Clinic: Contact Lenses [copied] Flashcards
1
Q
What is the expected corneal thickness of a normal patient vs one with mild KC?
A
Normal: 545um
Mild KC: 473um
2
Q
What is the expected corneal thickness of a patient with severe KC?
A
Severe KC: 415um
3
Q
What is a Fleisher’s ring, and in what condition does it occur?
A
Brownish partial or complete ring of iron deposition in the epithelium at the base of the cone in KC patients
4
Q
What is the main advantage of topography over keratometry for KC patients?
A
Toppography measures corneal thickness for the whole cornea whereas keratometry only measures 6% of the cornea
5
Q
What do axial maps measure?
A
Refractive/corneal power (they closely mimic K values)
6
Q
What do tangential maps measure? What condition is this particularly useful for?
A
shows location of any corneal irregularity. Useful for KC.
7
Q
What do elevation maps measure?
A
the highs + lows of the cornea (by subtracting a normal reference sphere)
8
Q
How do you interpret elevation maps?
A
red/yellow = cornea above ref. sphere
Blue/green = cornea below ref. sphere
9
Q
What do subtraction maps measure?
A
shape change in a cornea comparing before + after to see how it differs
10
Q
What are subtractive maps good for?
A
Observing change from orthoK lens or laser sx for myopic correction
11
Q
What do refractive power maps measure?
A
Based on snell’s law of refraction, which is the best estimate of corneal power
12
Q
Which topographical scale is most useful for KC monitoring? Standard power, K scale, or Normalised?
A
Normalised (39-60D)
13
Q
What does PEI measure?
A
Distance from centre of cornea (apex) to the steepest part of the cornea
14
Q
How is PEI used to ddx KC from PMD?
A
KC: PEI = 1.95mm
PMD: PEI = 3.5mm
15
Q
What is shape factor (P) and how does shape factor differ between KC and PMD?
A
Asphericity of the cornea (prolate = +ve, oblate = -ve, basically)
KC: Highly +ve (>0.50)
PMD: Usually -ve (<0.15)
16
Q
What is corneal eccentricity? What are the normal and abnormal values for it?
A
Rate of corneal flattening from centre to periphery
Normal < 0.65
Abnormal > 0.65
17
Q
For K values, what represents:
- Normal K value
- Steep normal K value
- Normative range for K values
- Steep pathological K value
A
Normal: 43D (7.8mm)
Steep normal: >45D (<7.5mm)
Normative range: Anything basically <46.9D
Steep pathological: >47D
18
Q
What does the IS Index measure? What is the abnormal value and what might this value suggest?
A
Difference between the average inferior power and avg superior power on the cornea.
(comparing the two hemispheres of the cornea)
Abnormal >1.40 (indicates inferior steepening, KC?)
19
Q
What does the SAI measure? What is the normal value?
A
Surface Asymmetry index: difference in superior vs inferior corneal powers at oblique perpendicular axes
Normal <1.0D
(therefore abnormal is >1D)
20
Q
What does SRI measure? What is the normal value?
A
Surface Regularity Index: assesses the smoothness of the central cornea. Measures local fluctuations in corneal power over the pupil
Normal <0.56D
21
Q
What is the main benefit of silicon hydrogel lenses (cf hydrogel) for early KC patients?
A
increased stiffness masks milder corneal irregularity
22
Q
List 4 corrective lens options for moderate/severe KC patients
A
Spherical RGPs (either tricurve or quadcurve)
Aspheric lenses (e.g. Gelflex/ACL)
Hybrid lenses (RGP centre, soft periphery)
Minisclerals (e.g. capricornia)
23
Q
What is the minimal tear layer thickness required for tear film to fluoresce?
A
20mm!!
24
Q
What is the ideal fluorescein clearance for an RPG lens for a KC patient? How does this appear on assessment?
A
Centre: 20-30mm
Edge: 70-90mm
Appears as “Feather Clearance” (3-point touch)
25
What does apical clearance suggest? What can this lead to? (4)
too steep (central curve vaults steepest part of cone)
- Less scarring/central staining
- reduced acuity (b/c lens flexure post blink)
- reduced peripheral tear exchange (less comfort)
- dimple veiling (if tear film >90um)
26
What does apical bearing suggest? What can this lead to? (2)
Too flat (substantial bearing on cone)
- epi staining/scar risk
- occasionally better acuity due to hard bearing + corneal compression
27
What is the ideal edge clearance for an RGP fitting of a KC patient?
0.6-0.8mm (corresponding to 70-90um deep)
(also remember CCE, centre, centration, edge)
28
When do you use small <9mm RGPs in KC? (3)
Advanced nipple; smaller cones; narrow palp aperture
29
When do you use larger >9mm RGPs in KC? (3)
Early KC; Oval/globus; Wider palp apertures
30
List 4 high dK materials (and their dK values) used as RGPs for KC patients
Paragon HDS-100 (100)
Boston XO (100)
Boston XO2 (141)
Acuity 200 (211)
31
What is a good ideal initial BOZR for a spherical RGP for a KC patient?
0.2mm steeper than K.
(then adjust it flatter with large 0.3-0.4mm steps until slight central touch, then go 0.1mm steeper after that)
32
What is one parameter of an RGP you can adjust if the lens drops down?
Try increasing diameter
33
What is the aim of an inferior tuck?
to reduce excessive edge clearance
34
How does the steepness of the periphery relate to clearance?
Steeper in periphery = less edge clearance (opposite of what happens in the centre)
35
What is the general rule explaining the relationship between OD (overall diameter) and BOZR to maintain constant sag?
For every 1mm increase in OD, you should increase BOZR by 0.1mm to maintain sag (and vice versa)
36
What is SAM FAP?
Steeper Add Minus
Flatter Add plus
refers to BOZR adjustment. I'll do calculations on paper for practice (from rgp fitting section)
37
How do you calculate tear film power?
336/BOZR(mm)
(basically we are converting from mm to D)
38
What is the approximation for tear lens power? (don't use this too often honestly, just use formula instead)
Delta 0.1mm BC = Delta 0.50D power change
39
For a gelflex/ACL limbal lift lens K7 series (corneoscleral), how do the peripheral quadrants control edge clearance?
K9 = up edge clearance
K4 = down edge clearance
i.e. K1 is lowest, K10 is highest
All cf to K7 which I assume is average/standard
40
How does the ACL limbal lift K7 series lens generally sit on the eye?
tends to ride up as a "lid attachment" design and vault over the sensitive limbal area
41
List 5 indications for RGPs
OrthoK/Myopia control
KC
High refractive error
Dryness with soft disposables
Prevoius RGP wearer
42
What is javal's rule and what is the formula for it?
An allowance of 0.50ATR astig from the posterior cornea not found on keratometry's delta K (diff between flat and steep K)
Rx Astig = Delta K + (-0.50x90)
Alternatively
Rx Astig = Delta K + (+0.50x180), if delta K is at 180 instead
43
What is the ideal BOZR fit for a spherical cornea?
Fit on K or tiny bit flatter (like 0.05mm)
44
How should you select an initial BOZR for a toric RGP?
Flat meridian - usually on K or slightly steeper (slightly steeper is best, e.g. 0.05mm steeper)
Steep meridian - slightly flatter than K
NB: Choose BOZRs so CL toricity is ≥ 1.50
45
List 10 indications for a toric RGP fit
Poor CL centration
Harsh bearing along flatter meridian
3+9 o'clock staining
Unsatisfactory vision (lens flexure, poor centratioN)
Lens warpage
Corneal cyl >2.50D for spherical GP
Delta K >/= 2D
DeltaK >/= 5D
Residual astig
46
What is a SPE bitoric?
When the dioptric difference in toric BOZR = dioptric difference in BVP (i.e. the astig)
47
Define Induced astig.
the astigmatic effect induced into the system every time a toric BOZ surface is used due to tear/lens refractive index differeence (is ~0.4xCL toricity)
48
What is the difference between a SPE and CPE bitoric?
CPE bitoric has RA (residual astig), whereas SPE has no RA (because all the astig is on the cornea for SPE wearers)
(RA exists because diff in toric BOZR =/= diff in bvp)
49
When do you use an alignment bitoric (a type of CPE)?
When the RA is within 25 degrees of one of the principle meridians of the cornea
50
When do you use a back surface toric?
When RA exists but is perfectly cancelled out by the iniduced astig (which btw is 0.4 x CL toricity)
51
When do you use a front surface toric/back surface spherical?
When there is no corneal astig (i.e it's all RA)
52
With what 2 steps do you calculate the new BVP of a final lens that has changed from the initial trial lens?
1: Apply trial over Rx to initial BVP
2: Compensate for tear film (find delta BOZR in dioptres), [SAM FAP can be used when added to your existing bvp value]
53
For bitoric lenses, how do you calculate the required power of the CL in each meridian?
BVPcl = Oc. Rx - BVPtears
(and just apply this formula to each meridian)
54
What is the formula for choosing the appropriate TD of a rgp lens? What about a soft lens?
RGP: TD = HVID - 2
Soft: TD = HVID + 2
55
What is the formula for choosing an appropriate BOZD? (in relation to TD)
BOZD = 0.75 x TD or 0.8 x TD
BOZD is usually 75-80% of TD
56
What are appropriate peripheral curves for a tricurve rgp?
1st BPR: flatter and within 0.5-1.1mm of BOZR (to avoid sharp transitions)
2nd BPR: flatter than BOZR by 1-1.5mm, and also flatter than 1st BPR.
Example: 1st BPR = +0.7, 2nd BPR = +1.5 (i.e. 0.8 flatter than 1st BPR)
57
What are appropriate peripheral curve widths for a tricurve rgp? [formula]
TD = BOZD + 2PCW1 + 2PCW2
Basically just subtract BOZD from TD then divide by 4 to get individual widths. You can keep each PCW the same or have PCW1 slightly thicker as it tends to be (as long as the formula holds true). Units are mm.
58
What is the alternate format of a final lens order for a tricurve bitoric rgp? [i'm using this one bc it's easier]
BOZR1/BOZR2 TD BVP1/BVP2 BOZD 1stBPR (PCW1) 2ndBPR (PCW2) Material Engravings Colour
Note: you could calculate the BPRs by adding on e.g. 0.7 to the bozd or you could just leave it as +0.7 which is way easier and still gets the point across.
59
How do you calculate 1st BPZD?
BOZD + 2PCW1
60
How do you calculate 2nd BPZD
BOZD + 2PCW2
61
What is the iso format of a final lens order for a tricurve bitoric rgp?
BOZR1/BOZR2 TD BVP1/BVP2 BOZD 1stBPR (1stBPZD) 2ndBPR (2ndBPZD) Material Engravings Colour
62
When selecting PCWs, what width should we avoid?
Avoid narrow curve widths (i.e. anything less than 0.25mm is too narrow)
63
When selecting a FOZD, should it be slightly larger or slightly smaller than BOZD? Why?
Should be slightly larger than BOZD to minimise visual disturbance (reduce flare)
64
What is an ideal initial BOZR for a spherical rgp for a spherical cornea? (non-kc)
TD 9-9.7mm: On K or 0.05mm flatter
TD 9.7-10.5mm: Flatter than K by 0.05mm-0.1mm
65
What is an ideal initial BOZR for a spherical rgp on a toric cornea?
cyl < 1.00: On flattest K
cyl > 1.00: slightly steeper than flattest K (0.05-0.1)
cyl >-2.50: fit a bitoric lens
66
What is the 'Jessen' Principle?
By fitting a myopic eye with a flatter BC than K, we can correct myopia to plano
(NB: use flat K as a reference if corneal toricity is present)
67
How can we adjust BC from ORx for an orthoK lens?
ORx = plano: BC correct!
ORx = -0.50: Flatten BC by adding 0.1mm (which is equivalent to 0.50)
ORx = +0.50: Steepen BC by subtracting 0.1mm
NB: BC adjustment for orthoK is NOT for fitting. It is for treatment (tx curve)
68
How does the reverse geometry of an orthoK lens work?
2nd curve (1stBPR) is steeper than BOZR (rather than flatter). This allows greater control of saggital depth as the periphery of the lens is closer to the cornea, allowing the BOZR to be much flatter (and therefore can provide tx)
69
Explain the forces exerted on a reverse geometry lens
+ve push force in the centre
-ve pull force in 1stBPR (aka the "reverse" curve)
70
List 5 advantages of reverse geometry
Improved centration
Rapid corneal flattening
Larger optic zones
More predictable results
Longer retention of tx effect
71
Define "Squeeze Film Force"
When the post-lens tear film is distributed unequally across the corneal surface (i.e. thinner in the centre, thicker in the periphery)
-- a pressure is created as the fluid tries to find equilibrium
-- the corneal epithelium responds to these pressures and causes the central prolate cornea to become more spherical over the treatment zone
72
What happens to corneal epithelial cells in the centre vs mid-periphery when wearing a reverse geometry lens?
Reduced central epithelium thickness
Increased mid-peripheral epithelium thickness
overall result is a flatter cornea
73
List the 3 current fitting methodologies for an orthoK lens
Laboratory Fitting
Topography fitting - great!
Inventory fitting (with a slide rule) - good for paeds
74
Describe the 3 key topographical outcomes for an orthoK lens
Bulls Eye: correct sag, well-centred tx zone
Central island: distortion within tx zone due to excess sag
Smiley face: lens decetres up due to insufficient sag
75
What is the ideal tear reservoir pooling for a CRT fitting of an orhtoK lens?
0.75-1mm
76
What are the 3 independent parameters of a CRT lens? Which 2 of the 3 parameters relate to the fit?
BC - tx zone
RZD (return zone depth) - sagittal depth control
LZA (landing zone angle) - tangent to cornea
RZD and LZA are the 2 fitting variables
77
When will cyl affect an orthoK outcome?
When it's limbal to limbal. Any cyl within the tx zone will be neutralised by the orthoK lens (b/c both meridians will be flattened to the same-ish level of flatness)
78
When will cyl affect an orthoK outcome?
When it's limbal to limbal. Any cyl within the tx zone will be neutralised by the orthoK lens (b/c both meridians will be flattened to the same-ish level of flatness)
79
What 2 parameters does the RZD of an orthoK determine?
Lens centration
Squeeze film force
80
What is the relationship between RZD and sag for an orthoK lens?
As you decrease, RZD, you also decrease sag by the same amount.
E.g. RZD reduction of 25 microns will lower sage by 25 microns
NB: normally change RZD in steps of 25 microns
81
What are the 3 RZD outcomes and how does this affect the orthoK?
Excess sag: bubble in tear reservoir
Correct sag: large zone of benign apical bearing
Insufficient sag: dcentration
82
How can we assess LZA?
Look at the edge lift to assess.
Excess edge lift = increase LZA to tighten
Insufficient edge lift = decrease LZA to loosen
83
What are the outcomes if the LZA is too flat, correct, or too steep?
LZA too flat: loose periphery may cause distortions
LZA correct: lens lands parallel to peripheral cornea
LZA too steep: tight periphery may restrict tear exchange
84
When can you only make a judgement of LZA?
If the lens is centred correctly
85
What is an ideal candidate for an orthoK in terms of refractive error?
Sphere: -1.00 to -4.00
Cyl: less than -1.50WTR, less than -0.75ATR
86
What is an ideal candidate for an orthoK in terms of ocular characteristics? (4)
Regular topography
Stable tear film
Absence of corneal or lid disease
Average pupil size
87
What is the relationship between BPR and AEL (axial edge lift)?
Increase BPR leads to increase in AEL
88
What is the usual value for AEL?
0.08-0.14mm
89
Name 3 methods to alter edge clearance in a lens (important)
Alter BPR or PCW of peripheral curves
Change number of peripheral curves
Alter BOZD
90
How can we change the number of peripheral curves to make an rgp flatter?
increase number of peripheral curves
91
List 5 general considerations for CL wear in a patient
Prev. or new lens wearer
Oc + systemic health (more problems if DED, lid tension, poor ocular hygeine, poor dexterity. Less tear volume if ibuprofen, oestrogen, antihypertensives)
Full time or part time wear
Visual needs
Realistic expectations
92
List 4 characteristics of good candidates for presbyopic CL wear
Emerging presbyopes
Motivated ptient
Existing CL wearer
Low-moderate uncorrected hyperopes
93
List 7 characteristics of more challenging candidates for presbyopic CL wear
Emmetrope with early presbyopia
High visual demander
Late presbyope with high near demands
High hyperope
Dry eye
Small pupils (if considering simultaneous design)
Astig >0.75DC
94
List 6 baseline data needed before fitting somebody with a CL
Ocular dominance
Pupil size (bright/dim)
Accurate refraction (push plus)
SL: tear film
Corneal topo
General CL fitting measures: palp aperture, HVID
95
List 5 disadvantages of bifocal CLs
Unsuitable if lower lid >1mm below limbus, or poor lid tension
Flat fitting lenses = less comfort
Possible lens rotation on downgaze can be a problem
Expense
Chair time
No near vision above primary gaze
96
What is the main disadvantage of a diffractive multifocal CL?
Some light is lost, leading to lower contrast acuity
97
Why is good centration critical in a multifocal CL?
because the optic zone can be very small (e.g. 2mm) and even 1mm decentration can lead to ghosting + blur
98
List 6 disadvantages of monovision CLs
Reduced binocularity and contrast sensitivity
Increased glare
Adaptation problems (asthenopia, poor tolerance/vis. quality)
May require 'enhancement' D and/or N glasses
Unsuitable if asymmetric VA (e.g. amblyope)
Contraindicated for pilots)
99
List 4 common indications and 5 other indications for CL wearing
Common: High Rx, Anisometropia, Presbyopia, Elective paediatric
Others: Corneal irregularity, Oc. disfirgurement, Post-keratoplasty, Paediatric, "Bandage"
100
Which eye is usually given the distance correction in a monovision CL?
The dominant eye
101
For up to what level of near add does monovision tend to be succesful?
Any add up to 1.75 (ideally want lower than this)
102
List the main advantage of DALK (deep anterior lamellar keratoplasty)
Preserves descemet's and endothelium, leading to greater structural integrity of the post-graft cornea and lesser chance of graft rejection
103
What determines the graft size in PK (penetrating keratoplasty)? (2)
size of recipient cornea
area of disease
104
Why are PK grafts usually 7.5-8.5mm in diameter? Why is it not ideal to go outside that range?
>8.5mm: up risk post-op increase in IOP, Ant. synechiae, vascularisation
<7mm: increased astig due to increased tension from host (esp. in KC)
105
What is the normal endothelial cell density for a 20yo?
2,800 cells/mm^2 (NB: this varies with race)
106
What is the normal endothelial cell density for a 80yo?
2,000-2,500 cells/mm^2 (i.e. they've lost 300-800 since 20)
107
At what endothelial cell density does corneal decompensation occur?
500-1000 cells/mm^2
108
List 5 complications following graft surgery
Fixed anterior chamber
acute post-op glauc
persistent epithelial defects
infection
fixed dilated pupil (due to ischemic atrophy of sphincter)
109
How is vision immediately after graft surgery?
Hazy due to folds in descemet's (this gradually improves over weeks-months)
110
When might we alter the number of sutures prematurely in a graft patient?
If excessive corneal astig on topo (>10-15D): may selectively remove or insert sutures to reduce astig (by changing the graft shape)
111
List 12 ways to assess grafts
Topo
Corneal toricity
Graft diameter
Location of graft
Check for scarring/haze
Check for BV ingrowth
Check for staining on host/graft
Check host corneal thickness
Check inferior graft edge to see if seperating/dehiscence
Check lids for papillae
Check endothelium for polymegathism
Check graft thickness
112
What does inactive BV ingrowth look like in a graft patient? How can we best assess them?
looks like ghost vessels (faint dark tracks that are hard to see). Best assessed with retro-illumination
113
What can cause irregular astig on the graft? (3)
Excess + variable suture tension
Poor suture alignment
Significant asymmetric host thinning
114
What is the aim of RGP lens fitting for a graft patient
Even bearing across the graft, with no intense areas of bearing, without excessive clearance and adequate but not excessive edge clearance
115
Name the 3 types of ACL limbal lift lenses and state what they are used for
E series: normal shaped corneas + relatively flat grafts + PMD + high ametropia (BOZD = 7.5mm)
K series: KC eyes (they have flatter peripheries)
G series: steeper/higher graphs (BOZD = 8mm). Has a reverse geometry design.
116
How does the E and G series numbering for the ACL limbal lift relate to steepness/flatness of the lens
E7/G7 = flatter edge
E5/G5 = standard edge
E2/G2 = steeper edge
As number goes up, so does edge flatness and clearance
(NB: G5 is steeper than E5)
so it's the same logic as K series actually, except K series has K7 as the standard.
117
What does RSVP stand for?
Graft rejection symptoms
Redness
Sensitivity to light
Vision changes
Pain
118
What is a khodadoust line? What is it a sign of?
KP (keratic precipitates) in a line advancing in from the peripheral cornea.
--- Presents with mild to moderate cells in ant. chamber and folds in descemet's and stroma
Is a sign of endothelial rejection.
119
List the 6 steps in the standard in office RGP disinfection protocol for a trial lens
Surfactant 30s
Rinse z sterile saline
Soak in Hydrogen Peroxide (3hrs), NaHypochlorite 0.4% 30 min or NaHypo 0.5% 5min
Rinse z sterile saline
Store dry
Surface clean + rinse prior to use
120
List the 6 steps in the standard care routine for an RGP
Hand washing
Surface clean 30s
Rinse
Disinfect
Protein Clen
CL case care
121
What are 2 contraindications for using abrasive surfactants like boston advance on rgps?
Hyper-dK
Plasma treated lenses
122
Is alcohol suitable as a surface cleaner for rgps? Why/why not?
No. While it removes mucin, it can alter lens parameters with repeated use, making the CL brittle.
123
What is the minimum dk/t to avoid corneal oedema for:
- daily wear lenses
- extended wear/overnight lenses
Daily wear: 24.1
Extended/overnight wear: 87.0
124
What is the minimum dk/t to avoid stromal anoxia in extended wear lenses?
125
125
What does "RSVP" mean in regards to contact lens wear?
A contact lens wearer should remove their contact lenses if they experience any of:
Redness
Soreness
Vision change
Photophobia
126
List 6 ocular effects of hydrogel extended wear
Limbal hyperemia (few hours in)
Epithelial microcysts (> 2mo hypoxia)
Stromal oedema (acute/reversible)
Stromal neovasc
Endothelial blebs (short term)
Endo polymegathism
127
What is CLARE?
Contact Lens Acute Red Eye. An immune response to acute or chronic hypoxia that is associated with high bacterial load (-ve or +ve) on the CL.
128
How might a tight fitting lens contribute to CLARE development?
no tear exchange to flush toxins
129
List 5 signs of CLARE
Painful red eye +/- photophobia [esp in morning]
Diffuse + focal infiltration
Corneal epithelial + stromal oedema
Conj + limbal edema, injection
Peripheral infiltrates
130
How can you manage CLARE? (2)
Cease lens wear
Consider steroid +/- AB (antibiotics)
131
Using PEDALS, describe an infectious inflammatory infiltrate
Pain - yes
Epi defect - full thickness
Discharge - possible
Ant chamber - cells/flare
Location - central/mid-peripheral
Size - > 1mm
132
What is the pass criteria for a disinfecting CL solution in a stand alone test? (2)
Bacteria reduction by 3 log units (99.9%) within MRDT
Mould and yeast reduction by 1 log unit (90%) within MRDT (+ no increase within 4xMRDT)
MRDT = minimum recommended disinfection time
133
List 6 benefits of miniscleral lenses
excellent stability
no debris entrapment
excellent comfort
minimal edge awareness
excellent vision esp at night
should permit reliable wearing times
134
List 4 contraindications for monovision contact lenses
>1.75 add
asymmetric VAs
occupation needing depth perception (e.g. pilot)
monocular patient
135
List the 3 steps involved in fitting a hybrid lens
Determine the vault
Determine skirt curvature
Determine lens power
136
What is the most common source of failure for hybrid lenses?
over vault
137
What is the aim for skirt curvature in hybrid lens, in terms of fluorescein pattern?
fluorescein thinning in ILZ
Gentle bearing in OLZ
138
List 3 indications for contact lens wear post graft surgery
ametropia
irregular astigmatism (usually VA < 6/12)
refractive anisometropia (diff of > 4.00D)
139
Are hybrid lenses suitable for corneal grafts?
no
140
For what type of scenarios is a hybrid clearkone lens considered a "priority choice"? (5)
Monocular corrections
Strong history of RGP intolerance
Part time lens wear
Dusty/windy occupational environment
Application for sport/recreational activities
141
How can you assess tear film thickness in a miniscleral lens? What is the ideal tear film thickness?
Compare the fluorescein stain in the tear reservoir layer to the dark band of a known value in microns (usually 300 microns). Aim to have a 1:1 ratio (meaning tear film thickness should be equally is thick as the dark band, to provide an ideal level of corneal clearance)
