Contact Lenses Flashcards
What is the recommended extension of a soft contact lens beyond the limbus?
1-2mm 360 degrees
Insufficient coverage can lead to ocular redness and irritation.
What indicates that a soft contact lens may be too flat?
Decentered contact lens
Steepening the base curve can improve centration.
What is the acceptable movement range for a well-fitting soft contact lens during a blink?
0.50 - 2.00mm
Too tight lenses can cause irritation, while excessive movement can lead to discomfort.
What does ‘sag’ refer to in soft contact lens evaluation?
The amount the lens drops during upward gaze
Excessive sag is considered over -1.5mm.
What is the maximum acceptable lag for a soft contact lens?
1.5mm
Less than this indicates the lens fits too tight.
What is the purpose of the push-up test in soft contact lens fitting?
To observe the lens’ resistance to movement
This helps evaluate the fit when other measurements are minimal.
How is lens rotation measured for toric soft contact lenses?
By matching the slit-lamp light beam to the cylinder marking
Record the direction of rotation (right or left).
True or False: A soft contact lens should not move at all during a blink.
False
Some movement is necessary for comfort and fit.
Soft contact lens evaluation protocol (7)
- Coverage
- Centration
- MOB ( movement on blink)
- Sag
- Lag
- Push-up test
- Rotation (Toric soft CL)
What is the significance of the contact lens fitting in terms of movement during blinking?
Loose fitting leads to excessive CL movement with blink
This can indicate poor lens centration and may cause discomfort or irritation.
What are the characteristics of a loose fitting contact lens?
- Excessive CL movement with blink
- Poor CL centration
- Lens edge bubbles
- Lens edge stand-off
- Superior eyelid irritation
A loose fitting lens may not provide adequate vision correction.
What are the characteristics of a tight fitting soft contact lens?
- Minimal CL movement with blink
- Indentation around limbus
- Injection around limbus
- Corneal edema
- No comfort complaints
A tight fitting lens may cause discomfort or corneal issues.
How can you adjust the fit of a contact lens?
- Increase sagittal height
- Decrease sagittal height
- Choose steeper base curve
- Choose flatter base curve
- Increase diameter
- Reduce diameter
Adjustments depend on the fit characteristics observed.
What effect does a steep contact lens have on the lacrimal lens?
Creates a positive lacrimal lens
This requires adding minus to adjust the power of the lens.
What effect does a flat contact lens have on the lacrimal lens?
Creates a negative lacrimal lens
This requires adding plus to adjust the power of the lens.
What is the formula to calculate the Tear Lens?
Tear Lens = Base Curve - Keratometry
This equation helps determine the effective power of the contact lens.
What is the mm to Diopter conversion formula?
mm = 337.5 / D
This is used to convert measurements for contact lens fitting.
What is the Vertexing Equation for contact lenses?
CL Power = Spectacle Power / [1 - (Vertex Distance * Spectacle Power)]
This equation adjusts spectacle power for contact lens fitting.
What is the formula for calculating Residual Astigmatism?
Residual Astigmatism = Total astigmatism of the eye - corneal astigmatism
This helps in determining the appropriate lens for astigmatism correction.
What does SAM FAP stand for in contact lens fitting?
Steeper Add Minus, Flatter Add Plus
This guideline assists in adjusting lens power based on curvature.
In the context of lens power adjustment, what does LARS stand for?
Left Add, Right Subtract
This indicates how to adjust the power based on the orientation of the lenses.
What does apical touch in a lens indicate?
The lens is too flat and likely has excessive edge lift.
Apical touch can cause distortion of the central cornea and, in extreme cases, corneal scarring.
How can apical touch be resolved?
By steepening the base curve of the lens or increasing the overall diameter of the lens or optic zone.
Steepening the base curve helps to prevent distortion and scarring.
What does apical clearance indicate?
The lens has been fit too steeply.
This fitting relationship can result in central pooling of sodium fluorescein with peripheral touch.
What can result from apical clearance?
Lens seal-off and lack of tear exchange.
Central pooling of sodium fluorescein indicates that the lens is not fitting properly.
How can apical clearance be remedied?
By flattening the base curve, decreasing the overall lens diameter, or decreasing the optic zone diameter.
These adjustments help ensure proper fit and tear exchange.
* apical clearance means it too steep, therefore fix it by making it flatter (flatten BC, decrease OAD and OZD
Fill in the blank: A lens that displays _______ is considered too flat.
apical touch
True or False: Increasing the lens diameter can resolve apical touch.
True
* apical touch means lens is too flat, therefore to fix it need to make the lens steeper by increasing the lens diameter
What is a common result of fitting a lens too steeply?
Central pooling of sodium fluorescein with peripheral touch.
What does this RGP fluorescein pattern indicate?
ATR astigmatism
* FL will pool along the right and left side of lens with central touch vertically 12 to 6 o’clock band
RGP FL pattern indicates?
WTR astigmatism
* bow tie on FL pattern
* FL will pool along top and bottom of lens with central touch across 3-9 o’clock band
What is flexure in the context of RGP lenses?
Flexure occurs when an RGP becomes distorted while it is on the cornea, but assumes its normal shape when it is no longer on the cornea.
How is irregular astigmatism measured with RGP lenses?
Irregular astigmatism is measured with keratometry or topography when the RGP is on the eye.
What happens to the measurement of irregular astigmatism when the RGP is off the eye?
When the RGP is off of the eye, irregular astigmatism will not be measured.
What typically causes flexure in RGP lenses?
Flexure is usually caused when the RGP is thin and so its shape is easily contorted.
True or False: Flexure can be confirmed by rubbing the RGP during cleaning.
True
Fill in the blank: Flexure occurs when an RGP becomes _______ while it is on the cornea.
[distorted]
FDA Classes of SCL
- Group 1 low water content, non-ionic
- Group 2 high water content, non-ionic
- Group 3 low water, ionic
- Group 4 high water, ionic
- Group 5 includes silicone hydrogels
Which FDA group of SCL is least likely to develop deposits?
Group 1: low water content and non-ionic
Dimple veiling
* tight RGPCL fit, bubbles get trapped under and imprint dimples into cornea which pool with FL when RGP is removed
Tx: fit into flatter RGP
What are mucin balls?
Small, white, pearl-like debris that develop behind the posterior surface of contact lenses
Mucin balls generally occur with silicone hydrogel lenses that are too flat and are used for extended wear purposes. They are not a threat to vision.
What can mucin balls cause on the corneal surface?
Indentations of the corneal surface
This can be mistaken for microcysts.
How can one distinguish between mucin balls and microcysts?
Microcysts are smaller and appear darker with indirect illumination
This is a simple way to differentiate the two.
What is dimple veiling?
A condition caused by incorrectly fit gas permeable contact lenses, trapping carbon dioxide bubbles underneath the lens
It leaves small circular indentations on the surface of the cornea.
How can dimple veiling be managed?
By flattening the base curve of the lens or decreasing the lens overall diameter
This adjustment can alleviate the condition.
What does protein deposition look like on contact lenses?
A superficial, white, slightly translucent deposition on the lens surface
Protein can become integrated into the lens matrix of high water-content lenses, but is typically limited to the lens surface.
What can protein deposits cause in patients?
Irritation of the upper eyelid and giant papillary conjunctivitis
This occurs due to the presence of protein on the lens surface.
What are jelly bumps?
Small white/gray deposits on the front surface of a contact lens
They are composed of calcium-protein complexes and are more commonly observed with silicone hydrogel soft contact lenses.
What type of lenses are more prone to jelly bumps?
Hydrophilic lenses
Scratched lenses, those with ridges, or polishing errors also increase the chances of jelly bump formation.
Who is at higher risk of developing jelly bumps?
Patients with parathyroid disorders, hypercalcemia, taking oral contraceptives, oral antibiotics, or high doses of hormones
Additionally, those who are breastfeeding or using medications that can alter calcium metabolism are also at higher risk.
Jelly bumps = lipids + calcium
When vertexing a spectacle Rx for CL, as one gets closer to the corneal plane does minus power increase or decrease?
Decreases in minus power
* therefore a minus powered CL will be LESS minus than the spectacle
And a plus powered CL will be MORE plus than the spectacle rx
