Flashcards in Sem 1 Deck (147):
Full time wear of contact lenses is when the patient wants to wear the lenses how often?
5-7 days per week
Part time wear of CLs is when the Px wants to wear the lenses how often?
occasional wear or only a few times per week
Daily wear is?
wearing CLs during waking hours only
Extended wear is?
wearing CLs continuously including overnight use
Disposable CLs are?
replaced (or diposed of) at greater frequency - daily, fortnightly, monthly
Conventional CLs are replaced how often?
Spherical RGPs fit what type of corneas?
near spherical cornea and any refractive error where the refractive astigmatism is mostly corneal
Spherical SCLs fit on what type of corneas?
most corneas and any refractive error with low refractive astigmatism
Front toric RGPs fit what type of corneas?
near spherical corneas and where there is significant refractive astigmatism
Toric SCLs fit what type of corneas?
most corneas and where there is significant refractive astigmatism
Bitoric/back toric RGPs fit on what type of corneas?
highly astigmatic corneas
Multifocal/Bifocal CLs have both RGP and SCL choices for patients requiring?
an ADD component
Other types of contact lenses? (6)
semi scleral RGPs
reverse geometry RGPs
A positive spectacle lens becomes more or less positive when brought to the ocular plane?
ΔK is the?
amount of corneal astigmatism
With ΔK, which K value is flat and which one is steep?
K1 @ _____?
K2 @ _____?
K1 = flat
K2 = steep
ΔK = ?
K1 - K2 x flat axis
Refractive astigmatism that is significant and primary due to corneal astigmatism in origin is optically best managed with a?
The corneal diameter is also known as?
Horizontal visible iris diameter
Corneal diameter is relevant in determining what parameter of a contact lens?
Which keratometer can measure along two principle meridians with one measurement?
Bausch and Lomb
Which keratometer can only measure one principle meridian at a time?
Keratometers are limited to what three things?
restricted to spherocylindrical surfaces
3mm central area only
greatly affected by user and focusing error
The range of a keratometer can be extended. For steep corneas a _____ trial lens is used and for fat corneas a _____ trial lens is used
Videokeratoscopes measure how much area?
If the rings on a placido disc are spaced further apart this means that the cornea is?
Normal axial patterns observed from topography maps? (5)
symmetric bow tie
asymmetric bow tie
What does thhe tangential display of a topographer do?
shows more discrete changes
The following characteristics apply to which topography map type?
- measures radius of curvature to a comparable sphere
- centers of rotation fall on the axis defined by videokeratoscope
- reasonably represents refractive
- involves smoothening
axial map/sagittal map
The following characteristics apply to which topography map type?
- Is not constrained by centers of rotation falling on axis of videokeratoscope
- more accurately depicts the irregularity and location
- shows local irregularity
tangential maps/instantaneous local map
Keratometers can be used to? (5)
measure normal variations
look at pathology/abnormal corneas
identify irregular corneas
monitor corneal shape changes
simulate RGP lens on eye fitting relationship
On a topography map, would you expect the hotter colors to run vertically or horizontally for a WTR astigmatic cornea?
Astigmatic corneas can be categorised into irregular astigmatism and regular astigmatism. What angle α is the boundary between the two?
ie α greater than 20 degrees = irregular astigmatism
α smaller than 20 degrees = regular
What is the term given to a flat cornea?
Pellucid marginal degeneration is thinning of which part of the cornea?
Spherical RGPs are good for?
a) High corneal cyl and low refractive cyl
b) High corneal cyl and high refractive cyl
c) Low to medium corneal cyl and low to medium refractive cyl (same axis)
d) Low to medium corneal cyl and high refractive cyl
Low to medium corneal cyl and low to medium refractive cyl (same axis)
Why are spherical RGPs not good for fitting eyes with low corneal cyl and high refractive astigmatism?
because the tear layer formed is insufficient to correct the refractive astigmatism
Why are spherical RGPs not good for fitting eyes with high corneal cyl and high refractive cyl?
the tear layer power formed would correct the refractive astigmatism however the spherical RGP would not fit the toric corneal shape well
Why are spherical RGPs not good for fitting eyes with high corneal cyl and low refractive cyl?
the tear layer power formed would over-correct the refractive astigmatism and the spherical RGP would not fit the toric cornea shape well - this is the LEAST DESIRABLE OPTION
What RGP parameters MUST you have to order a spherical RGP?
optic zone diameter
The diameter of a RGP should be how many millimeters smaller than the cornea?
An RGP that fits within the lids is called?
intra-palpebral fitting (usually small diameter lenses)
An RGP that stays more or less in the middle of the cornea and between the lids is called?
inter-palpebral fitting (usually medium diameter lenses)
An RGP that stays tucked under the upper eyelid and moves with the upper lid during blinking is called?
lid attachment fitting (usually large diameter lenses with respect to aperture size)
For a steep cornea (>47D) what lens diameter size would you choose?
For a flat cornea (<39D) what lens diameter size would you choose?
Med/large to large
A small diameter RGP is classified as?
<9.00mm (7.0 OZD)
A small/medium diameter RGP is classified as?
~9.0-9.3mm (7.5 OZD)
A medium/large diameter RGP is classified as?
~9.5-9.8mm (8.0 OZD)
A large diameter RGP is classified as?
>9.8mm (8.5 OZD)
When fitting a spherical RGP what are the parameters to select in order? (5)
1. Diameter of lens
2. Base curve
4. Lens design
5. Test lens on eye
How thick should the channel of tears be through the central portion of a RGP?
a 0.1mm change in radius is equivalent to how much dioptric change?
When there is moderate astigmatism what BC do you select for a spherical RGP?
slightly steeper than flat K
What is the ideal axial edge lift?
What does this mean:
7.85BC and 7.6 OZD
8.50 secondary curve and 8.2 diameter
11.0 peripheral curve and 9.0 diameter
Peripheral curve design - tangent designs uses the tangent cone concept and involves?
landing of midperiphery to allow even distribution of pressure and limit lens lateral drift
Dynamic fitting characteristics looks at?
how the lens acts naturally on the eye
Static fitting characteristics shows the?
lens-to-cornea curvature relationship
What is the minimum threshold needed in order to judge the presence of fluoroscein?
The optimum edge width is?
A RGP should decentre less than how many mm?
A RGP can be graded as flat and loose. The level of looseness is graded considering? The level of flatness is graded by the?
dynamic fitting characteristics
static fitting characteristics
Most loot fitting RGPs are caused by?
BOZR too much flatter than corneal curvature
Loose lenses can also be due to fitting ____ lenses without a ___ BC component
A lens that shows excessive central pooling, heavy mid-peripheral contact , narrow edge lift and width is?
steep and tight fitting lens
a slightly loose or tight RGP should have its BC changed by?
A visibly loose/tight RGP should have its BC changed by?
Very loose/right RGP should have its BC changed by?
An excessively loose RGP should have its BC changed by?
tight fitting lenses are most commonly caused by?
BOZR too much steeper than corneal curvature
Tight fitting RGPs can also occur when a lens fitting is too _____ leading to lens vaulting
What is wrong with a RGP if you see the following characteristics?
- typically ride centrally or low
- move quickly on blink
- vault over the central cornea too much
- produce bubbles in the central zone
- impinge on the peripheral cornea
BC too steep
What is wrong with a RGP if you see the following?
- typically ride high but eventually slip off to the side or downward
- float around on the eye
- have large excursions or hang beneath the lid
- have too much edge lift
BC too flat
What is wrong with a RGP if you see the following?
- irregular pressure over the cornea
- movement shows the lens to rock up and down or rock left and right
cornea too astigmatic
a lens movement that rocks up and down is associated with what sort of astigmatism?
with the rule
What is wrong with a RGP if it has the following characteristics?
- typically move freely on the eye or decentre easily
- may dislodge onto conj or pop off the eye with quick lateral gazes
diameter too small
Bigger lenses are naturally looser/tighter because of lower/higher sagittal height
Smaller lenses are naturally looser/tighter because of lower/higher sagittal height
What is wrong with a RGP with the following characteristics?
- excessive lens tightness
- restrict movement
- limit tear exchange
- can irritate limbal area = excessive tearing and lens awareness
lens too big
If a lens is made 0.5mm larger then the BC should be made how much flatter/steeper? (unless you want central clearance to change)
0.05mm flatter (0.25D)
What effect does increasing TD have on edge lift?
increases edge lift
What two things does a high edge lift cause?
causes lens to ride higher
produces more lid attachment
Reducing TD has what effect on edge lift?
decreases edge lift
What two things does a low edge lift cause?
causes lens to ride lower
less lid attachment
To increase edge lift, what parameters can you increase? (5)
SCR (most influential)
SCW (while reducing OZD)
PCW (while reducing OZD)
TD along with OZD
High plus powered RGPs have possible problems with inferior decentration. This can be managed through?
adding a minus carrier on the edge
High minus powered RGPs can have possible problems with lid interactions. This can be managed through?
adding a plus lenticular - resulting in wedge shape
What edge configuration has the best comfort level?
Wettability is measured with what three methods
Dk denotes the ______ of contact lens materials
Dk/t denotes what?
the transmissibility through a lens of given thickness
What lens parameters can you change for a lens that is too tight? (4)
increase peripheral curve (edge lift)
widen peripheral curve
What lens parameters can you change for a lens that is too loose? (4)
decrease peripheral curve (edge left)
reduce peripheral curve width
increase OZD (with TD)
Dimple veiling is caused if the lens is? (2)
too much edge lift
Constant high or low riding RGPs can lead to?
mechanically induced corneal warpage/moulding
The underlying associated cause of corneal warpage is?
What is often the cause of staining at 3 and 9o'clock?
lens edge miniscus causing local tear film thinning
What is characterised by an opaque, elevated mass at the nasal and/or temporal cornea adjacent the limbus
vascularised limbal keratitis
What is the etiology of blink related problems with a RGP?
lens edge awareness
sensation of any surface irregularity
excessive lens movement
The upper lid being subjected to higher frictional forces of intrinsic or extrinsic nature causes what?
Toric RGPs are used in what two situations?
to improve fitting on regular astigmatic cornea
to correct residual astigmatism that occurs when fitting spherical RGP
Calculated residual astigmatism (CRA) = ?
spectacle cyl - corneal cyl
What makes residual astigmatism significant?
Oblique when not corrected > ATR > WTR
large pupils with ATR astig
if the Px is habitually fully corrected with cyl
Bitoric and back-surface toric CLs are used to fit corneas with a minimum how many dioptres of regular astigmatism?
How do you choose the BCs for a back toric RGP?
select BC equal to flat K for r1
select BC 0.1mm flatter than steep K for r2 (note -0.50 tear layer)
Bennet's tricurve design SCR and PCR parameters?
SCR add 1.0-1.5mm to BC/0.3 wide
PCR add 1.5-2.0mm to SC/0.4 wide
The induced over-correction is a minus cylinder at the same axis as the _______ principle meridian. The magnitude is proportional to the degree of ________ and reractive index of ________
Back surface torics work for a patient that has refractive cylinder that is _____ greater than the corneal cylinder
Back surface toric ΔBVP (dioptres) =
ΔBC (dioptres) * material factor
What RGP works well with patients who have spectacle cyl = corneal cyl
SPE back toric RGP
Rotation of what toric RGP does NOT induce unwanted astigmatism?
SPE back toric
Sterilisation results in
all viable microbes to be eliminated; none can reproduce
Disinfection results in
substantial reduction in level of microbial contamination
The D-value of a solution is an index for what? Is a lower or higher D-value better?
index of disinfection rate
lower is better
Complete disinfection requires pressure of ____kPa applied to steam and temperature of _____ degrees and after _____ mins all bacteria, spores, fungi, and viruses killed
Name three buffers used in soft lens solutions
Methyl cellulose is what kind of agent and what does it do?
improve wetting time + comfort
Chelating agents e.g. sodium adetate have what function?
enhances actions of preservatives
You cannot use SCLs with what 3 eye drops?
Hydrogen peroxide systems aren't as good against which type of microbe?
PVA, PVP and PEG are all examples of what?
BAK, PHX, CHX are examples of what ind of agents?
RGP solution preservatives
What are the top RGP troubleshooting tips?
Polishing 6-12 monthly
Progent protein cleaner
AO Sept, omnicare
What lens is used when a normal spherical RGP gives good vision but uneven edge lift on a toric cornea
Adding what component to a lens makes it ionic?
methacrylic acid (MA)
An ionic lens causes what two things?
attracts more lysozyme but
makes material more wettable
low water + non-ionic
high water + non-ionic
low water + ionic
high water + ionic
higher water content of SCL usually correlates with
higher Dk/t (except SiHi)
How much O2 is needed to prevent oedema for DW and EW SCLs?
9.9% (Dk/t = 24) for DW
17.9% (Dk/t = 87) for EW
Thin or thick SCLs interact more with the lids and move more?
What effect does increasing water content do to lens movement?
decreases lens movement
The Da Vinci Code helped understand what concept?
corneal neutralisation in a bowl of water
Rene Descartes is known for what concept?
elongated fluid-filled tube to enlarge retinal image size
1946 Kevin Tuohy did what?
discovered first corneal lens
Dry eye etiologies with contact lenses? (3)
1. Dessication secondary to pervaporation (SCL)
2. Dessication secondary to exposure/poor tear consistency over cornea (RGP)
3. Bioincompatibility leading to tear instability
The lipid layer of the tear film has two phases which are? (2)
What is the function of the outermost layer of the lipid layer?
regulate transmission water rate, CO2, O2, ions
What is the function of the inner layer of the lipid layer?
provide enhanced stability
Which lipid layer is abundant in short chain saturated fatty acids?
Which FDA group material is the most lipid binding?
Which one has the least?
FDA group II materials (high water, non-ionic)
FDA group III
Local dry spots are caused by hydroph____ areas
Proteins deposit on what FDA group material which makes surface less hydrophilic which then attracts lipids
FDA group IV