Shorter the radius creates ↑ diameter
Steeper the curve creates ↑ saggittal depth
Stronger the power because of the PLUS lacrimal lens created
Tighter the fit distorts BEFORE the blink
A "steeper than K" fit
Key Concepts
Refractive Power of Cornea = 1.376
Refractive Power of Keratometer = 1.3375
Vertex Distance = Diopter/1-(mm-Diopter)
Reference Point: 7.50 mm = 45.00
31.0D flatter cornea 61.0D steeper cornea
Rule Of Thumb: When the power ↑ by .50D the curve ↓ by .10mm
Find mm: 337.5/Diopter ....................... Find Diopter: 337.5/mm
To find new power mm X .3775
Key Concepts
To extend the range of a Keratometer
Flat Cornea down to 31D use a -1.00 trial lens
Steeper Cornea up to 61D use a +1.25 trial lens
Longer the radius creates ↓ diameter
Shorter the curve created ↓ saggittal depth
Weaker the power because of the MINUS lacrimal lens created
Looser the fit distorts AFTER the blink
A "flatter than K" fit
Key Concepts
CARSA
CA (corneal stig) + RA (risdual astig) = SA (spectale astig)
Rigid Fit Rule
if CA is more than +3D order a back toric surface
if RA is more than +.50D order a front toric surface
if BOTH CA is more than +3D and RA is more than .50D order BITOIC rigid lens
Key Concepts
Soft Contact 1/3 Rule
Transpose if neccessary (All CL RX are in negative CYL)
if astig is ≤ .50D use SPH EQ, (compensate for VerDist if needed)
if astig is ≥ 1.50D use toric lens, (compensate for VerDist if needed)
if astig is between .50D and 1.50D divide SPH/CYL
if ≥ 3 use SPH EQ, (compensate for VerDist if needed)
if ≤ 3 use toric lens, (compensate for VerDist if needed)
spherical equivalent = SPH EQ ( sphere + 1/2 cyl)
vertex distance = VerDist
WTR - With the Rule = 30° of x90
ATR - Against the Rule = 30° of x180
Oblique - within x30 - x60 or x120 - x150
Key Concepts
Types of Bifocal Contacts Lenses
Annular aka Aspheric
Segmented aka Translating
Concentric
Key Concepts
Tear Index of Refraction = approx. 1.377
TBUT Test - Tear Break-up Time, tests the QUALITY of tears
Schirmer test - tests the QUANITY of tears with a strip of paper in 5min with a good tear collection of 15mm.
Key Concepts
Total Diamer of a Bicurve Lens
Base Curve + 2X peripheral curve
Example BC = 8.5 Peripheral curve = 1.2
total Diameter = 10.9
Key Concepts
ANSI = American National Standard Institute
RGP............................ SOFT
DIAMETER ........................................±.05mm......................±.25mm
BASE CURVE....................................±.05mm.......................±.20mm
CENTER THICKNESS.....................±.02mm .....................±.03mm
0.00-5.0D .............................................±.12D...........................±.25D
5.12D-10D............................................±.18D...........................±.25D
Key Concepts
FIT EVAUATIONS
Proper Fit
Keratometry- mires not distorted before or after a blink
Blinking Indicator - dips briefly and stabilizes right back
Steep Fit - Too Tight
Keratometry - mires distorted before blink, clear temporarily, then distort again
Blinking Indicator - barely budges across the blink cycle
Flat Fit - Too Loose
Keratometry - mires intially clear, distort after blink, and clear again
Blinking Indicator - jumps after blink, takes time to resettle altering vision
Key Concepts
VERTEX DISTANCE - Calculate if RX is ± 4.0D
Diopter / 1 - (mm - Diopter)
Rule of Thumb 10-10-1 or 10-5- .50
The effective power of a 10D lense is moved a distance of 10mm will change by approximately 1D. Or the effective power of a 10D lense is moved a distance of 5mm will change approximately .50D.
Remember: A minus lens gains power moved closer to the eye.
Remember: A plus lense looses power moved closer to the eye.
Key Concepts