Unit 3 Flashcards
Radius of curvature
Shorter radius - steeper curve
Longer radius - flatter curve
What is radius of curvature measure in?
mm
What does radius curvature convert too
Dioptric power
What does curvature determine ?
Corneal power
Conversion formula
P = ( n-1) / r
P- curvature power in diopters
N- 1.3375 (index of refraction for eye) never changes
R- radius of curvature in meters
Converting from mm to m what do u do
Divide by 1000
Corneal power
Average corneal power value - 43.50D
Steeper cornea- higher value
Flatter cornea- lower value
Keratomer
Measure ?
Measure- corneal curvature
Central 3 mm
- curvature of the two principal meridians (k readings) (horizontal and verticals)
Keratometer
Purpose?
-amount and type of asitgamistm
-Determine the initial contact lens to be place on the eye
-intraolcular lens implant section
-indicate a compromised cornea
-diagnose keratwconus
-asses tear layer / quality
Theory of keratometer
Cornea acts as a convex mirror
Reflection of an object (mires) a known size at a known distance is viewed using a short focus telescope
The corneal power that results from given radius
With the rule
Horizontal is the flatter meridian (less curved)
Against the rule
Vertical is the flatter meridian
If the minus signs don’t align vertically what does that mean?
Irregular astigmatism
Is minus signs horizontal or vertical?
Vertical power
Plus signs are ?
Horizontal power
Distorted mires are associated to what
Poor tear quality
Irregular asstigmatism
What is corneal topography
-Describes the shape of the corneal surface in all meridians
-Uses series of concentric targets (a placido disc)
-Allows both central and peripheral curvature to be determined
-Can detect gross irregularities in the corneal surface and very high astigmatism
The map displays the power distribution using colour coded displays
Normal corneas are represented by what colour
Greens and yellows
Topography
Flatter areas
Blues or cool colours
Topography
Steep areas
Reds or hot colours
Purpose of corneal topography
Screen for corneal changes (ectasia)
Monitor corneal ectasia
Fitting contact lens
Types of topographers
Placido disc
Slit scanning & scheimpflug technology
Types of maps
Axial
Tangential
Elevation
Corneal thickness
Axial map
Corneal power
Good overview
Tengential map
Power and curvature at individual points
Most accurate
Elevation
Placedo disc - calculate elevation
Scheimflug- actual elevation measure
Corneal thickness (pachymetery)
Scheimflug and scanning slit primarily
Elevation knob
Raise or lower the instrument until a bright ring refelection is centered in the cornea
Focusing knob
Clear the circular images
How to move the cross to the center of circle
Move the instrument or elevation knob
Contact Lens Equipment
- Keratometry
◦ Automated
◦ Manual - Slit Lamp
- Corneal Topography
- Dry Eye Assessment
◦ Tear Break Up Time (TBUT)
◦ Schirmer’s test
Manual Keratometry Process
- Handout: Procedure for Obtaining Keratometry Readings (Brightspace)
1. Calibrate instrument
2. Patient preparation
◦ Disinfect chin and head rest
◦ Occlude left eye
◦ Align reflected mires on patient’s cornea - Look into instrument
- Plus sign in lower right circle
- Focus mires
Adjust horizontal power until plus sign tips are near each other - Rotate telescope until tips of plus signs align
◦ Determine the principal meridians - Superimpose plus signs
◦ Horizontal power - Superimpose minus signs
◦ Vertical power
Manual Keratometry Documentation
- Record the power and axis for the horizontal meridian
- Record the power and axis for the vertical meridian
◦ You may see the readings recorded with only one axis identified since the other is assumed to be 90 degrees away.
◦ There is a smaller chance of error when both axes are recorded. - Record whether the mires are clear and regular (MCAR), D0
- Distorted mires are often associated with poor tear quality or irregular astigmatism
Manual Keratometry Documentation ex
43.50 @ 175 / 45.00 @ 085 D0
* Amount of astigmatism = 1.50 D
Slit-Lamp Biomicroscopy
- Plays an essential role in the preliminary assessment and follow-up care of contact lens wearers
- May also be used to assess:
◦ Posterior eye health
Vitreous, lens, retina
◦ Anterior chamber angle
◦ Intraocular pressure
Slit-Lamp Biomicroscopy
* Consists of:
◦ An illumination system (the slit lamp)
◦ Viewing system (the biomicroscope)
Joystick
◦ Focusing
◦ Lateral movements
* Height control
Illumination System
- Brightness is controlled by a rheostat or multiposition switch
◦ Can be adjusted to obtain balance between patient comfort and optimal visibility - Filters can be incorporated to enhance the visibility of certain conditions
◦ Green filter (neovascularization)
◦ Neutral-density filter
◦ Polarizing filter
◦ Diffusing filter
◦ Cobalt blue filter*
◦ Wratten (yellow) filter
Microscope
- Provides a clear image of the eye
- Sufficient magnification to view all structures of interest
◦ Up to 40x should be possible
Slit-Lamp Biomicroscopy
- Six areas of the anterior eye should be assessed in the preliminary examination ◦ Fig. 37.2 (Contact Lens Practice)
- A normal technique is to use a variety of illumination methods – and cobalt blue light for fluorescein staining
Recording Slit Lamp Findings
- Lids = clear, uneven lid margin
- Lashes = clear, Gr 1+ blepharitis
- Conjunctiva = clear, Gr 1+ GPC, Gr 2+ injection
- Cornea = clear, no staining, negative SPK, Grade 2+ SPK inferior
- Angles = Gr 4+ open, narrow angles
- Iris = flat and even, flat round pigment 1 mm at 6 o’clock
- A/C = negative for cells or flare; Gr 3+ flare
- Lens = Gr 1+ NS, clear, Gr 2+ brunescence
Asses lipid layer
- There are six clinical techniques:
Symptoms/history Dry-eye survey
Slit-lamp
Tear meniscus height Tear stability tests
Tear Break Up Time (TBUT) Ocular staining
Fluorescein or lissamine green Lid surface assessment
Blink rate
Tear secretion/quantity
Schirmer, Phenol red thread
Schirmer test
◦ Test 1 – basal and reflex (no anesthetic)
◦ Test 2 – basal (with anesthetic)
Slit Lamp Direct Illuminations
◦ Parallelepiped
◦ Optic section
◦ Specular reflection
◦ Conical
◦ Retroillumination (Retro)
* Indirect Illumination ◦ Retro
◦ Sclerotic scatter
