Soft Contacts Lens: Fitting and Management Flashcards
Material properties
safe
inert
nontoxic
biocompatible
chemically and physically stable
good wettability
deposit resistance
durable
easy to formulate and manufacture
good optical clarity
Materials
polymerization of different monomer combinations, creates a balance of:
- water content
- refractive index
- hardness
- mechnical strength
- oxygen permeability
silicone:
- a high Dk value (oxygen permeability)
- good comfort
- good wettability
- optical clarity
- deposit resistance
HEMA - first hydrogel lens to allow oxygen
DK value
how much oxygen allowed through contact lens
needs to be at least 84
silicone hydrogel allows higher oxygen permeability with proper hardness and mechanical strength
Final Evaluation
transparency
hardness and stiffness
tensile strength
modulus of elasticity
refractive index
wettability
ionic charge
hydration (water content)
oxygen permeability/oxygen transmission
Modulus of elasticity
silicon hydrogel > hydrogel material
pros: stiffer, resists deformation, holds shape, easier to handle, better VA maybe
cons: edge lift, flutting, superior epithelial arcuate lesions, mucin balls, GPC
Oxygen permeability
DK value
property of lens material itself
not affected by power
hydrogel - Dk value increases as water content increaes
silicone hydrogel - Dk value decreases as water content increases
oxygen transmission
Dk/t
Dk divided by average thickness of lens
considers how much oxygen for a given lens can reach the cornea
lens power indirectly affects Dk/t
holden and mertz
measured corneal swelling following lens wear and compared to measurements from without wearing lenses
Dk/t = 24 for daily wear to avoid corneal swelling
Dk/t = 87 for extended wear
water content and ionic charge for classification of lens groups
water content and ionic charge affect:
strength
refractive index
deposit resistance
Dk
interaction with contact lens solution
as water content increases:
- lens strength, deposit resistance, and refractive index decreases
- oxygen permeability increases
classification of lens group
low water content is defined as <50% water, high water content is defined at >50%
- group 1: low water content, nonionic polymers:
includes HEMA and hydrophobic monors, attracts less deposits, less oxygen permeability, stays clean longer - group 2: high water content, nonionic polymers: lower protein interaction due to nonionic polymer matrix, avoid sorbic acid and potassium sorbate
- group 3: low water content, ionic polymers - greater attraction towards tear proteins and lipids, therefore exhibit more deposits than nonionic group
- group 4: high water content, ionic polymers: strongest proteins attraction, avoid heat disinfection
silicone hydrogel
latest and greatest technology in soft cl
- very low water content
- high oxygen transmissibility
- beware of silicone allergy
- the higher the amt of silicone, the lower the water content = the more oxygen transmitte to cornea
- silicone material has higher modulus of elasticity (high resistance to deformation or stress), stiffer material compared to hydrogel
remember: hydrogel material has higher water content so it’s more fragile, therefore the modulus of elasticity is lower -> flimsier material
manufacturing methods
lathe cutting: PMMA, RGP, many soft CL
spin-cast: manufacture soft lens, hydrogel materials into a cast, spinning action determines final power
cast-molded: cost effective, reproducible, less labor intensive process for high-volume manufacturing
combination
lathe cutting
process begins with long, plastic cyl of material cut into lens buttons followed by:
- lathing: the hard, dry button is ground on a lath which cuts lenses into different front curves, base curves, and secondary curves when necessary
- polishing: removes any lathing marks, improves the optics, and smooths the edges
- hydration: for soft lenses - the brittle dry lenses are immersed in saline and become soft, flexible lens
- extraction: all unpolymerized chemicals or materials are removed
- tinting: if there is a handling tint
- finishing: full quality inspection
- sterilization: lenses are placed in autoclave for 20 minutes
spin casting
liquid lens polymer injected into spinning mold, followed by lens material being treated with heat and UV light (curing) -> causing liquid to solidify
front surface of lens determined by curvature of the mold
back surface is determined by combination of temp, gravity, centrifugal force, surface tension, amt of liquid in mold, rate of spin
high reproducible and thin, comfortable edges
small problem: often decenter on eye -> temporally or superior temporal
tends to be flat -> moves a lot (good an easy though)
Cast molding
make mold
liquid polymer poured into concave half of mold and convex portion of applied and clamped into place
material is cure with UV light and hydrated
contraindications
inflammation, disease of ant. seg., systemic disease that can be complicated by CL wear, poor hygiene, poor compliance, lack of motivation
irregular cornea (keratoconus, ocular trauma), autoimmune disease, immunocompromised patients, chronic allergies, chronic antihistamine use, GPC
indications
good tear quality and quantity
spherical RE
low astig.
low lenticular astig.
atheltes
unable to adapt to GP lenses
occasional/flexible wearer
desires tint to enhance or change eye color
previous GP adherence
previous 3 and 9 oclock staining with GP lenses
high motivation
soft lens advantages
excellent initial comfort
min. adaptation time
part-time wearing schedule possible
risk of corneal distortion min.
min. spectacle blur
dislocation uncommon
foreign-body sensation rare
ability to fit and dispence from inventory
low incidence of flare
low incidence of discomfort from excessive lens lag
ability to change or enhance eye color
simplicity of fit
rarely causes excessive tearing
disposable/frequence replacement possible
therapeutic use possible
soft lens disadvantages
reduced VA in uncorrected astig.
limited durability
oxygen transmission w/ hydrogels
deposit formation/ GPC possible
greater chance of bacterial contamination/infection
greater risks with noncompliance
more difficult to verify
limitation of corrections
quality of vision may be reduced
GPC
patient factors affecting material selection
RE
handling issues
deposit-prone patients
marginal dry eye
therapeutic use
ocular disease
age
aphakia
occupation
part-time wearers
refitting GP wearers
compliance
Refractive error selection
best candidates: spherical RE, low asitg, low lenticular astig
handling issue
first time wears will benefit from thicker lens or increased stiffness/modulus of elasticity
ex. sihy material better for first time wearers
handling tint
deposit-prone patient
soft lens are more prone to deposits than GP wearers
recommend frequent replacement - especially daily disposable
recommend hydrogen peroxide sol.
marginal dry eye
50% of CL wearers report dry eye
ocular factors affecting dry eye: wettability, dehydration, cl sol., poor tear film quality, environmental temp, time of day, humidity, wind, blink rate
therapeutic use
used for
corneal erosions, chronic epithelial defect, bullous keratopathy, mechanical trauma, dry eyes, filamentary keratitis
ocular disease
- diabetics benefit from daily disposables due to dec. wound healing
- daily wear also recommended in epithelial stained corneas, decreased tear film, chronic allergies
age
children may have diff. inserting large dia. lenses
-smaller dia. avail.
presbyopes may have difficulty viewing lens when cleaning or handling
- add visibility tint
- recommend thicker higher modulus lens
aphakia
higher oxygen transmission critical due to thick lens center, usually silicone based
limited parameters
lenses can be very expensive
refits
refit GP wearers to soft lenses
- inability to adapt to GP lenses
- chronic staining
- chronic adherence that doesn’t improve
reduce wear time until cornea re-stabilize
use disaposable trials until cornea stabilizes
compliance
patient are noncompliant with lens care and disinfection of lenses - daily disposables
patient sleep in their lenses - extended wear
patient eduction and information regarding adverse events
patients refuse to follow proper wear, care and handling steps should not be fitting into cl
lens design parameters
base curve
diameter
power
center thickness
tint
replacement schedule
base curve parameter
curvature of back surface of lens
optimize fitting relationship of lens to central and midperipheral cornea
flatter corneas -> flatter bc
steeper cornea -> steeper bc
flat k > 45 D; fit steeper bc
flat k is 41-45: fit medium bc
flat k < 41D: fit flatter bc
for k< 45D; fit flatter bc (8.7)
for k>45D: fit steeper bc (8.3)
bc in radians
BC radian dioptric conversion formula
- 5/ radius = diopter
- 5/diopter = radius
diopters: steepen a lens -inc. the value
flattening a lens - dec. the value
radius of curvature: steepen a lens - lowering the value
flattening a lens - inc. the value
diameter
diameter is measurement of full length of lens
overall diameter of lens should cover cornea 360 degrees and scleral - 0.5 mm in all direction
estimated dia: add 2 mm to HVID
increasing diameter, steepens/tightens the overall fit
power
predicted power determined by patient’s spectacle
-vertexed back to corneal plane if rx > 4 D
if pt has low amt. of astig, do spherocylindrical calc.
underminus if power na
4-6-8 rule
- 25 less if -4.00 to -5.75
- 50 less if -6.00 to -7.75
- 75 less if -8.00 to -9.75
vertex both meridians
residual astig.
CRA: calculated residual astig.
- amt of astig. comes through after cl placed on eye
- residual astig. for spherical soft lenses = refractive astig.
CRA = refractive astigmatism (TRA) - keratometric astig (K)
-in soft lenses: CRA = TRA
center thickness
minus power lenses range from 0.04 - 0.18 mm in center thicness
plus lens range from 0.2 to 0.7 mm in center thickness
thinner lens drape cornea more and move less with blink
thicker lens used for patients need assistance with lens handling
tint
enhancing
opaque tints
handling tints
replacement schedule
daily dispoables
weekly replacement
2 week replacement
monthly replacement
quarterly replacement
sagitta
sag is measure of lens curvature
base curve as size
sagitta is relationship diameter and base curve = how it fits overall
steeper curves = grater sag
larger dia. = greater sag = tighter fit
dec. sag will loosen a lens on the eye
- flattening a base curve
- reducing diameter
inc. sag will tighten a lens on eye
- steepening a base
- inc. dia
correcting sagitta
lens too tight:
-decrease sag: flatter BC, smaller dia
lens too loose:
-inc. sag: steeper bc, larger dia.
5 essential evaluation elements
CLAMO or CHAMP
Comfort Health Acuity Movement Position
CLAMO Coverage location acuity movement over-refraction
position eval
complete covage and extends 0.5 mm onto scleral
good centration
movement eval
ideal movement: 0.5 mm to 1.mm
push-up test: nudge lower lid to see if lens is moving
primary gaze: observe at 5 or 7 o clock pos.
superior gaze: lens should move with blink, more mvmt than primary gaze
lateral gaze: lens follow the movement
no movement and excessive movement can cause adverse affects - discomfort, awareness, and hypoxia
acuity eval
should be 20/25 or better
clear, sharp, constant, and acceptable per patient
over-refraction required
flexing phenomenon
health eval
lens condition must be assessed
lens is clear, cornea is clear, tears clear
comfort eval
overall these are believed most comfortable cl on market - rgp, hybrid, scleral
uncomfortable: debri under lens, excessive mvmt, edge stand off, surface or edge rip, material allergy, gpc, dehydration
chararcteristic of loose lens
position - superior or sup. temporal
mvmt - excessive
vision - inconsistent
healthy - mucus build up
comfort - lens awareness
characteristic of tight lens
position - inferior
mvmt - little to no mvmt
vision - flexing phenomenon
healthy - hypoxia
comfort - initially great
no tears to oxygen
other aux tests to check eval
over keratometry: quality of mires represent lens fitting
- loose len: distorted mires immediately after blink
- tight len: clear mires, then distorted
retinoscopic reflex
slide back test: slide lens off cornea either finger lower lid
- observe how much effort it took
- observe how fast lens returns to central pos.
- loose lens: slide easily return quickly cornea
- tight lens: slide off with more effort, will not return quickly cornea
tear smears
fluorescent dyes - fluorosoft
push up test
