Basic Contact Lenses COTCOMT Review O The optical

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 from O Atmosphere O Aqueous O 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 cause O Corneal edema O 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 RGP O Soft (hydrophilic)

O PMMA O Hard plastic O NOT rigid gas permeable O Poor oxygen transmission O Older model contact lenses

RGP O Base curve is the primary curve of the lens O Designed to contour the anterior corneal surface/tear film O RGPs may be fit “on K” “flatter than K” or “steeper than K” depending on amount of astigmatism

RGPs O Lens diameter should be such that the upper lid supports upper part of lens at all times O Diameter should be 2. 0 mm to 2. 5 mm smaller than the visible iris diameter

RGPs O 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

RGPs O 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 Lenses O Many different types/wearing schedules O 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)

SCLs O Many different sizes, powers, water contents O Diameter average 13. 8 -14. 5 mm O May be as small as 12. 5 or as large as 16 mm O Spherical and toric O Water content 50% or less is considered low to moderate O High water content is 50% or more

SCLs O Hydrogel material is primary material used in SCLs—makes the lens “hydrophilic” or water loving

SCLs O Base Curve O Traditionally based on K readings O Hydrophilic BCs range from 8. 0 - 9. 2 O Typical BC is 8. 5 -8. 8 O A lower number BC results in a steeper fit O So a BC of 8. 4 will fit steeper than a BC of 8. 8

SCLs O 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 excessive O No movement indicates the lens is too tight O Too much movement indicates the lens is too loose

Complications from Contact Lens Wear O Corneal neovascularization O Superior limbic keratitis O Sub epithelial infiltrates O Giant papillary conjunctivitis (GPC) O Acute red eye O Corneal ulcers O Superficial punctate keratitis (SPK)

Common Abbreviations/Terms O O O BC base curve CAC central anterior curve CPC central posterior curve Dk oxygen permeability ET edge thickness FAP flatter add plus I&R insertion and removal ICR intermediate curve LARS left add, right subtract OZD optical zone diameter SAM steeper add minus

Reference O Daniels, K. 1999. Contact Lenses. Slack Inc.