unit 5 Flashcards
◦ Optical parameters
Power
Optical quality
Scattering
Material properties
Water content
Modulus
Wettability
Surface friction
Oxygen permeability
Refractive index
Spectral transmission
Lens geometry
Diameter
Sagittal depth
Thickness
Radius of curvature
Edge shape
Optical Properties
* Power
◦ 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
- Factors that compromise optical quality are:
◦ Inhomogeneous material
◦ Surface scratches/deposits
◦ Lathe marks (from manufacture)
◦ Material inclusions
◦ Air bubbles
- Equipment
◦ Slit lamp
On patient’s eye
Holding lens
◦ Contact lens loupe
Material Properties
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
Water Content
- 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
Modulus
Stiffness of material
◦ “Young’s modulus”
* Softer lens is more comfortable
* Stiffer lens easier to handle
Wettability
- 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
Surface Friction
- 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
Oxygen Permeability and Transmissibility
- 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
Refractive Index
- 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
- Spectrophotometer
Measure % transmission of different wavelengths of light
- For CL that claim UV absorption:
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
Lens Geometry
- Diameter
- Sagittal depth
- Thickness
- Radius of curvature
- Edge appearance
Diameter
- 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
Sagittal Depth
- 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)
Thickness
- 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
Radius of Curvature
- 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
- Edge design
◦ Round
◦ Knife
◦ Chisel
* Can significantly influence on-eye performance
◦ Comfort
◦ Staining
* Poorly designed edges are often implicated in inferior comfort
- Edge defects
◦ Notch or chip
◦ Tear or cut
* Most disposable lenses undergo 100% edge defect inspection before being packaged
- Soft lens materials are divided into two main groups:
◦ Hydrogels (low-Dk materials)
◦ Silicone hydrogels (high-Dk materials)
- Polymer is an umbrella term that includes
◦ Plastics (PMMA)
◦ Fibres (nylon)
◦ Elastomers (rubbers)
◦ Hydrogels
Hydrogel Development
- 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
- Problems with hydrogel development
◦ Hypoxia (lack of oxygen)
◦ Solution toxicity
◦ Lens spoilage
Hydrogel Development
- How to increase oxygen transmission?
1. Create thinner (‘hyperthin’) lenses
2. Develop materials with higher water content
Higher equilibrium water content (EWC)
Higher Water Content Lenses- advantages
◦ Better comfort
◦ Longer wearing time
◦ Quick adaptation
◦ Easier to handle (thicker)
Higher Water Content Lenses
* Disadvantages
◦ Fragile and shorter life span
◦ Deposits (protein)
◦ Less reliably produced
◦ More solution problems
◦ Lens dehydration
Affects vision
Corneal dessication (drying of the cornea)
Examples of Hydrogel Materials
- 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
Silicone Hydrogel Development
- 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
First Generation Silicone Hydrogels
- 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)
2nd and 3rd Generation SiHy
- 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
Examples of SiHy Materials
- Balafilcon A
◦ Pure Vision - Lotrafilcon B
◦ Air Optix Aqua - Comfilcon A
◦ Biofinity
Comparison of Materials
- 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
