Lec 2 - Prefitting Considerations

Question 1

What’s the oxygen supply to the cornea

Answer 1

Atmosphere by tear film (main)

Capillaries of the limbus

Aqueous via corneal endothelium

Capillaries of palpebral conjunctiva

Question 2

What are the tears for

Answer 2

Optical - give cornea smooth surface

Metabolic support - supples cornea with nutrients

Protection

Lubrication

Question 3

What’s HVID and average value

Answer 3

Horizontal visible iris diameter which is the corneal diameter so determines overall diameter of CL

10-14mm 11.8

Question 4

What’s vertical palpebral aperture

Answer 4

Distance between upper and lower eyelid whole px in primary gaze and legatos and orbicularis is neutral

Question 5

Muscles involved in blink

Answer 5

Active orbicularis oculi contraction and passive downwards forces

Question 6

What’s muscles used to pull lid up

Answer 6

Increase in legatos palpebral activity

• greater distance so wide PA means bigger speed and force

Question 7

3 types of lid characteristics

Answer 7

Lid tension

Blink rate/type

Ethnic variation

Question 8

What’s lid tension and how is it evaluated

Answer 8

Tight medium or loose and evaluated by eversion

Question 9

What’s the average blink rate

Answer 9

12x a minute and can be full or partial

Question 10

Why is pupil important in contact lens

Answer 10

Optic Zone Diameter and its transition edge

Clear viewing zone of colour CL

Multifocal CL and placement of powers

Corneal reshaping CL treatment zone size

Question 11

How to measure pupil size for CL

Answer 11

Dim and distance target to get maximum size so get CL that won’t block view

Question 12

Mechanism of placido disk

Answer 12

Projects image of rings and this reflects on cornea

A shorter distance between rings means the cornea is curved more so it has more power

Question 13

How does the keratometry measure the central radius of curvature

Answer 13

The anterior corneal surface acts like a convex mirror and reflects light (mires) and then doubling principle is used to estimate how far mires are from each other to give curvature

Question 14

Disadvantages of keratometry

Answer 14

Restricted to sphero cyl surfaces (can’t do irregular astig)

3mm central area only

Affected by focusing error and user

Question 15

How to find curvature of cornea or power with vice versa?

Answer 15

337.5/ power or curve

Question 16

Advantage of axial/Sagittal map

Answer 16

Gives reasonable representation of refractive power

Useful for general shape overview and CL fitting

Measures radius of curvature for a comparable sphere with the same tangent

Question 17

Disadvantage of an axial/sagittal map

Answer 17

Smoother so less sensitive

Limited that all centre of rotation must fall on axis defined by optical axis of the keratoacope

Question 18

Advantage of tangential/Instaneous map

Answer 18

Accurately depicts local irregularity and location

Useful for complex lens fitting and interpreting optics

Finds radius of curvature for each point without the constraint centre of rotation must fall on axis of videoneratoscope

Question 19

Disadvantage of tangential/instaneous maps

Answer 19

May appear more noisy and irregular

Question 20

What is an elevation map

Answer 20

Height above or below a best fit sphere where anything above is a hotter red colour and anything below is cooler

So redder = higher =actually flatter

Question 21

What’s the 3 scale/colour coding options

Answer 21

Absolute = red 50D and blue 37D

Normalise = red is px steepest and blue px flattest

Keratometric scale = extended range above 50D where pale pink is 70D

Question 22

What’s topography used for

Answer 22

Finding if cyl corneal or lens

Finding DDX for irregular corneas Or why BCVA is reduced

Monitoring corneal shape/tear film changes

Simulating RGP fitting

Question 23

Difference between regular and irregular astig

Answer 23

> 20deg = irregular

Question 24

Difference between symmetric and asymmetric astig (2 branches of regular astig)

Answer 24

Symmetric is where difference between the bows is less than a diopter

Asymmetric is more than 1D