3. Spherical RGP Fitting I Flashcards
What are the 5 other clinical terms for rigid corneal lenses?
- RGP - Rigid Gas Permeable
- GP lens - Gas Permeable
- Rigid lens
- Corneal lens
- Hard lens
Compare and contrast RGP vs SCL in terms of material, durability, water content and Dk.
RGP
* Material: hard, rigid
* Durability: lasts about a year
* Water content: impermeable
* Dk (O2): 100+ (e.g. Boston XO)
SCL
* Material: silicone-hydrogel
* Durability: disposable (DD, FN, M)
* Water content: porous/ sponge
* Dk (O2): 100+ (e.g. Biofinity)
Describe the typical size and movement of RGP lenses
- Diameter tend to be smaller than the cornea (much smaller than SCL)
- Movement around 1-2mm with blinks (expect more movement compared to SCL)
Compare and contrast RGP and SCL in terms of manufacture type, available parameters, manufacture method and origins.
RGP
* Type: Custom made
* Parameters: Any BOZR, BVP, Dia
* Manufacture: Individual computer-lathe
* Origin: Melb, Brisb, Perth
SCL
* Type: Mass produced
* Parameters: Limited BOZR, BVP (depending on brands)
* Manufacture: Factory cast-moulding
* Origin: USA, Europe, UK
What are the 6 common indications for RGP?
- Orthokertology - myopia control
- Keratoconus
- High refractive error
- Astigmatism/ Grafts
- Dryness with SCL
- Previous GP lens wear
What are the 5 advantages of RGP lenses?
- Corrects irregular refractive conditions, give better optics & VA compared to specs & SCLs (keratoconus, graft)
- Larger range of correction
- Provides astigmatic & multifocal corrections
- OrthoK - myopia control
- Lenses durable & long lasting
What are the 4 precautions of RGP?
- Initial discomfort & adaptation time
- Lens ejection/ loss occasional
- Dirt/ FB may be trapped under lens
- Not suitable for part-time wear
What 4 px characteristic would complicate RGP wear?
- Previous SCL wear
- Part-time lens wear
- Contact sport or dusty environment → consider miniscleral CL
- Low corneal, high residual cylinder → consider front surface toric
What are the 3 pre-trial assessments that should be done for potential RGP wearers?
- Dexterity - ask px to try putting on lens - able to hold eyelids up & put lens in at the same time?
- Sensitivity - observe px response during trial fit
- Motivation - degree of vision improvement e.g. in keratoconus
Which reference point is used to select the initial back optic zone radius (BOZR) for fitting a spherical RGP? What is this rule called?
Flat K axis = reference axis = Cyl axis
Javal’s rule = you can predict the refractive error, particularly the cyl from the K readings
Determining BOZR for spherical RGPs
1. Measure central corneal curvature and identify the Flat K (less curved radius)
2. Calculate the corneal astigmatism = difference between flat and steep K (ΔK)
3. Calculate the initial base curve radius by referring ot the relevent selection chart for a given lens design/ diameter
What constitutes as an ideal RGP BOZR? (6 points)
- Near alignment to corneal Flat-K reading
- Allow proper tear exchange
- Allow appropriate lens movement
- Provide optimal lens centration
- Prevent corneal distortion
- Appropriate NaFl pattern
Surface tension = inverse square of post lens tear film thickness
* Tear film effect is similar to capillary action
* Force of attraction increases with alignment
* High attraction = less CL movement and more stable
Frictional forces relates to the volume of tear film layers
* High amounts of tears = reduce capillary action ∴ increase movement
* Low amounts of tears = increase capillary action ∴ reduce movement
What would the RGP lens power be equal to?
Lens power will be equal to the spherical component of the spectacle correction, corrected for vertex distance, expressed in minus cylinder format, for an “On-K” fit
How will the tear lens affect refraction/ power of the lens?
* Steeper than K
* On K
* Flatter than K
- Steeper than K = plus tear lens
∴ add minus power to the lens - Steeper Add Minus (SAM) - On K = plano
- Flatter than K = minus tear lens
∴ add plus power to the lens - Flatter Add Plus (FAP) - 0.25D for every 0.05mm change in BOZR
Determining BVP for RGPs
1. Begin with spectacle refraction. Convert to minus cyl form if required.
2. Correct for vertex distance if the spherical power if greater than 4.00D
3. If the BOZR differs from Flat-K, apply the rules of SAM or FAP to determine lens power. Change 0.25D for 0.05mm change in BOZR
4. Confirm BVP at delivery with over-refraction
Tear lens help correct corneal astigmatism. The tear lens can neutralise 90% of front surface astigmatism. Back surface corneal astigmatism neutralises remaining 10%. Hence, when GP lens is placed on the eye, resultant tear lens neutralises corneal astigmatism.
The keratometer measures front surface corneal radii but gives total corneal power. This is based on assumption that back surface is 10% of front surface astigmatism. The refractive index of a keratometer is slightly different compared to that of the cornea.
What is the solution for the following trial results:
* Trial lens BOZR: 7.80mm
* Trial lens BVP: -3.00DS
* Over-Rx: -1.00DS
* BOZR ordered: 7.70mm
If ordering 7.80mm, BVP should be -4.00DS according to over-Rx.
But currently ordering 7.70mm, therefore with rule: 0.25D for every 0.05mm change in BOZR, and SAM (decrease 0.10mm)
∴ -4.00 + 0.50D → BVP = -4.50D
Double check BVP at delivery with over-Rx
There is a limit to using the BVP approximation method. What is the limit of K? What should be done instead if it exceeds the normal range?
Limit to normal K range up to 46.0D (7.30mm)
Should calculate 336/old BOZR - 336/new BOZR
Or use tables.
Kv: 7.60 @90
Kh: 7.60@180
LE Spec Rx: -3.00DS (6/6)
LE Ocular Rx: same
Given that fit is on K. What BOZR (mm) and BVP (D) should be selected?
BOZR = 7.60mm
BVP = -3.00D
Kv: 7.60 @90
Kh: 7.60 @180
LE Spec Rx: -3.00DS (6/6)
LE Ocular Rx: same
Given that fit is steeper than K. What should the BOZR and BVP order be?
BOZR = 7.50D
BVP = -3.50D
Kv: 7.50 @90
Kh: 7.60 @180
LE Spec Rx: -3.00/-0.25x180 (6/6)
LE Ocular Rx: same
Given that fit is on slightly toric cornea. What should the BOZR and BVP be?
BOZR = 7.60mm
BVP = -3.00D
