KING SAUD UNIVERSITY
COLLEGE OF APPLIED MEDICAL SCIENCES
DEPARTMENT OF OPTOMETRY
OPTO 355 Questions & Answers
2nd TERM 2007/2008 (1428/1429)
COURSE NAME: Contact Lenses II
COURSE DIRECTOR: Dr. Ola M. Oriowo
DATE: June 2008
TIME ALLOWED: 1hr: 30 min
Instructions: Write your student number at the top right corner on all pages. Answer all questions. You can use the back of the page for answer.
1. List at least eight properties that an ideal contact lens material should have. 4p
Ans: 1 correct answer is 0.5pt.
– Oxygen permeability
– Surface wettability
– Low surface reactivity
– Dimensional stability
– Flexure resistance and recovery
– Surface hardness
– Machining and polishing capability
– Fracture resistance
– Material and quality control, i.e. different batches of the material should have the same chemical and physical caharacteristics and should behave in an identical manner as during the lens fabrication process.
– Wide range of tints and depths of tint.
2. Mention 5 advantages of gas permeable lens material over the Hard PMMA lenses.
5p
Answer:
The advantages of gas permeable lens material:
Over hard PMMA lenses:
- Fewer physiological problems such as central corneal clouding (oedema).
- Quicker adaptation.
- Few over-wear syndromes
- Less spectacle blur/post-wear refractive changes.
- Lenses may be fitted larger because anoxia due to corneal coverage by the lens is no longer of great significance.
- Extended wear becomes possible as transmissibility increases.
- Alternation of daily and extended wear becomes feasible.
- Less like hood of lens loss because RGP lenses can be fitted with minimal edge clearance
3. Mention 5 advantages of gas permeable lens material over the Soft Hydrogel lenses.
5p
Anser: The advantages of gas permeable lens material:
Over the soft hydrogel lenses:
- Good visual stability and better correction of astigmatism.
- Less deposit problems and easier lens maintenance.
- Less solution allergy or toxicity problems.
- Excellent tear interchange and lack of debris build-up behind the lens.
- Ability to be modified or repolished in most cases.
- Interchange between daily and extended wear without the fragility problems associated with extended wear soft lenses.
- Less likelihood of GPC.
- In fitting patients with RGP lenses the points made for fitting PMMA lenses in general still apply.
4. In order to ensure that the characteristics of a prescribed hard contact lens are appropriate for fitting a patient, certain parameters have to be measured. Mention these parameters.
2p
Answer:
One correct answer is 0.5p. Total (over all) power, power, base curve, Optic zone diameter.
5. Mention one instrument used for measuring the total diameter of RGP lens in contact lens fitting.
0.5p
Ans:
V-channel gauge OR
Measuring magnifier or magnifying loupe
6. Mention the instrument that you will use to measure the base curve of a hard contact lens. 0.5p
Ans:
Radiuscope
7. Mention the instrument that you will use to measure the power of a hard contact lens. 0.5p
Ans: Lensometer
8. State the reason why the optic zone diameter may be difficult to measure in a hard contact lens. 0.5p
Ans: Because of the Blend region, which is usually very small, and not clear to distinguish.
9. Select the correct answer. Peripheral curve width measurement is 0.5p
A) The sum of total lens peripheral curve and optic zone diameter.
B) The sum of total lens diameter and optic zone diameter.
C) The difference between the total lens diameter and optic zone diameter
D) The difference between the lens peripheral curve and optic zone diameter
Answer: C - The difference between the total lens diameter and optic zone diameter
10. Briefly discuss the use of and how to perform fluorescein staining in the assessment of contact lens fitting.
2p
Ans: After we fit the patient with the RGP lens, we put Fluoroscein on the conjunctiva, tell pt to blink, and observe with cobalt blue light. If fluorecein is evenly distributed it is optimum; if there is fluorescein pool in the center with apical clearance it is steeper than K; if pool is is at the mid-periphery and apical touch appearance it is flatter than K.
11. Mention 4 basic requirements of a good fitting contact lens
2p
Answer: 0.5pt for each point
Maintenance of corneal integrity (including integrity of the related ocular and extra- ocular tissues).
Maintenance of normal tear flow behind and over the lens.
Adequate vision.
Patient comfort.
Invisibility of lens.
12. The tear film provides oxygen exchange as the lens is moved. It also possesses lysozyme. Briefly state the role of lysozyme?.
Ans: 1p
Lysozyme is an enzyme that inhibits bacterial proliferation
13. Patients with normal tear film are more prone to infections compared to patients with a tear deficiency. Circle the correct answer.
0.5p
- True
- False
14. Mention four tests that should be performed on a patient being considered for RGP contact lens fitting.
2p
Answer: 0.5 for each correct point
- Rose Bengal test to stain tissue.
- Schirmer test with and without a local anesthetic.
- BUT (tear film breakup time).
- Corneal assessment for erosions. If the dryness is severe enough to cause pitting of the corneal epithelium, rigid lenses should not be used.
15. Calculate the contact lens power of an aphakic spectacle correction of +12.00 diopters at 12mm from the eye.
2p
Answer:
Δ = 12² x 0.012
Δ = 144 X 0.012
Δ =1.728 D
Thus, the power of the required contact lens corrected for vertex distance is
+12.00 +1.728= +13.728 D
16. Calculate the contact lens power of a spectacle correction of -8.00 diopters at 13 mm from the eye.
2p
Answer:
Δ = 8² x 0.013
Δ = 64 X 0.013
Δ = +0.832 D
Thus, the power of the required contact lens corrected for vertex distance is
- 8.00 + 0.832= - 7.168 D
17. Briefly describe what a patient will experience if the optic zone diameter of the RGP contact lens is too small for the size of the pupil.
1p
Answer: Flare (1pt for clear description)
- There may be flare since the optic zone diameter varies with the pupillary size, the lens diameter, and the palpebral fissure size.
- If the optic zone is too small for the size of the pupil, the person will experience flare in dim illumination when the pupillary aperture increases.
18. List 2 possible common findings in patients with poor blink habits. 1p
Answer: 0.5pt for each correct answer
The following are often found in persons with poor blink habits:
- Low-riding lenses
- Three and 9 o'clock staining
- Corneal edema
- Greasy or dry lenses with crystalline deposits
19. Choose the correct answer. The normal blinking rate is approximately…
- 15 to 18 blinks per minute and each blink lasts 3 seconds
- 15 to 18 blinks per minute and each blink lasts 5 seconds
- 10 to 12 blinks per minute and each blink lasts 3 seconds
- 20 to 25 blinks per minute and each blink lasts 3 seconds
0.5p
Answer: A - 15 to 18 blinks per minute and each blink lasts 3 seconds
20. Choose the correct answer. A toric lens is certainly required for astigmatism less than 3.00D.
0.5p
A. True
B. False
Ans: B False
A toric lens is required for astigmatism greater than 3.00 D.
21. Choose the correct answer. Patients with large, flat corneas (that is, a corneal diameter greater than 11.5 mm and flatter than 44.00 D) require a large lens.
0.5p
- True
- False
Ans: True
2- Lens diameter
The lens diameter may be determined in various ways.-
- Lenses over 4.00 D power usually need a diameter of 8.5 mm or more to avoid a high-riding lens. The greater the minus power, the larger the lens.
Rule 1
The lens diameter calculated in relation to the papillary size by adding 4 mm to the pupillary diameter.
- Large, flat corneas require a large lens, that is, a corneal diameter greater than 11.5 mm and flatter than 44.00 D. Conversely, small, steep cornea, for example, less than 11.5 mm and steeper than 44.00 D, requires a small lens.
22. Lenses over – 4.00D power usually need a diameter of 8.5mm or more to avoid a high-riding lens. The greater the minus power the larger the lens. 0.5p
- True
- False
Ans: True
Lens diameter
The lens diameter may be determined in various ways.-
- Lenses over 4.00 D power usually need a diameter of 8.5 mm or more to avoid a high-riding lens. The greater the minus power, the larger the lens.
23. For myopia, less minus power is required in the contact lens form as compared to the spectacle prescription, while for hyperopia more effective power is needed in the contact lens form.
0.5p
- True
- False
Answer: True
For myopia less minus power is required in the contact lens form as compared to the spectacle prescription. For hyperopia more effective power is needed in the contact lens form
24. When the base curve is steeper than the flattest central corneal curve (flattest K reading), a tear lens of minus power is formed.
0.5p
- True
- False
Answer: B - False
When the base curve is steeper than the flattest central corneal curve (flattest K reading), a tear lens of plus power is formed.
25. List 4 factors or forces that can considerably affect the fitting of RGP contact lens on the eye. 2p
Answer: (0.5 point for each factor correctly listed)
(A) Capillary attraction
(B) Gravity
(C) Tear meniscus
(D) Lid force and position
(E) Frictional forces
26. There are certain adaptive contact lens wearing symptoms which can occur initially. These symptoms should be temporary and last no longer than 1 to 2 weeks. Briefly mention 2 of these symptoms. 1p
Answer:
There are certain adaptive contact lens wearing symptoms which can occur initially, such as: -
Flare,
Sensitivity to light, and
Tearing.
These symptoms should be temporary and last no longer than 1 to 2 weeks.