Contact lenses 2 - rigid lenses

Question 1

How can RGPs be used to temporarily reshape the cornea? What does this achieve?

Answer 1

via orthokeratology (Ortho-K) or overnight vision correction (OVC). Allows the patient to see clearly during the day without need for glasses or CLs

Question 2

what are the advantages of soft contact lenses compared to corneal RGP?

Answer 2

-good initial comfort
-straight forward to fit
-higher replacement frequency so less lens deposits and its easier to change prescription
-good for both full time or occasional wear

Question 3

what are the disadvantages of soft contact lenses compared to corneal RGP?

Answer 3

-lead to a higher risk of infection and complication compared to corneal RGP
-they are not customisable

Question 4

what are the disadvantages of RGP compared to soft CLs?

Answer 4

-the fitting process is longer as lenses need to be ordered individually and manufactured
-adaptation is required to reduce corneal sensitivity and allow for increased wearing time which takes about 2 weeks
-initial lens cost is high so lens replacement is expensive
-need careful cleaning
-not for contact sports of dusty environment

Question 5

what are the advantages of RGP compared to soft CLs?

Answer 5

-rigid lens surface offers better quality of vision
-reduced risk of infection so less complications
-can be tailor made and fully customisable with a large range of parameters
-lenses are typically replaced every year so are a good value for money

Question 6

what type of back surface do rigid corneal lenses usually have?

Answer 6

multicurve or aspheric

Question 7

what are the 3 components to RGP lens designs? what are each of them and what is their selection based on?

Answer 7

BOZR - Back Optic Zone Radius
* Radius of curvature of the back of the lens
* Selected based on corneal curvature (keratometry readings)

BOZD – Back Optic Zone Diameter
* Diameter of the optic
* Selected based on pupil size in low light (max pupil)

TD – Total Diameter
* Total diameter of lens
* Selected based on Horizontal Visible Iris Diameter (HVID)

Question 8

how is a rigid lens fitted to a specific patient ? (answer in steps)

Answer 8

1. The lens Back Optic Zone Radius (BOZR) is selected to align with the corneal surface
2. This is so the curvature of the back surface of the lens mimics that of the front surface of the cornea
3. pressure is evenly distributed across the whole area under the lens
4. A thin tear lens is formed beneath the RGP
5. there is sufficient clearance at the edge of the lens to allow tear exchange and facilitate lens removal

Question 9

how do you care for rigid lenses?

Answer 9

Using a separate cleaner and conditioning solution

Question 10

what is rigid lens cleaner used for?

Answer 10

to emulsify or
solubilise cell debris, mucous,
lipid & protein- making soak
more effective

Question 11

what is rigid lens conditioning solution for?

Answer 11

-disinfecting and wetting the lens surface
-lens storage
-to lubricate the lens and cushion cornea and eyelids on insertion
-to render the lens surface hydrophilic to promote pre-lens tear film stability

Question 12

what kind of case is not suitable for rigid lenses?

Answer 12

barrel case

Question 13

how are RGP lenses stored so they can be kept clean?

Answer 13

in practise, lenses are put into a lens case with conditioning solution prior to collection to promote surface wetting. After, patients store RGP lenses in solution in a CL case

Question 14

how do you show patients how to wear and care for rigid lenses?

Answer 14

-explain the function of cl solution
-demonstrate lens and contact lens case cleaning
-importance of regular contact lens case replacement
-avoid water contamination

Question 15

what are the adaptation and wearing time recommendations for rigid lenses?

Answer 15

• 1-2 hours the first day
• Px should increase this by 1-2 hours each day, depending on comfort
• Encourage the patient to contact the practice if they’re struggling to
  adapt however
• After adaptation, it’s not uncommon for rigid lenses to be worn for 12-16 hours per day
• Max wearing time (WT) will be indicated by the practitioner

Question 16

how long after the trial period is aftercare typically booked?

Answer 16

typically 2 weeks after the teach because the patient needs time to adapt to the lenses and build up their wearing time

Question 17

what are the general steps to a patient teach

Answer 17

1. how to apply and remove lenses
2. advise on wearing and adaptation schedule
3. explain how to wear and care for lenses
4. advise the patient what to do if they have a problem
5. arrange a follow-up appt
6. provide a written copy of the advice 7. update the patient record `

Question 18

how do you assess corneal radius of curvature?

Answer 18

keratometry and topography

Question 19

what are the two types of keratometry?

Answer 19

one position (Bausch & Lomb type) and two position (Javal-Schiotz type)

Question 20

what part of the cornea does keratometry measure?

Answer 20

the central 3-4mm where radius of curvature is along the 2 principle meridians

Question 21

why and how does corneal radius differ in horizontal and vertical meridians

Answer 21

because the central cornea is toric so The flat (biggest) radius of curvature (r1) is along the horizontal meridian and the steep (smallest) radius of curvature (r2) is along the vertical meridian

Question 22

Why do we measure corneal curvature?

Answer 22

-allows a selection of appropriate radius of curvature rigid contact lens (NOT FOR SELECTING SOFT LENS BC)
-gives a measure of the amount of corneal astigmatism
-gives useful baseline measure to compare any future changes

Question 23

what do you need to write down when doing keratometry?

Answer 23

-the numbers you find
-the type of instrument you use
-the quality of the mires grade 0-4 where 0 is a clear image with no distortion and 4 gross distortion so reading is impossible

Question 24

what are the 3 parts of rigid lens edge design?

Answer 24

• EC = edge clearance
• REL = radial edge lift
• AEL = axial edge lift

Question 25

what is the EC compared to EL?

Answer 25

the gap between the cornea and the back surface of the peripheral curves (observed during fluorescein fit) whereas EL is a design characteristic of the lens and is definable in axial or radial form

Question 26

what has to happen for a rigid lens to be well fitting

Answer 26

• The back surface of the lens mimics the front surface of the
  cornea
• Pressure exerted on the cornea is evenly distributed across the
  whole area under the lens

Question 27

why do you need a rigid lens to fit well

Answer 27

• Limits the effect of the lens on the corneal surface
• Makes the lens comfortable
• Sufficient edge clearance to allow tear exchange and facilitate lens removal

Question 28

what can you use to check rigid lens fit?

Answer 28

fluorescein and cobalt blue or a burton lamp

Question 29

why check rigid lens fit with fluorescein?

Answer 29

• Allows visualisation of the tear lens
• Allows interpretation of how the back surface of the lens relates to the cornea

Question 30

check screenshots for rigid lens fit with fluorescein

Answer 30

ok

Question 31

what are the beneg

Question 32

why do RGPs have much less complications that soft CLs? what does this also mean?

Answer 32

Tears circulate under the lens in tear exchange but this also means RGPs move much more than soft lenses

Question 33

what does an RGP that’s too flat do?

Answer 33

moves a lot more and the periphery of the lens lifts away from the cornea

Question 34

why cant keratometry be used to fit soft CLs?

Answer 34

as they go over the cornea whereas RGPs fit the cornea exactly

Question 35

what us delta K?

Answer 35

the numerical difference between the flattest and steepest curve

Question 36

how much corneal astigmatism is 0.05mm delta K?

Answer 36

0.25DC corneal astigmatism

Question 37

when do you need a toric RGP?

Answer 37

when:
-There’s more than 2.50 D toric astigmatism
-There’s ≥0.75D lenticular astigmatism

Question 38

what shape are the central and peripheral cornea?

Answer 38

central is spherical/ toric

peripheral is aspheric

Question 39

what are the principle meridians of the cornea?

Answer 39

the flattest and steepest curves which are typically 90 degrees apart in regular astigmatism

Question 40

what makes up total ocular astigmatism?

Answer 40

corneal astigmatism + lenticular astigmatism

Question 41

how does BOZR need to be changed in relation to corneal astigmatism? why?

Answer 41

The greater the level of corneal astigmatism the greater we need to steepen the BOZR (hence reduce the number)

because reducing the BOZR reduces the clearance in the steeper meridian and so improves comfort and lens stability

Question 42

check google docs page 14 for filled out lecture handout

Answer 42

ok

Question 43

what is BOZD compared to pupil size and TD?

Answer 43

• Generally, 1.5mm larger than av/max. pupil size
• Generally, 1.5 - 2.0mm smaller than TD

Question 44

what is TD compared to HVID?

Answer 44

• Generally, 2mm smaller than HVID

Question 45

TD is smaller when?

Answer 45

if toric cornea >2.00DC

Question 46

what does increasing TD mean when selecting a trial lens?

Answer 46

generally stabilises the fit but also increases the extent of lid attachment