Assessment of Corneal Curvature - Week 1 Flashcards
Why measure corneal shape? (Indications)
Important for:
- screening tool
- contact lens practice
- stability of cornea and tear film
- aids in diagnosis/monitoring of corneal pathology
- gold standard for pre- and post- refractive surgery evaluation/orthokeratology (changes in corneal shape)
- estimating astigmatism when refractive results poor/unreliable
- IOL determination (for cataract surgery to calculate axial length)
Way to remember:
* screen contact stability is the gold standard for surgery evaluation and estimating astigmatism and IOL pathology
what does keratometry measure? And what does this accomplish?
Gives estimate of shape of cornea by measuring (anterior corneal) radius of curvature-
In keratometry you will see 3 circles, where do you need to put the ‘cross’? and what do you do next?
Position the cross in the middle of the bottom right circle, then focus the joystick so that all the mires are single
In keratometry, in what way do you want the 3 circles oriented?
The 3 circles should be aligned so that the +ve’s and -ves overlap (+ve and -ve are found on the sides of each circle, +ve on left/right and -ve on top.bottom)
Limitations of Modern Keratometry?
- assumes refractive index of cornea = 1.3375
- assumes cornea is spherical or toric
- assumes cornea is symmetrical all around
- estimates the average curvature of the central 3mm in 2 principle meridians (i.e. only measures ~6% of cornea)
- ———-> peripheral cornea is not measured
- limited capacity to measure + monitor irregular corneal surfaces (e.g. keratoconus)
What is a placido’s disc?
a. k.a keratoscope
- is an ophtalmic instrument used to assess the shape of the anterior surface of the cornea
- a series of concentric rings is projected onto the cornea and their reflection is viewed by the examiner through a small hole in the centre of the disc
What are the 2 types of keratoscope (attachments)?
- Photokeratoscope - gullstrand attached camera
2. Videokeratoscope - with a video attached
In keratoscope/placido disc: What is considered perfect? imperfect? pathology?
Perfect: Equally spaced concentric rings, reflection is perfect so the rings so the concentric rings are symmetrical
Imperfect: corneal astigmatism = oval shaped rings
Pathology: mires of irregular cornea with inferior steepening as in keratoconus (one side is steep with rings closer together, other side is flatter with rings further apart)
(*Actually: imperfect = needs to absorb 2 androids. perfect = has absorbed 2 androids and will now hold a martial arts competition)
What are the 2 systems in Videokeratoscopy?
- Placido disc system
2. Elevation based system
What do you see in placido disc system
- coloured topographical graph with warmer colours = steeper meridians, and cooler colours = flatter
What’s the difference between axial and tangential topography maps for measuring corneal shape?
Axial: measures the curvature at a certain point on the corneal surface in the axial direction relative to the centre
Tangential: measures curvature at a certain point on the corneal surface relative to the other points on the particular ring
True or False: the placido disc system measures both the anterior and posterior corneal shape
FALSE. it only measures anterior.
Limitations of Placido disc-based systems?
Curvature is reference specific
- changes in reference point or viewing angle results in changes of curvature (must have assoc. direction)
- consider axial versus tangential radii
- thus there are many potential solutions
No information about posterior surface
- placido disc reflects off anterior surface
- not good enough for lasik
What is Orthokeratology? And what does it do?
It is a hard lens that flattens the cornea. It is used for long periods of time to help slow the progression of myopia
How does the image look when performing keratometry on someone with dry eye?
Distorted, because the surface is not smooth
