Contact lens 1 Flashcards
what are the types of contact lenses?
rigid gas permeable (RGB)
soft contact lenses (SCL)
speciality
describe the properties of RGB?
. approx 9mm diameter
. re-usable 1-3 years
. clean after each week
.more fitted in eye
describe the properties of SCL?
. approx 14mm diameter
. disposable or re-usable
. covers the whole eye
why do we use speciality contact lenses?
they are used for specific purpose
e.g. complex corneal shape
they are re-usable
give examples of speciality contact lenses?
. hybrids ( 14mm ) RGB + SCL
. mini-sclerals (14mm) RGB
.ortho-keratology ( correct during sleep)
what does modalities mean?
. how someone wears contact lenses
what are the two types of modalities?
. replacement
. wear modality
what are the two types of replacement lenses?
. soft lenses
.RGB
what are the types of soft replacement lenses?
. daily disposables
. 2- weekly
. monthly disposables
. annual
what are the types of RGB replacement lenses
. replacement every 1-3 years
what are the types of soft wear modality lenses?
. daily (DW)
. 7 days and 6 nights (EW) extended wear
sleep with lenses
for people with long shifts
.30 days and 29 nights (CW) continuous wear
what are the types of RGB wear modality lenses?
. DW
. overnight
only with (ortho keratology ) used to correct short sight over night
what are types of prescription?
. spherical ( myopia , hypermetropia)
. toric (astigmatism)
.multifocal (reading , near)
what are the types of materials used for rigid contact lenses?
.PMMA ( polymethyl methacrylate)
. Gas permeable
what is PMMA?
. first material
. no oxygen permeability
. excellent vision
what is gas permeable material ?
. much better oxygen permeability
. healthier
what are the types of material used for soft contact lenses?
. hydrogel
. silicone hydrogel
what is hydrogel?
. contains water
. very comfortable
. don’t allow much oxygen around
what is silicone hydrogel?
. allows more oxygen than hydrogel
. allowed to use for EW/CW
. more expensive
which material is most popular in order?
- silicone hydrogel
- hydrogel
- GP
- hybrid
- PMMA
what baseline measurements are used for CL fitting?
- spectacle conversion
- convert to ocular prescription - measure curvature of cornea
- use keratometry - measure diameter of cornea
- measure horizontal visible iris diameter (HIVD)(IN essence its the diameter od cornea)
what is back vertex distance (BVD) ?
. how far away your glasses sit from your eyes
why do we convert spectacle prescription into ocular prescription ?
. this is because average BVD is usually 12mm
while contact lenses and eye distance is 0 so the power of contact lens must be different to spectacle to get some correction .
. same optical prescription
. conversion only applies to high prescription above 4
how do we convert spectacle prescription into ocular prescription ?
. using BVD table
what is astigmatism?
. two focal points at back eye
points are on two different meridians causing blurr
what are the two types of astigmatism due to?
. corneal astigmatism
. reticular astigmatism (lens)
how to correct astigmatism?
use two different prescriptions simultaneously in principle meridians
what is regular astigmatism?
. Rx in one meridian is 90 deg to Rx in second meridian
what is irregular astigmatism?
. meridian not 90 deg apart , due to trauma , disease
how to prescribe astigmatic spectacle prescription?
. different powers in two principle meridians
why do we have to prescribe different powers in two principle meridians for astigmatism?
to correct .blurry vision . headache . fatigue or eyestrain . distorted vision . light sensitivity
what are the two types of corneal astigmatism?
. with the rule
. against the rule
what is with the rule corneal astigmatism?
. vertical meridian is steeper ( more curved)
what is against the rule corneal astigmatism?
. horizontal meridian is steeper ( more curved )
how to write spectacle Rx for astigmatic prescription?
. most positive meridian is spherical power
. difference between two powers is cylinder power
. the axis is more positive reading
1st meridian = -7.50 along 25
2nd meridian= -5.50 along 115
-5.50/+2.00x115
convert -6.50/-1.00x10 spectacle Rx to ocular prescription?
11mm BVD
. first meridian = -6.50 . second meridian = -7.50 use BVD at 11mm . first meridian = -6.12 . second meridian = -6.87 difference is 0.75
what is a keratometry?
measurement of radii of anterior surface of cornea
. cornea acts as mirror since light is reflected from it
pros and cons of automated keratometry?
pros . quick .easy con . can't cope with irregular astigmatism . if someone focuses too much can have affect on result
why do we use keratometry?
determination of
. central corneal radius (mm) and power (D) in both principle meridian of cornea
.differentiate between regular and irregular astigmatism
. measurement of radius of contact lenses
. observe quality of tears
why do we measure (HVID) ?
. for total diameter CL
e.g . soft lenses = lens beyond size of HVID
RGP = we want lens to stay within
allows us to know what diameter of contact lenses we want to use
what do we measure (HVID) ?
. measure with PD ruler
or comparator scale
what is expected HVID ?
. expected 11.6 mm
. between 10.2 - 13.0 mm
How to write an astigamtic prescription?
difference between 2 meridians is 90 (between the axis)
- Use more positive power for sph
- Subtract sph to get negative cyl
- The axis is from the more positive reading.
