Week 5 - Correcting astigmatism RGP’s Flashcards
When to choose a toric RGP?
• When a lens with spherical front and back radi has not achieved adequate visual acuity
• When a spherical lens has not provided a suitable physical fit
What is residual astigmatism?
• Residual astigmatism - astigmatism that remains when a 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
What issues can arise when spherical CLs go on toric corneas?
• Poor vision
• Poor centration
• CL rocking on flat meridian
• Unstable CL fitting
• CL flexure (depending on tc and CL physical properties)
What issues can arise with the eye with spherical CLs on toric corneas?
• Corneal distortion
• Spectacle blur
• Discomfort
• Poor blinking
• Epithelial damage
• 3 & 9 o’clock staining
What are the 4 different types of toric RGP’s?
• Front surface Toric (FST)
• Back surface Toric (BST)
• Bitoric (bitor)
• Peripheral Toric
What is a back surface toric design?
• Corneal cylinder ≥ 2.00 D
• Physical compatibility with the cornea, good for corneal astigmatism
• Stable meridional orientation
A back-surface toric design is chosen to optimize the CL-to-cornea relationship that would be unsatisfactory with a spherical back surface shape
What is a front surface Toric design?
• Spherical back surface
• Cylindrical front surface (plus cyl on front surface)
• Circular design, or
• Truncated design
• Good for lenticular astigmatism
Explain Circular Design: with front surface toric designed lenses
• Optical zone is centred
• Base-down prism (1-1.5prism diopters)
• Easier manufacture and duplication/replication
• Good for when lower lid below limbus and large vertical palpebral aperture where truncated would not be suitable
- Increased comfort
- increased physiological performance
Explain Truncated Design: with front surface toric designed lenses
• Inferior zone of CL is truncated (linear or rounded)
• Rests against lower lid for ‘stability’
• Prism-ballast (some prism is lost, potentially -> stability)
• Optical zone is decentered superiorly
• Now uncommon
Explain Double Truncated Design: with front surface toric designed lenses
• A superior truncation can be added to 1 CL stability if a single truncation is insufficient
• However, comfort may be an issue
• Now very uncommon
What is a Bitoric?
Used when a front surface or back surface toric result in an unacceptable level of residual astigmatism
• Toric BS for physical fit
• Toric FS for full astigmatic correction
• Rotational stability
What is a Peripheral Toric?
• Spherical back optic zone
• Toric back peripheral curves
• Spherical front optic zone and peripheral curves
• Oval/ellipsoidal-shaped optic zone
A toric peripheral curve(s) CL design allows even bearing &/or clearance at the periphery, -> 1 improved centration & 1 comfort
What are the steps in selecting a first lens?
- Decision on modality of wear from H&S and slit lamp examination - as for spherical
- Oxygen
- Decision on material
- Choose a lens as a starting point
• BVP/BOZR/TD
• Also have to consider range of Rx available
• And cost - Refer to manufacturer’s guidelines for fitting guidance
What considerations are there on material selection for RGP’s?
• Physical stability
• O2 transmissibility
• Optical stability
• Ease of manufacture
What are advantages of Toric GP Cls?
• Stabilised CL fitting
• Increased CL-cornea fitting relationship
• Better corneal physiology than toric SCLs
What are disadvantages of Toric GP Cls?
• Relatively thick CLs
• Decreased control over the CL edge profile
• Possible misalignment of the corneal & spectacle Rx cylinder ax
Why can Trial banks be used for trial lens fitting?
• Means you can assess a variety of lenses at the initial fitting appointment
• Reduced delays in obtaining the correct lens
• Patient retention
What is empirical ordering?
The manufacturer then designs a lens based on this information. A common method.
• Rx Details
• Keratometry
• HVID
• Palpebral aperture size (PAS)
what are some pitfalls of empirical ordering?
• Inaccurate keratometry
• Limited value of keratometry data
• No knowledge of peripheral corneal shape
• Time delay for the patient if 1st CLs not successful
When are Bitorics fitted?
• Commonly used to prevent unacceptable residual astigmatism
• Back surface torics can induce astigmatism meaning you need a front surface correction anyway which is simply a bitoric
• Some of the commonly used toric RGPs are bitoric lenses e.g. Bausch and Lomb Maxim
What needs to be considered with Bitoric Fitting lenses?
• Calculate the refractive error along each meridian after allowing for the vertex distance.
- Use the BVP calculation from Tri A
• Aim for full alignment so in theory no tear lens effect
• Note down the BVP along the steep and flat meridian for ordi
Why and how is Rx adjusted for BVD?
• Obviously a CL is closer to the eye than glasses.
• We need to adjust for this as the Rx increases and treat each meridian separately if there is a cyl.
K= F/(1-dF), where F = spec Rx in Dioptres, & d = BVD, in metres.
Need to do for both meridians in toric lenses.
What are the optimum fitting characteristics?
• Alignment static central fluorescein pattern
• Inferior decentration not beyond the limbus
• Some post-blink CL movement is required
• Adequate pupil coverage
• Stable rotational position
How are Toric RGP’s results recorded?
• Same as for spherical RGPs
• BUT
• Need to add a comment on the lens rotation/stability of lens as we talked about in the soft toric lecture.
