j. perrigin opto 6374 spring 2017 · • allow lenses to “settle” approx 5 min if sph – 10-15...
TRANSCRIPT
Spherical Soft Contact Lens Fitting
J. Perrigin
OPTO 6374
Spring 2017
Reading Assignment
Bennett & Henry: Chapter 11 pages 270-286
Tables 11.1 &11.2
Some Advantages of SCLs versus
GPs
• Initial comfort, little adaptation
• Variable wearing schedule
• Availability of daily disposables
• Dusty environment not a problem
• High oxygen transmission in most newer
designs
• Ability to fit, dispense, & replace from
inventory
Cons of SCLs versus GPs
Greater risk of microbial contamination &
infection
• Greater risks with non-compliance
• Some parameter limitations but customized
available
• Possible vision fluctuations with high
astigmatic rxes but most do well with
contemporary designs
How to start
• Determine if any special features such as tints, UV block, overnight use, etc are desired or indicated
• Decide on most appropriate replacement schedule
• Decide on spherical versus toric
• For the lenses with the desired replacement schedule find the ones that come in the powers & characteristics you need
– Use Tyler’s Quarterly or other reference
Soft lens replacement schedule
• 1 day
• Biweekly
– Some (AV) may rec weekly if sleep in CLs
• Monthly
• Quarterly: more for custom lenses
• Yearly/conventional: we almost never Rx
for soft now
– replacement schedule determined by Dr.-not regulated
by FDA, but manufacturer usually recommends
specific schedule
– Typically best to use manufacturer recommendation
– More frequent replacement usually less $$ per box
Proven that more frequent
replacement decreases the number of
adverse events
• Today’s soft lens care products are not
designed to make lenses last a year
Lens selection
• Try to stay with state of the art healthiest
designs/materials when possible
• Consider:
– Pt goals: desired wt, activities during wear,
replacement schedule, tints, etc.
– Ocular health, RX, previous CL wear
– Always ask if desire anything special such as
tint, overnight wear, etc
DW/EW/CW/FW
lens type or wear schedule
• DW: remove and clean & disinfect nightly
• CW: continuous wear for up to 30 consecutive nights
• EW: no more than 6 consecutive nights without removal. Clean & disinfect each removal unless discarding
– Counsel pt re increased risk of serious ocular complications with overnight wear espec if std hydrogel
• FW: occasional overnight wear: not assoc with increase in incidence of infection
Wear schedules
• DW Lens: only FDA approved for DW (no overnight) use
– Can be low water and/or thick ct
– Durable, low evaporation rate
– Proclear, etc
• EW or CW Lens: Use for DW, FW, or EW
– Use when require or desire greater O2 transmission (DK values)
– Will either have thin ct or high water content or silicone component. Most effective is silicone.
Why are some high Dks approved
only for DW?
• Some CLs may have O2 permeability (Dk)
high enough for EW use but manufacturer
chose to only seek DW in FDA testing. So
can only market for DW unless do more
testing
• Some do this in order to get lens out into
market sooner & later do EW trials
After selection of the specific lens
product for your patient
• Select BC
• Select power
• Place on eye
• Evaluate movement, centration, corneal
coverage, vision, patient comfort
• Over refract with spheres
• If all OK, ready to teach I/R, dispense
Sag
• The BC/OAD combination produces the
sagittal depth which in turn controls the fit
• 8.6 BC/15.0 OAD would most likely fit
tighter than 8.6/14.0 in same material &
design
• Doesn’t always hold true between brands
and different (thinner) materials
Contact Lens Fitting
by Vishakh Nair
8.4
BC/14.0
OAD
8.7
BC/14.0
OAD
8.3
BC/13.8
OAD
8.4
BC/14.5
OAD
25% 25% 25% 25%
In theory, which will be the
steepest fit?
A. 8.4 BC/14.0 OAD
B. 8.7 BC/14.0 OAD
C. 8.3 BC/13.8 OAD
D. 8.4 BC/14.5 OAD
20
BC selection
• Corneal curvature measurements required by TX law but not always helpful with SCLs
• SCLs fitted significantly flatter (approx 4D) than patient’s flat K– 43.00K = 7.85 but if fit with 8.6BC (39.25)
• Follow manufacturer’s recommendations
• Typically only 1 to 2 choices within a particular lens series
• Most SCL BCs fall within 8.4mm to 9.2mm range
Reality
• Most current scls offer one BC and one
OAD
– Fits most patients regardless of Ks
• Manufacturer did many trials to find
parameter combination / sag that would fit
the majority of patients
If you have a BC choice
• In general but not always:
• Flat K 41-45D: start with median BC
• Flattest K > 45.00: start with steepest BC
available for that lens
• Flattest K < 41: start with flattest BC
available for that lens
• Follow manufacturer recommendations
SCL BC
• More significant in thicker, less flexible SCLs like
Ciba Night & Day & custom designs
• Almost irrelevant in thin, very flexible SCLs
– Many designs can fit approx 80% of population
with a single BC
• Steepens with on-eye dehydration so PM f/u good
• A particular BC in 1 brand may not fit the same as
in another brand or material
OAD
• Most brands only offer 1 choice
• Need OAD that provides at least 0.5mm extension onto sclera in all directions
• Typical OADs range from 13.8mm - 15.0mm with most around 14.0mm.
• Larger OADs with torics or high RXs to improve centration & stability & for large corneas
• Custom lenses with larger or smaller OAD available for difficult fits
Choosing lens power
• Vertex BOTH meridians of best spectacle
RX to corneal plane
• For low amounts of cyl,
– Rx vertexed spherical equivalent
– Power needed varies slightly between
brands/designs
– CONSIDER toric if -0.75 cyl or > after
vertexing both meridians
For trial fits
• May not have exact power needed in dx set
– DXs usually in only 0.50D steps
• Go with closest and may be good enough to
wear home temporarily
• At f/u recheck OR and order best power for
year supply
Perform OR• Don’t do OR if poor fit: waste of time
• OR = refracting while patient wearing CL
– Only do spherical if good VA obtained
• Can begin OR with “plussing out” or scope over CLs
– Note regularity/quality of retinoscopic reflex & power found
• Question patient re stability & clarity of vision D & N, particularly following blink
Lens power needed
• OR= over refraction. Perform spherical refraction over the trial CLs using either
– Loose lenses outside phoropter
– Phoropter
Add OR results to lens power of lens on the eye
– This is ideally the power needed. If refracting in a very short exam room may need to add -0.25DS to results or reck at > distance. Don’t overplus! If OR LIP or loose lens > 4 need to vertex
Residual astigmatism with a soft
spherical lens
• All of the manifest cyl in the best spectacle
refraction after vertexing
• Front surface aspherics may provide better
acuity but don’t actually “mask” cyl
– Minimize spherical abberation
– Only helps if low illumination or large pupils
Desirable fit
• Good comfort (>8/10)
• Crisp, clear, stable vision D & N
• Full corneal coverage in all gaze positions
• Minimum acceptable overlap onto sclera = 0.5mm 360 degrees. 1mm ideal
– Edge shouldn’t cross limbus with blink
Centration
• Due to large OZWs of
SCLs, if overlap is
adeq, pupil coverage
not usually a prob
except with opaques or
very high Rxs, or
torics
What is not acceptable re
centration
• Corneal exposure or edge continually passing over
limbus
• Limbal blood flow restriction
• Limbal or peri-limbal irritation
• Inflammation
Desirable lens movement
• Post-blink movement approx 0.5mm N & T
– Slightly less ok for some SHs
– Text says .5 to 1.0 but 1mm usually too much
unless in upward gaze
• No resistance & smooth recovery from
push-up
• Lag on change of gaze
Slit lamp eval of movement • Allow lenses to “settle” approx 5 min if sph
– 10-15 min or more for torics & bifocals
– Look again before pt leaves
• Lens movement: Primary function is removal of debris during tear exchange– Traditional meth: observe vertical mvt with NORMAL
blinking in primary & upward gaze. Expect approx 0.5mm N & T in primary & 0.75mm to 1.5mm in upward gaze. Prefer @ least 2 of 3 be in range
– Slightly less movement may be acceptable with higher DK/l (high O2 transmission) lenses if good push-up
Inside out CLs
• Edge stand off (fluting) &/or decentration
• Excessive movement, lens awareness, may fall out
• If high modulus such as Night & Day may cause corneal molding especially if inside-out
• Look for inversion markers to help patient
– For thin lenses and for patients with tight lids
not showing adeq movement wi traditional
meth eval by:
• Lens lag on gaze change
– Observe degree of lag & speed of
recentering
– 0.3-0.7mm is desired (just estimate)
• Resistance to push meth (push - up test)
– Observe ease of moving lens from its
static position & speed of recovery
Push-up test
Change of gaze
Tight lids
• May pull lens superiorly
– Traditional movemt may not be seen or
possible. For these pts, if lens moves easily
with the lids, BC to cornea relationship is
usually acceptable.
Precautions in Eval of Movement
• Edge indentation may be > with loose conj
• Flexure of loose conj may give false impression of movement
• Loose conj may decrease movement by enveloping lens edge
• Incomplete blinks: SCL won’t move adequately giving false impression of tight fit
EMR Recording of Fit
Click on yellow box marked observation for choices:
Movement/centration good
Movement good, minimal, or none
Centration good, superior, inferior, nasal, or temporal
For pushup record as no resistance to push or as resistance to push if CL is tight
or could use scale 0-5
For change of gaze can record in mm change with gaze change or just as good lag with change of gaze
Factors Governing Fit
• Sag of lens versus corneal-scleral sag &
asphericity
• Anterior ocular topography
• Lid position & tightness
• Blinking characteristics
• Tear film quantity & quality
• Wear environment
• Physical properties of lens
– Material: stiffer (higher modulus) moves more
– RX
– Thickness: overall, ct & midperipheral t
– Design
More factors influencing fit
Unacceptable fits• Corneal exposure
• Limbal impingement
• Complete lack of movement
• Blink-related visual disturbances
• Conjunctival indentation @ lens edge
In General
• To loosen a SCL fit, while keeping other
parameters constant:
– Decrease OAD
– Flatten BC
– Select CL with greater ct or midperiph
thickness or higher modulus (stiffer)
• To improve centration: tighten fit if movemt still adequate
– Increase OAD or steepen BC
– Try different brand
• To tighten a SCL fit while keeping other parameters constant:
– Increase OAD
– Steepen BC
– Decrease ct or overall thickness
Blinking/Tear Quality• May have more effect on lens positioning &
movement than BC or OAD
• Instead of changing lens fit, try artificial
tears, punctal plugs, lid hygiene etc.
Blinking & quality of vision
• If too steep:
– Vision is clearest immediately after a blink
– Not always seen if very thin CL
• If too flat:
– Vision is blurred immediately after a blink
Effect of lens thickness on fit
• Thicker lenses show > movemt than thin
– Interact more with lids
– >post-lens tear volume
• Except for silicone/hydrogels oxygen
transmission decreases as thickness
increases
Excessively Tight SCLs May:
• Be comfortable initially
• Cause hypoxia, edema, neo, injection,
conjunctival indentation after several hrs
wear
• Trap debris or bubbles behind the CL
• Be difficult to remove- ask patient
• Possibly show “conjunctival drag”
• Increase tendency for “myopic creep”
– Controversial
Excessively Loose SCLs May:
• Cause discomfort/lens awareness
• Cause blurring, fluctuating vision, &
injection
• Cause lens loss, decentration, edge standoff
• Verify not inside out or defective especially
if only 1 eye appears loose
Remember
SCL performance in response to parameter
changes not as predictable as with RGPs
Different brands with same parameters may
not fit the same & F needed may vary
Marginal Fits
• Optimize fit as much as possible by choices
of BC, OAD, material, thickness, O2, etc.
• If best fit still not optimal but you feel the pt
should be wearing SCLs:
– Consider past SCL Hx, presence of neo, edema
– Limit wt to appropriate level
– Rx ATs on regular basis
– Monitor more frequently
– Advise pt & note in record