unit 5

Question 1

◦ Optical parameters

Answer 1

Power
Optical quality
Scattering

Question 2

Material properties

Answer 2

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

Question 3

Lens geometry

Answer 3

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

Question 4

Optical Properties
* Power

Answer 4

◦ 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

Question 5

• Factors that compromise optical quality are:

Answer 5

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

Question 6

• Equipment

Answer 6

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

Question 7

Material Properties

Answer 7

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

Question 8

Water Content

Answer 8

• 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

Question 9

Modulus

Answer 9

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

Question 10

Wettability

Answer 10

• 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

Question 11

Surface Friction

Answer 11

• 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

Question 12

Oxygen Permeability and Transmissibility

Answer 12

• 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

Question 13

Refractive Index

Answer 13

• 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

Question 14

• Spectrophotometer

Answer 14

Measure % transmission of different wavelengths of light

Question 15

• For CL that claim UV absorption:

Answer 15

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

Question 16

Lens Geometry

Answer 16

• Diameter
• Sagittal depth
• Thickness
• Radius of curvature
• Edge appearance

Question 17

Diameter

Answer 17

• 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

Question 18

Sagittal Depth

Answer 18

• 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)

Question 19

Thickness

Answer 19

• 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

Question 20

Radius of Curvature

Answer 20

• 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

Question 21

• Edge design

Answer 21

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

Question 22

• Edge defects

Answer 22

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

Question 23

• Soft lens materials are divided into two main groups:

Answer 23

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

Question 24

• Polymer is an umbrella term that includes

Answer 24

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

Question 25

Hydrogel Development

Answer 25

* 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

Question 26

* Problems with hydrogel development

Answer 26

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

Question 27

Hydrogel Development

Answer 27

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

Question 28

Higher Water Content Lenses- advantages

Answer 28

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

Question 29

Higher Water Content Lenses * Disadvantages

Answer 29

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

Question 30

Examples of Hydrogel Materials

Answer 30

* 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

Question 31

Silicone Hydrogel Development

Answer 31

* 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

Question 32

First Generation Silicone Hydrogels

Answer 32

* 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)

Question 33

2nd and 3rd Generation SiHy

Answer 33

* 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

Question 34

Examples of SiHy Materials

Answer 34

* Balafilcon A ◦ Pure Vision * Lotrafilcon B ◦ Air Optix Aqua * Comfilcon A ◦ Biofinity

Question 35

Comparison of Materials

Answer 35

* 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