Test 12  (Contact lenses)

1. The power of a contact lens is determined by its:

a. thickness

b. posterior curvature

c.diameter

d. oxygen permeability

e. refractive index
 

Answera.T   b.T   c.F    d.F   e.T
The power of a contact lens is determined by its:
a. anterior curvature
b. posterior curvature
c. thickness
d. refractive index

 

2. The following are true about a contact lens with the following 
    numbers 8.9/13.8/-4.25:

a. the contact lens has a base curve of 8.9 mm

b. the contact lens has a diameter of 8.9 mm

c. the power of the contact lens is 13.8 D

d. the base curve of the contact lens is 13.8 mm

e. the contact lens has a power of -4.25 D
 

Answera.T   b.F   c.F   d.F   e.T
The numbers represent base curve/diameter/power respectively.
 

3. The base curve of a contact lens:

a. refers to its posterior central curvature 

b. is usually measured in dioptres

c. is measured with a radiuscope 

d. determines its movement with blinking

e. for a particular patient is choosen according to the central keratometry measurement
 

Answera.T   b.F   c.T    d.T   e.T
The posterior surface of a contact lens determines its fitting relationship with the cornea. Known as the base curve or central posterior curve, its dimensions are usually expressed in mm radius of curvature and sometimes in dioptric power. 
The radiuscope is the traditional instrument used by practitioners and laboratories to measure the radius of spherical base curves.
The base curve determines how tight the lens is fit to the cornea and hence its movement with blinking. To choose a base curve for a particular patient, central keratometry measurement is used.

4. With regard to contact lenses:

a. they are usually tinted to make them more visible for 
    handling

b. only rigid gas permeable contact lens can correct high 
    astigmatism

c. tear lens can neutralize astigmatism

d. truncation of a contact lens is used to prevent lens 
    rotation

e. piggyback contact lens involves the use of two soft 
    contact lenses one above the other
 

Answera.T   b.F   c.T    d.T   e.F
Tint in contact lenses make them more visible for easy handling. Both rigid gas permeable spherical contact lens and soft spherical contact lens can correct astigmatism. However, because the soft contact lens conform to the shape of the cornea, there is little tear lens ie. the tear film between the cornea and the posterior surface of a contact lens to allow neutralization of high astigmatism. Soft spherical contact lens can correct up to 1.00D of lens. Soft contact lens with toric surface can be used to correct higher astigmatism. Truncation involves cutting off the lower part of the spherical lens and prevent lens rotation. Piggback contact lens involves the use of two lenses, the soft contact lens provides the fitting surface for the rigid gas permeable lens. 
 

5. Compare with spectacles, the contact lenses:

a. increase the field of vision

b. magnify images in hypermetropia

c. minify images in myopia

d. reduce aneisokonia

e. reduce optical aberration
 

Answera.T  b.F  c.F  d.T  e.T
Spectacle magnifies and minifies images in hypermetropia and myopia respectively. As contact lenses tend to return the images to near normal size, the images are minifid in hypermetropia and magnified in myopia. Aneisokonia ie. differences in image size is reduced with contact lenses. Contact lens allow the patient to look through the optical centre in all direction of gaze, optical aberration is reduced compared with spectacles.
 

6. Compared with glasses, the contact lenses:

a. increase the amount of convergence needed in myopes

b.increase the amount of accommodation needed in myopes

c. decrease the amount of convergence needed in hypermetropes

d. decrease the amount of accommodation needed in hypermetropes

e. cause eyestrain in presbyopic patients at an earlier age
 

Answer
a.T   b.T  c.T   d.T  e.T
Because of the prismatic effect of glasses with reading 
(base in in myopes and base out in hypermetropes), the myopes need less convergence and accommodation than when using the contact lenses, the converse is true for hypermetropes. Because of the additional accommodation and convergence required, contact lenses can cause eyestrain in presbyopic at an earlier age.
 

7. The following are true about corneal warpage:

a. corneal oedema is a feature

b. it refers to a change in the corneal curvature associated with contact lens wear

c. it is more common with rigid gas permeable contact lens

d. it interferes with the result of biometry

e. it is a reversible condition

Answer
a.F   b.T  c.T  d.T  e.T
Corneal warpage refers to change in the corneal curvature 
associated with contact lens wear. Corneal oedema is absent. It is commoner with rigid gas permeable contact lens than soft contact lens.  The result is reversible. Biometry or refractive surgery should be avoided
until the cornea returns to its normal shape otherwise the
results of the measurement or surgery will be inaccurate.
 

8. With regard to contact lenses:

a. three and 9 o'clock staining is commoner with soft than RGP lens

b. bacterial corneal ulcer is more common with soft than RGP contact lens

c. soft contact lens is more suitable for occasional wear than RGP lens

d. giant papillary conjunctivitis is more common with soft contact lens than RGP lens

e. ptosis is more common with RGP than soft contact lens
 

Answer
a.F   b.T  c.T  d.T  e.T
Three and 9 o'clock staining is a common complication 
of RGP and is caused by peripheral corneal desiccation. It is believed to be initiated by excessive upper lid 
margin sensation, leading to abortive blinking, which in 
turn initiates a cascade of events that include drying and accumulation of mucous debris on the surface of the lens, limited lens movement, loss of the lens edge tear
meniscus, and desiccation of the exposed cornea.
Rigid gas permeable (RGP)contact lens has a smaller 
diameter than the cornea. The soft contact lens typically has a diameter than extends beyong the limbus.
Bacterial corneal ulcers, although rare, are potentially 
the most devastating complication of contact lens wear. Their occurrence is more common in soft lens wearers, 
and extended wear increases the incidence 10- to 15-fold.
RGP usually requires a period of wear to get use to and occasional wear may be uncomfortable for the patient.
Giant papillary conjunctivitis is more common with soft than RGP lens. It is thought to be caused by deposits of denatured proteins.
Ptosis is more common with RGP lens. It is thought to be related to the way the RGP lens is removed ie. temporal stretching of the lids combined with forceful lid closure.
 

9. A contact lens wearer complains that his vision is blurred 
    immediately after blinking. Slit-lamp examination reveals excessive 
    slit-lamp movement. To reduce the movement, you may:
 

a. increase the oxygen permeability of the contact lens

b. decrease the diameter of the contact lens

c. increase the thickness of the contact lens

d. increase the base curve of the contact lens

e. reduce the wearing time
 

Answer
a.F   b.F  c.T  d.F  e.F
To reduce the lens movement, you may increase the contact lens diameter, decrease the base curve of the contact lens or increase the thickness of the contact lens.

 

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