A/B. Overall/Optical Zone Diameter
Dependent Upon:
*Average OAD/OZD = 9.2/7.8mm
Corneal Cylinder
FitPl – 0.50D 0.50D – 0.75D Flat
0.75D – 1.00D 0.25D – 0.50D Flat
1.25D – 1.50D On "K" – 0.25D Flat
1.75D – 2.00D 0.25D Steep
2.25D – 2.50D 0.50D Steep
2.75D – 3.00D 0.50D – 0.75D Steep
As a result of the more anterior center of gravity of plus lenses, hyperopes should be fitted 0.25D – 0.50D steeper than the BCR’s recommended for myopic patients.
•remove debris/wastes
•prevent adhesion phenomenon
•Ensure adequate tear pump
•Debris removal
•Adequate Fit
•Eliminate Bearing
•Edge Clearance – the actual distance from the lens edge to the cornea; it is dependent upon:
Edge Lift – geometrical calculated values from lens edge to cornea; it is most often used as it is more easily determined. It approximates edge clearance although typically greater.
Axial Edge Lift – vertical distance from lens edge to an extension of the BCR
Radial Edge Lift (rarely used) – distance from lens edge perpendicular to an extension of the BCR
•Excessive edge lift/clearance acts in a funnel-like manner – drying out the surrounding tear pool
•"Lid Gap" may further exaggerate process
•Steepening PC’s
•Decreasing Bevel Width
•Increase # of Curves (width unchanged)/aspheric
•Recommended peripheral curve close to 11mm (not 12 – 12.25mm as with PMMA)
•Recommended peripheral curve width = .2-.3mm (not .4-.6 as with PMMA)
•Recommended several peripheral curves (not one)
Tetracurve Design (8.8mm + OAD)
SCR = BCR + 0.8mm/0.3mm
ICR = SCR + 1.0mm/0.2mm
PCR = ICR + 1.4mm/0.2mm
Example:
BCR = 8.01mm
SCR/W = 8.80mm/0.3mm
ICR/W = 9.80mm/0.2mm
PCR/W = 11.20mm/0.2mm
Tricurve Design (8.80mm OAD)
SCR = BCR + 1.0mm/0.3mm
PCR = SCR + 2.0mm/0.3mm
Example:
BCR = 8.01mm
SCR/W = 9.00mm/0.3mm
PCR/W = 11.0mm/0.3mm
•Can be light, medium and high
•RGP lenses should be blended (medium preferred) to:
Power(D)
Dk Value-1.00 .18 .20
-2.00 .16 .18
-3.00 .14 .16
-4.00 .14 .16
-5.00 .13 .15
-6.00+ .12 .14
One – Increase center thickness .02mm for High Dk materials
Two – Increase center thickness .02 for each diopter of corneal astigmatism
Center Thickness – Effect on Equivalent Oxygen Percentage
(Hill) – Increasing the center thickness by .04mm will increase mass by 24%, but will only decrease EOP by less than 1%
Parameter
Minus Power Plus PowerBCR Flatter than "K" Steeper than "K"
OAD Smaller (8.8-9.6mm) Larger (9.2-9.8mm)
CT Lesser (<0.20mm) Greater (>0.20mm)
Edge Design Thicker = +lenticular Thinner = -lenticular
For all (>-6D) necessary for all powers
powers; - lenticular
with low (-1.50D)
powers
Fitting Pearls
•Remember: the cornea gradually flattens from center to periphery; the base curve does not
To maintain the same lens-to-cornea fitting relationship:
•Flatten base curve 0.25D for every 0.5mm increase in optical zone
•Steepen base curve 0.25D for every 0.5mm decrease in optical zone
•Change OAD/OZD by a minimum of 0.3mm
•Change BCR by a minimum of 0.50D
•Change CT by a minimum of .03mm
•Change PC radius by a minimum of 1.0mm
•Change PC width by a minimum of 0.2mm
Dx Group = 77.8% within 5 weeks
Empirical = 38.1% within 5 weeks
Dx Group = 9 lenses
Empirical = 20 lenses (BCR in 12 cases – usually to a flatter BCR)
PMMA Vs. RGP
OAD 8.4-9.2 8.8-9.8
BCR Steep Flat
PC’s Flat/Wide Steep
CT Thin Thick
Edge Consistent Inconsistent
a-b. Low Dk, High Dk, 20 lenses
BCR – 40.75D-45.50D
OAD/OZD – 9.2/7.8mm
Tricurve
Three Rules:
One: 2 most important factors are vertex distance and tear layer power (LLP)
Two: SAM (steep add minus) and FAP (flat add plus)
Three: Remember, unless significant residual astigmatism, all tear lens power calculations pertain to "K" and refractive sphere value
Example One:
If flat keratometry reading is 42.00D and base curve radius is 42.50D = add –0.50D
Example Two: (attached)
Example Three:
Rx = -4.50 –1.00 x 180
Keratometry = 44.00 @ 180, 45.00 @ 090
Dx CL = 7.71mm (43.75D)
Predicted CL Power = Vertexed Sphere (-4.25D)
+(Compensated) Tear Lens Power (+0.25D)
= -4.00D
Example Four:
Keratometry: 42.00 @ 180; 43.25 @ 090
Diffraction: -3.00 – 1.25 x 180
Dx Lens: BCR = 8.0mm(41.75D); -3.00D
Predicted Over Refraction: +0.25D
Typical Lens Order:
OAD = 9.2mm
OZD = 7.8mm
BCR = 8.08mm
SCR/W = 8.9mm/0.3mm
ICR/W = 9.9mm/0.2mm
PCR/W = 11.3mm/0.2mm
RX = -2.75D
CT = .14mm
Blend = Medium
Tint = Blue
Special Instructions: Dot OD
Material: Fluoroperm 30
Examples Five and Six: UM-St. Louis Fits (attached)