This document discusses guidelines for assessing the fit of trial contact lenses. It recommends assessing lens centration, position, movement, and edge characteristics after 5 minutes and 3-4 hours. Acceptable decentration ranges from 0.2-0.75mm in primary gaze. Lens fit is assessed by observing movement on blink, with lateral gaze, and with lower lid push up tests. Tight fits show little movement while loose fits show more movement or lag. Corneal coverage in all gazes is important. The document provides criteria for acceptable versus unacceptable lens fit characteristics.

1. Farrukh Mahmood
LecturerVision Sciences
Soft Contact Lens Fitting

2. Trial Lens Settling Period
 An assessment of the trial lens should be recorded after a 5
minute
 Settling time may depend on water content, BVP and
material chemistry.
 Water content can influence lens parameters and therefore,
the lens fit.
 High water and high power lenses take longer to settle
(equilibrate with their environment)
 Can be done after 3 - 4 hours or at the progress evaluation
stage after lenses have been dispensed.

3. What to Assess During Lens Trial
 Lens centration.
 In the primary gaze position, decentrations of 0.2 – 0.75 mm
are acceptable.
 Position and movement of the lens
 Outcome of a lower lid push-up test in primary gaze
position.
 Edge buckling or lifting.

4. Primary Gaze Position

5. Lateral Gaze, Lateral Versions
 Directing fixation laterally or requesting the fixation
alternate rapidly (e.g. between right and left) can reveal
much about the fitting behaviour of a lens.
 Of greatest interest are any tendencies for the lens to
decentre or for the lens position and movement to lag behind
that of the eye’s.
 Excessive decentration or lag may indicate the lens is too
loose or flat.
 No lag or decentration could mean that the lens is too tight.

6. Lateral Gaze

7. Lens decentration and/or lag
 Primary gaze (while 0.2 to 0.5 mm is more common, 1.0 mm is
probably excessive, depending on lens flexibility).
 31% of tightly fitting lenses showed > 0.3 mm decentration
 0.3 mm was the optimum cut-off point for identifying loose
lenses.
 Upgaze (up to 1.5 mm acceptable provided it is consistent).
 Lateral eye movements (up to 1.5 mm acceptable providing it is
consistent).
 Very flexible lenses move little due to conformity, the resulting
thin post-lens tear film and the larger shear forces thin films
induce.
 Corneal coverage under all reasonable circumstances is still
required.This needs to be ascertained.

8. The Need for Corneal Coverage
Optical
 Centration (soft lenses often decentre up and sometimes up
and out)
 Shape regularity
 A decentred lens, when conforming to the anterior eye
shape, may affect the optical performance
 Post-lens tear film uniformity
 A decentred, conforming soft lens will have some localized
thinning of the postlens tear film in the regions of heaviest
touch (greatest bearing pressure).

9. The Need for Corneal Coverage
Mechanical
 Prevent trauma of Cornea, Limbus and Conjunctiva.
 Comfort
 A decentred lens, especially a mobile decentred lens, may be
less comfortable than a centred lens exhibiting optimal soft
lens movement.

10. The Need for Corneal Coverage
Physiological
 Corneal exposure can lead to discomfort
 Corneal dessication in the areas exposed
 Any loss of the aqueous phase of the tears by evaporation may
lead to corneal desiccation.
 Tear film disturbance, lens and eye wettability.
 Loss of the aqueous phase may lead to lipid contamination of
the mucin layer potentially affecting the eye’s wettability.
 About 1 mm symmetrical overlap, and in all positions of
gaze, is considered ideal.

11. Effect of a Blink
 Too steep:
 −Vision clears immediately after a blink and then quickly
reverts to a lesser quality.
 The pressure of the lids during a blink forces the lens to
conform to the central cornea.This results in a transient
improvement in the vision quality.
 However, this is very unlikely as all soft lenses are fitted
flatter than the cornea.

12. Too flat
 Vision is generally good in the primary gaze position but
blurs immediately after a blink.
 It may then revert to its previous better quality.
 A loose fitting lens will tend to decentre.
 Regardless of the direction, the decentred lens will be forced
to conform to the shape of the anterior eye.

13. Lens Movement
 A slight inferior lag in the primary position may assist debris
dispersal by lens movement. Lowgrade inflammation may be
a consequence of excessive tightness.

14. Lens Movement
 Lens type
 Lens design
 Physical properties of lens material
 Fitting relationship Optimal, steep, flat
 Lid factors.
 Anterior eye topography.
 The presence or absence of a significant corneo-scleral
junction angle (e.g. Japanese eyes have almost none, making
soft lens fitting different to other racial groups).

15. Assessment of Mobility
 Movement on a blink
 Lag following eye movement
 Movement on up-gaze blink
 Lower lid push-up test

16. Primary Gaze

17. Upgaze

18. Lateral Gaze

19. Lower Lid Push Up Test
 When performing this test it is important to keep the square
rear lid margin apposed to the globe so that the lens can be
engaged usefully.
 When a finger is applied to the centre of the lower lid there
is a natural tendency for the lower lid to‘gape’ away from the
globe thus rendering this test impossible.

20. Lower Lid Push Up Test
 What is desired is an easy displacement of the lens across or
over from the cornea while the eye is in the primary
position.
 Once lid manipulation is ceased, it is expected that the lens
will rapidly and reliably recentre on the cornea.
 Should the lens be difficult to displace, and then be sluggish
to recentre, it can be assumed that the lens fit is tending
towards tight.
 If the lens fails to return at all then the fit is tight to very tight.

21. Lower Lid Push Up Test

22. Ranking Assessment Methods
 The following list, ranked from the most predictive to the
least, is taken fromYoung, 1996.The factor to which each
test is more applicable is presented in brackets.
 Lower lid push-up test (tight > loose).
 Post-blink movement (tight).
 Lateral version lag (loose).
 Up-gaze lag (tight).
 Comfort and centration are of some value in loose fits only.

23. Supplementary Tests
 Regularity of retinoscopic reflex
 Regularity of keratometer mires
 Effect of a blink on both
 Stability of vision
 Placido disc
 Video topography analysis

24. Assessment of lens tightness
 Movement in mm
 Post blink movement of 0.25mm to 0.75mm
 Slightly less movement may be acceptable with high Dk/tc
lenses
 0 tightness
 40-60%
 40-50 is ideal
 100%

25. Assessment of lens tightness
 60% acceptable if lens:
 − is thin
 − is flexible
 − has a high water content.
 • 30 - 40% acceptable if lens:
 − is comfortable
 − provides good and stable vision
 − gives full corneal coverage.

26. What is unacceptable
 Lens edge curling/wrinkling
 Conjunctival indentation
 Excessive decentration leading to corneal exposure
 Excessive movement producing visual disturbances, e.g. poor
vision or fluctuating vision
 No movement even if a‘push-up’ test is satisfactory
 Corneal exposure under any circumstances
 Fits which result in blink-related visual disturbances

27. Tight fitting lens
 Lens is immobile even when lower lid push-up manipulation
is attempted.
 Conjunctival indentation at the lens edge.
 Blood flow constriction in peri-limbal conjunctival vessels
under lens periphery.
 Low-grade inflammation
 Vision is better immediately after a blink

30. Flat fitting lens
 If extreme, lens will not stay in eye
 • Excessive lens movement
 •Wrinkling/buckling of lens edge
 •Vision variable but worse immediately after blink if
decentred
 - if lens centred blink may be little or no difference

31. Flat fitting lens
 Inferior lens lag even in primary eye position
 Lens may slip off cornea in up-gaze and/or up-gaze blink
 Subjective discomfort due to lens mobility

32. Final decision
 BVP for Spherical SCLs:
 4:1 Rule:As a very general rule, if the sphere component of an
astigmatic Rx is ≥ 4X the cylinder component, there is a
reasonable chance that the vision with the Rx’s best sphere result,
will be acceptable.This rule-of thumb may not be applicable to
cases where the magnitude of the cylinder is considerable ( ≥ 1.50
D).
 ReducedV/A may be acceptable for some‘social’, occupational
and other occasional wear situations.
 Other occupations may require high acuities and compromises
may be unacceptable.
 Some wearers have higher expectations than others and
compromises may be unacceptable to them.

33. Final decision
 If the best sphere fails to provide the acuity required, a toric
lens will be required if contact lenses are to be pursued
further.
 •The final BVP should be close ( ±0.25 D, ±0.50 D at the
most) to the ocular Rx (sphere or best sphere).