toric rigid corneal lenses Flashcards
how is astigmatism caused?
. astigmatism is caused when the cornea or the lens develops into irregular shape and distorts the light as it enters the eye causing the images to focus at different distances from the retina
how much does astigmatism cover of all refractive errors ?
. astigmatism is responsible for 13% of all refractive errors
what is the prevalence of astigmatism with age ?
. 0-6 months: steep cornea - high prevalence ( 20%)
- varying results on predominance of WTR and ATR
. 1-3 years - the cornea flattens - prevalence of high degrees of astigmatism which is (>1DC) deceases ( 4.8%)
- WTR astigmatism more common
. 20 to 30 years - around 63% exhibit > 0.25 DC with only few >1DC
. > 40 years - axis shifts from WTR to a predominance of ATR, probably due to changes in the corneal curvature
what is the prevalence of astigmatism based on ethnicity ?
. east asian higher prevalence
. approx 19% of Singaporean children and 21% of preschool Chinese children show > 1DC astigmatism
. native American people have an increased prevalence of high levels of astigmatism
( >1D) due to underlying factors such as hereditary and nutrition
what are the types of astigmatism ?
. corneal . lenticular . combination - which can cancel out . regular - astigmatism where principal meridians are 90 degrees apart . irregular
what is spectacle astigmatism ?
. can occur either from corneal and /or lenticular astigmatism. measured in the spectacle plane
what is ocular astigmatism ?
. taken directly from the spectacle astigmatism after adjusting for BVD
what is corneal astigmatism ?
. astigmatism arising just from the cornea and can be measured using a keratometer
what is residual astigmatism ?
. residual astigmatism is another name for lenticular astigmatism
. when a spherical RGP lens is placed on the cornea , it will correct the corneal astigmatism , but not the lenticular astigmatism . This lenticular astigmatism becomes known as residual astigmatism
what is induced astigmatism ?
. term which is used to indicate astigmatism that is created when using a back surface toric RGP lens due to the difference between the refractive indices of the contact lens material and the tear film beneath
what is refractive astigmatism ?
. refractive astigmatism = corneal + lenticular
. refractive astigmatism from spectacle Rx
. corneal astigmatism from keratometry or topography
how is corneal astigmatism calculated ?
. the difference between the K readings in both principal meridians
. every 0.05 mm difference between the K represents a quarter of a diopter corneal astigmatism along the axis of flattest k
. rule of thumb : 0.05 mm difference = 0.25D
what are the advantages of a spherical base curve on toric cornea ?
. simple
. inexpensive
what are the disadvantages of spherical base curve on toric cornea ?
. fluctuations in vision ( there may be excessive movement or lens may not centre well )
. may cause corneal distortion
. lens flexure
explain the fitting of spherical base curve on toric cornea with 3 D corneal astigmatism ?
. fit the lens on the flattest K
. horizontal band of alignment on WTR corneas
. edge stands off 12 and 6 o’clock
. this caused an excessive movement and rocking on blinking
. lens tend to ride high if held up be eye lid or drops and rides low
. lens may be easily lost from eye due to lid catching lens edge
how to improve spherical lens fitting on toric corneas ?
. smaller diameter - to help minimise the exaggeration between the two different meridians on the corneal surface which helps to avoid the excessive clearance in the steeper meridian
. aspheric lens design- which gives a reduced edge clearance along the steeper meridian which encourages the lens to centre more accurately and give a better visual result
. thin lens -
. steeper BOZR - BOZR should be decreased by 0.05 mm for each 0.50 D that the corneal astigmatism exceeds 1.50D
when to use a thin RGP ?
. when a patient shows WTR corneal astigmatism and ATR lenticular astigmatism use a thin RGP which will flex and correct the residual or the lenticular astigmatism
how to fit a spherical base curve on toric cornea with
- 00 mm at 180
- 40 mm at 90 ?
- you can use the average K which is 7.70 mm ( this is fitted to steep )
2 . you can use the best compromised fit
rule of thumb : this is when you choose 0.15 mm steeper than kf ( 7.85mm )
what is the criteria of toric lenses ?
. improve the physical fit
. improve the visual acuity
what is the principle of toric lens design systems ?
. different powers in two mutually perpendicular meridians
. different methods and designs for stabilization
when does poor centration occur in toric lenses ?
. occurs if ATR astigmatism is present causing the lens to decentre horizontally
. WTR astigmatism the lens will rock along the steeper meridian or decentre upwards or downwards
when is corneal moulding induced ?
. this is induced when the differential bearing effect of a spherical lens on a large astigmatic corneal leads to unwanted alteration on the refraction cornea after lens is removed
what does corneal moulding cause ?
. corneal moulding causes spectacle blur ?
what causes reduced comfort in toric lenses ?
. area of alignment is reduced; excessive edge clearance leads to unwanted lid interaction
what causes 3 and 9 o’clock staining in toric lenses ?
. lack of lens movement caused by reduced edge clearance
what is the choice of contact lenses in rigid corneal lens ?
. spherical CL :with CA up to 2.50 D
. back surface toric: if CA > 2.50 D
. front surface toric : for any remaining significant lenticula astigmatism ( >0.75D)
. bitoric = front and back toric combined in cases of high CA plus significant lenticular astigmatism
when is the choice of contact lenses in SCL ?
. toric CL to correct any astigmatism
. SCL needs to be stabilised to avoid rotation
how to fit a back surface toric in practice ?
. measure the steepest and flattest corneal radii using keratometry
. calculate ocular prescription
. specification TD should be based on the same criteria used for a spherical lens design
what are some clinical tips for back surface toric in practice ?
. back surface torics over-correct the corneal astigmatism due to induced astigmatism
. an aligned fitting back surface toric lens should not need any stabilisation
. a rotating back surface toric will only cause visual problems if it has been corrected for residual astigmatism ( alignment bitoric )
. computer programs are available to save time with lengthy and laborious calculations
. empirical fitting with back surface torics will give a high standard of first time success
. toric lenses are thicker than spherical lenses - it is recommended to use a higher Dk material
how to fit a front surface toric in practice ?
. a front surface toric is a spherical rigid corneal lens with a toroidal front surface due to significant residual astigmatism
. fit a standard spherical rigid corneal lens ( with dot ) on Kf
( the lens is dotted for diagnostic purposes )
. if pleased with the fit , over refract the sphere and cyl
. estimate lens rotation of the dot by
how do we stabilise front surface toric or bitoric lenses ?
. prism ballast
. truncation
how to use prism ballast to stabilise toric lenses ?
- 1.5 to 3.0 prism
- prism orientates downwards
- usually marked to assist observation
- tendency for 5-10 degrees nasal rotation on blinking due to lid tension and eyelid position
how to use truncation to stabilise toric lenses ?
- used with or without prism ballast
- single or double
- chord of 0.5 to 1.0 mm removed
- truncation should sit on lower lid
what are the disadvantages of prism ballast ?
. the thicker lower edge may cause the lens to be more uncomfortable and may cause lens to drop
what are the disadvantages of truncation ?
. ineffective with small TD or lid below limbus
. TD increased by 0.5 mm to assist stability
what are clinical tips for front surface toric ?
- front surface torics may be required to improve VA where residual astigmatism is present
- if only one eye requires a front surface toric , prism ballast stabilisation may disturb binocular vision
- front surface stabilisation methods may induce flare due to inferior decentration
- toric lenses are thicker than spherical lenses , it is recommended to use a higher Dk material
what is the best method for fitting toric rigid corneal cls ?
. send details to manufacturing lab ( provide spectacle Rx , keratometry / topography readings and BVD )
. use a software programme
. use a topographer CL programme
what are clinical tip for fitting toric lenses in practice ?
. k readings should be carefully checked
. adaptation may be required even for existing RGP wearers due to the extra thickness , particularly if prism ballast or truncation is used
. manage patient expectations
. more appointments
. make use of the technical support of the manufacturers
. there will be situations where soft toric lenses are the better option
