FITTING TECHNIQUE
Diameter
Selecting Total Diameter
• Individual practitioner preference
• Independent variables to consider:
⎼ How much clearance is required- larger diameter lenses offer
larger sagittal height
⎼ Greater bearing area on the sclera/conj therefore less pressure
on conj – better comfort
⎼ Smaller lenses “sink into the cornea more – also less movement
of the lens
⎼ Larger lenses tend to decenter more temporally (due to flatter
nasal sclera)
⎼ Smaller lenses are easier to handle
SA LENSES - GUIDE
LENS
HVID(mm)
CLLSA: 2-Spec/SSF
REC DIAMETER(mm)
15.6 (SSF)
16.7 (SW)
17.0
21.0
CLLSA – DF
CLLSA:
Rose –K2 XL
Danker Lenses: Zenlens
INNOCON: Eye Space
Scleral
From 15.8
11.8
(1.3-1.5>)
13.6-15.6
Average 14.60
<11.7
16.00
>11.8
17.00
< 11.5
16.50
>= 11.5
17.50
FITTING TECHNIQUE
Optical Zone/Clearance Zone Diameter
• Fixed optic zone diameters –may not be possible to adjust
this parameter in some lens designs.
• Consider: Pupil diameter taking into account the depth of
the AC and possible lens decentration
• The clearance zone area (optical zone + limbal zone)
should be chosen approx. 0.2mm larger than the corneal
diameter
• If the optical zone diameter cannot be adjusted – change
the overall lens design/larger diameter lens
FITTING TECHNIQUE: CLEARANCE
How to determine corneal clearance
• no rules - typically a minimum of 100-150 (vs 300)
microns is desired
• Some cases - lenses can be fitted with a minimum of
20-50 microns of clearance
• In larger lenses 200-300 microns is considered
sufficient however this can be as much as 500-600
microns
• Keratoconics need higher sag than a post-graft or a
post-refractive surgery.
• ocular surface disease - larger sag
FITTING TECHNIQUE: CLEARANCE
• Start with a low/high sag height – adjust until you have
sufficient clearance – approx. 150-300 microns
• Fill lens cup with preservative-free saline & plenty
fluorescein
• No air bubbles
• An ideal optical zone fit is an equal green fluorescein pattern
with no areas of bearing (no black patches)
• Fluorescein becomes visible to the human eye at a minimum
tear lens thickness of 20 microns.
• “Black” indicates insufficient clearance/bearing
FITTING TECHNIQUE: CLEARANCE
FITTING TECHNIQUE: CLEARANCE
• If central corneal “bearing” is visible this means
the sag of the lens may be too low.
•
If bubbles appear under the lens (not from
incorrect insertion) – maybe excessive corneal
clearance – lens edge lifting
• Bubbles also from nonsymmetrical shape of the
ant. ocular surface.
• Small bubbles that move may be acceptable
provided they do not cross the pupil
• large stationery bubbles are not acceptable. The
ideal fit – no bubbles.
FITTING TECHNIQUE: CLEARANCE
• Excessive clearance, of >500 microns -decrease in VA &
increased visual disturbances.
• keratoconus - higher sagittal height depending on cone
height
• “feather touch” with marginal bearing on the cornea - Fitting
goal is minimal sagittal height, vault the cornea with
marginal or no corneal bearing.
• Minimal bearing appears to be better tolerated than with
corneal RGPs – minimal lens movement less irritation to
corneal apex.
• The lenses need a minimum settling time of approx. 3040min before evaluating
FITTING TECHNIQUE: CLEARANCE
• The lenses may “sink” up to 100
microns - therefore allow
adequate settling time before
assessing
• Fenestrated lenses may sink
more than non-fenestrated
lenses
• Use the optic section at 45º
angle, on your slit lamp to
evaluate the post-lens corneal
tear film thickness – white light
FITTING TECHNIQUE: CLEARANCE
Peripheral/Limbal Clearance
• BC slightly flatter than the flattest K value helps to create
clearance in the peripheral/limbal zone
• flattening the BC will reduce sagittal height as well - need to
compensate by increasing the sag height slightly.
• Most manufacturers factor this in by default
• Sag height is also dependent on the lens diameter. Increasing
the diameter for a particular BC will result in an increase in
the sag height. - manufacturers compensate for this
FITTING TECHNIQUE: CLEARANCE
• No bearing on limbus - limbal stem cells
• Limbal stem cells – regeneration of corneal epithelium
• Clearance of approx. 100 microns – NO staining over
limbus
• corneo-scleral lenses do not always clear limbus
adequately, however some lens designs come with
different transitions zone profiles.
• Persistent bubble in limbal zone - eliminate by flattening
the BC or fitting a lower profile limbal zone curve.
An OCT can be extremely useful in determining a more precise
clearance and aiding in fitting.
A
B
SCLERAL LANDING ZONE( SLZ)
• The scleral landing zone is closely
related to the amount of clearance
that is created beneath the lens.
• If the SLZ is steep it will lift the lens
creating more clearance.
• If there is marked central corneal
touch the landing zone will be
elevated off the ocular surface –
edge lift
• The goal is to create alignment with
the sclera/corneo-scleral zone
SCLERAL LANDING ZONE
• The alignment can be assessed using a slit lamp or using an OCT
• Once the trial lens has settled assess the fit based on how the
landing zone bears on the sclera/conj.
• A ring of bearing on the inner aspect of the landing zone indicates
that the SLZ is too flat. The presence of air bubbles or frothing in
the periphery also indicates this- heel blanching
• Bearing on the outer aspect of the SLZ indicates a steep fit and
will increase the overall vault of the lens – toe blanching
• Areas of “whitening” indicate blanching of conjunctival vessels
due to compression from the landing curve.
Blanching of conjunctival vessels on the outer aspect of the
landing zone i.e. toe blanching
Anterior segment 5-line raster images show a scleral lens edge that lands too flat and
lifts away from the conjunctiva (A); good, stable landing aligned with conjunctiva (B);
and impingement into the conjunctiva with a steep landing zone edge (C).
• Assess for blanching in all directions of gaze.
• Conj compression does not necessarily result in conjunctival
staining on lens removal but can cause rebound hyperaemia.
• If the lens is ‘pinching’ on the conjunctival tissue this
impingement and will stain on lens removal. Long term
impingement can result in conjunctival hypertrophy (thickening).
• The larger the lens diameter the more lens weight is distributed
over a larger area of the sclera. This results in the scleral lens
vaulting the cornea more.
• High vault may allow for limited movement of the lens.
LENS EDGE
• Too much edge lift may cause discomfort and increased
lens awareness
• Low edge profiles can cause impingement on the
conjunctiva and obstruct blood flow
• Observe the edge using white light. Check whether it sinks
into the conjunctiva or whether it lifts.
• Edge lift will be observed as a dark ring around the edge of
the lens
• Impinging will be observed as conjunctival staining on lens
removal.
• Most lens manufacturers allow for a degree of variation in
the landing zone
LENS EDGE
Non-rotationally Symmetrical Lens Design
Fitting toric scleral lenses
• As mentioned previously the anterior ocular surface of
the eye is non-rotationally symmetric.
• This implies that one or two aspects of the
sclera/conjunctiva are compressed more when a scleral
lens is placed on the eye.
• Different methods of alleviating this
compression/impingement have been used/suggested
e.g. truncating the lens
• Back toric or quadrant specific lens designs are an
alternative to alleviating this problem, improving the
lens fit and comfort.
LENS EDGE
• The toric or quadrant specific curve is applied only to the SLZ
• Makes the lens more susceptible to flexure and therefore
should be used if the anterior ocular surface shows significant
irregularity.
• Toric scleral lenses allow for more equal distribution of
pressure over the corneal surface, Visser et al (2006)
• The difference in the sag height between the two curves can
vary between 100-1000 microns and is usually determined by
the lab/manufacturer
• Evaluation of a toric scleral lens is the same as for a spherical
lens.
• Once an optimal fit is obtained perform an over-refraction.
Fitting quadrant specific lenses
• Usually an empirical approach is used to fit quadrant
specific designs
• Use a standard fitting set, determine the area of lift off at
the edge using an optic section and a reference such as the
central corneal thickness.
• Theoretically the lens should find its own orientation on
the cornea however since these lenses don’t move much
lens markings help to insert the lens correctly
• If more than one quadrant is altered then the location of
the specific quadrants need to be indicated.
• If needed/desired front optics can be applied, as with toric
RGPs
Fitting front toric scleral lenses
• If the over refraction requires a cyl while no toric back
surface is needed, the lens is considered a true front toric
lens and needs to be stabilized some how. Prism ballast
/double slab-off ballasting is used to stabilize the lens.
• As with standard lenses use the LARS rule to assess the
rotation of the lens.
• Scleral lenses typically don’t move however larger lenses
may move slightly more than smaller lenses
Movement• The lens should be mobile when pushed up slightly
• Changing the lens edge does not seem to have too much
impact on lens movement.
• If the lens has too little apical clearance it may vault across
the corneal apex or along the flat meridian on a toric ocular
surface.
• Optimize the lens fit first before performing an overrefraction.
• If the over rx exceeds 4.00 D convert for the vertex distance
correction
• Trial frame over-refraction