RGP CLs for Astigmatism (Toric Fitting) Flashcards
1
Q
When should you choose a toric RGP?
A
- When lens w/ spherical front & back radii has not achieved adequate VA
- When spherical lens has not provided suitable physical fit
2
Q
Describe residual astigmatism?
A
- Astigmatism that remains when lens has been used to correct ametropia (e.g. under corrected cyl or lenticular astigmatism or mis-located lens)
- Residual astigmatism = Total astigmatism – Corneal astigmatism (difference between specs & cornea)
o E.g. spec rx of -2.00/-2.00x180
o K’s of 8.00 @180 & 7.80 @90 indicates -1.00x180
o Therefore residual = -2.00x180 – (-1.00x180)
= -1.00x180 (or +1.00x90) -> toric lens needed
3
Q
Describe spherical CLs on toric corneas?
A
- As refractive &/or corneal astig ↑, likelihood of achieving acceptable spherical GP CL fitting ↓
- Some possible problems:
o Poor vision – may be due to residual astig, unstable fitting characteristics & CL flexure
o Poor centration – may be due to lack of physical compatibility between CL & cornea
o CL rocking on flat meridian – can’t sit stably – can cause greater CL awareness & visual instability
o Unstable CL fitting – poor physical compatibility between CL & cornea will cause CL to move excessively & to centre poorly
o CL flexure – dependent on tc & CL physical properties – losing perfect shape of lens – as cornea becomes more toric, spherical GP CL is more likely to flex to conform to corneal shape -> this can ↓ quality of vision
o Corneal distortion
o Spectacle blur
o Discomfort
o Poor blinking
o Epithelial damage
o 3 & 9 o’clock staining
4
Q
What are the types of toric RGPs?
A
- Front-surface toric (FST)
- Back-surface toric (BST)
- Bitoric (Bitor)
- Peripheral toric
5
Q
Describe back surface toric (BST) RGP design?
A
- Corneal cylinder ≥2.00D
- Not relying on tear lens to cancel out the Rx – are giving actual toric correction on back surface
- Physical compatibility w/ cornea, good for corneal astigmatism
- Stable meridional orientation
- BST design is chosen to optimise CL-to-cornea relationship that would be unsatisfactory w/ a spherical back surface shape
6
Q
Describe front surface toric (FST) RGP design?
A
- Spherical back surface – no issue with shape of cornea
- Cylindrical front surface – plus cyl on front surface
- Circular design:
o This is the favoured option
o Optical zone is centred
o Base-down prism (1-1.5 prism dioptres)
o Easier manufacture & duplication/replication
o Good for when lower lid below limbus & large vertical palpebral aperture where truncated would not be suitable
o ↑ comfort
o ↑ physiological performance - Truncated design:
o Inferior zone of CL is truncated (linear or rounded)
o Rests against lower lid for ‘stability’
o Prism-ballast (some prism is lost, potentially ↓ stability)
o Optical zone is decentred superiorly
o Now uncommon
o N.B:
A superior truncation can be added to ↑ CL stability if a single truncation is insufficient. However, comfort may become an issue. Now very uncommon - Good for lenticular astigmatism – cornea is spherical and cyl is further back in lens
7
Q
Describe bitoric RGP design?
A
- Used when front surface or back surface toric result in unacceptable level of residual astigmatism
- Toric BS for physical fit
- Toric FS for full astigmatic correction
- Rotational stability
8
Q
Describe peripheral toric RGP design?
A
- Spherical back optic zone
- Toric back peripheral curves
- Spherical front optic zone & peripheral curves
- Oval/ellipsoidal-shaped optic zone
- A toric peripheral curve(s) CL design allows even bearing &/or clearance at periphery -> ↑improved centration & ↑ comfort
- Used when cannot get a satisfactory fit with other options if have more unusual shaping in periphery
9
Q
What are the steps for selecting a 1st RGP lens?
A
- Decision on modality of wear from H&S and slit lamp examination – as for spherical
- Oxygen
- Decision on material – based on the oxygen
- Choose a lens as a starting point
a. BVP/BOZR/TD
b. Consider range of Rx available
c. Consider cost - Refer to manufacturer’s guidelines for fitting guidance
10
Q
Describe material selection: considerations in toric RGPs?
A
- Physical stability:
o CL must maintain shape to ensure physical compatibility between back surface & cornea is maintained
o With some materials, degree of back-surface toricity may vary over time & as a result the quality of vision may deteriorate with wear - O2 transmissibility:
o Toric lenses are thicker so moderate to high O2 transmissibility is important to ensure cornea’s physiological requirements are satisfied - Optical stability:
o Stable CL material minimises risk of CL warpage
o Irregular change in shape of CL would result in a ↓of quality of vision - Ease of manufacture:
o Some GP CL materials are difficult for manufacturer to cut & polish
o It is prudent to ask CL lab about materials they use & to ask them to suggest which materials best combine requirements of strength & durability w/ ease of manufacturing
11
Q
What are the advantages and disadvantages of toric GP CLs?
A
- Advantages:
o Stabilised CL fitting
o ↑CL-cornea fitting relationship
o Better corneal physiology than toric SCLs – due to high level of O2 that can be supplied to cornea by highly permeable materials & significant tear exchange that occurs with each blink - Disadvantages:
o Relatively thick CLs - ↑CL awareness, ↑levels of 3 & 9o’clock staining, ↓O2 permeability
o ↓control over CL edge profile
o Possible misalignment of corneal & spectacle Rx cylinder axes
12
Q
Describe trial lens fitting in toric RGPs?
A
- Choosing lens from trial bank in practice/HES
- Means can assess variety of lenses at initial fitting appt
- Reduced delays in obtaining correct lens
- Px retention – explain that it may take a while to get right as it must fit them individually
13
Q
What is empirical ordering in toric RGPs?
A
- Requirements:
o Rx details
o Keratometry
o HVID
o Palpebral aperture size (PAS)
Manufacture then designs a lens based on this info – a common method
The 1st CL therefore becomes the trial CL from which a 2nd, improved CL design can be derived - Potential Pitfalls:
o Inaccurate keratometry – can result in poor physical fitting on cornea
o Limited value of keratometry data – as only central corneal curvature is measured, there is v little useful info to help guide selection of peripheral curves & optic zone diameter
o No knowledge of peripheral corneal shape – significant variation in corneal toricity can occur from central to peripheral zone
o Time delay for px if 1st CLs not successful – greater success can be achieved by trial fitting the px & basing CL order on results of a fitting assessment
14
Q
Describe bitoric fitting in toric RGPs?
A
- Commonly used to prevent unacceptable residual astigmatism
- BSTs can induce astigmatism meaning need a front surface correction anyway which is a bitoric
- Some of commonly used toric RGPs are bitoric lenses e.g. Bausch & Lomb Maxim
- Calculate refractive error along each meridian after allowing for vertex distance
o Use BVP calcs from TriA - Aim for full alignment so in theory no tear lens effect
- Not down BVP along steep & flat meridian for ordering
Example:
* Spectacle Rx: -2.00/-4.00 x 180
* BVD = 12mm, -2.00 is unchanged and -6.00 becomes -5.50 so ocular Rx is -2.00/-3.50 x 180
* Keratometry: 8.00@180 and 7.30@90 (indicates corneal cyl of 3.50 and the 180 is the flat meridian)
* Therefore order
o -2.00D at 180 with a BOZR of 8.00
o -5.50D at 90 with a BOZR of 7.30
Adjusting Rx for BVD: K=F/(1-dF) –> for Rxs above 3/4D – needs to be done for both meridians in toric lenses
15
Q
Describe the optimum fitting characteristics in toric RGPs?
A
- Alignment static central fluorescein pattern
- Inferior decentration not beyond the limbus
- Some post-blink CL movement is required
- Adequate pupil coverage
- Stable rotational position
- CL Rotation: Measurement:
o Two Mnemonics if see rotation:
LARS (Left rotation Add to the axis, Right rotation Subtract to the axis)
CAAS (Clockwise, Add; Anti-clockwise, Subtract)
o This adjustment should be made to spec Rx (not CL Rx) & order a new Rx based on that
