Lec 2 - Prefitting Considerations Flashcards
What’s the oxygen supply to the cornea
Atmosphere by tear film (main)
Capillaries of the limbus
Aqueous via corneal endothelium
Capillaries of palpebral conjunctiva
What are the tears for
Optical - give cornea smooth surface
Metabolic support - supples cornea with nutrients
Protection
Lubrication
What’s HVID and average value
Horizontal visible iris diameter which is the corneal diameter so determines overall diameter of CL
10-14mm 11.8
What’s vertical palpebral aperture
Distance between upper and lower eyelid whole px in primary gaze and legatos and orbicularis is neutral
Muscles involved in blink
Active orbicularis oculi contraction and passive downwards forces
What’s muscles used to pull lid up
Increase in legatos palpebral activity
- greater distance so wide PA means bigger speed and force
3 types of lid characteristics
Lid tension
Blink rate/type
Ethnic variation
What’s lid tension and how is it evaluated
Tight medium or loose and evaluated by eversion
What’s the average blink rate
12x a minute and can be full or partial
Why is pupil important in contact lens
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
How to measure pupil size for CL
Dim and distance target to get maximum size so get CL that won’t block view
Mechanism of placido disk
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
How does the keratometry measure the central radius of curvature
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
Disadvantages of keratometry
Restricted to sphero cyl surfaces (can’t do irregular astig)
3mm central area only
Affected by focusing error and user
How to find curvature of cornea or power with vice versa?
337.5/ power or curve
Advantage of axial/Sagittal map
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
Disadvantage of an axial/sagittal map
Smoother so less sensitive
Limited that all centre of rotation must fall on axis defined by optical axis of the keratoacope
Advantage of tangential/Instaneous map
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
Disadvantage of tangential/instaneous maps
May appear more noisy and irregular
What is an elevation map
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
What’s the 3 scale/colour coding options
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
What’s topography used for
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
Difference between regular and irregular astig
> 20deg = irregular
Difference between symmetric and asymmetric astig (2 branches of regular astig)
Symmetric is where difference between the bows is less than a diopter
Asymmetric is more than 1D
