lid
  • ‘Friction angle’ method is the simplest way to measure friction
    ◦ Sample material is placed on a flat plate and the plate is tilted slowly ◦ Static and kinetic friction
  • Want a low coefficient of friction
  • Add surface coatings and plasma treatments to reduce friction
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12
Q

Oxygen Permeability and Transmissibility

A
  • Oxygen permeability - how well oxygen passes through a specific material
    ◦ Dk (D = diffusivity and k = solubility)
  • Oxygen transmissibility – how well oxygen passes through a contact lens with a specific material and thickness
    ◦ Permeability divided by the lens thickness
    ◦ Dk/t (t = thickness)
    ◦ Is converted to permeability by multiplying it with the thickness, as oxygen flow can be measured through a CL
  • Research by Irving Fatt
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13
Q

Refractive Index

A
  • Abbe refractometer - preferred choice of instrument for measuring refractive index of hydrogel CLs
    ◦ Measures the critical angle of incidence for total internal reflection, which is directly correlated to refractive index
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14
Q
  • Spectrophotometer
A

Measure % transmission of different wavelengths of light

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15
Q
  • For CL that claim UV absorption:
A

Class 1 Absorbers
Less than 1% UVB transmittance
Less than 10% UVA transmittance
◦ Class 2 Absorbers
Less than 5% UVB transmittance
Less than 50% UVA transmittance

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16
Q

Lens Geometry

A
  • Diameter
  • Sagittal depth
  • Thickness
  • Radius of curvature
  • Edge appearance
17
Q

Diameter

A
  • Outside lens diameter
    ◦ Generally, between 14.0 and 15.0 mm
    ◦ Can be measured on-eye using a slit-lamp
    ◦ Power-mapping instruments
  • Optic zone diameter
    ◦ Usually not visible by observation
    ◦ May see outer edge on high powered lenses
18
Q

Sagittal Depth

A
  • Height of the contact lens as measured as the vertical distance from the lens edge to the apex of:
    ◦ The front surface (front sag)
    ◦ The back surface (back sag)
19
Q

Thickness

A
  • Of most interest is the center thickness
  • Generally specified for a -3.00 D lens for comparison purposes
    ◦ Varies with lens power
  • Information conveyed by
    ◦ Single point
    ◦ Line graph
    ◦ Thickness profile
20
Q

Radius of Curvature

A
  • 3 most commonly referenced radii of curvature are:
    ◦ Front optic zone radius (FOZR)
    ◦ Back optic zone radius (BOZR)
    ◦ Average base curve of back surface (BC)
    On the contact lens package
21
Q
  • Edge design
A

◦ Round
◦ Knife
◦ Chisel
* Can significantly influence on-eye performance
◦ Comfort
◦ Staining
* Poorly designed edges are often implicated in inferior comfort

22
Q
  • Edge defects
A

◦ Notch or chip
◦ Tear or cut
* Most disposable lenses undergo 100% edge defect inspection before being packaged

23
Q
  • Soft lens materials are divided into two main groups:
A

◦ Hydrogels (low-Dk materials)
◦ Silicone hydrogels (high-Dk materials)

24
Q
  • Polymer is an umbrella term that includes
A

◦ Plastics (PMMA)
◦ Fibres (nylon)
◦ Elastomers (rubbers)
◦ Hydrogels

25
Q

Hydrogel Development

A
  • First pHEMA (“soft”) lens in 1961
    ◦ Produced by Wichterle
    Not successful…
    ◦ Refined by Bausch & Lomb in 1971
    Increased popularity in USA
    –Increased comfort, reduced adaptation and easier fitting procedures
26
Q
  • Problems with hydrogel development
A

◦ Hypoxia (lack of oxygen)
◦ Solution toxicity
◦ Lens spoilage

27
Q

Hydrogel Development

A
  • How to increase oxygen transmission?
    1. Create thinner (‘hyperthin’) lenses
    2. Develop materials with higher water content
    Higher equilibrium water content (EWC)
28
Q

Higher Water Content Lenses- advantages

A

◦ Better comfort
◦ Longer wearing time
◦ Quick adaptation
◦ Easier to handle (thicker)

29
Q

Higher Water Content Lenses
* Disadvantages

A

◦ Fragile and shorter life span
◦ Deposits (protein)
◦ Less reliably produced
◦ More solution problems
◦ Lens dehydration
Affects vision
Corneal dessication (drying of the cornea)

30
Q

Examples of Hydrogel Materials

A
  • Polymacon
    ◦ Biomedics 38, SofLens 38
  • Nefilcon A
    ◦ Dailies AquaComfort Plus
  • Etafilcon A
    ◦ Acuvue 2, 1-Day Acuvue Moist,
  • Ocufilcon D
    ◦ Biomedics 55
  • Omafilcon B
    ◦ Proclear
31
Q

Silicone Hydrogel Development

A
  • Limit to how much oxygen permeability can be attained by increasing the EWC of hydrogel materials
    ◦ Had to come up with a new kind of material
    ◦ Silicone
  • Work from 1970s on incorporating silicone into hydrogels
  • Challenge
    ◦ Silicone is hydrophobic
  • Solutions
    1. Polar groups inserted into material to improve hydrophilic properties
    2. Macromers added to lenses
32
Q

First Generation Silicone Hydrogels

A
  • Material surfaces are very hydrophobic, so would be unsuitable for wear without further treatment
    ◦ Plasma coated to improve wettability
    ◦ Despite these modifications, wettability problems were reported
    ◦ Generally accepted that silicone hydrogel lenses have inferior wettability compared with hydrogels
  • Material has a higher modulus (they are ‘stiffer’)
    ◦ Easy to handle
    ◦ Created corneal complications (epithelial lesions, mucin balls, CLPC)
33
Q

2nd and 3rd Generation SiHy

A
  • 2nd and 3rd generation lenses have emerged as a result of manufacturers improving on mechanical and surface characteristics of 1st generation lenses
  • Main advantage of newer SiHy lenses:
    ◦ Increased water content
    ◦ Reduced moduli (less stiff)
    ◦ Do not require surface treatments
    ◦ May be lower incidence of CLPC
34
Q

Examples of SiHy Materials

A
  • Balafilcon A
    ◦ Pure Vision
  • Lotrafilcon B
    ◦ Air Optix Aqua
  • Comfilcon A
    ◦ Biofinity
35
Q

Comparison of Materials

A
  • Efron Contact Lens Practice
    ◦ Hydrogel
    As water content increases, Dk increases but is never as high as silicone hydrogel lenses
    ◦ Silicone hydrogel
    As water content increases, Dk decreases but is still higher than hydrogel lenses

Contact Lens year 1 (9 decks)

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