Week 1 - RGP fitting 1 Flashcards
What are the 4 types if RGP lenses?
• Corneal (7-12mm) - our focus
• Corneal-scleral (12.1-15mm)
• Mini-Scleral (15.1-18mm)
• Scleral (>18mm)
what are the advantages to RGP’s? (1-3)
- Robust
- Can be re-polished (if not surface treated
Longer lens life (i.e. years) - Easy to clean
- Soaking solutions stronger
- Fewer Solution problems (constituents don’t soak into lens) - Easy to handle (relatively)
what are the advantages to RGP’s? (4-8)
- Stable vision
Can avoid need to fit toric lenses (tear lens) - Good oxygen supply
- Dehydration less problematic (compared to soft lens materials)
- Customised designs
- Good tear exchange
- Aids metabolism
Disadvantages of RGP lenses?
- Physical comfort
- Edge sensation present to begin with
- Need to build up tolerance
- Poor for intermittent wear - Greater movement of lens
- May become dislodged - FB entrapment
When do you chose an RGP over SCL?
• Moderate corneal astigmatism
• High complex prescriptions
• History of allergies
• Problem with SCLs
• Financial reasons
• Poor compliance
• High Visual demands
When should you avoid using RGP lenses?
• Contact sports
- Rugby, judo, karate, boxing etc
• Water Sports
- May get washed out
• Dusty environments
What is the definition of BOZR and BOZD??
• BOZR : Back optic zone radius
• BOZD : Back optic zone diameter
What are the parameters for BOZD and FOZD, centre thickness and edge lift?
• BOZD: 1-1.5mm smaller than TD. Large enough to cover pupil in different light conditions.
• FOZD: 0.5mm larger than the BOZD
• tc: Centre thickness: Too thin a lens leads to flexure which can lead to breakage and residual astigmatism
Edge lift: to prevent mechanical pressure on the cornea, allow tear exchange and enable lens removal
What are the 3 types of tear lenses?
• Flat fit creates negative tear lens
• Alignment fit creates neutral tear film
• Steep fit creates positive tear lens
What are the tear lens rules of thumb?
• Rigid lens: Tear lens power increases about +0.25D for each 0.05mm that the BOZR of lens is steeper than corneal radius
• For the BVD of the rigid contact lens needs to be changed by -0.25D for each 0.05mm that the BOZR is made steeper, to compensate for the extra positive power of the liquid lens
- If the lens BOZR is made flatter by 0.05mm, the BVD needs to be changed by +0.25 D
British standards - what is the minimum specification?
• BOZR/ TD / BVP
e.g. 7.55/9.20/-5.25
Then lens type and material
What is spherical vs aspherical
• Spherical - spherical back optic zone with spherical peripheral zones
• Aspheric - elliptical lens design, gradual flattening from the centre out
What steps are needed for selecting a 1st RGP lens?
- Find out about the eye
- Shape, size, pupils and refractive error
- Health (slit lamp, history & symptoms) - Choose the appropriate lens type
- Work out the shape you want
- Find the nearest available - Once you know what shape you are going to order, work out the power you need - BVP calculations from Tri A
What steps need to be considered before selecting an RGP material? (1-2)
- Corneal Oxygen Requirements
- Dk (Oxygen transmissibility)
- Thin lens - more O, to lens
- Thick lens - easier to handle - Centre thickness of lens (esp. high Plus)
- Consider edge form too - can be like a knife if manufacturer trying to maximise oxygen permeability
- Maximum wearing time required
What steps need to be considered before selecting an RGP material? (3-5)
- Wettability - ability of the tears to form a stable layer on the surface of the material
- Flexure
• Can cause residual astigmatism on toric corneas
• Especially true of thinner lenses (less than 0.15mm) - Compliance
If likely to be poor go for simple to care for materials
What are common RGP Materials?
• Silicone acrylate - more prone to lens flexure
• Fluorosilicone acrylate - better wettability, fewer deposits. But can be brittle if too thin.
• Fluorosiloxanyl styrene
Common trade names are Boston and Optimum
What is considered before Fitting the lens?
• Pupil size
Make sure optical portion >scotopic pupil
• Environment
- Atmosphere
- Dusty? Dry?
- Activity (sports etc)
So how do we know if the RGP lens is suitable to fit depending on factors?
• Check manufacturers advice
- if possible, chose first lens according to manual
How do we fit the BOZR if different in K is ≤ 1DC?
• If difference in K readings is ≤ 1DC
- Fit on flattest K (round to 0.05mm)
• Positive lenses tend to drop a little, (central thickness) consider fitting 0.05mm steeper to stabilise
How do we fit the BOZR if different in K is > 1DC?
• Can still be worth trying a spherical RGP BOZR:
• Using the GENERIC Formula
BOZR = FlatK - (FlatK - SteepK)/3
• Round to nearest 0.05mm, this will give you a lens slightly steeper than flattest K
• You can use this formula for any level of toricity - if toricity minimal, this will round to Flat K, so you don’t need to remember lots of formulae!
What are the different sources if deciding a BOZR?
• Contact lens manual
• Contact kens practice- depends on BOZD, Flattest K - 0.05 for standard BOZR
How is BOZD determined?
• Often predetermined by the laboratory
• But you need to know if the standard lens won’t work due to pupil size
• Can also consider a smaller BOZD in a toric cornea to minimize the area of mismatch
Small: <7.30mm (need a steeper BOZR)
Medium: 7.30-7.90mm
Large: >7.90mm (need a flatter BOZR)
How is TD (total diameter) decided?
Consider pupil size
• BOZD > (scotopic) pupil by 1- 2 mm (roughly)
• TD < HVID (Horizontal Visible Iris Diameter) By ~2mm (roughly)
• TD > BOZD By ~2mm (roughly)
• Remember these are STARTING points
• Consider availability
• Most lenses are between 9.20 to 9.80mm in diameter
How are the pupil diameters measured?
• Measured with a ruler
• Need maximum (dim light) and minimum (bright light)
• Can use Burton lamp to help with this
• If no Burton lamp then have the room as dim as possible for maximum that still allows you to see the pupil margin
• Can use slit lamp for minimum
