CLM - Optics of Contact Lenses II - Week 3

Question 1

Define the tear lens. How thicc is it?

Answer 1

When a rigid lens is placed on the eye, a thin fluid layer is trapped between the posterior surface of the contact lens and the anterior surface of the eye
Thickness is 10-20μm

Question 2

Is the tear lens involved in the optical system to produce a retinal image?

Answer 2

Yesd

Question 3

Comparing the BOZR to the K readings of the cornea, what is the power of the tear lens?

Answer 3

BOZR > K, then negative tear lens
BOZR = K, then plano tear lens
BOZR < K, then positive tear lens

Question 4

For every what difference between the BOZR and K, there is a what change of tear lens power?
When does this approximation fail (2)?

Answer 4

For every 0.05mm difference between the BOZR and K, there is 0.25D of tear lens power
Very high (steep) radii like keratoconus
Large changes in BOZR

Question 5

Define over-refraction.

Answer 5

Determination of a residual error of refraction of the eye while the patient is wearing spectacles or contact lenses

Question 6

If the BOZR of the trial lens is the same as which you are going to order, what is the power of the ordered lens (formula)?

Answer 6

BVP (CL) = BVP (trial) + over-refraction

Question 7

Consider astigmatism with RGP lenses. What percentage of the front surface astigmatism of the cornea will be neutralised by the back surface of the tear lens? What results in neutralisation of the remainder?

Answer 7

90%
The radii of the back surface of the cornea are slightly smaller than the front surface, resulting in a mild neutralisation of approximately 10%

Question 8

Consider that sperical rigid lenses might optically correct astigmatism. Does it always fit the cornea adequately? What levels of astigmatism does it become a problem?

Answer 8

It might not always fit the cornea adequately
For astigmatic corneas >2.00D, toroidal lenses are required

Question 9

Do RGPs correct for all the astigmatism in an eye? Explain (5).

Answer 9

No, there may be residual astigmatism
Residual astigmatism = ocular astigmatism (prescription) - corneal astigmatism
Can be lenticular, retinal, or both
RGPs neutralise all of them except residual astigmatism

Question 10

Define resigual astigmatism.

Answer 10

The astigmatic prescrition that remains after a rigid contact lens is placed on the eye

Question 11

What are two benefits of using a toroidal lens?

Answer 11

Improved fit in cases of high astigmatism
Stabilises lens when correcting residual astigmatism

Question 12

What does making the BOZR toric do to the tear lens? What must be introduced to the RGP as a result? What is it equal to?

Answer 12

Alters the degree of astigmatism in the tear lens (will reduce it)
A cylindrical component must therefore be incorporated, equal to the degree by which the astigmatic component of the tear lens has been reduced

Question 13

Is the tear lens of a soft contact lens focal or afocal?

Answer 13

Afocal

Question 14

What is the formula for BVP (CL) with soft contact lenses?

Answer 14

BVP (trial) + over-prescription

Question 15

Is corneal astigmatism masked by a soft contact lens?

Answer 15

No

Question 16

In what kind of soft contact lenses can the tear lens cause changes to lens power? Is it minor or major?

Answer 16

High plus, causing minor changes

Question 17

List three things that can cause a change in lens power for soft contact lenses (aside from power).

Answer 17

Lens flexure
Changes in temperature
Evaporation

Question 18

Can you use a minus trial lens for a hyperope and vice versa (soft contact lenses)?

Answer 18

No, must use minus for myopes and plus for hyperopes

Question 19

Within what power should a trial soft contact lens be of their prescription?

Answer 19

Within 3.00D

Question 20

Define lens flexure.

Answer 20

When a soft contact lens is placed on the eye, it takes up the same curvature as the central cornea
This change compared to normal is the lens flexure

Question 21

Is lens flexure important when prescribing soft contact lenses? Explain.

Answer 21

Not very important and generally insignificant except where the lenses are of high positive power

Question 22

What happens to a soft contact lens when there is a change in temperature (especially when placing it on the eye)? Explain (2).

Answer 22

With the contact lens being ~20C and the eye being ~37C, the change in temperature can cause a slight steepening of the contact lens and a slight increase in the negative power

Question 23

List two ways that evaporation can affect the lens power of a contact lens.

Answer 23

Small increase in refractive index
Steepening of the lens
Both tend to increase the negative power of the lens

Question 24

Should the tear lens of a soft contact lens be focal or afocal?

Answer 24

Should be afocal

Question 25

Between plus and minus soft contact lenses, which can entrap a greater volume of tears? What effect does this have?

Answer 25

Minus entrap low volume and minimal power
Plus lenses can entrap a greater volume and significant power
Plus tend to lose power when placed on the eye