Contact Lenses

For a GP wearing patient with moderate peripheral cornea desiccation (3-9) staining, what contact lens parameter adjustments can you make to minimize this?

• Most common complication with RGP is peripheral corneal dessication (3-9 staining)

• Often due to excessive edge lift of contact lenses

  • Result in areas of tear film disruption → areas of dryness and desiccation of the corneal epithelium 

• Decrease edge lift and increase the lid apposition to the peripheral cornea 

  • Steepen the base curve and/or steepen the peripheral curves

  • Thinning the CL edge and lenticulating higher minus also helps

• 3-9 staining is purely fit related, so changing lens material won’t make a difference 

If a patient is wearing a +5.00 D soft CL and does NCT, how would you expect the pressures to be affected? 

• NCT over plus SCL (greater than +3.00 D) → IOP overestimated

  • Due to increased center thickness & increased rigidity

• NCT over minus SCL (greater than -6.00D) → IOP underestimated 

The Dk/t value of a contact lens is referred to as what?

A. Oxygen permeability

B. Oxygen transmissibility

B. Oxygen transmissibility

• Oxygen transmissibility is directly proportional to the oxygen permeability of a material (Dk) and inversely proportional to the avg thickness of a lens material (t). 

• Cl companies use a standard -3.00 CL rx to calculate this value 

What type of toric GP lens design is this? 

BC: 7.58 mm and 7.84 mm 

CLP: -1.00D and -2.50 

First step: Convert BC from mm to Diopters 

• BC 1 = 7.58 mm = 337.5/7.58 = 44.50 D 

• BC 2 = 7.84 mm = 337.5/7.84 = 43.00 D 

• Change in BC = 1.50 D 

Second Step: Calculate Change in CLP 

• Change in CLP = 1.50 D 

Differences in Changes

• BC = CLP means SPE Bitoric 

• 3/2 BC = CLP means Base Curve Toric 

• 3/2 BC does not = CLP means CPE Bitoric 

When fitting a scleral contact lens, how much clearance over the steepest area of the cornea is considered ideal? 

100 - 300 microns

What is the power of the tear lens created by a GP? Round to the nearest 0.12 D 

• BC = 42.87 D 

• Placed on a cornea with a spherical curvature of 43.25 D

Tear Lens (TL) = Base Curve (BC) - Keratometry (K)

• When the base curve is flatter than the k values, the tear lens will be a negative value 

• TL = 42.87 - 43.25 = -0.37 D 

Increasing silicone content and decreasing water content has what effect on the oxygen transmissibility of soft contact lenses? 

• Oxygen transmissibility increases 

• But this combination increases chance of lipid deposition and increases the modulus of the material 

• More water content = less oxygen transmissibility

• More silicone = more oxygen transmissibility

How do you calculate the SCw for a GP? 

OAD = OZ + 2(SCw) + 2 (TCw) 

In a minus SCL, does the periphery of the lens have a higher or lower Dk/t? What about a plus lens?

• Minus

  • Lower Dk/t in the periphery

  • Minus lens is thickest in the periphery

• Plus

  • Lower DK/t in the center

  • Plus lens is thickest in the center

What happens to the accommodative demand and convergence demand for a +10.00 going from spectacles to CLS? How about a -10.00?

Hyperopes (+10.00)

• Less accommodative demand

  • More (+) power in contacts

• More convergence demand

  • When hyperopes look at near targets, their eyes will converge and move inward (Imagine prisms with its bases facing each other)

  • They will be looking through BO prisms which moves the images inward and increases the convergence demand even more

Myopes (-10.00)

• More accommodative demand 

  • Minus spectacles reduce near accommodative demand due to vertex-distance effects.

  • Minus contacts do not.

• Less convergence demand ??? ( To double check) 

What parameter changes do you do to loosen a tightly fitting GP?

• Flattening the base curve (most common) 

• Decrease optic zone 

• Decrease the overall diameter (OAD) 

• Widen the peripheral curve system 

• Flatten the peripheral curve system 

What parameter changes do you do to tighten a loosing fitting GP?

• Steepen the BC 

• Increase the OAD  

• Increase the optic zone 

• Steepen the peripheral curve system 

• Narrow the width of the peripheral curves