cl Flashcards
Modern rigid gas-permeable (GP) contact lenses are usually fitted to adhere with which of the following parameters?
Spare the corneal apex from undue pressure and rubbing during wear
Maximize lens movement
Maximize tear exchange for appropriate re-supply of oxygen
Align with the anterior corneal surface and maintain tear exchange
Align with the anterior corneal surface and maintain tear exchange
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.
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 index of refraction decreases The thickness increases The contact lens power becomes more plus The overall diameter increases The base curve steepens
The base curve steepens
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.
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?
Spherical Front Surface (F1) Toric Toric Base Curve CPE Bitoric SPE Bitoric
Front Surface (F1) Toric Correct Answer
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.
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?
NCT should never be performed over soft contact lenses
The measured intraocular pressure will be falsely lowered
The measured intraocular pressure will be falsely elevated
The measured intraocular pressure will remain unaffected
The measured intraocular pressure will be falsely elevated Your Answer
ou 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?
- 00 / -5.25
- 00 / -4.50 4
- 50 / -3.75
- 00 / -3.75
- 50 / -4.50
- 50 / -5.25
45.50 / -5.25 Correct Answer
Explanation
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)
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 increase 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 not change
The equivalent diopter sphere of the over-refraction will become minus Your Answer
Explanation
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.
A 24-year old female wears soft contact lenses with a Dk/t of 175 and admits to sleeping in her lenses. She is very satisfied with both the comfort and the vision of her lenses. Biomicroscopy reveals mucin balls under her lenses bilaterally that leave impressions in her central corneas upon removal of her lenses. Which of the following actions would BEST help to eliminate the formation of mucin balls?
Altering the power of the contact lens but maintaining the same lens material
Maintaining the same lens material but changing to a steeper base curve Your Answer
Changing her multi-purpose solution
Instructing the patient to increase her blinking frequency
Explanation
Mucin balls appear as small, white, pearl-like debris that occur behind the posterior surface of contact lenses. They generally occur with silicone hydrogel lenses that are fit too flat and are used for extended wear purposes. Mucin balls do not actually pose a threat to vision and do not generally compromise the integrity of the cornea. However, if they are severe enough, there are several options available to clinicians to combat their formation. An easy way to decrease generation of mucin balls is to steepen the base curve of the lens. Alternatively, one can decrease the amount of extended wear or add re-wetting drops to the patient’s contact lens regimen. Upon removal, mucin balls will cause pooling of sodium fluorescein but will not cause staining of the cornea.
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?
- 20 mm
- 70 mm
- 40 mm
- 60 mm
0.60 mm Correct Answer
Explanation
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.
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?
- 00 / -8.75
- 00 / -9.00 Your Answer
- 00 / -6.00
- 00 / -9.00
- 00 / -8.75 Correct Answer
- 00 / -6.00
Explanation
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.
Note: An easy way to remember this is SAM FAP (Steepen Add Minus; Flatten Add Plus)
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?
Increase the overall diameter Flatten the peripheral curves Increase the optic zone diameter Decrease the optic zone diameter Your Answer Steepen the base curve
Explanation
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.
Which 2 of the following statements are TRUE in regards to the center thickness of rigid gas permeable contact lenses? (Select 2)
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 increase Correct Answer
As the base curve of the contact lens becomes steeper, the center thickness of the contact lens should decrease Your Answer
As the overall diameter of the contact lens increases, the center thickness of the contact lens should increase Your Answer
Explanation
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.
Logo PRACTICE QUESTIONS TIMED EXAM REVIEW & STUDY TOOLS DIANA PART I Practice Questions Answer Correct 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?
plano -0.75 x 020 Your Answer plano -0.75x 110 \+0.75 -0.75 x 110 \+0.75 -0.75 x 020 \+0.75 DS -0.75 DS
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Explanation 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:
Explanation 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:
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”?
- 50 D
- 50 D Correct Answer
- 00 D
- 00 D Your Answer
Explanation
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’.
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?
\+6.00 DS Correct Answer \+5.75 DS Your Answer \+5.50 DS \+5.00 DS 0.
Explanation
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 +6.0
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?
Omega III fish oil capsules Correct Answer
Preservative-free artificial tears
Restasis ophthalmic emulsion Your Answer
Punctal occlusion
Explanation
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.
