Toric RGP lenses Flashcards
what are the two criteria for fitting topic RGP lenses
to:
improve the physical fit
- to avoid lens rocking on cornea, so improving the radii of the lens
improve the visual acuity
- when the spec rx and k readings don’t match, indicating lenticular astigmatism
when is residual astigmatism expected
when the spectacle astigmatism doesn’t equal the corneal astigmatism
when is residual astigmatism found
when placing a spherical lens on the cornea
what is residual astigmatism
it is the uncorrected astigmatism from the crystalline lens
which type of astigmatism is residual astigmatism generally
against the rule astigmatism (Horizontal meridian is steeper)
when can you maybe get away with not correcting residual astigmatism with contact lenses
if it is in the non-dominant eye, it may not be noticed by the px
when is induced astigmatism expected
when using toric back surface - otherwise the lens is rocking about
why is induced astigmatism found
because of the difference in refractive indices (RI) between the lens material and the tears
hence the px does not see what you expect them to see/as well as they’re supposed to
how can you sort out the flourescein patten for a induced astigmatism for a back surface toric lens
- can compromise by changing the cylindrical element of the back surface
or - put some cyl on the front, so won’t have cyl on the back
= bitoric lens
list 5 different reasons/patients you will want to fit a toric RGP on, for a improved physical fit
- difference between principle meridians is greater than 0.6mm (3.00D of corneal astigmatism) - to avoid the spherical lens rocking around
- when spherical lens is unstable
- when spherical lens decentres
- when the spherical lens gives unacceptable bearing areas
- when the cornea is more toric towards the periphery - by looking at the FP
list 2 different reasons when you will want to fit a toric RGP for improved visual acuity
- residual astigmatism >1.00D, put power on the front
- induced astigmatism >1.00D, it is induced by the back but put the power on the front
list 2 non-toric RGP lens designs
- spherical
- aspheric
simply and less costly for the patient
list 4 types of toric RGP lenses
- front surface toric
- toric periphery
- back surface toric - mostly used
- bitoric - mostly used
how can a spherical lens be fitted on a toric cornea and why
fitted with flat fitting lenses to minimise the lens flexure
use diagnostic of a specific material - so make sure you don’t use too soft of a material
how do you observe the flexure of a spherical lens on a toric cornea
observe flexure with the keratometer mires
- when the patient blinks, the mires should always look clear
- if the lens is flexing, then the mires will distort and then go clear
when will the flourescein pattern be acceptable, when there is a spherical lens on a toric cornea
if there is adequate edge clearance along the flattest meridian and no excessive stand off in the steeper meridian
can be useful to use a small TD design
which lens parameter is best kept small when fitting a spherical RGP on a toric cornea
small TD - as gets more toric as you go away from the centre
why may an aspheric lens provide better fitting on a toric cornea than a spherical lens
better fitting characteristics because of the close alignment to the cornea - fits the shape of the cornea more closely
what is the ideal fit of an aspheric lens on a toric cornea
alignment or slightly flatter than flattest k
what are the two advantages of using an aspheric lens on a toric cornea
- minimises lens flexure
- maintains good acuity
what sort of flourescein pattern will an aspheric lens have on a toric cornea
the mainly aspheric designs produce the least peripheral stand off in the steeper meridian
they produce less edge clearance than spherical lenses
and don’t need so much tear flow under the lens as the material helps with the oxygen going through
which power cyls of corneal astigmatism are aspheric most useful for
CA between 3.00 and 4.00D, so try this as a first option before toric lenses
when may a aspheric lens not work on a toric cornea, hence needing to use a toric lens as a solution
if the lens decentres on the cornea
what are the three methods of stabilisation used on toric lenses
- prism ballast
- truncation
- back surface design
which type of toric lenses do you need to use one or more of the stabilisation methods
for front surface toric or toric periphery
which method of stabilisation is used if a lens doesn’t click into place on a front surface toric lens
prism ballast
list how a prism ballast is used as a method of stabilisation
- 1.5 to 3.0 prism base down
- usually marked to help observation
- prism is at periphery of lens as no aspect for it is for vision
- it is only used for weighing down the lens as its thicker base on the bottom and it is kept away from the optic zone
- has a tendency for 5-10 degrees nasal rotation on blinking due to lid tension and eyelid position, but is only irritating on dominant eye or on one eye, otherwise not
name a disadvantage to the prism ballast method of stabilisation
has a tendency for 5-10 degrees nasal rotation on blinking due to lid tension and eyelid position, but is only irritating on dominant eye or on one eye, otherwise not
list how truncation is used as a method of stabilisation
- used with or without a prism ballast
- single or double - can cut off just the bottom or top and bottom
- chord of 0.5-1mm removed
- truncation should sit on lower eyelid
- ineffective with small TD or if lid is below the limbus
- TD is increased by 0.5mm to assist the stability
when will a truncation method of stabilisation be ineffective, and how is the problem generally solved
with a small TD or if the lid is below the limbus
TD is increased by 0.5mm to assist the stability
which type if eyelid will you want to have a truncation but off at the top and bottom
on a tight eyelid
what else can a truncation be used with or without
a prism ballast, it varies with the rx and lid tension
as well as used in toric lenses, when else is a truncation method of stabilisation used
in bifocal CL’s to weight the lens
list how a back surface design is used as a method of stabilisation
- the lens will click into place if the curve is chosen appropriately
- will help when fitting a bitoric
what is a front surface toric lens
- a lens with a spherical or aspheric back surface
- the cylinder is worked on the front
why does a front surface toric lens need stabilising
to keep the cyl orientation in the right place/axis
what type of lens may be more appropriate over a front surface toric lens
soft lens
when may a soft lens be more appropriate over a front surface toric RGP lens
- if k readings show a toric cornea
- but the spec rx is near spherical
- then lenticular cyl becomes the reason for a toric design
if you use a thin SCL, it will take up the shape of the cornea and it will come onto the front of the soft lens, which will be continued to be neutralised by lenticular astigmatism, so SCL is better than rigid lens optically
why are toric periphery lenses not generally used
due to aspheric lenses being used instead
what design is a toric periphery lens
- spherical BOZR
- toric BPORs - peripheral curves are toric
- oval optic (BOZR oval)
what can you use with a toric periphery lens is there is manufacturing problems
a spherical last curve
what is available with toric periphery lenses
fitting sets, to try on, but used more in hospital practice
what design is a back surface toric lens
- full back toric surface
- front spherical surface
- no extra stabilisation needed, as back surface toric will usually click into place
what three ways can a back surface toric lens be fitted by
- toric fitting set
- spherical fitting set
- fitting my calculation
how is a toric fitting set used to fit a back surface toric lens
- the trial lens chosen is based on keratometry
- flattest curve is chosen along Kf (flattest K)
- steepest curve is 0.10mm flatter than Ks (steepest K)
- FP = spherical fitting, if the k readings are correct, as have matched the shape of the eye
- over refraction gives the BVP along the flatter meridian
how is a spherical fitting set used to fit a back surface toric lens
- flattest meridian is worked out the normal way
- steepest meridian is decided from the steepest k = 0.10 flatter than Ks
- the power is worked out using the steepest k, using the normal power calculation: BVP: F = n-1/r
if deciding to fit a back surface toric lens by fitting by calculation, which 4 details must be sent to the lab to do so
- spectacle rx
- BVD
- keratometry readings needed
- refractive index of material (as it will have implications as it can alter how much astigmatism is induced)
why does the induced astigmatism need to be considered, in the lab when fitting a back surface toric lens, via fitting by calculation method
because if the induced astigmatism corrects the residual astigmatism (i.e. the difference between spec rx and k’s) then the front surface remains spherical
a thin air gap separates all surfaces = between the tears-lens and lens-cornea, must all be considered with calculations
what is the formula for induced astigmatism
I = n-n’/rs - n-n’/rf
n = RI of tears n' = RI of CL material r = k readings
how is a bitoric lens designed
it has a toric: BOZR (central), BPOR (peripheral) and FOZR
the cylinder is worked on the front as there is enough induced astigmatism which causes the problem
when is a bitoric lens needed
when induced astigmatism is a visual problem
- when corneal astigmatism = ocular astigmatism - hence no residual astigmatism
- when corneal astigmatism is less than the ocular astigmatism:
- so lenticular astigmatism is revealed so a front surface toric lens is needed to compensate as the resulting astigmatism is increased when a rigid lens is fitted
what 3 details need to be sent to the lab when making a bitoric lens
- refraction
- keratometry
- HVID
- use a computer programme
- use a topographer CL programme
what will a good fitting toric lens look like
an alignment spherical fit
