5.1.3 Chooses, fits and orders rigid lenses. Flashcards
Advantages of RGP wear
- Extra sharp vision
- Correct high levels of astigmatism
- Less chance of infection
B+L, Boston Advance Formula Cleaner + Conditioner Sol
- Conditioner
- Disinfectants → Polyhexanide (Preservative), Chlorhexadine Gluconate (Preservative), Disodium EDTA (chelating)
- Others → Sodium chloride (tonicity), Sodium phosphate (buffer), Polyvinyl alcohol (viscosity agent)
- Cleaner
- Alkyl sulphate surfactant (8.0%)
- Sodium chloride (tonicity)
- Sodium phosphate (buffer)
Types of RGP’s
Tetracurve (Maxim) & Aspheric (Quantum) (better for steeper corneas) (matches shape of cornea better so better for unusual corneas)
Tear lens?
Sometimes px can have tear lens and it isn’t a problem e.g. because it’s correcting astigmatism. E.g. OR may be plano but they have a tear lens and that might be correcting the astigmatism that px isn’t aware of, so don’t bother changing the lens even if steep fit in this case due to tear lens.
Materials:
Fluorosilicone acrylate RGP materials tend to be more wettable and less susceptible to protein
deposition than silicone acrylate materials.
Higher Dk RGP means increased deposition, more susceptible to scratches, thicker lenses and lower
modulus; lower modulus means reduced rigidity, and therefore leads to poorer correction of
astigmatism when using a spherical RGP on a toric cornea.
Although PMMA might still be used to manufacture rigid lens trial sets, they have little place in
modern contact lens practice because of their inadequate oxygen permeability (Dk effectively zero).
HVID, TD, Pupil size, VPA
- TD = HVID - 2
- BOZD = 1mm greater than max pupil size (ideally)
Why do we have edge lift of lens
tear exchange
K’s (manual)
- Difference in K’s is 0.15-0.25= then steepen(reduce) by 0.05
- Difference in K’s is 0.3-0.45, then steepen(reduce) by 0.10
- Difference 0.5 or greater= back toric/ bitoric/ smaller diameter RGP
POWER of RGP?
same as patient’s refractive error unless +/- 4.00D (BVD!)
Rules of thumb:
- Every +0.50mm increase in TD, increased BOZR by 0.05mm
- Every 0.05mm lens is flatter than corneal radius = -0.25D tear lens = +0.25 OR
- Every 0.05mm lens is steeper than corneal radius = +0.25D tear lens = -0.25 OR
Dynamic fit:
- Centration - optic zone alignment important!
- VPA - lit hitch or attached?, interpalpebral?
- Movement on blink - should be enough for tear exchange & debris removal but not too much that it causes discomfort from lid interaction & vision problems
- Coverage - should not cross at limbus otherwise 3 & 9 staining & other mechanical issues
What is Apical clearance
central vault over corneal apex & bearing toward the paracentral cornea causing some NaFl to build up underneath but not as excessively as a steep fit
Ordering RGP - What to put on the order form?
- Manufacturer
- Lens name
- Material
- Measurements - BOZR (write their K’s for bitoric as it’s being fitted directly to their corneal curvature), TD, Power
The most common rigid design
The most common rigid design used in this country is a tricurve back surface design. Bicurve designs are occasionally used with smaller designs (<8.40mm) while tetracurve designs are sometimes used with larger designs (>9.50mm).
Aspheric designs are often used with branded, stock designs (eg Quantum, Conflex) as opposed to
custom designs. The perceived advantages of aspheric designs are:
* Thinness
* Comfort due to smooth back surface
* Aspheric optics (but this also gives induced astigmatism on decentration).
Higher DK RGP means
increased deposition, more susceptible to scratches, thicker lenses and lower modulus, therefore, poorer correction of astigmatism
