Contact lens

Question 1

ou are designing a rigid gas permeable (RGP) contact lens for your patient who presents with keratometry reading of 44.25 x 41.00 @ 180, and manifest refraction of -2.50 -3.00 x 180. With a spherical RGP diagnostic lens placed on the eye, the over-refraction is +0.50 -0.25 x 180. Which of the following RGP lens designs is MOST appropriate for this patient?

Front Surface (F1) Toric

Toric Base Curve

Spherical

SPE Bitoric

CPE Bitoric

Answer 1

The following chart represents guidelines for lens selection when prescribing rigid gas-permeable (RGP) contact lenses. This will aid in determining which type of RGP lens is best for your patient, depending on the patient’s corneal toricity and residual over-refraction cylinder values (when a spherical GP diagnostic lens is placed on the eye).

The above patient presents with 3.25 diopters of corneal cylinder, which is typically above the threshold for a spherical RGP contact lens. Although a spherical lens would provide a low cylinder over-refraction (as shown when a diagnostic lens is placed on the eye), the fit of a spherical lens on a cornea with over 2.50 D of toricity would not be ideal. Because the above patient has high amounts of corneal astigmatism and a very low amount of cylinder on over-refraction, a spherical power effect (SPE) bitoric lens would be the most appropriate option. An SPE bitoric lens will improve the fit of the contact lens with a toric surface but will not change the cylinder amount on over-refraction.

The spherical power effect lens has a toric curve on the front surface of the contact lens to compensate for the astigmatic power induced by the toric back surface of the lens. Since the lens has a spherical power effect, the patient does not see any changes in vision with blinking if the lens rotates on the eye.

Question 2

Your 50 year-old patient currently wears soft contact lenses as monovision with the right eye distance and left eye near. Manifest refraction is OD: -2.00 -0.50 x 180 and OS: -2.25 -0.50 x 180. If the patient requires a +1.75D add, what would you expect the over-refraction to be if spherical soft lenses were placed on the eyes to allow for this type of vision?

Correct answer OD: +0.25 -0.50 x 180 OS: -1.50 -0.50 x 180

OD: -1.50 -0.50 x 180 OS: +0.25 -0.50 x 180

OD: +2.00 -0.50 x 180 OS: +0.25 -0.50 x 180

OD: +0.25 -0.50 x 180 OS: +2.00 -0.50 x 180

Answer 2

In the above case, the patient wishes to wear monovision contact lenses, with right eye distance and left eye near with a +1.75 effective add.
Because the right eye is to be used for distance, her over-refraction should be plano equivalent sphere (EDS) for the right eye. She does show -0.50 cylinder on refraction, so it will also likely present in the over-refraction as well. Therefore, in order to give the best possible distance vision in this eye, we would expect the over-refraction to be +0.25 -0.50 x 180 (plano EDS).
The left eye is to be used for near and the patient requires a +1.75 add power to achieve her best reading vision. Therefore, in order to achieve this, we would expect the over-refraction to be -1.75 equivalent sphere (EDS). However, she also has -0.50D of cylinder in her left eye that will need to be taken into consideration; this will then produce an expected over-refraction of -1.50 -0.50 x 180 (-1.75 EDS).

Question 3

or a gas-permeable lens, what happens to the following parameters as the oxygen permeability of the lens is increased (Dk)?

The wettability increases, the durability decreases, the lens flexes/warps more easily, the ability of the lens to resist scratches increases

The wettability increases, the durability increases, the lens flexes/warps less easily, the ability of the lens to resist scratches decreases

The wettability decreases, the durability increases, the lens flexes/warps less easily, the ability of the lens to resist scratches increases

Correct answer The wettability decreases, the durability decreases, the lens flexes/warps more easily, the ability of the lens to resist scratches decreases

Answer 3

As one increases the Dk of a gas-permeable lens, the amount of oxygen that is able to reach the cornea increases; however, the wettability of the lens decreases. This hurdle can be overcome by adding fluorine, which helps to increase the wettability and increase deposit resistance. As the Dk increases, the lens also becomes less durable and can become prone to scratching, flexure, and warping.

Question 4

Which of the following BEST describes the proper method of measuring the contact lens “sag” on your soft contact lens-wearing patient?

Correct answer Have the patient move from primary gaze to superior gaze; measure the amount the soft contact lens drops

Have the patient look in primary gaze without blinking; measure the amount of inferior decentration of the soft contact lens

With the patient looking in primary gaze; measure the amount of movement of the contact lens occurs when it is gently pushed up with manipulation of the lid

Have the patient move from primary gaze to lateral gaze; measure the amount the soft contact lens moves relative to the cornea

With the patient looking in primary gaze, have them blink and measure the amount of contact lens movement that occurs during the blink

Answer 4

• The following describes the proper procedure for measuring certain aspects of the fitting relationship of soft contact lenses:
• Sag: Have the patient move from primary gaze to superior gaze; measure the amount the soft contact lens drops
• Centration: Have the patient look in primary gaze without blinking; measure the amount of decentration (if present) of the soft contact lens; this may be in any direction
• Movement on Blink: With the patient looking in primary gaze, have them blink and measure the amount of contact lens movement that occurs during the blink
• Lag: Have the patient move from primary gaze to lateral gaze; measure the amount the soft contact lens moves relative to the cornea
• Push-up test: With the patient looking in primary gaze; measure the amount of movement of the contact lens occurs when it is gently pushed up with manipulation of the lid

Question 5

You are fitting a toric soft contact lens to your patient’s right eye. The patient’s manifest refraction is -2.00 -1.50 X 095. You apply a -1.75 -1.25 X 085 diagnostic toric soft contact lens. It fits well, and the prism base down marking consistently locates halfway between the 6 o’clock and 7 o’clock hours. What axis should you order?

100 degrees

70 degrees

95 degrees

80 degrees

Correct answer 110 degrees

Answer 5

Applying LARS to compensate for lens rotation, since the lens is rotated to the Left, you would Add the amount of left rotation to the manifest refraction axis. Every hour on the clock dial would translate to 30 degrees rotation. In the above example, the lens is rotated to the doctor’s left by 15 degrees (between the 6 and 7 o’clock hours). Add the amount of rotation (15 degrees) to the cylinder axis of the manifest refraction (95 degrees). This results in a cylinder axis order of 110 degrees.

Question 6

Your 32 year-old patient wearing rigid gas-permeable contact lenses comes in to your office complaining of mild redness and irritation with his current set of contact lenses. On slit-lamp examination you notice moderate peripheral cornea desiccation (3-9 staining). Which 2 of the following adjustments could you make to the contact lens parameters in order to minimize his signs and symptoms? (Select 2)

Flatten the peripheral curves

Change lens material to higher Dk

Correct answer Steepen the base curve

Flatten the base curve

Correct answer Steepen the peripheral curve

Answer 6

he most common complication associated with rigid gas-permeable contact lens wear is peripheral corneal desiccation, also known as 3-9 staining. In most cases, the patient reports only mild symptoms of interpalpebral redness and slight irritation. Slit lamp examination will reveal superficial punctate staining that can vary from small areas of non-coalescing SPK to larger coalescing areas of SPK, with deep fluorescein staining that can result in dellen formation. These signs are often due to excessive edge lift of the contact lenses which results in areas of tear film disruption, leading to areas of dryness and disruption of the corneal epithelium. In order to minimize these signs and symptoms, adjustments can be made to the fit of the contact lenses in order to decrease the edge lift and increase the lid apposition to the peripheral cornea. Steepening the base curve of the contact lens and/or steepening the peripheral curves of the contact lens will aid in achieving this objective. Additionally, thinning the contact lens edge and lenticulating higher minus lenses will also help. Studies have shown that the 3-9 staining is purely fit related, and changing the material of the lens has no benefit in reducing corneal signs.

Question 7

Increasing silicone content and decreasing water content will have what effect on the oxygen transmissibility of soft contact lenses?

The oxygen transmissibility will not be affected

Correct answer The oxygen transmissibility will increase

The oxygen transmissibility will decrease

Answer 7

Increasing silicone content and decreasing water content will increase overall oxygen permeability and transmissibility; this combination might also increase the propensity for lipid deposition and may increase the modulus of the material. Silicone hydrogel materials tend to have an inverse relationship between Dk and water content

Question 8

You are recording the parameters of your patient’s rigid gas permeable lenses into his record that you have obtained from a prescription from his prior optometrist. The overall diameter (OAD) is noted to be 9.0mm, the optic zone (OZ) is 7.4mm, and the tertiary curve width (TCw) is 0.2mm. The secondary curve width (SCw) is not available, but based on the above information you calculate it to be which of the following?

1. 20 mm
2. 70 mm

Correct answer 0.60 mm

1.40 mm

Answer 8

The overall diameter of the rigid gas permeable contact lens is equal to the optic zone diameter, 2x the secondary curve width, and 2x the tertiary curve width. One may rearrange the equation to calculate the necessary unknown.
https://www.optoprep.com/simboards/pics/CMLYPIZDMJVGXT115784.PNGhttps://www.optoprep.com/simboards/pics/CMLYPIZDMJVGXT115784.PNG

Question 9

The Dk/t value of a contact lens refers to which of the following terms?

Correct answer Oxygen transmissibility

Oxygen content

Oxygen permeability

Water transmissibility

Water permeability

Water content

Answer 9

The Dk/t value of a contact lens (hard or soft) refers to the oxygen transmissibility of a particular contact lens. Oxygen transmissibility is therefore directly proportional to the oxygen permeability of a material (Dk) and inversely proportional to the average thickness of a lens material (t). In most cases of published Dk/t values, contact lens companies will use a standard -3.00D contact lens prescription to calculate this value.

Manual of Contact Len Prescribing and Fitting (3rd edition). Hom, M.M., Bruce, A.S. 2006. Butterworth Heinemann Elsevier. St. Louis, MO.

Question 10

Which of the following toric gas-permeable lens designs is MOST appropriate for a patient with keratometry values of 43.50 @ 100 / 43.00 @ 080 and subjective refraction data of -2.00 -2.50 x 095?

Thin-flex

Correct answer Prism ballast front surface (F1) toric

CPE bitoric

Back surface (base curve) toric

SPE bitoric

Answer 10

A front surface, prism ballast, toric gas-permeable contact lens is indicated in a patient that presents with a low amount of corneal toricity (typically less than or equal to 1.00D) and, when tested with a spherical rigid lens, reveals an unacceptable amount of residual astigmatism in the over-refraction (usually greater than or equal to 1.00D). These findings indicate the presence of residual or lenticular astigmatism that will need correcting in order to optimize visual acuity. Because the corneal curvature is essentially spherical, a gas-permeable lens with back surface toricity will not align properly on the cornea; therefore, the most appropriate way of overcoming the uncorrected cylinder issue is to order a toric front surface contact lens for the patient.

Question 11

Your 54 year-old female patient wishes to wear soft contact lenses as monovision. She currently wears distance contact lenses in both eyes, is right eye dominant, and requires a +2.00 add power. Based on the following data, which of the following contact lenses would you prescribe for each eye?
Current distance CL Rx: OD: -2.50 OS: -1.00
Distance over-refraction: OD: plano OS: +0.25 -0.50 x 180

OD: plano OS: +1.00

OD: plano OS: +2.00

Correct answer OD: -2.50 OS: +1.00

OD: -2.50 OS: +2.00

Answer 11

Because the above patient is right eye dominant, her right eye would be considered the distance eye and the left eye would be used for near vision.
The patient is currently wearing distance Rx in both eyes, and distance over-refraction is plano in the right eye; therefore we would keep the right contact lens prescription the same in this eye (-2.50).
For the left eye, the patient is also wearing distance Rx, and the distance over-refraction has a plano equivalent sphere. In order to determine the near Rx for this eye, we would simply add the near requirement to her current distance Rx; -1.00 +2.00 = +1.00.

Question 12

Your rigid gas-permeable contact lens patient presents with dimple veiling of the cornea. What modification can be made to the contact lens in order to decrease the occurrence of this finding?

Flatten the peripheral curves

Steepen the base curve

Increase the overall diameter

Correct answer Decrease the optic zone diameter

Increase the optic zone diameter

Answer 12

Removing the contact lens for several hours will typically allow for resolution of dimple veiling and any associated symptoms. Long-term treatment, however, will require modification of the contact lens parameters. Dimple veiling is a result of poor tear exchange under the gas permeable contact lens, which leads to the entrapment of bubbles of carbon dioxide under the central curvature of the lens. Decreasing the overall diameter of the contact lens and flattening the base curve of the lens may help prevent the formation of dimple veiling, but the treatment of choice is to decrease the diameter of the optic zone or in some manner reduce the vault of the lens. This allows for better tear flow that may have been impeded by a tight mid-peripheral junction of the optic zone and intermediate and peripheral curvatures of the contact lens.

Question 13

Your rigid gas-permeable contact lens patient presents with dimple veiling of the cornea. What modification can be made to the contact lens in order to decrease the occurrence of this finding?

Flatten the peripheral curves

Steepen the base curve

Increase the overall diameter

Correct answer Decrease the optic zone diameter

Increase the optic zone diameter

Answer 13

Removing the contact lens for several hours will typically allow for resolution of dimple veiling and any associated symptoms. Long-term treatment, however, will require modification of the contact lens parameters. Dimple veiling is a result of poor tear exchange under the gas permeable contact lens, which leads to the entrapment of bubbles of carbon dioxide under the central curvature of the lens. Decreasing the overall diameter of the contact lens and flattening the base curve of the lens may help prevent the formation of dimple veiling, but the treatment of choice is to decrease the diameter of the optic zone or in some manner reduce the vault of the lens. This allows for better tear flow that may have been impeded by a tight mid-peripheral junction of the optic zone and intermediate and peripheral curvatures of the contact lens.

Question 14

Which 3 of the following typically occur when the oxygen permeability (Dk) of a rigid gas-permeable (RGP) lens increases? (Select 3)

Correct answer Less resistant to protein deposits on the lens

Correct answer Lenses are more likely to scratch

Durability of the lens increases

Lenses are less likely to warp/flex

Correct answer Wettability of the lens decreases

Answer 14

A high Dk rigid gas-permeable (RGP) contact lens will allow greater oxygen permeability, thereby allowing more oxygen to reach the corneal surface. This is beneficial in that corneal physiology is typically superior with a higher Dk lens, and there tend to be fewer complications that can result from lower amounts of oxygen (inflammation, neovascularization, etc.).
However, there are disadvantages to high Dk RGP contact lenses. High Dk lenses tend to be less durable than lower Dk lenses. Thus they are more prone to warpage and flexure and are more likely to scratch. Because of this, these lenses often need to be replaced on a more regular basis. Additionally, higher Dk lenses are also more likely to develop protein deposits and typically have worse wettability than a lens with a lower Dk value.

Question 15

Which of the following BEST describes the design function of a fused crescent bifocal rigid gas-permeable (RGP) contact lens?

Annular

Simultaneous

Concentric

Correct answer Alternating

Aspheric

Diffractive

Answer 15

Fused crescent bifocal rigid gas-permeable (RGP) contact lenses are fit as an alternating design. The appearance of the contact lens is similar to that of a bifocal spectacle lens in which there is a segment positioned in the inferior portion of the lens that posses a higher index of refraction than that of the main portion of the contact lens. The effectiveness of this type of lens is based on the idea that as the patient views in primary gaze, the pupil will be in the area of the distance Rx; further, as the patient looks down, the lens catches on the lower lid and stays in position while the pupil moves down into the add portion. In order for this to work properly, these lenses are typically prism ballasted so that to segment does not rotate, and truncated so as to catch on the lower lid. With this idea, the patient is “alternating” viewing through the distance and near portion of the lens as desired.

Question 16

Your 38 year-old male patient wears rigid gas-permeable contact lenses. The following parameters are for his right eye:
Keratometry: 45.50 @ 090 x 44.00 @ 180
Base curve of RGP: 44.00 (apical alignment)
Over-refraction: +0.50 -1.00 x 180
Which 2 of the following would you expect to occur to the over-refraction if you suspect that his contact lens is warped by 0.50D? (Select 2)

The equivalent diopter sphere of the over-refraction will become plus

The amount of with-the-rule astigmatism in the over-refraction will decrease

The amount of with-the-rule astigmatism in the over-refraction will not change

Correct answer The equivalent diopter sphere of the over-refraction will not change

Correct answer The amount of with-the-rule astigmatism in the over-refraction will increase

The equivalent diopter sphere of the over-refraction will become minus

Answer 16

When a rigid gas permeable contact lens warps or flexes on the eye, the over-refraction will change.
- If the corneal toricity is WTR and the original over-refraction is WTR, the amount of WTR astigmatism in the new over-refraction will increase (by the amount of warp)
- If the corneal toricity is ATR and the original over-refraction is ATR, the amount of ATR astigmatism in the new over-refraction will increase (by the amount of warp)
- If the corneal toricity is WTR and the original over-refraction is ATR, the amount of ATR astigmatism in the new over-refraction will decrease (warp/flexure actually helps here)
Additionally, the equivalent diopter sphere (EDS) of the over-refraction will remain the same

Question 17

Which of the following alterations will help to loosen a tightly-fitting gas-permeable lens?

Steepen the peripheral curve system

Increase the overall diameter

Correct answer Reduce the size of the optic zone

Reduce the width of the peripheral curve system

Steepen the base curve of the lens

Answer 17

There are a multitude of alterations that can be made when a lens is fitting too tightly, many of which can be done in-office if a modification unit is available. If a gas-permeable lens is fit too tightly, the most commonly altered parameter is flattening of the base curve. One can also decrease the optic zone, decrease the overall diameter (OAD), widen the peripheral curve system, or flatten the peripheral curve system. In order to modify a lens that is fitting too loosely, simply reverse all of the above: steepen the base curve, increase the OAD, increase the optic zone, steepen the peripheral curve system, and narrow the width of the peripheral curves.

Question 18

our 26 year-old patient wears rigid gas-permeable contact lenses with the following base curve and power, respectively:
OD: 44.37 / -1.00DS and OS: 44.00 / -1.50DS
He reports that after he put his contacts on this morning, his vision was blurred at distance, especially out of the left eye. You suspect that he may have accidentally switched the lenses. If this were the case, what would you expect the over-refraction (OR) to be for each eye? (Assuming that the over-refraction is plano with the proper lenses and fit is apical alignment OD, OS)

OD: -0.87DS OS: +0.87DS

OD: +0.37DS OS: -0.37DS

OD: -0.37DS OS: +0.37DS

OD: -0.50DS OS: +0.50DS

Correct answer OD: +0.87DS OS: -0.87DS

OD: +0.50DS OS: -0.50DS

Answer 18

OD: 44.37 / -1.00DS OS: 44.00 / -1.50DS
With these lenses, fit is apical alignment and over-refraction is plano.

OD: 44.00 / -1.50DS (switched).
This lens will create apical touch by 0.37DS; this would add -0.37D created by the negative tear meniscus.
+0.37D would be manifested in the OR to offset the minus power created.

The switched lens is -0.50D overcorrected in power and will require +0.50D in the OR to offset this.

Expected OR: +0.87DS

OS: 44.37 / -1.00DS (switched).
This lens will create apical clearance by 0.37DS; this would add +0.37D created by the positive tear meniscus.
-0.37D would be manifested in the OR to offset to plus power created.

The switched lens is -0.50D undercorrected in power and will require -0.50D in the OR to offset this.

Expected OR: -0.87DS

Question 19

You place a soft toric contact lens on your patient’s right eye. You let the lens settle for several minutes and then assess the fit with the slit-lamp. You note that coverage is full and the lens is centered, but the inferior laser lens marking is rotated so that it sits at the 7 o’clock position. Which of the following BEST describes the amount of rotation of this contact lens in degrees?

10 degrees

15 degrees

5 degrees

20 degrees

Correct answer 30 degrees

Answer 19

Each hour of the clock represents 30 degrees. Therefore, if the inferior vertical laser marking is rotated to the 7 o’clock position (1 hour away from the desired 6 o’clock position), one would quantify the amount of rotation as 30 degrees (left).

Question 20

You place a diagnostic rigid gas permeable contact lens on your patient’s right eye with a base curve of 44.75D and a power of -3.00D. Analysis of the fit shows that the lens is apical touch by 0.75D, and an over-refraction reveals -1.25 -0.50 x 170. If you desire an apical alignment lens fit and plano equivalent spherical over-refraction, which of the following contact lenses would you order that would MOST likely provide this?

44.00 / -4.50

Correct answer 45.50 / -5.25

44. 00 / -5.25
45. 50 / -4.50
46. 50 / -3.75
47. 00 / -3.75

Answer 20

In order to find the power of the contact lens, one would add the diagnostic contact lens power (-3.00D) to the equivalent spherical value of the over-refraction (-1.50D), which equals -4.50D. If there was no need to alter the base curve of the contact lens, this would be the power that would provide an equivalent spherical plano over-refraction. However; in this case, the base curve needs to be adjusted to provide an apical alignment fit. Because the diagnostic contact lens shows apical touch by 0.75D, the base curve needs to be steepened by 0.75D (to 45.50). When a base curve is steepened, the tear lens becomes more plus (or less minus), thus an equal amount of additional minus must be added to the contact lens power to adjust for this. Therefore, adding -0.75D to -4.50D means that the new contact lens will require a power of -5.25 in order to provide a plano equivalent spherical over-refraction.
Note: An easy way to remember this is SAM FAP (Steepen Add Minus; Flatten Add Plus)

Question 21

our 22 year-old patient with a manifest refraction of OD: +2.00 -2.00 x 086 and OS: +2.50 -1.75 x 092 wishes to try soft contact lenses. After discussion, you decide to try Air Optix Toric monthly lenses. Which of the following prescriptions would you try first for her right eye?

+1.75 -2.00 x 090

Correct answer +2.00 -1.75 x 090

+2.00 -2.00 x 090

+1.75 -1.75 x 090

+1.75 -2.25 x 090

+2.00 -2.25 x 090

Answer 21

When determining the initial toric contact lens prescription to try on a patient, it is first important to know the contact lens parameters that are available for the specific brand of lenses that you would like to use. Most soft toric contact lenses all have very similar parameters; 0.25D steps in the spherical component (0.50 steps with higher Rxs), cylinder component is in 0.50 steps (starting with 0.75, up to 2.25 or 2.75 depending on brand), and cylinder axis is typically in 10-degree steps.
For the above patient, the cylinder component is -2.00D, but this is not available in Air Optix toric lenses. The next closest cylinder prescriptions available would be -1.75D or -2.25D, and the rule is to typically use the lower cylinder value. Therefore, for this patient, we would use -1.75D cyl. When it comes to axis determination, this patient is between 080 and 090 but is closer to 090 degrees, so we would begin with a 090-degree axis lens. It is also appropriate to keep the same sphere power of +2.00D, as we are only adjusting the cylinder power by 0.25, and this is not enough to need to adjust the sphere power as well.

Question 22

he non-silicone hydrogel lens type that tends to absorb the LEAST protein is which of the following?

High water, ionic (group 4)

High water, non ionic (group 2)

Correct answer Low water, non ionic (group 1)

Low water, ionic (group 3)

Answer 22

The current FDA classification system for soft contact lenses is currently under review. A new material classification will include silicone hydrogels. Group 1 lenses tend to absorb the least amount of protein.

Question 23

Given the following parameters, what is the power of the tear lens formed (in diopters) by a rigid gas-permeable lens with a base curve of 42.00 D and a power of +3.00 DS that is placed upon a cornea with a keratometry reading of 43.25 DS?

Correct answer -1.25 DS

+4.25 DS

+1.25 DS

+1.75 DS

Answer 23

The tear lens is formed by the front surface of the cornea and the posterior surface of a contact lens. Tear lenses come into play mostly with gas-permeable lenses since they result in a thicker tear lens that will alter the overall power of the lens if it is not compensated for in some manner. The tear lens, also called the lacrimal lens, can be viewed while evaluating the fluorescein pattern of a gas-permeable lens. The tear lens power is calculated by subtracting the keratometry readings from the base curve of the gas-permeable lens in diopters (or both can be converted to millimeters). In the above example, 42.00-43.25= -1.25 DS.

Question 24

When analyzing a rigid gas-permeable (RGP) contact lens, you measure base curves of 7.30 (46.25) and 7.54 (44.75) with a radiuscope, and -3.75 and -6.00 on lensometry. What type of toric gas-permeable contact lens design do you have?

Spherical

Correct answer Toric Base Curve

CPE Bitoric

Front Surface (F1) Toric

SPE Bitoric

Answer 24

The first measurement that should be made when analyzing RGP contact lenses is lensometry. Lenses should be placed concave side down on the aperture stop, and the power and axis wheels should be rotated until mires are clear and lined up. If cylinder is present, powers should be recorded in both major meridians (record gross amounts). After lensometry, lenses should be analyzed using the radiuscope. Again, once the sharpest focus of mires is found, both base curves should be measured and recorded. Base curves should be converted from millimeters to diopters. The steep meridian is recorded over the flat meridian, and the most plus meridian goes with the flat meridian.

For the above patient, the measurements are as follows:
46.25 -6.00
44.75 -3.75
The next step is to determine the values of the difference in base curves and the difference in contact lens power. For this patient, the difference in base curves is 1.50D and the difference in contact lens power is 2.25D.
- If the difference in base curve is multiplied by 3/2 and that value is equivalent to the contact lens power then you have a Toric Base Curve lens (with spherical front surface)
o For this patient (3/2) x (1.50) = 2.25 and difference in CL power = 2.25
o 2.25 = 2.25 therefore, this is a Toric Base Curve lens design
o Note that “equivalent” is defined as less than or equal to 0.50D
- If the difference in base curve is equal to the difference in contact lens power then you have a spherical power effect (SPE) bitoric lens design
- If the difference in base curve is not equivalent to the difference in CL power, and 3/2 x the difference in base curve is also not equal to the difference in CL power then you have a cylinder power effect (CPE) bitoric lens

Question 25

Ultraviolet (UV) protection is very important for people who wear contact lenses. Which of the following is a drawback to adding UV protection to a gas-permeable lens?

It decreases the oxygen (Dk) permeability of the lens and compromises the epithelial integrity

It compromises the optics of the lens

Correct answer It can interfere with the interpretation of the sodium fluorescein pattern

It greatly decreases the comfort and wettability of the lens

Answer 25

UV protection can be added to gas-permeable lenses; however, it interferes with the sodium fluorescein pattern and can make fitting a gas-permeable more challenging. Therefore, it is advisable to add UV protection to the lens after all of the lens parameters have been finalized and the patient is comfortable with the lens. Remember, contrary to what patients believe a UV inhibitor that is added to the lens does NOT replace sunglasses.

Question 26

A back surface toric (spherical front surface) gas-permeable (GP) contact lens is ordered with base curve radii of 7.85 mm (43.00 D) and 8.44 mm (40.00 D). When verifying this lens with a lensometer you would expect to find approximately how many diopters of “induced cylinder”?

3.00 D

Correct answer 4.50 D

2. 00 D
3. 50 D

Answer 26

Assuming the 1-2-3 rule is correct, a base curve toric GP lens with a spherical front surface when analyzed will exhibit a difference in lensometry readings that are 3/2 the amount of the base curve difference measured in diopters. In the above case, the difference in the two measured base curve meridians is 3 diopters; therefore, if there is no toricity on the front surface (that is, this is not a bitoric GP); optically, the difference in the two raw powers measured by lensometry will be 4.50 diopters. For example, the powers could be measured to be 1.00 D in one meridian and -5.50 D in the meridian 90 degrees away, or +2.00 D and -2.50 D. Keep in mind that the 1-2-3 rule is based on the index of refraction (n) of the lens material. Most of today’s GP lenses have ‘n’ values that are in the 1.40 to 1.48 range. This range of ‘n’ will result in less difference in the measured lensometry powers. For our example of a 3.00 D base curve toric GP lens, the difference in lensometry powers might be 4.00 D for a GP lens fabricated in a material with a lower ‘n’.

Question 27

You place a diagnostic rigid gas permeable contact lens on your patient’s right eye with a base curve of 46.50 D and a power of -3.00D. Analysis of the fit shows that the lens is apical touch by 1.50D, and an over-refraction reveals -4.50DS. If you desire an apical alignment lens fit and plano over-refraction, which of the following contact lenses would you order that would MOST likely provide this?

45. 00 / -6.00
46. 00 / -9.00
47. 00 / -6.00
48. 00 / -8.75
49. 00 / -9.00

Correct answer 48.00 / -8.75

Answer 27

In order to find the power of the contact lens, one would add the diagnostic contact lens power to the equivalent spherical value of the over-refraction. Remember though, if the over-refraction is greater than 4.00D, this must be vertexed. Therefore, in this case, one would add the contact lens power of -3.00 to the vertexed over-refraction of -4.25, which equals -7.25. If there was no need to alter the base curve of the contact lens, this would be the power that would provide a plano over-refraction. However; in this case, the base curve needs to be adjusted to provide an apical alignment fit. Because the diagnostic contact lens shows apical touch by 1.50D, the base curve needs to be steepened by 1.50D (to 48.00D). When a base curve is steepened, the tear lens becomes more plus (or less minus), thus an equal amount of additional minus must be added to the contact lens power to adjust for this. Therefore, adding -1.50D to -7.25D means that the new contact lens will require a power of -8.75 in order to provide a plano over-refraction.

Question 28

What is the power of the tear lens created by a gas permeable contact lens with a base curve of 42.87D placed on a cornea with a spherical curvature of 43.25D? (round to the nearest 0.12D)

-1.12D

+0.67D

Correct answer -0.37D

+0.37D

-0.67D

+1.12D

Answer 28

In order to find the power of the tear lens created by the back surface of a rigid gas permeable contact lens (base curve) and the anterior surface of the cornea (keratometry reading), the following equation may be used: 
Tear lens (TL) = base curve (BC) - keratometry (K) 
TL = 42.87 - 43.25 
TL = -0.37 D 

When the base curve is flatter than the keratometry values, the tear lens will be of negative value.

Question 29

You are fitting a toric soft contact lens to your patient’s left eye. The patient’s manifest refraction is +3.00 -1.00 X 050. You apply a +3.00 -0.75 X 060 diagnostic toric soft contact lens. It fits well, but the horizontal lens markings are consistently located at the 4 o’clock and 10 o’clock positions. What axis should you order?

030 degrees

020 degrees

090 degrees

Correct answer 080 degrees

Answer 29

Applying LARS (Left Add, Right Subtract) to compensate for lens rotation, since the lens is rotated to the Left, you would Add the amount of left rotation to the MANIFEST refraction axis. Every hour on the clock dial would translate to 30 degrees of rotation. In the above example, the lens is rotated to the doctor’s left by 30 degrees (1 clock hour away from horizontal). Add the amount of rotation (30 degrees) to the cylinder axis of the manifest refraction (050 degrees); this results in a cylinder axis order of 080 degrees.

Question 30

Your gas-permeable contact lens patient has an alignment fluorescein pattern with a 9.0 mm overall diameter / 7.5 mm optic zone diameter rigid contact lens in place. However, the patient experiences symptoms of flare with this design. If he is refitted to a 9.5 mm overall diameter / 7.5 mm optic zone diameter design, what base curve change is necessary to maintain the alignment fluorescein pattern?

Steepen the base curve by 0.25 D

Flatten the base curve by 0.62 D

No change in base curve is indicated

Steepen the base curve by 0.62 D

Correct answer Flatten the base curve by 0.25 D

Answer 30

Increasing the size of the optic zone or overall lens diameter increases the sagittal height of the lens, thereby steepening the fitting relationship on the cornea. To compensate for this steepening, you prescribe a flatter base curve. The clinical rule-of-thumb generally used for gas-permeable (GP) lenses is that you change the base curve by 0.25 D for every 0.5 mm change in optic zone or overall diameter. If you increase the diameter, you flatten the base curve; if you decrease the diameter, you steepen the base curve. Keep in mind that when you change the base curve you will need to apply the rule SAM-FAP rule (Steep Add Minus-Flat Add Plus) to the contact lens power. Therefore, in the above case, you would add +0.25 D to the contact lens power.

Question 31

What is the most effective “procedure” for removing lipid deposits from an existing soft contact lens?

Enzyme treatment

Switch to heat-based care systems

Correct answer Rub with surfactant cleaner

Avoid solutions containing thimerosal

Switch to hydrogen peroxide-based care system

Answer 31

Rubbing with a cleaner or multi-purpose solution containing a surfactant will remove lipid deposits that are not bound to the lens surface. Lipids tend to readily deposit on today’s silicone hydrogel lenses.

Thimerosal, a mercurial antibacterial effective against Gram-positive bacteria, was a component of some soft lens care solutions from the 1970’s and 1980’s. There were many reports of cytotoxicity to the corneal epithelium, leading to Type IV hypersensitivity and patient discomfort.

Enzyme treatment utilizing papain- or pancreatin-based products attacks denatured tear proteins accumulated on the lens surface or in the lens matrix. Heat-based care systems tended to denature proteins onto the lens surface.

Most current hydrogen peroxide-based lens care systems do not contain a rubbing component to the procedure. Rubbing is instrumental in removing lipid deposition.

Alternatively, if the deposits are numerous, one can replace the lens with a brand new one.

Question 32

Which of the following parameters contributes the MOST to the vision achieved through a soft toric contact lens?

Lens material

Correct answer Overall stability

Dk/t

Lens markings

Answer 32

A toric lens MUST offer stability if it is to perform optimally. Lens markings help to determine lens movement and orientation, but they in themselves do not affect vision. Because toric lenses deliver power in a specific meridian, it is essential that the lens does not rotate or change orientation, as this will alter acuity. Lens stability becomes more significant with higher levels of astigmatism. A lens that is meant to correct 1.25 D of astigmatism that is rotated 15 degrees off axis will induce roughly 0.62 D of astigmatism. A lens that is meant to correct 4.00 D of astigmatism if it is rotated 15 degrees off axis will induce approximately 2.00 D of astigmatism!

Question 33

You obtain the following data during your GP contact lens evaluation:

OD Diagnostic CL: 43.37 / -2.87
OD Distance OR: -0.50-0.25 X 90
OD Fluorescein Pattern: Alignment

OS Diagnostic CL: 42.87 / -3.25
OS DIstance OR: -0.75 DS
OS Fluorescein Pattern: Flat by 0.37 D

It is decided that monovision is the best option for this 51-year old patient who requires a +1.75 D add. Testing reveals that the right eye is dominant. You prescribe the rigid gas-permeable (RGP) contact lenses to fit alignment on both eyes. What contact lens powers would you order?

OD= -3.50 D; OS= -2.25 D

OD= -3.50 D; OS= -1.62 D

Correct answer OD= -3.50 D; OS= -2.62 D

OD= -0.50 D; OS= -4.00 D

OD= -2.87 D; OS= -1.25 D

Answer 33

he right eye’s trial lens fitting relationship is already alignment, so you would add the equivalent sphere of the OR (-0.62 D) to the power of the trial lens (-2.87). This results in a power of -3.50 D. Since the right eye is dominant and will be correcting distance vision and the fitting relationship of the trial lens is alignment, you do not need to further adjust the lens power.

For the left eye, adding the trial lens power (-3.25 D) to the OR (-0.75DS) results in a power of -4.00 D. You would order this power if you were prescribing a 42.87 D base curve for distance viewing. However, you desire to prescribe a 43.25 D base curve to achieve an alignment fitting relationship. To compensate for the steeper base curve, you will need to adjust the power by -0.37 D, resulting in a distance correcting power of -4.37 D (SAM-FAP). To prescribe a near monovision lens for the left (non-dominant) eye, you add the add power (+1.75 D) to the -4.37 D value. The final contact lens power for the left eye will be -2.62 D.

Question 34

Your 29 year-old rigid gas-permeable contact lens patient requests a new prescription as she lost her current contacts. Her keratometry readings are 44.62 @ 180 / 43.12 @ 90 and refraction is +2.75 -1.00 x 095 for her right eye. Given this data, which of the following diagnostic rigid contact lenses would provide a predicted fluorescein pattern of apical alignment and predicted spherical over-refraction of plano, respectively? (Assume that overall diameter of the lens is 9.0mm, and optic zone is 7.4mm)

Base curve: 44.62 D / Contact lens power: +1.25 D

Correct answer Base curve: 43.12 D / Contact lens power: +2.75 D

Base curve: 42.37 D / Contact lens power: +0.50 D

Base curve: 45.37 D / Contact lens power: +3.50 D

Base curve: 43.87 D / Contact lens power: +2.00 D

Answer 34

A common fitting philosophy for gas-permeable contact lenses is to use the average keratometry value minus 0.75 diopters. This is based upon the idea that the cornea is an aspheric surface with an eccentricity of close to 0.50. Therefore, to achieve an apical alignment fit on a surface with this type of curvature, the base curve of the contact lens must be flatter than the central cornea curvature (if the cornea was spherical, an alignment base curve would be equal to the average keratometry value). Assuming a corneal eccentricity of 0.50 and a contact lens optic zone diameter of 7.4mm, the base curve of an alignment-fitting lens should be close to 0.75 diopters flatter than the average keratometry reading. Another common fitting philosophy is to fit the contact lens so that the base curve is equal to the flat keratometry value (this would provide the same answer in the above case).

Therefore, in the above patient, the base curve of a diagnostic contact lens that would provide a predicted fluorescein pattern of apical alignment would be 43.12 D.
Average K= (44.62 + 43.12) / 2 = 43.87
Average K -0.75D = 43.87 - 0.75 = 43.12

When determining the contact lens power that would provide a predicted spherical over-refraction of plano, the spherical portion of the subjective refraction, flat keratometry value, and base curve of the contact lens are utilized for calculations. First, one must determine the effect of the tear layer created between the anterior corneal surface and back surface of the contact lens using the flat keratometry and contact lens base curve values. If the contact lens is fit flatter than K, a minus tear power is created; therefore, a corresponding change in plus power is necessary. On the other hand, if the lens is fit steeper than K, a plus tear lens is created that must be compensated by adding minus to the contact lens power.

In the above case, the contact lens base curve for ideal apical alignment was determined to be 43.12, which also corresponds to the patient’s flat K reading. When the contact lens is fit “on K,” or equal to the flat keratometry reading, the predicted lens power will be equal to the spherical portion of the subjective refraction (this must be vertexed if above 4D). Therefore; in order to provide a predicted over-refraction of plano, a diagnostic contact lens with a base curve of 43.12 should have a power of +2.75D.

Question 35

Your patient undergoes pre-testing and reports that she was wearing her +5.00 D soft contact lenses while non-contact tonometry (NCT) was performed. How would you expect this finding to alter the test results?

The measured intraocular pressure will be falsely lowered

Correct answer The measured intraocular pressure will be falsely elevated

NCT should never be performed over soft contact lenses

The measured intraocular pressure will remain unaffected

Answer 35

Studies have demonstrated that NCT performed over soft contact lenses generally does not affect the results unless the power of the lens is greater than +3.00 D due to increased center thickness (and therefore increased rigidity). Lenses greater than +3.00 D cause a falsely elevated intraocular pressure (IOP) when NCT is performed over the contact lenses. Research has also purported that when NCT is performed over minus-powered soft contact lenses (greater than -6.00 D), the IOP tends to be underestimated, with a direct relationship between increasing degree of myopia and increased underestimated values.

Question 36

Which of the following rigid gas-permeable (RGP) lens designs is MOST appropriate for a patient presenting with corneal toricity of 0.50D at axis 090, and 1.75D of residual cylinder axis 090 on over-refraction (when a spherical RGP diagnostic lens is placed on the eye)?

Correct answer Front Surface (F1) Toric

SPE Bitoric

CPE Bitoric

Toric Base Curve

Spherical

Answer 36

The following chart represents guidelines for lens selection when prescribing rigid gas-permeable (RGP) contact lenses. This will aid in determining which type of RGP lens is best for your patient, depending on the patient’s corneal toricity and residual over-refraction cylinder values (when a spherical GP diagnostic lens is placed on the eye).

The above patient presents with a minimal amount of corneal toricity, so a lens with a toric back surface is not required. A spherical contact lens; however, will not provide https://www.optoprep.com/simboards/pics/KVKADAONIWRBGF121322.PNGadequate vision, as there is significant residual cylinder (lenticular) on over-refraction. In these cases, a Front Surface Toric (F1 toric) lens is most appropriate as the base curve will be spherical, and the front surface will be toric to account for the residual cylinder.

Question 37

Which one of the following bitoric GP contact lenses would NOT induce cylinder if rotated to a misaligned position on the eye?

7. 46 mm / -4.25 D
   - ——————–
8. 13 mm / -1.75 D
9. 54 mm / +1.50 D
   - ——————–
10. 99 mm / +2.75 D

All of the options listed would induce cylinder if rotated off axis

7. 58 mm / -5.37 D
   - ——————–
8. 18 mm / -0.50 D

Correct answer 7.63 mm / -1.50 D

8. 11 mm / +1.12 D

Answer 37

Cylinder power effect (CPE) bitoric and base curve toric (with a spherical front-surface) gas-permeable (GP) lenses will induce unwanted cylinder if the lens rotates off axis. The resulting cylinder is due to cross-cylinder effects. However, a spherical power effect (bitoric) will not induce unwanted cylinder regardless of lens rotation. To determine whether a GP lens is a spherical power effect (SPE) or cylinder power effect (CPE) bitoric, measure the two base curves using a radiuscope and the two raw contact lens powers using a lensometer. If the difference between the two base curve meridians in diopters is the same as the difference between the two raw powers, the lens is an SPE bitoric. This is the case for only one of the above answers. Converting mm of base curve radius to diopters results in 7.63 mm = 44.25 D and 8.11 mm = 41.62; a difference of 2.62 D. The difference between the two raw powers of +1.12 D and -1.50 D is also 2.62 D. Therefore, this lens is a spherical power effect (SPE) bitoric GP contact lens.

Question 38

Your 23 year-old rigid gas-permeable contact lens wearer returns to your office for a contact lens follow-up after wearing his new lenses for about 2 weeks. During slit-lamp evaluation you notice several circular, well-demarcated indentations of the central cornea that pool with fluorescein. What is the name of this finding?

Dellen

Superficial punctate keratitis

Correct answer Dimple veiling

3-9 staining

Answer 38

Dimple veiling is a finding that occurs almost exclusively in rigid gas-permeable contact lens wearers (but can occur with scleral lenses due to mucin ball formation). Several circular, well-demarcated indentations are found in the cornea, either central or peripheral, that pool with fluorescein (they do not stain, as they do not represent breaks in the corneal epithelium). Dimple veiling is caused by a sub-optimal fitting relationship of a rigid gas-permeable contact lens in which tiny air bubbles that become trapped beneath the contact lens and are then mechanically compressed by the lens, indenting the cornea. Patients are typically asymptomatic; however, if sufficient in number, the dimples may interfere with vision, producing glare, hazy vision, and/or a loss of contrast sensitivity.

Dimple veiling is transient and will resolve within a few hours if the contact lens is removed from the cornea. Longer-term treatment involves modification of the fitting relationship of the contact lens, such as flattening the base curve, decreasing the optical zone diameter, blending the junction between the base curve and peripheral curve, or steepening the peripheral curves to allow for better tear exchange beneath the lens.

Dimple veiling has also been shown to occur in a few cases of soft contact lens wear. In these cases, mucin balls composed of mucus, lipids, and proteins can build up on the contact lens and create a similar corneal finding.

Question 39

A patient walks into your office and would like to be fit with soft contact lenses. Your subjective refraction reveals: OD: +5.50 DS, OS: +4.75 DS. If the phoropter was placed 13 mm from the corneal plane, which of the following contact lens prescriptions would be the BEST choice for the right eye of this patient?

+5.00 DS

+5.50 DS

+5.75 DS

Correct answer +6.00 DS

Answer 39

To determine the proper power, you must calculate the power at the cornea rather than the spectacle plane by utilizing the vertex power formula. Vertexing is important when the power in any meridian exceeds +/- 4.00 D. In order to properly vertex, one must use the following formula: Fc= Fs/1-dFs where Fc= the dioptric power at the corneal plane, Fs= the dioptric power at the spectacle plane, and d= the vertex distance in meters. In general, the vertex distance of the phoropter from the corneal plane is 13 mm unless otherwise specified. Solving for Fc, Fc=+5.50/1- (0.013)(+5.50), Fc = +5.50/1 -(0.0715), Fc= +5.50/0.9285, Fc=+5.93; rounding to the nearest 0.25, Fc is +6.00.

Question 40

Which 2 of the following statements are TRUE in regards to the center thickness of rigid gas permeable contact lenses? (Select 2)

Correct answer As the base curve of the contact lens becomes steeper the center thickness of the contact lens should increase

Correct answer As the overall diameter of the contact lens increases, the center thickness of the contact lens should increase

As the overall diameter of the contact lens increases, the center thickness of the contact lens should decrease

As the base curve of the contact lens becomes steeper, the center thickness of the contact lens should decrease

Answer 40

Rules of thumb for the central thickness of a rigid gas permeable contact lenses state that as the overall diameter of the contact lens increases, the central thickness increases in order to maintain an adequate edge thickness. Additionally, as the base curve of a contact lens increases, the center thickness of the contact lens should increase as well.

Question 41

Soft lenses are ideal for sports because they tend to dislodge less than gas-permeable lenses. Which of the following occurs when a soft lens dehydrates?

The thickness increases

The overall diameter increases

The contact lens power becomes more plus

Correct answer The base curve steepens

The index of refraction decreases

Answer 41

When a soft contact lens dehydrates, the contact lens power increases in minus, the overall diameter and the thickness decrease while the index of refraction increases. Clinically, it is important to monitor the base curve because as a contact lens dehydrates, its base curve steepens, especially in people with dry eyes. When a contact lens steepens, it can tighten up and can seal off the cornea, causing edema as well as eye infections. In contrast, when a soft contact lens is hydrated, its base curve flattens, the overall diameter and the thickness increase, the contact lens power increases in plus, and the index of refraction decreases.

Question 42

Soft lenses are ideal for sports because they tend to dislodge less than gas-permeable lenses. Which of the following occurs when a soft lens dehydrates?

The thickness increases

The overall diameter increases

The contact lens power becomes more plus

Correct answer The base curve steepens

The index of refraction decreases

Answer 42

When a soft contact lens dehydrates, the contact lens power increases in minus, the overall diameter and the thickness decrease while the index of refraction increases. Clinically, it is important to monitor the base curve because as a contact lens dehydrates, its base curve steepens, especially in people with dry eyes. When a contact lens steepens, it can tighten up and can seal off the cornea, causing edema as well as eye infections. In contrast, when a soft contact lens is hydrated, its base curve flattens, the overall diameter and the thickness increase, the contact lens power increases in plus, and the index of refraction decreases.

Question 43

What is the power of the tear lens created by a gas permeable contact lens with a base curve of 43.50D placed on a cornea with a keratometry reading of 44.25 x 43.50 @ 020?

-0.75 DS

plano -0.75x 110

+0.75 DS

+0.75 -0.75 x 110

Correct answer plano -0.75 x 020

+0.75 -0.75 x 020

Answer 43

In order to find the power of the tear lens created by the back surface of a rigid gas permeable contact lens (base curve) and the anterior surface of the cornea (keratometry reading), the following equation may be used: 
Tear lens (TL) = base curve (BC) - keratometry (K) 
If the keratometry values are not spherical, this equation must be calculated in each meridian. The easiest way to do this is to place the base curve and keratometry values on an optical cross and subtract K from BC. Then convert from the optical cross to a lens power. This is shown below:

Question 44

A 53-year old female soft contact lens patient is seen at your office reporting symptoms of dry eyes while wearing her contact lenses. Her current medications include anti-hypertensive pills and blood thinners. Which of the following dry eye treatments should NOT be recommended to her?

Correct answer Omega III fish oil capsules

Preservative-free artificial tears

Restasis ophthalmic emulsion

Punctal occlusion

Answer 44

In general, all of the above choices would be acceptable options; this is especially true because the patient is taking medications to help regulate her blood pressure (these medications can have the side effect of dry eye symptoms). Patients taking blood thinners should check with their primary care doctor before beginning an omega III regimen. Omega III fish oil can cause an increase in blood clotting time, which can be dangerous when used in combination with blood thinners.

Question 45

When designing a rigid gas-permeable lens, which type of edge contour is MOST desirable in order to achieve maximal comfort?

Correct answer A plus edge

A blunt edge

A V-cut edge

An A-cut edge

A knife edge
.

Answer 45

The edge profile is essential in achieving patient comfort. Remember, every time the patient blinks, the eyelids are going to move across that lens, and if the edge is uncomfortable or excessively sharp, the patient will definitely feel it. There are two types of edge profiles that ensure maximal comfort, a round edge and a plus edge. Both of these edges are smooth and slightly round allowing for a smooth transition of the lid over the lens upon blinking. A blunt edge is a straight edge that one would find on an uncut lens. A V-cut occurs when the lens is placed concave side down upon a cutting tool. An A-cut edge results when a lens is placed concave side up on a cutting/finishing tool. A knife edge is very thin and sharp.

Question 46

When analyzing a gas-permeable lens, you measure base curves of 7.58 and 7.84 with a radiuscope, and -1.00 and -2.50 on lensometry. What type of toric gas-permeable contact lens design do you have?

Correct answer Spherical power effect (SPE) bitoric

Thin-flex

Cylinder power effect (CPE) bitoric

Back surface (base curve) toric

Front surface (F1) toric

Answer 46

In order to determine which design of toric gas-permeable contact lens you have once the lens has been analyzed, the difference in base curve (BC) values and contact lens power (CLP) readings must be calculated first.

BC1 = 7.58 = 337.5/7.58 = 44.50
BC2 = 7.84 = 337.5/7.84 = 43.00
Change in BC = 1.50 D

CLP1 = -1.00
CLP2 = -2.50
Change in CLP = 1.50 D

The differences in base curves and contact lens powers for the above gas-permeable contact lens are equal (both 1.50 D), indicating that the design of the lens is a spherical power effect (SPE) bitoric type.

If the change in BC does not equal the change in CLP, the lens type may either be a base curve toric or cylinder power effect (CPE) bitoric. The way to tell these two apart is that if 3/2 change in BC = change in CLP, then it may be considered a base curve toric.

Question 47

Hypoxia associated with hydrophilic (soft) contact lens wear can result in which of the following?

3/9 staining, scarring and pseudoptyergium

Blepharitis

Correct answer Corneal swelling

Corneal decompensation

Answer 47

Hypoxia can cause corneal swelling (edema) acutely and corneal thinning chronically (by mobilization of glycosaminoglycans), can lead to secondary cornea neovascularization, both superficial pannus and occasionally deep stromal vessels, and endothelial changes including polymegathism and decreased cell numbers. Contact lens hypoxia, however, does not lead to corneal decompensation, blepharitis or peripheral 3/9 lesions which are more related to chronic rigid lens-induced exposure keratitis.

Question 48

A soft contact lens with a power of -7.00 D states on the package that it has a Dk/t equal to 35. However, given your knowledge of Dk/t, you know that the lens actually possesses which of the following parameters?

Because the lens is minus in power, it is thickest in the periphery; thus the periphery of the lens has a Dk/t higher than that stated on the package

Because the lens is minus in power, it is thickest in the center; thus the center of the lens has a Dk/t higher than that stated on the package

Correct answer Because the lens is minus in power, it is thickest in the periphery; thus the periphery of the lens has a Dk/t lower than that stated on the package

Because the lens is minus in power, it is thickest in the center; thus the center of the lens has a Dk/t lower than that stated in the package

Answer 48

The Dk/t of a lens is a measurement of the oxygen permeability for a given thickness. Generally, this measurement is given for a lens with a power of -3.00 D. Therefore, a lens that possesses a power higher than -3.00 D will actually display a lower Dk/t, especially towards the edges, because a minus lens is thickest in the periphery. A high plus lens will display a lower Dk/t, especially towards the center, because the lens is thickest in this area.

Question 49

Your patient has a refractive error: +10.00 DS OU. What patient education should you provide regarding his visual system when switching from spectacles to contact lenses?

He will experience no effect on accommodation and no effect on image magnification

Correct answer He will need to accommodate less and will experience less image magnification

He will need to accommodate more and will experience greater image magnification

He will experience no effect on accommodation and will experience greater image magnification

He will need to accommodate more and will experience no effect on image magnification

Answer 49

Image size magnification through +10.00 D spectacles is approximately 19%, whereas image size magnification through a +11.50 D (Rx at corneal plane) contact lens is approximately 4%.

Question 50

An aphakic patient is seen at your office and wishes to be fit with contact lenses. What is an important contact lens parameter that MUST be considered in this patient’s care?

Contact lens material

Correct answer Ultraviolet (UV) inhibitor

Edge thickness

Contact lens solution

Answer 50

Because this patient is aphakic, their retinas no longer receive the UV protection that is naturally provided by the crystalline lens. Although all of the above options should be included when deciding which type of lens to order, it is essential that you provide a UV inhibitor on the contact lens as well as sunglasses for this patient. When the contact lens power will be a high plus prescription, one should order a lenticular lens design to reduce lens thickness, help enhance centration, increase comfort as well as increase the Dk/t of the contact lens.

Question 51

Which of the following ophthalmic instruments is MOST commonly used to evaluate the edge profile of a rigid gas-permeable contact lens?

Radiuscope

7x comparator

V-Channel gauge

Lensometer

Correct answer Shadowscope

Answer 51

A shadowscope is an instrument commonly used with rigid gas-permeable contact lenses in order to inspect several aspects of the lens. It provides about 20x magnification and projects the contact lens on a glass screen with a reticule for measurements. Shadowscopes are often used for evaluating the contour and thickness of the edge of the contact lens. It is also commonly utilized to measure the overall diameter of the lens, as well as the width of the intermediate and peripheral curves. The shadowscope is also useful in showing any scratches, cracks, or nicks in the edge of the lenses.

• A radiuscope is used to measure the base curve of a rigid gas-permeable contact lens
• A V-channel gauge aids in measuring the overall diameter of the contact lens
• A 7x comparator is a handheld measuring magnifier that is useful for measuring the overall diameter, optic zone diameter, and peripheral curve diameter, as well as overall inspection of the lens for scratches, etc.
• A lensometer is used to measure the power of the contact lens

Question 52

A new patient to your office presents wearing rigid gas-permeable contact lenses. You begin to analyze the lenses and notice that on lensometry both lenses have a toric prescription and prism. Which of the following lenses does the patient MOST likely currently wear?

Toric Base Curve

SPE Bitoric

CPE Bitoric

Correct answer Front Surface (F1) Toric

Spherical

Answer 52

Front surface (F1) toric rigid gas-permeable (RGP) contact lenses are the only toric contact lenses that always have a prism-ballast design. The back surface of an F1 toric RGP lens is spherical, due to the fact that a patient wearing this design will have a mostly spherical cornea (usually less than about 1.00D). Patients will also have a significant amount of cylinder on over-refraction, so the toricity is placed on the front surface of the lens. In order for the lens to sit properly on the eye so that the lens power is oriented properly, F1 toric lenses have a prism-ballast design, making them heavier at the bottom. These are the only toric RGP lenses that always have this design. Therefore, one can determine that if prism is present, the lens design is most likely an F1 toric. Prism is rarely added to other lens designs to aid in lens centration. For all other toric RGP lenses, one must also use a radiuscope to gather more information on the base curves before being able to determine the exact lens design.

Question 53

You are designing a rigid gas permeable (RGP) contact lens for your patient who presents with keratometry reading of 44.25 x 41.00 @ 180, and manifest refraction of -2.50 -3.00 x 180. With a spherical RGP diagnostic lens placed on the eye, the over-refraction is +0.50 -0.25 x 180. Which of the following RGP lens designs is MOST appropriate for this patient?

Correct answer SPE Bitoric

Toric Base Curve

CPE Bitoric

Spherical

Front Surface (F1) Toric

Answer 53

The following chart represents guidelines for lens selection when prescribing rigid gas-permeable (RGP) contact lenses. This will aid in determining which type of RGP lens is best for your patient, depending on the patient’s corneal toricity and residual over-refraction cylinder values (when a spherical GP diagnostic lens is placed on the eye).

The above patient presents with 3.25 diopters of corneal cylinder, which is typically above the threshold for a spherical RGP contact lens. Although a spherical lens would provide a low cylinder over-refraction (as shown when a diagnostic lens is placed on the eye), the fit of a spherical lens on a cornea with over 2.50 D of toricity would not be ideal. Because the above patient has high amounts of corneal astigmatism and a very low amount of cylinder on over-refraction, a spherical power effect (SPE) bitoric lens would be the most appropriate option. An SPE bitoric lens will improve the fit of the contact lens with a toric surface but will not change the cylinder amount on over-refraction.

The spherical power effect lens has a toric curve on the front surface of the contact lens to compensate for the astigmatic power induced by the toric back surface of the lens. Since the lens has a spherical power effect, the patient does not see any changes in vision with blinking if the lens rotates on the eye.

Question 54

Modern rigid gas-permeable (GP) contact lenses are usually fitted to adhere with which of the following parameters?

Maximize lens movement

Spare the corneal apex from undue pressure and rubbing during wear

Maximize tear exchange for appropriate re-supply of oxygen

Correct answer Align with the anterior corneal surface and maintain tear exchange

Answer 54

Modern GP CLs are usually fitted to align with the corneal surface and maintain tear exchange, but there is no need to maximize tear exchange for oxygen supply reasons as the lenses themselves are permeable to oxygen. Lenses for keratoconic eyes are often fitted to spare the corneal apex; this is not the normal paradigm for non-keratoconic eyes.