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Optometry > Contact Lenses > Flashcards

Contact Lenses Flashcards

1
Q

What is Regular Astigmatism?

A

This is where the refracting surfaces of the cornea and crystalline lens are toroidal, not spherical.

It is independent of the axial length of the globe and results in a point of light being refracted as 2 separate focal lines perpendicular to each other, rather than a single focal point.

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2
Q

What does a Keratometer measure?

A

The curvature of the anterior cornea which gives an indication of astigmatic refractive error if present.

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3
Q

What is the relationship between radius and astigmatism?

A

Every 0.01mm of difference in radius between the two principal meridians equates to approximately 0.50DC of Astigmatism.

Any additional astigmatism in the refractive error is usually of lenticular origin.

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4
Q

What is a major disadvantage of Keratometers?

A

It measures the corneal curvature in a 3mm region around the central cornea and assumes a spherical profile.

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5
Q

What would be a more accurate alternative to Keratometers?

A

Topographer

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6
Q

How does corneal curvature typically change from childhood to adulthood?

A

From With the rule Astigmatism to Against the rule Astigmatism

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7
Q

What to look for on a successful CL fit?

A

1) Centration
2) Full Coverage
3) Smooth, uninterrupted movement on Push-up
4) Slit lag on upward gaze and on blink

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8
Q

What is Prism-Ballast?

A

Most common approach in toric soft lens fitting.

The lens increases in thickness from top to bottom, with the top lid squeezing the thicker portion to the lower lid.

This results in decreased comfort

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9
Q

What are Eyelid Stabilisation Designs?

A

These have relative thinned superior and inferior zones and the lids squeeze across the 4 active zones within the palpebral aperture to control orientation.

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10
Q

What are the 2 methods to calculate the NEW AXIS?

A

LARS

CAAS

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11
Q

What are the 2 methods to calculate the NEW AXIS?

A

LARS:

  • Left: Add
  • Right: Subtract

CAAS:

  • Clockwise: Add
  • Anticlockwise: Subtract
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12
Q

What gives the best lens rotational alignment and stability for a soft toric CL?

A
  • Low lid tension
  • Low Myopia
  • Small Palpebral Aperture
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13
Q

What is Modulus of Elasticity?

A

Measure of the stiffness of an elastic, isotropic material.

Ratio of Stress : Strain

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14
Q

What is Youngs Modulus?

A

The response to linear strain

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15
Q

What is Bulk Modulus?

A

The response to a uniform pressure from all directions

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16
Q

What is Shear Modulus?

A

Concerned with the deformation of a solid when it experiences a force parallel to one surface, while its opposite face experiences a directly opposing force.

17
Q

What does Low Modulus mean?

A

Increased flexibility

18
Q

What is Coefficient of Friction?

A

Measure of Resistance to movement of one material over another

In the case of CLs, this relates to the ability of the eyelid to slide over the lens surface with each blink.

CoF is often referred to as lubricity - where low CoF = high lubricity.

19
Q

What are 2 indicators of excessive friction?

A

1) Lid Parallel Conjunctival Folds

2) Lid Wiper Epitheliopathy

20
Q

What is Lid Parallel Conjunctival Folds?

A

Folds in the lateral, lower quadrant of the bulbar conjunctiva, parallel to the lower lid margin and graded by the number present.

21
Q

What is Lid Wiper Epitheliopathy?

A

The result of frictional trauma sustained from repeated lid movement over a resistant area.

22
Q

What are the effects of inadequate oxygen delivery to the cornea?

A
  • Limbal and Conjunctival Hyperaemia
  • Corneal swelling with oedema, striae and endothelial folds
  • Epithelial microcysts
  • Corneal Vascularisation
  • Endothelial blebs and Polymegathism
  • Central thinning of the epithelium and stroma
23
Q

Monovision

A

CLS which are commonly used to correct one eye of a Presbyope for distance and the other eye for near nabling the patient to alternate between the two.

The unused eye is largely suppressed and Diplopia does not occur.

Monovision may interfere with Stereopsis but the patient can still uses Monocular Cues to Depth and have relatively good depth perception.

24
Q

What advice should be given to Pxs trialling Monovision CLs?

A

Reduction in Depth Perception that could interfere with driving safely

Optometry (41 decks)

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