Chapter 12: Contact Lenses

Question 1

Why are contact lenses optically superior to glasses

Answer 1

Eliminate aberrations associated with high refractive spectacles.
Reduce aniseikonia caused by anisometropia and high degrees of astigmatism.

Question 2

Apart from to correct visual impairments, what are the other uses of contact lenses

Answer 2

• Bandage contacts
• Provide artificial iris for aniridia
• Can also be used for ERG
• Can be used to see fundus and trabecular mesh work

Question 3

Describe torsion of a contact lens and how it can be prevented

Answer 3

With a toric lens, some small vision fluctuation with a blink is expected. This occurs because, during a blink, the lid causes mild torsion of the lens on the eye.

Torsion can be prevented by incorporating a 2D base down prism to weigh down the lower pole of the lens or truncating the lens, so that it sits on the edge of the lower eyelid.

Question 4

Define the posterior central curvature of a contact lens

Answer 4

PCC is also known as the base curve.
It is the radius of curvature in mm
It can also be measured by its dioptric power in air.

Question 5

To ensure that a contact lens closely conforms to the cornea, what shape must it be?

Answer 5

Aspheric, like the cornea

Question 6

What 2 factors determine how well a contact lens fits the cornea

Answer 6

Contact lens base curve
Contact lens diameter
This will also determine how easily the contact moves when you blink

Question 7

How do contact lenses facilitate the circulation of the tear film?

Answer 7

They have can contain fenestrations

Question 8

What is the main drawback of a high power minus contact lens and how is this issue mitigated?

Answer 8

Heavy and thick. They catch on the upper eyelid and tend to sight high up. This can be counteracted by adding a bevel.

Question 9

What is the main drawback of a high power plus contact lens and how is this issue mitigated?

Answer 9

Heavy and thick. Due to the weight, tends to sit low down.
This is counteracted by by a minus peripheral carrier zone

Question 10

Apart from nourishing and lubricating the corneal surface, what other benefit does the corneal tear film provide

Answer 10

It has approx the same refractive index as the cornea. As a result, it smoothens out irregularities in the cornea without distrusting the refractive index of the eye.

Question 11

Define the tear lens

Answer 11

The tear film between the posterior surface of the contact lens and the anterior surface of the cornea.

Question 12

If the tear lens has uniform thickness, what is the resultant power

Answer 12

Plano power

Question 13

What happens to the power of the tear lens when the contact lens base curve is made steeper?

Answer 13

Tear lens axial height increases, making the power more positive

Question 14

What happens to the power of the tear lens when the contact lens base curve is made less steep?

Answer 14

Becomes more negative

Question 15

How can we use a contact lens to neutralise corneal astigmatism?

Answer 15

Ensure that the base curve of the lens matches the flattest meridian of the cornea. The tear lens fills in the gaps in the steeper corneal meridian and is thicker, thus neutralising the astigmatism.

Nb in contacts, only spherical components can be prescribed and are expressed in terms of negative cylindrical power.

Question 16

True or false, soft contact lenses can correct high degrees of astigmatism

Answer 16

False, because they are soft, they tend to conform to the shape of the cornea and so wont neutralise the steepest axis. As a result they wont neutralise more than 1D

To neutralise high astigmatism, you need hard contacts as they will maintain their shape and so the tear film will fill in the gaps, thus neutralising the astigmatism.

Question 17

What phenomena occurs when a dilated pupil looks through a hard contact lens

Answer 17

The patient will see halos due to refraction through peripheral zones of the lens

Question 18

Why do hypermetropic patients reduce their field of view with glasses when compared to contacts? How does this differ from myopic patients?

Answer 18

Spectacle lens periphery causes prismatic effect with the base towards the visual axis.

The opposite happens in myopic patients, glasses increase field of view as prismatic effect is with the base away from the visual axis.

Question 19

Why might a patient who is myopic who has developed presbyopia feel like their vision is straining switching from glasses to contacts

Answer 19

When looking from distance to near, myopic glasses have a base in prismatic effects, reducing the amount of accommodation convergence needed. This phenomena is eliminated when wearing contacts which may cause straining

Question 20

What is the maximum prism power that can be incorporated into a corneal contact lens and why? What is a major disadvantage to them?

Answer 20

3 diopters. More prism power will make it too thick and heavy.

Drawback: The weight of the contact rotates the contact so that it is always base down. This makes horizontal correction impossible and limits the prism to one lens.

Question 21

What are the benefits of carefully fitted scleral contact lenses?

Answer 21

They can be used to incorporate six prism dioptres of either vertical or horizontal power divided between both contacts (corneal contacts only allows three prism dioptres vertically)

Question 22

Why might you incorporate a tint into contacts

Answer 22

So they are easier to see and thus handle/ retrieve (using blue tint).
Can also make eyes looks more colourful

Question 23

What is monovision?

Answer 23

One eye (usually the better eye) fitted with a contact that allows distance vision.
The other eye is fitted with contact that allows near vision. Binocularity and stereopsis is reduced.

Question 24

Describe how annular bifocal contact lenses work

Answer 24

Made of concentric rings. Central circle is for distance vision. When the patient looks down to read, the contact lens slides up slightly, bringing the near vision portion of the contact in front of the visual axis.

Question 25

What are the disadvantages of anular contact lenses

Answer 25

When pupil dilated, light will be focused by both distance and near section of lens, producing 2 images. Patient needs to learn to focus on the correct one.

If pupil is small, the peripheral zones of annular contact may not meet the visual axis when looking closely, thus not forming a focused image

Question 26

describe segmental bifocals

Answer 26

Near vision section is at the bottom of the contact lens. When the patient looks down, the near segment is at the visual access and is used to focus the image on the retina. It is very important that the contact is not rotated, which is why the contact is often truncated.

Question 27

True or false, images formed by diffraction contact lenses are often les bright compared to other forms of bifocal contacts

Answer 27

True