12. Contact Lenses Flashcards
why are CLs better in some cases for patients
- reduce or elminiate the aberrations assocaited with glasses used in the correction of high refractive errors
- reduce aniseikonia associated with anisometropia and high degrees of atigmatism
examples of CLs used in ophthal
- bandage contact lenses - precroneal membranes
- used in electroretinography
what determines the power of the optical (central) zone of the contact lens
- anterior curvature
- posterior curvature
- thickness
- refractuve index
when is a CL described as spherical
same radius of surgace curvature in each meridian
when can cyclindrical refractive erros be corrected by contact lenses
front surfac,e, back surface or both are toric
how is torsion of a toric CL prevention
either
incorporating an up to 2.00 D base down prism to weight the lower pole or lens
OR
removing the lower 0.5-1mm of the lens (truncation) to allow it sit on the edge of the lower eyelide
what is the base curve of a CL
posterior surface of the optical zone is defined by the posterior central curvature
how is a correct fit ensured for CL
base curve should conform closly to the aspheric surface of the cornea
how is an aspheric shape created
encircling the optical zone with one or two concentrix zones of increasing radius of curvate to produce a bircurve or tricurve C;
how are the junctions between teh zones made smooth
blneding
or computer controlled production
corneal contact lenses have a smaller diameter than the cornea on which they are supported - T/F
true
sclearal contact lenses
rarely used
have a peripheral rim which is supported by the sclera
how is the corneal surface oxygenated
by the tear film
CL are made of gas-imperable materials
thf incorporate fenestrations slot or grooves to facilitate the circulation of tears behind the lens
why do high power concave (minus) lenses ride high
tendency of the upper lid to grip the thick upper edge of a high power lens
countered by a periheral bevel
why do high power convex (plus) lenses drop lower
due to weight
counted by a minus peripheral carrier postion which tends to be lifted by the upper lid
refractive index of the precorneal tear film
1.333
refractive index of the cornea
1.3375
refractive index of precorneal tear film = cornea
YES
tear film helps to reduce what
corneal surface irregulalrity
deinfe the tear lens
tear film between the posteruor surface of the CL and anterior surface of the eye
if the tear lens has a uniform thickness it has
plano power
a steeper base curve of the CL (i.e more vaulted CL) does what
increases the axial ehigh of the tear lens making it more strongly positive
what does the tear lens help
allows a speherical CL to neutralise corneal astigmatism
the base curve whould be what in regards ot the corneal surface
be the same as the corneal surgface curvature at the FLATTEST meridian
so that were the cornea is steepr the tear lens is thicker thf neutrlisies atigmatism
why does only the sphercial component need to be prescribed
tear lens neutrlaises astigmatism
how much astigmatism can soft CLs corrected
1.00D
rigid CLs can correct more stigmatism - T/F
truse
astigmatism arising from crystalline lens or implanted IOL can only be corrected by
the front surface of a toric lens
why do CL give good vision in all positione of gaze
the cl moves with the eye
when may a ridig contact produce a halo effect
when the pupil is dilate
because of refraction through the peripheral zone of the lens or adjacent tear film
why do myoptic patients have a reduced field of veiew
the lens peripheray has a primatic effect with the base towards the visual axis
why do hypermetropic patients have an increased field of veiew
primatic effect with the base away from the visual axis
aniseikonia is
reduced with CLs when compared to glasses
CL give image mag/minification for myopic
magnification
CL give image mag/minification for hypermetropia
minification
CL and oblique aberration
is minimised as you can’t look through the non-axial portion of a well fitting CL
why does a change to CL for myopes cause eye straing
a change to CL increased the deman for greater convergnece and accomation which may cause eye strian
how much prism power can be incoportaed in corneal CL
up to 3 dioptres
what orientation is the prisms
base down thf can’t do horizontal primatic correction
a scleral lens can incorporate up to how much dioptre corection
6 prims in vertical or horizontal split between two lenses
times when more than one object distance needs to be corrected
presbyoic, pseudophakic and aphakic
because their accomodation is reduced or absent
options for CL w/o accomodation
1/ glasses and cl
2/ biofocal CL
3/ monofocal vision
described monofocal vision
one eye for distance 9usually better eye)
one eye for near
reduced binoculalirty and steropsis
options for bifocal or multifocal lensses
annular aspheric segmental and diffrative
annular biofocal CL
- central zone corrects for distance
- annular one for near
- in down gaze the CL rises relative to the cornea placing the nar portion in front of the visual acis
Peripheral portion of the lens is not as effective when the pupil is msllaer
Aspheric multifocal CL
- central part corrects for distance
- gradual trasnition in power to the peripheral zone which corrects for near
- onle a small ampint of light focuses on the retina
Segmental biofocal CL
- incorportae the near additiona over the lower portion of the lens
- inin down gaze the CL rises relative to the cornea placing the nar portion in front of the visual acis
- must be preventied by rotation –> truncation or ballasting with a base down prims
Diffractive bifocal LC
- concentric diffraction rings on their posterior surface which are desined t focus equal amoutns of lght from distand and near objects
keratonconus
increased corneal surface curvature causes increasing myopia and irregular astimaticm
short term problems with CL
- if posterior surface is too flat, it will move
- movement during blinking cayses changes to vision
- small changes occur when lens is in use -
evaopration of water increases the refractive index and increse in temperature increase the curvatre
incresa the negative power of the lens
long term problems with CL
warpage is the change of corneal curvature induced by CL
regresses after lens is removd over hours or day
more prondouc in long term wear
CL
