Anatomy: Nomenclature & Terminology Flashcards
What does on flat “K” mean?
Alignment fitting relationship matches the flattest meridian
Where do you want a RGP to land?
At 3 and 9 o’clock
What does a darker green area of staining indicate?
Less sodium fluorescein, thinner tear film
What does brighter green staining mean?
More room between lens and epithelium; thicker tear film
What type of patient’s are excellent candidates for RGP lenses?
Those with a slight (~1.50D) WTR astigmatism
How many microns should the lens lift off the vertical meridian at the edges?
About 40 microns
How thick should the staining/tear film be under the RGP lens?
20 microns, this is about the normal thickness of a tear film without lenses.
Where would a RPG land for a ATR astigmat?
12 and 6 o’clock
What is the posterior optical zone (OZ)?
The OZ is the chord diameter over which the base curve extends (measured in mm)
What does ‘cord diameter’ mean?
Since the definition of a diameter is a chord passing through the center of the circle, A segment of a straight line joining two points on a circle is called a chord (i.e. a line going through the 2 and 10 o’clock on a clock)
What is sagittal depth?
Distance between the flat plane of a given diameter and the highest point of a concave surface of the CL
If you have a larger OZ, will the fitting relationship be steeper or flatter?
Steeper (i.e. greater sagittal depth)
If you have a smaller OZ, will the fitting relationship be steeper or flatter?
Flatter (i.e. less sagittal depth)
Overall lens diameters usually come in 0.5mm increments: 8.5, 8, 9.5, etc…
Free card- this one is for Matt
What are the names of the two other curves on a RGP lens? (found near the edge)
Secondary Curve (width is about 0.4mm) and Peripheral Curve (width is about 0.3mm). These curves help the lens land more comfortably for the patient and help with the fit I’m assuming as it gives an aspheric lens design
If you have an OZ of 7.6mm, a secondary curve width of 0.4mm, and a peripheral curve width of 0.3mm, what would be your optical zone diameter?
9mm, remember that each side of the lens has a secondary and peripheral curve
Normal cornea flattens in the periphery, also remember that according to Pat, ATR astigmats are just bad people haha
Free card
The secondary curve (between OZ and peripheral curve) is flatter radius of curvature adjacent to the base curve. Approximately how much flatter than base curve (BC) radius?
0.70 mm to 1.50 mm (i.e. BC 7.90 mm, secondary curve 8.90 mm)
The width of the secondary curve is how big?
0.2 to 1 mm
Why do you want clearance on the peripheral curve?
The clearance will help the fit on the cornea stay fairly nice when the patient looks out of primary gaze.
What is the outer most curve on the posterior surface of the lens?
Peripheral Curve, it is designed to clear the peripheral cornea and limbus. The width of the peripheral curve radius is between 0.1 and 0.4 mm.
Lens Diameter is the overall width of the lens from edge to edge. What are the GP diameter range?
7.0 mm to 11.0 mm (without a specialty order I’m assuming)
What 3 things do you need for a CL Rx?
BC, Diameter, Power
As a general rule for fitting RGPs, if the lens is steeper, how do you change the Rx? Also if lens is flatter?
SAM and FAP
Steeper, it acts a plus lens so you need to add minus power to compensate. Flatter, it acts as a minus lens so you need to add plus power.
How does a RGP lens correct corneal astigmatism despite it being a spherical lens?
The tear film underneath will act as a refracting surface so the front of the lens will act now as a perfect spherical ‘front’ for incoming light.
We talked about a lens center of gravity. Where is the center of gravity for a plus lens vs a minus lens?
A plus lens is more anterior, this in effect will tend to make the CL ride down. A minus lens is adversely more posterior, which will somehow make the lens ride up.
What is the ‘Anterior Optical Zone Radius’?
Radius of curvature on the anterior lens surface which determines the refractive power. Has both radius of curvature (power) and diameter (in mm). Note- we can ask the lab to increase this if the patient needs it (i.e. their eyes dilate more than usual at night)
Describe ‘Lenticular Flange’
Refers to an increase or decrease in the anterior edge thickness to aid in lens positioning.
What is Myo-Flange?
An INCREASE in peripheral lens thickness to aid in raising low riding lenses.
How does a myo-flange help a lens ride higher?
Additional edge thickness creates GREATER lid/lens interaction thus allowing the upper lid to grab and position the lens higher on the cornea.
What is a Hyper-Flange or CN Bevel?
Decrease in peripheral thickness to aid in lowering high riding lenses. Less lid/lens interaction will occur allowing it to sit lower on cornea.
Describe edge design
Represents junction between anterior and posterior lens surfaces. The edge profile is instrumental to overall comfort and optometrists used to do this in office quite frequently. Refer to notes for what a good edge vs a bad edge looks like.
What is the purpose of a lens ballast?
Helps keep orientation of the lens on the eye, usually a CL design with a thicker inferior portion of the lens periphery.
How does a lens ballast help orientation?
Upper lid squeezes lens downward, “watermelon seed effect”
What is truncation and what specialty CLs are they used in?
Removal of inferior portion of a lens to aid in meridional orientation (stabilization) of a lens. Use in some toric and bi-focal lens designs.
Why are fenestrations used?
Holes in CLs to help prevent suction, easier for removal and possible to increase tear flow.
