COMMON CONTACT LENS MISCONCEPTIONS
Contact Lens B****cks
Noel A Brennan, Brennan Consultants, Melbourne Australia
Graeme Young, Visioncare Research Ltd, Farnham, United Kingdom
Myths and misconceptions abound in contact lens (CL) practice - this course will attempt to set
some of the record straight.
PR E- FIT T ING AS S E S SM ENT
Myth:
Previous failure with CLs is a contra-indication for refitting with CLs.
Reality: There are many different reasons for CL failure not least of which is the prescribing of
inappropriate lenses. The eye-CL interaction is complex and different CLs have many
diverse properties - varying CL type may solve the previous problem.
Myth:
K-readings help to predict the appropriate soft lens base curve.
Reality: Both K-readings and base curve apply only to the central cornea and lens - the fit of a lens
results from many more factors than these alone.
Myth:
The only methods for assessing the tear film in clinical practice are the Schirmer test and
tear break-up time.
Reality: Symptoms are the best indicator of dry eye. The following slit lamp signs provide further
important clinical information: Tear film debris, shallow tear meniscus, irregular tear prism,
lid parallel conjunctival folds, abnormal lid margins, slow fluorescein mixing,
hypofluorescence, fluorescein banding, desiccation staining, thin aqueous or lipid layer,
rapid drying time.
LE N S M AT ER I AL AN D D E SI G N
Myth:
Dk/t is a direct index of corneal oxygenation – i.e, doubling Dk/t doubles the oxygen.
Reality: The amount of oxygen passing through a lens is only directly proportional to Dk/t in the lab.
On eye, the law of diminishing returns applies and oxygen delivery plateaus.
Myth:
More oxygen gives better comfort.
Reality: There is no scientific evidence to support this contention. Silicone-hydrogels may cause
greater or lesser comfort than traditional hydrogels and the factors leading to this are lens
modulus, surface chemistry and edge design - but not Dk/t.
Myth:
Silicone-hydrogels are the ‘healthiest’ CLs.
Reality: The ultimate measure of CL safety is the associated risk of infection. Rigid lenses and 1day disposable lenses, which are currently made from hydrogel materials, give the lowest
risk of infection and thus could be said to be the ‘healthiest’ lenses to prescribe.
AAO-07 CL Bollocks;12Jan07
Myth:
PMMA lens wearers need not be refitted with RGPs unless experiencing problems.
Reality: The following potential problems arise when refitting PMMA wearers: Corneal curvature
changes, refractive changes, decreased corneal sensitivity, lens flexure, lens durability,
deposits. The following advantages result from refitting PMMA wearers: The obvious (
oedema), improved end-of-day comfort ( O2), less flare ( FOZD), less irritation from FBs
( EC)
Myth:
Back surface toric soft lenses are more stable.
Reality: The differences in lens sagittal depth in the major meridians are generally too small to result
in any significant orientational force. Toric lens stability is influenced by lens fit, front surface
profile but primarily by the action of the lids during the blink and whether these are
complementary or conflicting.
LE N S S EL E CT IO N AN D F IT T ING
Myth:
The optimum rigid lens BOZR is usually steeper than flattest K by 1/3 the difference
between Ks.
Reality: With astigmatic corneas, rigid lenses touch along the flattest meridian. Fitting significantly
steeper than flattest K will only serve to increase central clearance and reduce peripheral
clearance in the steeper meridian, making for a less stable fit. The first trial lens should be
based only on flattest K whether the cornea has low or high astigmatism.
Myth:
Toric RGPs are difficult to fit.
Reality: Toric RGPs can be fitted using spherical trial lenses and simple mental arithmetic.
Myth:
Flatter soft lenses show more movement.
Reality: Very large changes in base curve are required to effect a change in lens fit and lenses
across a wide range of radii can show similar amounts of movement while lenses of similar
base curve but different design can show different fitting characteristics
Myth:
The labelled parameters of soft lenses allow comparison between lens brands.
Reality: The fit of soft lenses is governed, not just by base curve, but by the materials’ rigidity, the
lens’ thickness and design. The on-eye diameter of soft lenses is governed by the
material’s temperature sensitivity and the lens sagittal depth as well as its off-eye diameter.
Myth:
Thicker soft lenses mask astigmatism.
Reality: One theoretical calculation suggests that, to correct just 0.25D on a cornea with 2.00D of
astigmatism, a soft lens would have to have 0.6mm centre thickness. Not surprisingly,
therefore, several clinical studies have found no evidence of masking of astigmatism.
Myth:
The soft lens spherical power of choice (sphere equivalent) for astigmats = sphere + ½ cyl.
Reality: Since we live in a world of verticals, with-the-rule astigmats (-ve cylinder) need no additional
spherical power when correcting with spherical lenses.
Myth:
The required toric soft lens power can only be determined by over-refraction due to
unpredictable ‘fudge factors’.
Reality: With most toric fittings, the correct power should be predictable from the refraction and lens
orientation. Toric lens calculators are, therefore, only helpful as a last resort and only when
the lens power is known with some certainty.
LE N S W E AR I NG AD V I CE
Myth:
CL wearers should interrupt lens wear one day per week.
Reality: Why?
Myth:
CL wearers should leave lenses out for at least 24 hours prior to a refraction for spectacles.
Reality: Modern lenses cause minimal refractive change but, in any case, most CL wearers want
spectacles for use immediately after CL wear.
Myth:
Most CL wearers are non-compliant.
Reality: ALL CL wearers are non-compliant. Since a typical soft lens care regimen involves
approximately 50 steps per day, it would be astonishing if patients were perfectly compliant.
The key is to ensure that patients don’t skip any of the critical steps.
AFT ER - C AR E
Myth:
CLs cannot be verified without specialist equipment.
Reality: It is true that many CL parameters are not easily measured in the laboratory, let alone in
practice. However, two of the most useful parameters to check can easily be measured
using standard office equipment: soft lens BVP and rigid lens BOZR.
Myth:
The slit lamp technique of choice for observing corneal oedema is sclerotic scatter.
Reality: Any corneal oedema visible by sclerotic scatter is also visible by every other form of slit
lamp illumination.
Myth:
Soft lenses should be left out after using fluorescein.
Reality: Because fluorescein sodium is a long-chain molecule, there is a concern that molecules can
become trapped within the matrix of some polymers, however, most materials in use
nowadays do not trap fluorescein. Also, most residual fluorescein can be easily washed out
of the eye prior to reinsertion.
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