toric rigid corneal lenses

Question 1

how is astigmatism caused?

Answer 1

. 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

Question 2

how much does astigmatism cover of all refractive errors ?

Answer 2

. astigmatism is responsible for 13% of all refractive errors

Question 3

what is the prevalence of astigmatism with age ?

Answer 3

. 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

Question 4

what is the prevalence of astigmatism based on ethnicity ?

Answer 4

. 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

Question 5

what are the types of astigmatism ?

Answer 5

. corneal 
. lenticular 
. combination - which can cancel out 
. regular - astigmatism where principal meridians are 90 degrees apart 
. irregular

Question 6

what is spectacle astigmatism ?

Answer 6

. can occur either from corneal and /or lenticular astigmatism. measured in the spectacle plane

Question 7

what is ocular astigmatism ?

Answer 7

. taken directly from the spectacle astigmatism after adjusting for BVD

Question 8

what is corneal astigmatism ?

Answer 8

. astigmatism arising just from the cornea and can be measured using a keratometer

Question 9

what is residual astigmatism ?

Answer 9

. 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

Question 10

what is induced astigmatism ?

Answer 10

. 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

Question 11

what is refractive astigmatism ?

Answer 11

. refractive astigmatism = corneal + lenticular
. refractive astigmatism from spectacle Rx
. corneal astigmatism from keratometry or topography

Question 12

how is corneal astigmatism calculated ?

Answer 12

. 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

Question 13

what are the advantages of a spherical base curve on toric cornea ?

Answer 13

. simple

. inexpensive

Question 14

what are the disadvantages of spherical base curve on toric cornea ?

Answer 14

. fluctuations in vision ( there may be excessive movement or lens may not centre well )
. may cause corneal distortion
. lens flexure

Question 15

explain the fitting of spherical base curve on toric cornea with 3 D corneal astigmatism ?

Answer 15

. 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

Question 16

how to improve spherical lens fitting on toric corneas ?

Answer 16

. 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

Question 17

when to use a thin RGP ?

Answer 17

. 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

Question 18

how to fit a spherical base curve on toric cornea with

8. 00 mm at 180
9. 40 mm at 90 ?

Answer 18

1. 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 )

Question 19

what is the criteria of toric lenses ?

Answer 19

. improve the physical fit

. improve the visual acuity

Question 20

what is the principle of toric lens design systems ?

Answer 20

. different powers in two mutually perpendicular meridians

. different methods and designs for stabilization

Question 21

when does poor centration occur in toric lenses ?

Answer 21

. 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

Question 22

when is corneal moulding induced ?

Answer 22

. 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

Question 23

what does corneal moulding cause ?

Answer 23

. corneal moulding causes spectacle blur ?

Question 24

what causes reduced comfort in toric lenses ?

Answer 24

. area of alignment is reduced; excessive edge clearance leads to unwanted lid interaction

Question 25

what causes 3 and 9 o’clock staining in toric lenses ?

Answer 25

. lack of lens movement caused by reduced edge clearance

Question 26

what is the choice of contact lenses in rigid corneal lens ?

Answer 26

. 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

Question 27

when is the choice of contact lenses in SCL ?

Answer 27

. toric CL to correct any astigmatism

. SCL needs to be stabilised to avoid rotation

Question 28

how to fit a back surface toric in practice ?

Answer 28

. 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

Question 29

what are some clinical tips for back surface toric in practice ?

Answer 29

. 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

Question 30

how to fit a front surface toric in practice ?

Answer 30

. 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

Question 31

how do we stabilise front surface toric or bitoric lenses ?

Answer 31

. prism ballast

. truncation

Question 32

how to use prism ballast to stabilise toric lenses ?

Answer 32

• 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

Question 33

how to use truncation to stabilise toric lenses ?

Answer 33

• used with or without prism ballast
• single or double
• chord of 0.5 to 1.0 mm removed
• truncation should sit on lower lid

Question 34

what are the disadvantages of prism ballast ?

Answer 34

. the thicker lower edge may cause the lens to be more uncomfortable and may cause lens to drop

Question 35

what are the disadvantages of truncation ?

Answer 35

. ineffective with small TD or lid below limbus

. TD increased by 0.5 mm to assist stability

Question 36

what are clinical tips for front surface toric ?

Answer 36

• 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

Question 37

what is the best method for fitting toric rigid corneal cls ?

Answer 37

. send details to manufacturing lab ( provide spectacle Rx , keratometry / topography readings and BVD )
. use a software programme
. use a topographer CL programme

Question 38

what are clinical tip for fitting toric lenses in practice ?

Answer 38

. 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