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.