KING SAUD UNIVERSITY
COLLEGE OF APPLIED MEDICAL SCIENCES
DEPARTMENT OF OPTOMETRY
OPTO 354 Questions & Answers
2nd TERM 2006/2007 (1427/1428)
COURSE NAME: Contact Lenses I
COURSE DIRECTOR: Dr. Ola M. Oriowo
DATE: June 07
TIME ALLOWED: 1hr: 30 min
INSTRUCTIONS: Answer all questions. Write your number on all pages. Circle the correct answer, and fill in the blank spaces where appropriate. You can write on the back of the page.
1. Briefly describe the 3 main types of contact lens materials. 6p
Answer:
Polymethylmethacrylate (PMMA), Silicone, and Hydroxyethylmethacrylate (HEMA).
Polymethylmethacrylate (PMMA),
Polymethylmethacrylate (PMMA) is an organic plastic compound stable at room temperature. The water content is 0.5%. It is easily worked material and has refractive index of 1.48 – 1.50. It is used to produce hard contact lenses.
Silicone
This is a polymer of dimethyl-silicone, permeable to oxygen and glucose. The water content is 0%. Its refractive index is 1.43. Example – Dow Corning (silicon) lens.
Hydroxyethylmethacrylate (HEMA)
These are soft or hydrophilic contact lenses that are characterized by the ability to absorb water, elasticity and flexibility. HEMA is hydrophilic because it contains a free Hydroxyl group that bounds with water. The water content range from 38% to 60%. Its refractive index is 1.43. Example – Bausch & Lomb (Soflens).
2. Briefly explain the difference between daily wear and extended wear soft contact lenses.
Answer:
Daily wear
These are lenses worn on daily basis for 12 to 14 hours and removed before bedtime for cleaning and disinfection. Methods of disinfection are heating, chemical and oxidation (hydrogen peroxide).
Flexible/Extended wear
In contact lens practice, it means lenses given to patients who wear lenses overnight only on an occasional basis such as weekend, and should be cleaned and disinfected upon removal. Most practitioners now recommend lens wear without overnight removal for 3 to 7 days, depending on the lens type.
3. In questions 3 to 6, fill the blank spaces with appropriate answers. 2p
Write the correct description for the Soft lens groupings according to FDA.
Group I lenses: _____________, ______________
Group II lenses: _____________, ______________
Group III lenses: _____________, ______________
Group IV lenses:______________, _____________
Answer:
SOFT LENS GROUPINGS USED BY FDA
- Low water content (less than 50% H20)
- High water content (more than 50% H20)
- Ionic versus non-ionic
Group I: Low H20 non-ionic polymers e.g. 100% HEMA
Group II: High H20 non-ionic polymers
Group III: Low H20 ionic. They contain MA
Group IV: High H20 ionic. H20 content from 55-60%.
4. The corneal shape and physiology are very important in contact lens design because the cornea obtains the oxygen supply from the __________ and that the ________ is the_____.
3p
Answer: each blank is 0.5pt
It is known that corneal shape and physiology are critical in contact lens design. The cornea obtains the bulk of its oxygen supply from the air and that the medium of exchange is the tears. As a result the sclera lenses were abandoned and hard contact lenses were redesigned.
5. During sleep, the eyelids block oxygen from the atmosphere, and most of the oxygen in the tears diffuses from the blood vessels of the________ and the ______________. This reduces the amount of oxygen in the tear film to approximately __________.
3p
Answer: each point is 0.5pt
During sleep, the eyelids block oxygen from the atmosphere, and most of the oxygen in the tears diffuses from the blood vessels of the limbus and the palbebral conjunctiva. This reduces the amount of oxygen in the tear film to approximately one third.
6. Fill the blank spaces. Tear pumping is the major source of ________________with PMMA lenses, since these lenses have virtually no oxygen permeability. However, the disadvantage of PMMA lenses is that tear pumping mechanism alone is ____________ to provide adequate amounts of oxygen to the cornea. 2p
Answer: each point is 0.5
Tear pumping is the major source of corneal oxygenation with PMMA lenses, since these lenses have virtually no oxygen permeability. However, the disadvantage of PMMA lenses is that tear pumping mechanism alone is insufficient to provide adequate amounts of oxygen to the cornea.
For questions 7 through to 10, the sentences are either true or false. Write the correct answer as True or False.
7. The permeability coefficient (Dk) and oxygen transmission of contact lens material are affected by thickness and increase of temperature. ( True ) 0.5p
8. In contact lens, the greater the plus power required the more the minus power needed to correct the refractive error. ( False ) 0.5p
Answer: For questions 7 to 8, each true response is 0.5pt
9. In base curve selection for soft contact lens fitting , a K-reading of 45.00D and above is an indication to select flat base curve range of 9.0mm and above.
0.5p
- True
- False
ans: B: False
10. In soft contact lens fitting, the lens must completely cover the cornea. However, large diameter like 15 mm lens tends to tighten on the cornea and may result in certain complications.
0.5p
- True
- False
Answer: A: True
The cornea has no blood vessels of its own to supply it with oxygen necessary for normal metabolism. Therefore the cornea must obtain most of its required oxygen from the tear film. The tear film supplies the cornea with oxygen from the atmosphere when the eyes are open.
11. The permeability of a material is expressed as a permeability coefficient denoted by Dk. Explain this statement.
Ans: each point mentioned is 0.5p
The permeability of a CL material is expressed as a permeability coefficient denoted by Dk.
Explanation: Permeability is a natural function of the molecular composition of any material. In order for oxygen to pass through a contact lens material, the oxygen molecules must first dissolve into the material and then travel through it.
Permeability is the product of the diffusion coefficient (D) and the solubility coefficient (k).
2p
12. Briefly define DK/L and how to calculate it. 2p
Ans: Definition is 2p. Explanation on How to calculate is 1p.
The term oxygen transmissibility, denoted by DK/L is the rate at which oxygen will pass through a specific contact lens of a given thickness.
Explanation on How to calculate: To calculate oxygen transmissibility of a given contact lens, the Dk value for the material is divided by the lens thickness (L).
13. Explain why contact lenses with higher Dk values are often made thicker? 2p
Answer:
For hydrophilic polymers, increasing the water content increases oxygen permeability. However, this often increases the lens fragility and makes the material more prone to deposit formation. see Pg 43.
To answer the question above, it means that lenses with higher Dk needs to be made thicker because increasing the water content will make the contact lens to more fragile. Therefore to compensate for the increase in fragility, the lens should be made thicker.
Alternate answer:
Although lenses with higher water content typically have higher Dk values, they often must be made thicker than lower water content lenses because:
- They dry out, or dehydrate more rapidly in thin designs, leading to corneal drying which is observed as corneal desiccation staining.
- High water content lenses are generally more fragile than thin designs.
14. For contact lens fitting, patient tear quality and quantity should be tested, because wearing contact lenses on dry cornea can cause poor tear circulation, corneal edema, blurry vision and burning sensation. Describe two tests commonly used for tear analysis.
6p
Answer:
Tear quality (Tear break-up time (TBUT)
- Blinking helps in distributing tear over the cornea, immediately after a blink, evaporation begins and tear film begin to thin. Therefore, the tear break-up time is often used an index for an abnormal tear formation.
- Tear break-up time is the interval time between a complete blink and the first randomly distributed dry spot.
- If an eye is kept open without blinking for 15-34 seconds, the tear will show dry spot areas. When fluorescein applied to these dry areas appear black when examined with ultraviolet light. Observing any dry areas occurring in less than 10 seconds is considered a negative factor in patient selection for CL fitting.
Schirmer Test
- It is used to evaluate the rate of tear flow thus provide information on hypo and hyper secretion of tears.
- A special filter paper (5x35mm) is used; this paper has an indentation at the upper 5mm of its length.
- After 5 minutes, the paper is removed and the length moistened by tear is measured with a ruler. Normal tear secretion moistens 10-15mm of the strip, but the older the patient the less the reading.
15. Mention two possible complications that a lens with too large diameter can cause. 1p
Answer: Ulcers or neo-vascularisation.
Lens must completely cover cornea.
Most soft CLs are available from diameter of 13.5m to 15mm. However, large diameter e.g. 15 mm tends to tighten on the cornea and may result in complications e.g. ulcers or neo-vascularisation.
16. If a patient comes to your contact lens practice for fitting, and his refraction (spectacle) prescription is –6.00 +1.00 x 90 with a vertex distance of 13mm. Calculate the approximate contact lens prescription. Show the step of your calculations.
3p
Answer:
Conversion should be correct ( 0.5p) if not deduct 0.5 from final answer even if correct.
Correct Formula = 1p
Steps of calculation and correct final answer = 2p
Selection of lens power
- Refraction prescription must be converted to minus cylinder form:
- If cylinder in refraction is <0.50D, power = spherical component
- If cylinder in refraction is between 0.50D – 1.00D, power = spherical equivalent (i.e. spherical component + ½ Cyl).
- If overall spherical component in 1 and 2 is greater than ±4.00D, compensate for vertex distance using either method 1 or 2:
Method 1
Fc = Fs / 1- dFs
Where:
Fc = power of CL
d = distance between spectacle lens and CL in meter.
Fs = power of spectacle lens (D)
For example:
Refraction: –6.00 +1.00x90 (plus cyl form)
–5.00 – 1.00 x 180 (minus cyl form)
Vertex distance = 13mm (in meter, vertex distance = 13/1000 = 0.013m)
Add ½ of cyl to sphere power –5.00D
This is greater than ±4.00D, so compensate for vertex distance e.g. 13 mm (calculations or using Table)
Solution: using Fc = Fs / 1- dFs
Fc = 5.50 / 1- (0.013 x 5.5) = 5.5 / 1-0.0715 = 5.5 / 0.9285 = 5.924 ≈ –6.00D
17. Tight fitting contact lens does not appear to have any movement on the cornea following blinking or after eye movement in any direction. List 4 tests you can use to diagnose a tight fitting contact lens.
2p
Ans:
The following tests are diagnosis of tight fit:
- Unstable vision: with the best subjective over correction vision is slightly blurred
- Patient will note temporary clearing in vision immediately following blinking
- Retinoscopy: the retinoscope reflex shows a dark, shadowy area in the central portion of the papillary area, which disappear immediately after blinking.
- Keratometry: performed with the CL in place, the mires appear distorted but distortion may be reduced immediately following blinking.
- Blanching of peripheral vessels: during blinking it may be noted that the lens movement also pulls the vessels and near by conjunctiva in the same direction.
18. Briefly mention at least 4 characteristics of good fitting lenses.
2p
Answer: Each point is 0.5p
Characteristics of good fitting
There are general characteristics of good fitting for soft contact lens:
- Comfortable and well fitting in all directions of gaze
- Gives complete corneal coverage and appears properly centered
- Normal blinking results in up to 0.5mm of vertical movement when the eye is in the primary position.
- The lens lags by no more than 1.00mm on upwards gaze or lateral movements of the eye.
- Vision is good remaining stable on blinking.
- Refraction gives a precise end-point, correlating with the best visual prescription of the spectacles.
- The retinoscopy reflex is crisp and sharp both before and after blinking.
- The slit-lamp shows no irritation of the limbal vessels or compression of the conjunctiva.
- The keratometry of the front surface of the lens on the cornea shows the mires to be stable and undistorted.