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Basic Contact Lenses
COT/COMT Review
O The optical quality of the corneal is
dependent on the corneal epithelium O With contact lens wear, the integrity
of the epithelium can be disrupted, leading to increased or decreased nerve sensitivity
O The corneal endothelium is responsible for keeping the cornea dehydrated and clear
O With contact lens wear, the cornea can be under stress and the endothelial pump system can be affected, which can lead to corneal edema
O With the eye open, the cornea receives oxygen fromO AtmosphereO AqueousO Limbal blood vessels
O With a contact lens on, the eye receives oxygen primarily from aqueous
O When the eye is closed, the cornea does not have atmospheric oxygen and relies more on limbal blood vessels
O Reduced oxygen to the cornea can lead to corneal hypoxia
O Corneal hypoxia can causeO Corneal edemaO Corneal neovascularization (abnormal
blood vessel growth onto the peripheral cornea)
O The Dk value of the contact lens refers to the oxygen permeability of the lens
O A higher Dk value means more oxygen is able to be transmitted through the contact lens
O The Dk value will vary based on lens material, thickness
Contact Lens Design O PMMA O RGPO Soft (hydrophilic)
O PMMAO Hard plasticO NOT rigid gas permeableO Poor oxygen transmissionO Older model contact lenses
RGPO Base curve is the primary curve of
the lensO Designed to contour the anterior
corneal surface/tear filmO RGPs may be fit “on K” “flatter than
K” or “steeper than K” depending on amount of astigmatism
RGPsO Lens diameter should be such that
the upper lid supports upper part of lens at all times
O Diameter should be 2.0mm to 2.5mm smaller than the visible iris diameter
RGPsO Flourescein patterns
O Optimal alignment is when even flourescence is visible between posterior lens surface and anterior cornea
O A flat fit will show a dark area centrally, where the RGP is touching, with pool of flourescence peripherally
O A steep fit will show pooling of flourescein centrally
RGPsO Spectacle blur
O Caused by corneal edema, lens over-wear, lens-induced corneal molding
O Causes vision fluctuation throughout the day, and an inability to see well through spectacles
O May also have glare/halos around lights
Soft Contact LensesO Many different types/wearing
schedulesO Conventional (6 months) Frequent
replacement (1-3 months) Disposable (1 day to 2 weeks)
O Daily wear (remove at night) Flexible wear (remove alternate nights) Extended wear (remove after 6 nights)
SCLsO Many different sizes, powers, water
contentsO Diameter average 13.8-14.5mmO May be as small as 12.5 or as large as
16mmO Spherical and toricO Water content 50% or less is
considered low to moderateO High water content is 50% or more
SCLsO Hydrogel material is primary
material used in SCLs—makes the lens “hydrophilic” or water loving
SCLsO Base Curve
O Traditionally based on K readingsO Hydrophilic BCs range from 8.0- 9.2O Typical BC is 8.5-8.8O A lower number BC results in a
steeper fitO So a BC of 8.4 will fit steeper than a BC
of 8.8
SCLsO Since the diameter of the SCL is so
large, it will fit beyond the limbus (unlike RGPs, which are smaller than the cornea)
O Once the lens is fit, there should be some movement of the lens with each blink, but it should not be excessiveO No movement indicates the lens is
too tightO Too much movement indicates the
lens is too loose
Complications from Contact Lens Wear
O Corneal neovascularizationO Superior limbic keratitisO Sub epithelial infiltratesO Giant papillary conjunctivitis (GPC)O Acute red eyeO Corneal ulcersO Superficial punctate keratitis (SPK)
Common Abbreviations/Terms
O BC base curveO CAC central anterior curveO CPC central posterior curveO Dk oxygen permeabilityO ET edge thicknessO FAP flatter add plusO I&R insertion and removalO ICR intermediate curveO LARS left add, right subtractO OZD optical zone diameterO SAM steeper add minus
Reference
O Daniels, K. 1999. Contact Lenses. Slack Inc.