1. Basic Contact Lenses
COT/COMT Review

2. The optical quality of the corneal is dependent on the corneal epithelium
With contact lens wear, the integrity of the epithelium can be disrupted, leading to increased or decreased nerve sensitivity

3. The corneal endothelium is responsible for keeping the cornea dehydrated and clear
With contact lens wear, the cornea can be under stress and the endothelial pump system can be affected, which can lead to corneal edema

4. With the eye open, the cornea receives oxygen from
Atmosphere
Aqueous
Limbal blood vessels
With a contact lens on, the eye receives oxygen primarily from aqueous
When the eye is closed, the cornea does not have atmospheric oxygen and relies more on limbal blood vessels

5. Reduced oxygen to the cornea can lead to corneal hypoxia
Corneal hypoxia can cause
Corneal edema
Corneal neovascularization (abnormal blood vessel growth onto the peripheral cornea)

6. The Dk value of the contact lens refers to the oxygen permeability of the lens
A higher Dk value means more oxygen is able to be transmitted through the contact lens
The Dk value will vary based on lens material, thickness

7. Contact Lens Design
PMMA
RGP
Soft (hydrophilic)

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

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

10. RGPs
Lens diameter should be such that the upper lid supports upper part of lens at all times
Diameter should be 2.0mm to 2.5mm smaller than the visible iris diameter

11. RGPs Flourescein patterns
Optimal alignment is when even flourescence is visible between posterior lens surface and anterior cornea
A flat fit will show a dark area centrally, where the RGP is touching, with pool of flourescence peripherally
A steep fit will show pooling of flourescein centrally

12. RGPs
Spectacle blur
Caused by corneal edema, lens over-wear, lens-induced corneal molding
Causes vision fluctuation throughout the day, and an inability to see well through spectacles
May also have glare/halos around lights

13. Soft Contact Lenses Many different types/wearing schedules
Conventional (6 months) Frequent replacement (1-3 months) Disposable (1 day to 2 weeks)
Daily wear (remove at night) Flexible wear (remove alternate nights) Extended wear (remove after 6 nights)

14. SCLs Many different sizes, powers, water contents
Diameter average mm
May be as small as 12.5 or as large as 16mm
Spherical and toric
Water content 50% or less is considered low to moderate
High water content is 50% or more

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

16. SCLs Base Curve Traditionally based on K readings
Hydrophilic BCs range from
Typical BC is
A lower number BC results in a steeper fit
So a BC of 8.4 will fit steeper than a BC of 8.8

17. SCLs
Since the diameter of the SCL is so large, it will fit beyond the limbus (unlike RGPs, which are smaller than the cornea)
Once the lens is fit, there should be some movement of the lens with each blink, but it should not be excessive
No movement indicates the lens is too tight
Too much movement indicates the lens is too loose

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

19. Common Abbreviations/Terms
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

20. Reference
Daniels, K Contact Lenses. Slack Inc.