Toric RGPs Flashcards
What are the types of astigmatism?
Regular
Irregular
Oblique
With-the-rule
Against-the-rule
Residual/lenticular
Induced
Corneal
What is regular astigmatism?
Meridians are at 90 degrees to each other
What is irregular astigmatism?
Meridians are not at 90 degrees to each other
(Normally due to disease, injury or congenital causes)
What is oblique astigmatism?
Meridians are more than 30 degrees from 90 or 180
What is with-the-rule astigmatism?
Vertical meridian has higher rx, steeper Ks (axis: 180 degrees +/- 30)
What is against-the-rule astigmatism?
Horizontal meridian has higher rx, steeper Ks (axis: 90 degrees +/- 30)
What is residual/lenticular astigmatism?
‘Leftover’ astigmatism when corneal astigmatism corrected by tear lens created by spherical RGP
What is induced astigmatism?
Astigmatism which is induced by a back surface toric RGP on a toric cornea - caused by differing refractive indices of RGP and tear film
What are the possible combinations of astigmatism?
Spherical cornea/spherical refraction
Spherical cornea/astigmatic refraction
Toric cornea/spherical refraction
Toric cornea/astigmatic refraction
Where is the majority of the astigmatism in spherical cornea/astigmatic refraction?
Lenticular
Where is the majority of the astigmatism in toric cornea/spherical refraction?
Corneal and lenticular astigmatism equal and opposite to each other (cancel each other out and give spherical refraction)
Where is the majority of the astigmatism in toric cornea/astigmatic refraction?
Corneal
Who should you fit with a toric lens?
Cyl over -0.75DC
If induced or residual astigmatism
Toric cornea which a spherical RGP fits poorly
What are the types of toric RGP?
Spherical
Back surface toric
Front surface toric
Spherical centre/toric periphery
Bitoric
What can a spherical RGP be used for in px’s with astigmatism?
To correct corneal astigmatism up to -2.50DC
When can a spherical centre/toric periphery RGP be used?
To correct astigmatism via the tear lens through spherical BOZR, toric periphery ensures more optimal fit on a toric cornea and allows even edge clearance
When can a back surface toric be used?
Toric cornea/spherical refraction
Back surface should mimic corneal surface
What should the fluorescein fit look like on a back surface toric?
Spherical alignment (bc tear lens spherical due to matching shape to cornea)
When can a bitoric be used?
High levels of mixed astigmatism, if front or back surface torics produce lots of induced or residual astigmatism
How does a bitoric work?
Both surfaces toric
Back surface helps physical fit
Front surface corrects astigmatism
When can a front surface toric be used?
Spherical back surface
Toric front surface which corrects residual astigmatism
What combination of astigmatism does a front surface toric correct?
Spherical cornea/astigmatic refraction
Does a front surface toric need stabilisation?
Yes
What are the types of stabilisation?
Toric back surface
Prism ballast
Truncation
Dynamic stabilisation
