Week 10: Eye Protection & Anisometropic Patient Flashcards
Who Requires Eye Protection?
Mechanical Hazard:
- Sports player
- Motorcyclist
- Occupations such as; handyman
Chemical Hazard:
- Dealing with chemical solutions
Thermal/Infra-Red Hazard:
- Firefighter
- People dealing with molten metal
Radiation Hazard:
- Welding
- Laser
Microbiological Hazard:
- Preventing from a virus
- People in high risk environment/jobs (e.g. nurse)
How does lens fail due to stress?
- Elasticity is a measure of capability of stretching or deformation in response to a force, such that it will return to its original shape or size when the force is released
- Stress is a measure of force put on an object/area
- Strain is change in length of an object, original length or dimension of the object
- If strain of object increases, elastic limit of object will permanently deform or fracture
Describe Hooke’s Law
- For small deformations, object returns to original shape when force is removed
F = k△L, where △L: change in length
Smaller the k, easier it is to stretch an object
Describe Tensile stress
Rod is stretched a length △L when a force is applied parallel to its length
Describe Compressive stress
Same rod is compressed by forces with the same magnitude in opposite direction
Describe Elastic region
- Zone in which a material bends under strain and is able to return to its original shape when the force is released
- Beyond the elastic region, additional strain will result in material failing via fracture
- Charted using a stress strain curve
Describe the different Hoop Stress
- Tensile stress
- Focus per unit area which pushes the molecules apart - Compressive stress
- Force per unit area which pushes molecules together - Lens failure
- Compressive stress greater than tensile stress will occur in lens cracking
Flex Induced Back Surface Initiated typically occurs for;
- Moderate velocity particles
- Moderate mass objects
- More of an issue for minus lenses
Flex Induced Flattening Of Lens typically occurs for;
- Large mass objects
- Slow moving objects
- More of an issue for positive lenses
Flex Induced Flattening Of Lens typically occurs for;
- Large mass objects
- Slow moving objects
- More of an issue for positive lenses
How does Shock affect lens failure?
- Fracture will start at a peripheral flaw
- Flaw usually a poor quality bevel
- Large mass object
How does Plastic Permanent Deformation occur?
- Typically occurs if a lens is too thin
- It can go through the lens
How can glass be strengthened with Thermally Tempered Glass?
Production process:
1. Oven to raise to just below softening point (650o)
2. Time depends on volume (weight)
3. Air quenching so surface cools rapidly & sets hard
4. Internal body will cool more slowing & contract
5. A reinforced structure – in order to break the glass, the compressive stress skin must be neutralised
How can glass be strengthened with Chemically Tempered Glass?
- Glass is an Na+ in Si matrix
- Immerse in a K+ salt bath at temperature > 400oC by osmosis Na+ diffusion in
- Ask K+ is a larger ion than Na+, this results in compressive stresses over entire surface & edge of the lenses
- A reinforced structure – in order to break the glass, the compressive stress skin must be neutralised
- Used when heat tempering cannot be used
How can glass be strengthened with Laminated Glass?
Layer glass sheets with interlayers of plastic that holds the fragments together
What frame materials protect from mechanical hazards?
- Acrylic Frames (plastic)
- Nylon, cellulose acetate, polycarbonate
- One piece moulded with lenses
- Frame separate to lenses - Metal Frames
- Electric hazard – not recommended for people who work with electricity
- Screws work loose
What are the labelling requirements for Mechanical Eye Protection?
I = passes medium velocity IMPACT projectiles
- It must include lateral (side protection)
V = passes high VELOCITY impact projectiles
- It must be a face shield
D = provides DUST protection
- It must include a seal between the face & eye protector
What are the labelling requirements for Chemical Protection?
C = SPLASH proof
G = GAS tight
What are the labelling requirements for Thermal Protection?
H = rated for HIGH temperature protection
M = provides protection from MOLTEN metal & hot solids
What are some options for prescription wearers when eye protections are required and describe briefly?
- Fit Over = must ensure that the eye protector is larger & can fully encase the spectacles
- Prescription Insert = prescription fits behind eye protector
- Prescription Eye Protector Spectacles = conventional spectacles are only suitable for low impact protection
What are the advantages vs disadvantages of fit over?
Advantages:
- Relatively low cost
- People who need to regularly replace eye protectors to damage
- People who only require occasional eye protection
- Wider cross-section across the brow that prevents the eye protector from contacting the prescription lens
- Preventing scratching of rx lens
Disadvantages:
- Wearing eye protector over own spectacles may be uncomfortable
- Only a limited range designs available
- Vision may be poorer introducing increased reflections or aberrations
What are the advantages vs disadvantages of Prescription Insert?
Advantages:
- Better fit across bridge of nose & behind ears compared to fit over
- Insert can be fitted with different lens types
- Damage will be sustained to the eye protector
- Less expensive to replace than rx lenses
Disadvantages:
- Cost is higher than fit over
- Only a limited range designs available
- Some rx lenses may not fit in the insert due to thickness
- Dirt may come between insert & eye protector
- Vision might be poorer looking through two sets of lenses
- Combined weight of insert & eye protector may be higher than rx spectacles
What does welding eye protection protects against?
Protects against:
- High intensity visible light
- Ultra-radiation
- Infrared radiation
Explain Welding Personal Protective Equipment (PPE)
- Require other PPE against molten, metal, gas, dust
- Whole body requires protection
- Different shade numbers
Shade n.o. 3 (not very dark – used for gas welding)
Shade n.o. 14 (very dark – used for plasma arc welding)
Explain Welding Personal Protective Equipment (PPE)
- Require other PPE against molten, metal, gas, dust
- Whole body requires protection
- Different shade numbers
Shade n.o. 3 (not very dark – used for gas welding)
Shade n.o. 14 (very dark – used for plasma arc welding)
Describe Monovision
- One emmetropic eye and one myopic eye is a case of natural monovision
- Monovision is when patient’s use one eye for distance viewing and one eye for near viewing
- Patient’s have to learn to supress the blurry image from the eye which is not focussed
Describe Amblyopia
In children, during the critical period of visual development, the visual system can maladapt to blur, learn to suppress image from one eye, resulting in amblyopia
Describe Differential Prismatic effects
- Different powers will result in different prismatic effects for the same distance from the optic centre (typically at Near Visual Point)
- The brain must work harder to try and fuse the images
Describe Spectacle magnification
Different powers will result in different magnifications of objects imaged, so the brain must work harder to fuse the images
Failure of fusion results in either;
- DIPLOPIA (double vision) or
- SUPPRESSION (seeing only the percept of one eye)
When to Correct Vertical Differential Prism?
- Consider for correcting vertical meridian difference > 1.50D difference
- Must for correcting vertical meridian difference > 2-3D difference
What happens with Special Case of Anisometropia?
- When an anisometrope looks away from the OC of the lens in any direction, they experience different prismatic effects (differential prism) caused by the different lens powers
- You must be able to calculate the differential prism
What is the solution for single vision anisometropes?
- Encourage anisometropes to look through the OCs
- Decrease the vertex distance as this will mean that for the same visual angle, they will be looking through a part of the lens which is closer to the OC than if they had a farther vertex distance
- For single vision near spectacles, align the OCs with the line of sight when viewing at near
What does Slab-off provide?
- Provides base up prism
- Will always be applied to lower part of the lens that is naturally inducing the most base down, or least base up prism when viewing through a position below the distance OC
- Hence, most minus or least plus
What does Reverse slab provide?
- Will always be applied to lower part of the lens that is naturally inducing most base up, or least base down prism when viewing through a position below distance OC
- Hence, most plus or lease minus
Describe Different Sized Segments
- Different sized segments with equal seg heights will result in more base down prism than the other at the near visual point
- Consider whether this is cosmetically desirable to have two differently sized segment diameters
What is Fresnel Press-On Prism?
Are made of soft, thin plastic which consists of successive and adjacent small prisms
How do you fit Fresnel Press-On Prism?
How to Fit:
- Cut it down (using scissors) to the shape of the segment & attach it to segment with base down prism required
- Stick on to the back surface and this effectively simulates slab-off
Disadvantages of Fresnel Press-On Prism?
- Clarity is poor through the Fresnel prism
- Cosmetically unappealing
- Can come loose – temporary solution or trial before ordering permanent slab-off
What is Franklin Split Bifocals?
Total prism control in the distance and near zones
Pros of Separate Pairs in anisometropes
- Total prism control in the two pairs but decreased convenience switching between pairs
- To use this solution, centre the readers so that patient views directly through the OCs when reading
Anisometropes and PALs
- Try a shorter progressive corridor when reading with the progressive
- Try Slab Off for PALs
