Clinic: Contact Lenses [copied] Flashcards
What is the expected corneal thickness of a normal patient vs one with mild KC?
Normal: 545um
Mild KC: 473um
What is the expected corneal thickness of a patient with severe KC?
Severe KC: 415um
What is a Fleisher’s ring, and in what condition does it occur?
Brownish partial or complete ring of iron deposition in the epithelium at the base of the cone in KC patients
What is the main advantage of topography over keratometry for KC patients?
Toppography measures corneal thickness for the whole cornea whereas keratometry only measures 6% of the cornea
What do axial maps measure?
Refractive/corneal power (they closely mimic K values)
What do tangential maps measure? What condition is this particularly useful for?
shows location of any corneal irregularity. Useful for KC.
What do elevation maps measure?
the highs + lows of the cornea (by subtracting a normal reference sphere)
How do you interpret elevation maps?
red/yellow = cornea above ref. sphere Blue/green = cornea below ref. sphere
What do subtraction maps measure?
shape change in a cornea comparing before + after to see how it differs
What are subtractive maps good for?
Observing change from orthoK lens or laser sx for myopic correction
What do refractive power maps measure?
Based on snell’s law of refraction, which is the best estimate of corneal power
Which topographical scale is most useful for KC monitoring? Standard power, K scale, or Normalised?
Normalised (39-60D)
What does PEI measure?
Distance from centre of cornea (apex) to the steepest part of the cornea
How is PEI used to ddx KC from PMD?
KC: PEI = 1.95mm
PMD: PEI = 3.5mm
What is shape factor (P) and how does shape factor differ between KC and PMD?
Asphericity of the cornea (prolate = +ve, oblate = -ve, basically)
KC: Highly +ve (>0.50)
PMD: Usually -ve (<0.15)
What is corneal eccentricity? What are the normal and abnormal values for it?
Rate of corneal flattening from centre to periphery
Normal < 0.65
Abnormal > 0.65
For K values, what represents:
- Normal K value
- Steep normal K value
- Normative range for K values
- Steep pathological K value
Normal: 43D (7.8mm)
Steep normal: >45D (<7.5mm)
Normative range: Anything basically <46.9D
Steep pathological: >47D
What does the IS Index measure? What is the abnormal value and what might this value suggest?
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?)
What does the SAI measure? What is the normal value?
Surface Asymmetry index: difference in superior vs inferior corneal powers at oblique perpendicular axes
Normal <1.0D
(therefore abnormal is >1D)
What does SRI measure? What is the normal value?
Surface Regularity Index: assesses the smoothness of the central cornea. Measures local fluctuations in corneal power over the pupil
Normal <0.56D
What is the main benefit of silicon hydrogel lenses (cf hydrogel) for early KC patients?
increased stiffness masks milder corneal irregularity
List 4 corrective lens options for moderate/severe KC patients
Spherical RGPs (either tricurve or quadcurve) Aspheric lenses (e.g. Gelflex/ACL) Hybrid lenses (RGP centre, soft periphery) Minisclerals (e.g. capricornia)
What is the minimal tear layer thickness required for tear film to fluoresce?
20mm!!
What is the ideal fluorescein clearance for an RPG lens for a KC patient? How does this appear on assessment?
Centre: 20-30mm
Edge: 70-90mm
Appears as “Feather Clearance” (3-point touch)
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)
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
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
When do you use small <9mm RGPs in KC? (3)
Advanced nipple; smaller cones; narrow palp aperture
When do you use larger >9mm RGPs in KC? (3)
Early KC; Oval/globus; Wider palp apertures
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)
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
What is one parameter of an RGP you can adjust if the lens drops down?
Try increasing diameter
What is the aim of an inferior tuck?
to reduce excessive edge clearance
How does the steepness of the periphery relate to clearance?
Steeper in periphery = less edge clearance (opposite of what happens in the centre)
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)
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)
How do you calculate tear film power?
336/BOZR(mm)
basically we are converting from mm to D
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
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
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
List 5 indications for RGPs
OrthoK/Myopia control KC High refractive error Dryness with soft disposables Prevoius RGP wearer
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
What is the ideal BOZR fit for a spherical cornea?
Fit on K or tiny bit flatter (like 0.05mm)
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
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
What is a SPE bitoric?
When the dioptric difference in toric BOZR = dioptric difference in BVP (i.e. the astig)
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)
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)
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
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)
When do you use a front surface toric/back surface spherical?
When there is no corneal astig (i.e it’s all RA)
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]
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
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
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
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)