OPTICS: SEMINARS - keratoprosthesis, lenses, filters, clarity Flashcards
What is the role of limbal stem cells? [Keratoprosthesis #1]
Involved in:
- rapid, functional healing of the corneal epithelium
How does injury of the cornea affect the limbal epithelium? [Keratoprosthesis #1]
When corneal wounding occurs, the limbal epithelium gets removed, leading to vascularisation and conjunctivalisation
How can stem cells restore epithelial function, and for how long? [Kratoprosthesis #1]
Expansion of limbal stem cells in vitro with transplantation to the central cornea can restore epithelial function for up to 10 years
What situations/conditions result in limbal stem cell deficiency (LSCD)? [Keratoproshesis #1]
Results due to:
- genetic disease
- chemical burns
Define penetrating keratoplasty. [Keratoprosthesis #1]
Where the entire cornea is removed and replaced completely with a donor one
Define keratoprosthesis, how does it differ to keratoplasty [Keratoprosthesis #1]
Transplantation with an artificial cornea
whereas keratoplasty is transplantation with a donor cornea. Try keratoprosthesis if plasty has failed
List 4 types of keratoprosthesis [Keratoprosthesis #1]
- Boston Kpro types I and II
- AlphaCor
- OOKP
- KeraKlear
What are the indications for Boston Keratoprosthesis (Boston Kpro) [Keratoprosthesis #1]
- failed corneal graft with poor prognosis for future grafts
- vision less than 6/60 in affected eye
- NO end-stage gluacoma or retinal detachment
- alternative when corneal limbal stem cell failure
What are 3 types of keratoplasty? [Keratoprosthesis #2]
- Full thickness grafts (penetrating keratoplasty)
- Anterior lamellar grafts (superficial or deep anterior lamellar keratoplasty)
- Posterior lamellar grafts (descement’s membrane endothelial keratoplasty; DMEK)
List the indications for keratoplasty {Keratoprosthesis #1]
- keratoconus
- corneal dystrophies (i.e. fuch’s corneal endothelial dystrophy)
- severe injury or trauma (mainly chem. burns)
- infectious keratitis
- autoimmune disease
- corneal ulcerations
- a repeat graft
List the contra-indications for keratoplasty [Keratoprosthesis #1]
- previous corneal graft failure
- young px
- stromal revascularisation
- pre-operative inflammation, glaucoma or anterior segment surgery
- risk of astigmatism
List the complications of keratoplasty [Keratoprosthesis #1]
- persistent epithelial defect
- re-infection
- graft melting
- graft rejection
- glaucoma or ocular hypertension
What is the difference between penetrating and non-penetrating keratoprosthesis? {Keratoprosthesis #2]
Penetrating: full thickness of cornea is removed
Non-penetrating: only the anterior cornea is removed (descement’s membrane + endothelium still intact)
Which of the following keratoprosthesis are non-penetrating techniques? {Keratoprosthesis #2]
- Boston Type 1
- Boston Type 2
- Osteo-Odonto
- AlphaCor
- KeraKlear
Out of these, only KeraKlear is non-penetrating. Rest are penetrating.
List the general indications for keratoprosthesis [Keratoprosthesis #1]
- VA worse than 6/60
- previous further grapts with poor chance of success with further PKP
- a functioning retina
- Absence of advanced glaucomatous optic neuropathy
- chemical or thermal injury
- congenital abnormalities e.g. aniridia
- absence of inflammation or infection (e.g. herpetic keratitis)
How is optical clarity affected by keratoprosthesis? [Keratoprosthesis #3]
Optical clarity is maintained through using a corneal graft
List some complications for keratoprosthesis [Keratoprosthesis #3]
- retroprosthetic membrane formation (most common complication)
- infectious endophthalmitis
- glaucoma
- necrosis and tissue melt
- sterile vitritis (type of uveitis/endophthalmitis)
What is ‘tarsorrhaphy’ and which kerotoprosthesis patients underwent it? [Keratoprosthesis #3]
Tarsorrhapy = the joining the upper + lower eyelids to partially or completely close the eyelids
Boston type II px’s underwent this
Which keratoprosthesis is the ideal? [Keratoprosthesis #2]
Melbourne Type 3 is the ideal KPro
Order the refractive indices for the following lens materials from lowest to highest [Lenses #3]:
- Crown glass
- CR39 (hard resin)
- Polycarbonate
- Trivex
- Tribrid
CR39 = 1.49 Crown Glass = 1.523 Trivex = 1.530 Polycarbonate = 1.586 Tribrid =1.600
(CR39 - Crown Glass - Trivex - Polycarbonate - Tribrid)
Which lens material has the greatest scratch-resistance out of [Lenses #3]:
- Crown Glass
- CR39
- Polycarbonate
“Crown glass” has the highest scratch resistance
(followed closely by cr39)
(note: polycarbonate has the lowest scratch resistance)
Order the impact resistance from highest to lowest for the following lens materials: [Lenses #3]
- Trivex
- Polycarbonate
- High index glass
- CR39
- Glass
Polycarbonate = most impact resistant High index CR39 Glass Trivex = least
Which lens materials are easy to tint? Which are hard to tint? [Lenses #3] Out of: - Trivex - Tribrid - Glass - CR39 - Polycarbonate
Easy to tint = Trivex, Tribrid, CR39
Hard to tint = Glass, Polycarbonate
Which lens was the first plastic lens to be developed?[Lenses #1]
CR39, first made in 1947
When were polycarbonate lenses developed? [Lenses #1]
In the 1970s
Why was polycarbonate considered an alternative to CR39 and glass lenses? [Lenses #1]
Due to polycarbonate’s toughness (high impact resistant) and light weight (however trivex is lighter)
What is the earliest discovered ‘lens’? [Lenses #1]
“Nimrud Lens” - it is very ancient
When were Trivex lenses commercially introduced? [Lenses #1]
2002
When were Tribrid lenses commercially introduced? And what is their intended purpose? [Lenses #1]
Introduced in 2012 as a high-index material
How many types of hard contact lenses are still in use today? [Lenses #1]
Only one. Gas-permeable contact lenses
When were hard contact lenses originally developed? What about soft contact lenses? [Lenses #1]
Hard = 1888 Soft = 1961
What are the characteristics of an “ideal” spectacle lens? [Lenses #2]
- no abberations
- no reflections
- as thin and light as possible
- scratch and impact resistant
- easily tinted and can retain coatings
- easy + cheap to manufacture
Define curve variation factor [Lenses #2]
indicates the degree of lens flattening achieved by using material of higher refractive index
(i.e. if you have a high refractive index material, you don’t need the lens to bend as much to achieve the same refraction)
Define Abbe Number [Lenses #3]
is denoted by symbol ‘V’
Is a measure of a lens’ dispersion; describing the level of chromatic aberration for a given lens material
Higher Abbe number = less light dispersion = less chromatic aberration
Lower Abbe number = more light dispersion = more chromatic aberration
In terms of Abbe number, what values constitutes high light dispersion vs low light dispersion? [Lenses #3]
High dispersion = v<39
Low dispersion = v>45
What is the general relationship between Refractive Index and Abbe Number? [Lenses #3]
The higher The R.I, the lower the Abbe Number; vice-versa
Describe the relationship between Refractive Index and Reflection {Lenses #3]
As R.I increases, there is more light lost due to reflection at the surface (surface reflectance can be calculated with fresnel’s reflection factor)
Which has the most light reflectance: Crown Glass, CR39, or high index materials? [Lenses #3]
High index materials (~8% of light reflected)
Crown Glass and CR39 have similar reflectance (~4% of light reflected)
Which lens material has the best optics? [Lenses #3]
Glass
List the optical parameters that characterise lens materials [Lenses #2]
- specific gravity
- refractive index
- abbe number
- reflectance and transmission
- impact resistance
What patient profile information should be considered when suggesting lens materials for prescription? [Lenses #4]
Age - more durable lenses Activities of glasses wearer e.g. sport Strength of prescription and type Possible hindrance by chromatic aberration Sensitivity to weight and facial profile
List and describe 5 kinds of colour filters [Filters]
- Absorptive Filters - blocked light is absorbed and contained within the filter
- Monochromatic Filters - a.k.a monochromator: mechanically selects a narrow band/range of wavelengths to pass through
- Interference (Dichroic) Filters - use thin film interference to produce colours
- Long-pass filters: reduce short wavelengths, transmit long
- Short-pass filters: reduce long wavelengths, transmit short
List the advantages of Absorptive Filters. [Filters]
Advantages:
- not very angle sensitive; filter maintains transmission + optical properties at many angles
- cheap
- good blocking + transmission
List the disadvantages of Absorptive Filters. [Filters]
Disadvantages:
- poor transmission slope
- not suitable for precision applications
- heat-sensitive; breakdown after prolonged use
- requires a necessary thickness
How are the layers in interference filters created? [Filters]
By vacuum deposition
List advantages of Interference/Dichroic Filters [Filters]
- easily create filters for any pass band frequency
- NOT heat sensitive
- Longer lifespan
- is thin
- precise transmission of light
- high laser damage threshold
List disadvantages of Interference/Dichroic Filters [Filters]
- made of glass, therefore fragile + expensive
- lower blocking properties
- increased angle sensitivity
- ineffective at blocking wide wavelength ranges
- reflects + transmits only 20nm light on each side of transition point
Do commercial coloured filters improve red-green colour vision deficiency? [Filters]
No! They do not!
In a study in 2016, how did amblyopic subjects playing a binocular Ipad game with filtered lenses compare to subjects with traditional patching? [Filters]
Binocular ipad game amblyopic subjects showed greater improvement in V.A after 2 weeks
How can yellow filters help low vision patients? Use diseases as examples (e.g. RP, pathological myopia, DR, maculopathy, glauc, amd, albinism) [Filters]
- Retinitis Pigmentosa - 26% mean reduction in visual disturbance
- Pathological Myopia & Diabetic Retinopathy - significant improvement in contrast sensitivity
- Maculopathy & Glaucoma - half the patients improved contrast sensitivity
- AMD and albinism: NO IMPROVEMENT
What type of damage can blue light exposure cause? [Filters]
photoCHEMICAL damage
remember prasan’s seminar on photochem vs thermal
How do we use coloured filters clinically? [Filters]
- as a confirming tool: duochrome test
- as a dissociative tool: worth 4 dot test, hess screen
- as a highlighting tool: monochromatic fundus photography, ophthalmoscopy, fluoroscein angiography
How are the following colour filters useful in ophthalmoscopy? [Filters]:
- Blue Filter
- Red-free filter/Green Filter
- Red Filter
Blue filter: can see NFL patterns (also useful for corneal + conjunctival defects and in tonometry)
Red-free filter: Good contrast of blood vessels, good overall fundus view
Red filter: low contrast of blood vessels, choroidal structures are highlighted
Define Fluorescence Angiography [Filters]
a technique for examining the circulation of retina and choroid using a fluorescent dye (fluorophore) that tags and tracks the fluids
How does pupil size affect diffraction and aberration? [Optical Clarity]
Smaller pupils have greater diffraction and reduced aberration
State the typical refractive indices for the following parts of the eye: - anterior chamber - cornea - lens - vitreous [Optical Clarity]
anterior chamber: 1.33
cornea: 1.38
lens: varies between 1.39 - 1.41
vitreous: 1.34
What proportion of light refraction happens at the cornea? [Optical Clarity]
80%
Describe the pathway of light as it travels through the air to the vitreous. [Optical Clarity]
air (RI = 1) – cornea (1.38) – anterior chamber (aqueous humour) (1.33) – lens (1.40ish) – vitreous (1.34)
Light comes in and strongly bends towards the normal when entering the cornea, then slightly bends more away from the normal entering the lens, and then slightly bending more towards the normal again when entering the lens, then bending away entering vitreous
State the refractive power from air to the front corneal surface. How does this differ for the back corneal surface to aqueous humour? [Optical Clarity]
air to front corneal surface: +46 diopters
back corneal surface to aq. humour: -5 diopters
What range of wavelengths does the aqueous humour transmit? [Optical Clarity]
265nm to around 2000nm
Does the range of wavelengths transmitted by the aqueous humour help protect against photochemical damage? Explain. [Optical Clarity]
Photochemical damage is mainly due to short-wavelength light. Aqueous humour doesn’t transmit wavelengths less than 265nm, therefore protects against UV related photochemical damage.
However photochemical damage from the short wavelength part of the visible spectrum still gets through [note that most of the damage comes from the short end of visible spectrum]
What is the purpose of the lens and what is required for it to achieve this? [Optical Clarity]
Purpose is to project a sharply focused, undistorted image of the visual surround onto the neural retina
- therefore it needs to be transparent and have refractive index matching between lens membrane and cytosol
How is transparency of the lens accomplished? [Optical Clarity]
- absence of nuclei and other light-scattering organelles located within the pupillary
- close apposition of fibre cells (lacking organelles in the nucleus)
- densely packed lens crystallins [provide a refractive index gradient, allows for transparency of the lens]
- no blood supply
List the components of the vitreous [Optical Clarity]
- 98-99% water
- Hyaluronic acid
- Collagen type II mainly (also V, IX, XI)
What 3 ways does the vitreous ensure optical clarity? [Optical Clarity]
- very low macromolecular solutes concentrations
- small diameter collagen fibres
- hyaluronic acid separating collagen fibres and acting as a physicochemical barrier
How do muller cells influence the propagation of light through the eye? [Optical Clarity]
It has been suggested that they guide the pathway of light and reduce scatter by doing so
Where in the retinal layer are muller cells? [Optical Clarity]
spans across retinal layer
Describe the refractive index of muller cells in comparison to the surrounding tissue, are the muller cells RI higher or lower? [Optical Clarity]
Higher
Name 4 corneal dystrophies [Optical Clarity]
keratoconus (it’s me!)
fuch’s dystrophy
lattice dystrophy
map-dot-fingerprint dystrophy
Briefly describe the pathology of keratoconus [Optical Clarity]
- non inflammatory thinning of corneal stroma
- protrusion
What is Hyphema? How can it affect vision? Is it painful? and what is the major cause for it? [Optical Clarity]
Is a pooling or collection of blood inside the anterior chamber of the eye, can block vision partially or completely. Is usually painful.
Major cause: trauma
What do cells and flare in the anterior chamber typically indicate? [Optical Clarity]
Anterior uveitis
How does anterior uveitis affect scattering of light? [Optical Clarity]
Increased proteins in cells in anterior chamber therefore increased scattering
How does ageing affect scatter in the anterior chamber? [Optical Clarity]
increases it
Other than scatter, how else does ageing affect the anterior chamber? [Optical Clarity]
breakdown of blood-aqueous barrier
Describe the effects of a senile cataract [Optical Clarity]
- increasing light scatter
- decreasing elasticity
- changes to lens crystallins
- membrane bound bodies disturb the regular refractive index distribution
How might forward light scattering in the lens affect vision? [Optical Clarity]
- reduced contrast sensitivity
- increased glare
- reduced visual acuity
How does ageing affect the vitreous structure? [Optical Clarity]
- vitreous structure gets denser
- vitreous liquefication
Name the symptoms of a posterior vitreous detachment (PVD) [Optical Clarity]
- floaters (sudden increase in amount)
- flashes (usually on the side)
note: higher prevalence of PVD when flashers and floaters onset together
Describe the process of a posterior vitreous detachment. How does it occur? [Optical Clarity]
- gel shrinks and becomes liquid, eventually cannot fill the whole vitreous cavity
- gel separates from retina
- gel keeps condensing and shrinking
- clear vitreous fluid fills up the empty space in between
How can diabetic retinopathy affect the vitreous? [Optical Clarity]
Vitreous haemmorhage
