Week 3: Rigid Lens Fitting
RGP Movement: post-blink movement of how much is normal?
1-2mm
Corneal cyl is equal to approximately how much BOZR?
1D ~ 0.16 mm
Fluorescence becomes effectively invisible below how many microns of tear thickness?
20 microns tear thickness
fluorescence is at its maximum at how many microns of tear thickness?
60 to 80 microns
fit on K or slightly flatter than K reading for lens diameters of what?
9.0 to 9.7mm
fit flatter than K readings by about 0.05 to 0.1 mm for lens diameters of what
9.7 to 10.5mm
what is the Lens prescription format for RGPs?
BOZR/LD/BOZD/PCRx PCW/BVP
how does a wratten filter work?
Blocks lower blue wavelengths and transmits yellow and higher wavelengths, This makes NaFl more visible, Improves rigid lens fit assessment and detection of subtle surface staining
Astigmatic K readings: How do you fit an RGP If Corneal cyl 1.00D-3.00D?
Fit lens BOZR about 1/3 the difference steeper than the flattest K reading
Astigmatic K readings: How do you fit an RGP If Corneal cyl < 1.00 D?
Fit on the flattest K
Astigmatic K readings: Corneal cyl is <1.00D, K90=47.00D 7.18 mm, K180=46.25D 7.30 mm. what BOZR do you fit?
Fit on the flattest K, fit 7.3mm BOZR
Fitting in the mid-periphery: How do you reduce mid-peripheral bearings?
Flatten the inner peripheral curves, Reduce the BOZD and add extra peripheral curve in this region
How does a Steeper BOZR improve a Low Riding RGP Lens?
If lens is too flat, the steeper BOZR will create more central pooling and mid-peripheral bearing, this makes the lens centre better and move less with blink, flatter fits ride low and move more
back optic zone radius (BOZR) influences what?
Influences alignment, centration, comfort, movement, tear exchange
what are the steps in RGP Fitting?
Keratometry or videokeratoscopy, Trial lens fitting, Over-refraction on optimal trial lens, Lens order (Customise lens design (if needed), Calculate back vertex power (BVP), Select material)
why is a larger diameter lens an option to improve a Low Riding RGP Lens?
Larger LD sits under top lid and improves chances of upper lid traction fit, lens held in position by upper lid grabbing top edge of lens
why is a larger diameter lens the first option to improve a Low Riding RGP Lens?
Larger LD sits under top lid and improves chances of upper lid traction fit, lens held in position by upper lid grabbing top edge of lens
how do you improve a Low Riding RGP Lens?
Larger diameter (first option), Steeper BOZR, Flatter peripheral curves, Thinner lens
Rigid lens diameter and centration: what tend to be more comfortable?
Larger diameters and lid traction fits
Fitting in the mid-periphery influences what?
Lens centration, Tear exchange
Edge Design influences what?
Lens comfort, Mechanical interaction with cornea, Mechanical interaction with lids
which lens fit/centration causes increased 3-9 staining Probably as a result of increased incomplete blinking?
Low riding fit
Axial edge clearance is predominantly determined by what?
Peripheral curve radius, Peripheral curve width
Appropriate edge lift is necessary for what?
Stop mechanical abrasion, Promote tear exchange, Reduce 3-9 o'clock staining
How does a Thinner lens improve a Low Riding RGP Lens?
Thinner lens reduces mass and reduces interaction between eyelid and lens edge during blink, used for interpalpebral fits, less likely to be pushed down by upper lid during blink
Astigmatic K readings: If Corneal cyl < 1.00 D a) Fit on the flattest K b) Fit on the steepest K c) Fit lens BOZR, about 1/3 the difference steeper than the flattest K reading d) Use toric back surface
a
Axial edge clearance is predominantly determined by Peripheral curve radius, Peripheral curve width. Small clearance causes a) pressure/rubbing on blink b) discomfort on blink and 3-9 o'clock staining
a
BOZR Selection: BOZR Selection depends upon the size of the optic zone (OZD) of the lens. Larger optic zones require... a) flatter BOZR b) steeper BOZR
a
For Lens Diameters 9.0 to 9.7 mm... a) fit on K or slightly flatter than K reading. b) fit flatter than K readings by about 0.05 to 0.1 mm c) fit slightly steeper than K readings.
a
If a lens has BOZR 7.7mm and corneal K reading of 7.8mm, what is the fit a) steep b) flat c) alignment
a
Lens Diameters 9.0 to 9.7 mm: a) fit on K or slightly flatter than K reading. b) fit flatter than K readings by about 0.05 to 0.1 mm c) fit slightly steeper than K readings.
a
Peripheral curves are... a) flatter (longer radius) than BOZR b) steeper (shorter radius) than BOZR
a
QUESTION 3 For a cornea with a radius of 7.8 mm fitted with an RGP of 7.7 mm, the power of the tear fluid lens would be approximately: a) +0.50 D b) +1.00 D c) -0.50 D d) -1.00 D
a
QUESTION 9 Which type of RGP fit tends to be the most comfortable for the wearer? a) Lid traction b) Flat c) Low riding d) Interpalpebral
a
Rigid lens diameter and centration: what tend to be more comfortable? a) larger diameter b) smaller diameter
a
True or false? Rigid lens trial fitting set BOZR varies, BVP and LD usually are fixed a) true b) false
a
fit flatter than K readings by about 0.05 to 0.1 mm for.... a) Lens Diameters 9.7 to 10.5 mm b) Lens Diameters 9.0 to 9.7 mm
a
true or false? Dimensions of a lens designed to closely follow the average corneal profile a) true b) false
a
true or false? Peripheral curves are flatter (longer radius) than BOZR a) true b) false
a
true or false? Ptosis occurs following rigid lens wear a) true b) false
a
true or false? flatter fits ride low and move more a) true b) false
a
The BOZR is a spherical approximation of what?
an aspheric (elliptical) corneal profile
Fitting in the mid-periphery: The peripheral curves are designed to do what?
approximate the changing shape of the cornea
Astigmatic K readings: How do you fit an RGP If Corneal cyl 1.00D-3.00D? Fit lens BOZR about 1/3 the difference... a) flatter than the flattest K reading b) steeper than the flattest K reading
b
Axial edge clearance is predominantly determined by Peripheral curve radius, Peripheral curve width. Large clearance causes a) pressure/rubbing on blink b) discomfort on blink and 3-9 o'clock staining
b
BOZR Selection: BOZR Selection depends upon the size of the optic zone (OZD) of the lens. Smaller optic zones require... a) flatter BOZR b) steeper BOZR
b
Fitting in the mid-periphery: Which is correct? a) Steeper curves are required in the mid-periphery (compared with the central BOZR) b) Flatter curves are required in the mid-periphery (compared with the central BOZR) c) Equivalent curves to BOZR are required in the mid-periphery
b
For larger lens diameters... a) fit on K or slightly flatter than K reading. b) fit flatter than K readings by about 0.05 to 0.1 mm c) fit slightly steeper than K readings.
b
If a lens has BOZR 7.9mm and corneal K reading of 7.8mm, what is the fit a) steep b) flat c) alignment
b
Lens Diameters 9.7 to 10.5 mm: a) fit on K or slightly flatter than K reading. b) fit flatter than K readings by about 0.05 to 0.1 mm c) fit slightly steeper than K readings.
b
QUESTION 8 If the optic zone of an RGP is increased in size, to obtain the same fit the BOZR should be: a) Halved b) Increased c) Kept the same d) Decreased
b
Rigid lens diameter and centration: what tend to be more comfortable? a) no lid traction fits b) lid reaction fits
b
fit on K or slightly flatter than K reading for... a) Lens Diameters 9.7 to 10.5 mm b) Lens Diameters 9.0 to 9.7 mm
b
which is correct? a) steeper fits ride low and move more b) flatter fits ride low and move more
b
what is the most important determinant of fit for RGP lenses?
back optic zone radius (BOZR)
BOZR Selection: BOZR Selection depends upon the size of the optic zone (OZD) of the lens. why do larger optic zones require flatter BOZR?
because the cornea is flatter in the periphery
Fitting in the mid-periphery: why are Flatter curves are required in the mid-periphery (compared with the central BOZR)?
because the cornea is flatter in the periphery
Fluorescence becomes effectively invisible when?
below 20 microns tear thickness
Astigmatic K readings: If Corneal cyl 1.00 - 3.00D a) Fit on the flattest K b) Fit on the steepest K c) Fit lens BOZR, about 1/3 the difference steeper than the flattest K reading d) Use toric back surface
c
If a lens has BOZR 7.8mm and corneal K reading of 7.8mm, what is the fit a) steep b) flat c) alignment
c
QUESTION 10 A cornea with a radius of 7.8 mm is fit with an RGP with BOZR of 8.0 mm. This contact lens is most likely to fit: a) Optimally b) Steep c) Flat d) Tight
c
QUESTION 2 Your patient has K readings of 7.90 and 7.60, and you are using a lens diameter of 9.5 mm. Empirically, what BOZR should you try? a.7.60 mm b.7.70 mm c.7.80 mm d.7.90 mm
c
QUESTION 4 Fluorescein is invisible when tear thickness is below: a. 5 microns b. 10 microns c. 20 microns d. 40 microns
c
QUESTION 5 Your patient has K readings of 7.65 and 7.60, and you are using a lens diameter of 10.0 mm. Empirically, what BOZR should you try? a. 7.50 mm b. 7.60 mm c. 7.70 mm d. 7.80 mm
c
QUESTION 6 How should the BOZR be selected when fitting a cornea with between 1-3 D of astigmatism? a) On K or slightly flatter b) Use a toric back surface c) 1/3 difference steeper than the flattest K d) On K for an alignment fit
c
RGP Movement: post-blink movement of how much is normal? a) 0.25mm b) 0.5mm c) 1-2mm d) 3-5mm
c
Rigid lens diameter and centration: what is Optimal centration of an RGP lens? a) central only b) high c) central or slightly high d) low e) low or central
c
Rigid lens diameter and centration: what is Optimal centration of an RGP lens?
central or slightly high
What is the first option to improve a Low Riding RGP Lens?
choose Larger diameter
The degree of fluorescence depends on?
concentration of fluorescein used, the tear flow, the wavelength of the blue light, fluorescein on the lens front surface, the brand of fluorescein used, the pH of the saline used, the thickness of the anterior tear layer (~5 microns)
Corneal cyl >2.5 D then a) Fit on the flattest K b) Fit on the steepest K c) Fit lens BOZR, about 1/3 the difference steeper than the flattest K reading d) Use toric back surface
d
QUESTION 1 To reduce mid-peripheral bearing, you might: a. Reduce the BOZD b. Increase the number of peripheral curves c. Increase the BOZR d. All of the Above
d
QUESTION 7 Which alteration would help to correct a low riding RGP lens? a) Steepen BOZR b) Increase lens diameter c) Decrease lens thickness d) All of the above
d
What can you alter to influence RGP centration? a) back optic zone radius b) Peripheral curvature radius c) Peripheral curvature width d) Lens diameter e) back optic zone diameter
d
what RGP lens centration is uncomfortable and lead to incomplete blinks and 3-9 staining? a) central b) high riding c) central or slightly high d) low riding
d
which lens fit causes increased 3-9 staining Probably as a result of increased incomplete blinking? a) Lid traction fit b) Interpalpebral fit d) Low riding fit
d
Astigmatic K readings: Corneal cyl is 1.00D-3.00D, K 90=47.50D, 7.11 mm, K 180=45.00Dm, 7.50mm. what BOZR do you fit?
fit 7.35 to 7.4mm BOZR (Fit lens BOZR, about 1/3 the difference steeper than the flattest K reading (e.g. 1/3 difference=0.13 mm)
For Lens Diameters 9.7 to 10.5 mm, how should the RGP be fit?
fit flatter than K readings by about 0.05 to 0.1 mm
A RGP Lens Diameter is 9.0 to 9.7 mm, for K readings = 7.6 mm, what BOZR Selection would you make?
fit lens 7.6 or 7.65 mm BOZR
A RGP Lens Diameter is 9.7 to 10.5 mm, for K readings = 7.6 mm, what BOZR Selection would you make?
fit lens 7.65 or 7.7 mm BOZR
For Lens Diameters 9.0 to 9.7mm, how should the RGP be fit?
fit on K or slightly flatter than K reading
Lens fit terminology: If a lens has BOZR 7.9mm and corneal K reading of 7.8mm, what is the fit?
flat fit (0.1mm flatter than K), loose fit
Low riding lenses cause increased 3-9 staining, Probably as a result of what?
increased incomplete blinking
How does a Flatter peripheral curves improve a Low Riding RGP Lens?
increases edge lift and improves chances of upper lid grabbing top edge of lens, negative carrier used for plus designs
how do you select initial trial lens for RGPs?
keratometry/videokeratoscopy readings
Alter _____________ to influence centration.
lens diameter
What can you alter to influence RGP centration?
lens diameter
There is an approximately what relationship between 20 to 80 microns thickness and fluorescence?
linear relationship
what RGP lens centration is uncomfortable and lead to incomplete blinks and 3-9 staining?
low riding lenses
A wratten filter blocks lower blue wavelengths and transmits yellow and higher wavelengths, what benefit does this have?
makes NaFl more visible, Improves rigid lens fit assessment and detection of subtle surface staining
What is 3-9 o'clock staining why does it occur?
occurs due to too much edge lift on lens, causes discomfort with each blink and patient begin incomplete blinking due to the discomfort, resulting lack of lubrication from tears outside the edge of the edge of the RGP
what is normal RGP Movement?
post-blink movement of 1-2 mm is normal, lens should recentre consistently and quickly
why is some lens movement (post-blink movement of 1-2 mm) good?
promotes post-lens tear exchange
what is the most comfortable edge shape for RGPs?
round round
what is the most uncomfortable/painful edge shape for RGPs?
square square, square round
Lens fit terminology: If a lens has BOZR 7.7mm and corneal K reading of 7.8mm, what is the fit?
steep fit (0.1mm steeper than K), tight fit
BOZR Selection: BOZR Selection depends upon what?
the size of the optic zone (OZD) of the lens
what is bad about RGP Low riding lenses?
uncomfortable and lead to incomplete blinks and 3-9 staining
