Random Flashcards
Contact lenses VS spectacles
Advantages and disadvantages
cosmetically more appealing no weight less spec mag no fogging wider fov - no Peripheral aberration Good for Sports refractive alsometropes keratoconus no differential prism- as lens moves with eye no abberations/ Surface reflections increased VA CL - bandage good for hyperopes for near vision - less accommodation
Disadvantages can't wear in water less oxygen infection risk restricted RX Allergies / Hygiene FB time AC/ EE costly can become lost or damaged Good level of manual dexterity I & R
Colour defect
most common
Deuteranomaly
Detective green pigment
what is deuteranomaly
Defective green pigment
colour defect
what is Protanopia
struggle to see Dark reds reds blue green looks grey
cover Test
most useful binocular assesment anomalies
only best to differentiate between Strabismus ( hetertropia )from heterophoria
6m distance
33cm near
hirschberg Test
Gross binocular uncooperative kids moderate or large mainfest deviations only fixate on Pen torch @ 33 cm @ eye level reflections on cornea and any asymmetry esotropia = reflection temporal exotropia = reflection nassally
20 base out Prism tests
6 month and above
Positive response demonstrates an adequate convergent fusinal range and absense of global Supression
look @ fixationstick 20 Prism placed in front of each eye In turn Prism causes image to shift should cause eye to adduct to keep focus on Image other eye should also move
if global suppression Present when prism placed infront no movement shift of image undetected.
near point vergence accomodation formula’s
near point vergence = B = K-A Age= 13+( 13 - A ) 4. ( Duanes formula ) near point = b = 1/ B Accomodation = A = K-B Add hsp - ( 2/3 A )
B near point vergence
b =near point
Amp= Amplitude of accomodation
A / ACC = Accomodation
K= k = k ' = K ' = Fe=
K = ocular refraction vergence required at the eye
k = far point - a point conjugate with the macular in the unaccomodated and uncorrected eye
k ‘ = axial length of eye
k’ = Droptric length vergence required after refraction in order to focus the image on the retina
Fe =Power of the reduced surface
k’ = K + Fe ( image Vergence = object vergence +refractive power )
Depth of field
the retinal Image is only Sharp when the object Position is conjugale with the retina.However very Small amounts of blurring are not noticed by the eye so the object can be moved a small distance nearer or further away without appearing to be out of focus. This range of object Positions is the linear depth of field.
As the depth of field depends on blur dimensions which is a function of pupil size , it is increased with miosis (pupil constriction ) and decreased with mydrasis ( pupil dilation)
How emsley modified Gullstrands Simplified schematic eye to produce the reduced eye
the simplified schematic eye has 3 Surfaces
A Single surface Cornea and 2 Surface crystaline lens
Emsley calculated the single surface that would be equivalent to these using refractive indices of4/3 for Aqueous and vitreous and 1.416 for the lens
Define Anisometropia
Anisometropia is the condition where the right and left eyes have a different Ocular ametropias
if one hypermetropia and other Myopic = Antimetropia
Low = 0.25 - 0.75 D
medium = 1.00 - 2.00D
High = 2.25 Or more
can cause vertical differential Prism
can cause eyes to focus unevenly Subjects vision may blur due to different size images through each eye
can result in strabismus and amblyopia
Spec Mag
S M defined as a ratio
Retinal image size in the corrected eye h ‘
__________________________________________
uncorrected retinal Image size h ‘ u
value calculated using
SM =K
—–
Fsp
SM = h ‘ c
—— Thus h ‘ c = h ‘ u x S M. change as a % ( Sm - 1.00 ) x 100
h ‘ u
h ‘ e = 1000 x tan w x k
‐——————- —-
k ‘ Fsp
Definition of Spectacle magnification and the derivation of the formula
SM = K
—–
Fsp
The relationship assumes that the Spectacle lens is thin and that the object is at infinity
SM important - calculates the change in retinal Image size experienced by the eye when Spec lens is placed in front of it
influenced by vertex distance + by the form and thickness of the lens
L A S E R
Light Amplification Stimulated Emission Radiation
laser produces light which is
Monochromatic - targeted at tissue which absorb Particular wavelengths
Coherent -all particals in phase - arrive same place Same time
Directional - can be accurately positioned
can be Switched on and off rapidly - allow precise amounts of energy to be delivered
Contact lens
HVID
Horizontal visible Iris Diameter ( average = 11. 5mm)
contact lens
VPA
vertical Palpabral Aperture ( Top + bottom eyelid )
Contact lens
Pupil Diameter
in low and bright conditions ( 1 mm- 8mm)
lid Position and tension
Pupil reflex ( looking for nerve Problems )
Rigid lens Materials
Silicone acrylates - allow more oxygen to pass through the material , good dimensional stability and Scratch resistance _ good stable vision
however attract Protien deposits from tears can be brittle don’t wet well + Produce various Ocular problems to dry eye
flurosilicone acrylates - addition of fluorine into lens Increases amount of oxygen which passes through lens Improves Wettability of the lens Surface + increases deposit resistance
However difficult to manufacture dimensionally unstable brittle of too thin.to overcome this Crosslinking molecules are often added to materials
advantages + Disadvantages High water content
advantages
increase amount of oxygen reaching the cornea so reduce the risk of ocular changes
Allow increased wearing + flexible wear
can give betterComfort,material more flexible
Enable eye to adapt to the presence of the CL more quickly
often easier to handle due to increased thickness
Disadvantages
fragile + have shorter life span
more prone to deposits
lens dehydrates more in dry environments - uncomfortable- more Solution reaction problems material absorbs chemicals
Silicon Hydrogel
lenses have Silicone added to the soft lens material in Order to increase the amount of oxygen reaching the eye
high oxygen transmission
can be Proveto lipid deposits
Silicone is a stiffer material than hydrogel lenses
biometric - designed to mimic human tissue and after use components which are naturally occuring in the body produce less Ocular changes + fever deposits suitable for dry eyes
soft lens materials
ordered in relation to amount of water they absorb
Iow water = 50 % or less
high water = 50% and over
classified in relation to their lonicity . this term describes the electrochemical charge properties of the material
An Ionic material attracts more protien deposits from the tears than a non - Ionic maternal would do
CL 4 Groups
low water content Ionic
low water non Ionic
High water content Ionic
high water content - non ionic
Advantages low water
Disadvantages low water
Advantages less dehydration on the eye longer life span Stronger less likely to break less solution reaction problems lens can be made thinner easy to manufacture
Disadvantages
lower water allow less Oxygen to reach comea
Producing ocular changes Such as oedema
Oxygen Transmission
different materials = different oxygen transmissions
Normal soft lens hydrogel materials have oxygen transmission which varis with water content
Soft lenses cover entire cornea = almost all oxygen getting to the cornea comes through lens maternal
Thicker lens let’s less oxygen through
Silicone hydrogel
Silicone added to let more oxygen through to the eye than hydrogel materials
A thicker lens made in Silicone hydrogel will allow o higher oxygen transmission
Modern GPlens materials have high oxygen materials
Do not cover whole comea- Oxygen reaches uncovered cornea
tear exchange provided by tear pump on blinking gets more Oxygen to the comea
Scleral lenses
made from PMMA need to be fenestrated to all some oxygen to reach the cornea.If made from GP Oxygen will pass through the lens and reach comea
no tear exchange no oxygen to reach this way as lens covers comea
Advantages of Silicone
Increase amount of oxygen reaching the cornea so reduce risk of ocular changes
Some Suitable for extended Wear
Rapid adaptation
Good dehydration characteristics
easier handling
wide choice of type of Prescription + modalities
Disadvantages of Silicone
more Surface deposition
Increased risk of papillary conjunctivitis induced by the contact lens
Potenti al allergic response in Some Patients
more expensive
Greater incidence of arcuate Staining
stiffer materialfor current hydrogel Wearers
Scleral contact lenses
Size ( diameter) 12-25mm
material - PMMA - doesn’t cause reactions - can be RGP material
Comfort - Doesn’t touch comea - good Comfort
vision - Good optic Zone,held in place by eyelids Brilliant vision ( tear inbetween acts as lens = Corrects Astigmatism)
Prescription availability - Almost anything
Scleral contact lens advantages
Easy to handle easy to maintain unlikely to be dislodged or lost large apic Zone -reduces flare Therapeutic use + cosmetic options FB - unlikely can be modified Solution toxicity allergy unlikely
Scleral contact lenses - Disadvantages
low oxygen supply as Covers entire cornea + often not Permable material
Ocular problems related to corneal hypoxia
I + R issues
long adaptation Periods
long chair time
use of fitting sets ( sterilised)
moulding can be uncomfortableor daughting for cx
cost higher than soft lenses
Hard Rigid CL
Corneal lenses
Size Material comfort vision Perscription availability
size - 8-10. 5 mm total Diameter 2mm Smaller than HVID
Material -RGP- Sometimes PMMA ( fitting lens bleached after use)
comfort - feel Something there,Wear 30mins first day build up wear each day as much as Possible,
vision -Good VA,tear lens Corrects astigmatism,come in multifocals
Rx availability - Most limited Prism 2 base down max
Hard Rigid CL advantages
Good oxygen Supply due to tear exchange and pump
few deposits- easy to maintain
Robust -long life
Easy I + R
many designs + modifications possible( eg ortho Keratocones (flatens Cornea overnight- myopia reduces)
Solution toxicity ( does not absorb)
allergy unlikely
Hard Rigid CL - Disadvantages
can be lost Or displaced Slow adaptation long chair time FB Risk flare due to smaller optic Zone 3 + 9 O clock Staining ( Prevents tear flow) lens adherence cleaning regieme more complicated initial cost higher
ocular structures
which are observed
Lids + Lashes Cornea conjunctiva Anterior chamber Iris Limbus
Lids + lashes= blethaitis Cornea = conjunctiva = Smooth eyelids Anterior chamber = blood Cells Iris = Melanoma Limbus = Redness
