Fitting Guides > CAD Thin Lens
Fitting Guides
CAD Thin Lens
Fitting Guide
Patient acceptance of rigid contact lenses has dramatically improved in recent years. This has been made possible through the simultaneous merging of three technologies which include:
- Modern computer controlled manufacturing techniques.
- Advances is GP lens material and
- refinements in GP lens design
The new CAD Lens, by Valley Contax, is simply the product of these three evolutionary advances. The design and fitting techniques represent years of collaborative research between Valley Contax and Patrick J. Caroline and Mark P. Andre at the Oregon Health & Sciences University in Portland, Oregon. The unique fitting system simplifies the lens design process and optimizes patient comfort with GP lenses.
Advanced GP Lens Manufacturing
The story of the CAD Lens begins with Valley Contax's ongoing investment
in modern, state-of-the-art lens manufacturing equipment. Valley's computer
controlled manufacturing process, has resulted in parameter consistence never
before possible. Today, it is possible to produce lenses in which base curve
accuracy, edge contour, peripheral lens design and lens thickness are
computer controlled to levels measured in microns. This consistency allows
practitioners to focus their efforts on the fit of the lens rather than the
quality of each lens parameter.
Advances in GP Material
A collaborative venture between Paragon Vision Sciences and the National Aeronautics
and Space Administration (NASA) has resulted in a revolutionary polymer process
called HDS™ - Hyper purified Delivery System. Through this process, a new generation
GP plastic has emerged called Paragon HDS™. This material incorporated the physiological
benefits of a mid-range 58 Dk materials. The Paragon HDS™ material was chosen for the CAD
Lens due to it's enhanced surface properties and it's ability to maintain rigidity and stability
in the thin lens design
Refinements in GP Lens Design
The quest to improve initial and long term comfort of GP lenses has resulted in refinements
throughout every parameter of the lens. In March of 1995 the Casey Eye Institute at the Oregon Health &
Sciences University in conjunction with Valley Contax began a series of controlled studies to determine
the design factors most responsible for GP comfort. A single lens parameter - lens thickness - continually
emerged as one of the primary comfort factors. The studies clearly showed showed that minimizing lens thickness,
while maintaining optimum peripheral lens design and edge contour, resulted in comfort levels far superior
to traditional lens designs.
The CAD Lens
The collaborative effort between the Casey Eye Institute and Valley Contax has resulted in the advent of a
revolutionary HP lens called the CAD Lens. The lens brings together the state-of-the-art production techniques
of Valley Contax, the superior clinical performance of the Paragon HDS™ material and clinically engineered
lens parameters to optimize initial and long term patient comfort.
One of the primary goals in developing the CAD Lens was to dramatically simplify the fitting procedure. To this end,
we designed the lens with controlled parameters (i.e., posterior optical zone diameter, peripheral lens design and
thickness), thereby baking base curve, power and diameter the only considerations for the practitioner. In conjunction
with the design we have developed a simplified three step fitting guide and a unique corneal diameter measuring gauge
which together have resulted in a greater than 80% first fit success rate.
The CAD Lens is available exclusively from Valley Contax in a wide range of parameters:
Base Curves: Any base curve radius
Powers: +20.00 to -20.00
Diameters: 8.5 mm, 9.0 mm, 9.5 mm
We feel that the comfort and ease of fit of the CAD Lens will make this product the benchmark for modern GP lens designs.
| Fitting Guide | |
| Step One - Lens Diameter Selection | |
| 1. | Measure the Horizontal Visible Iris Diameter (HVID) of the cornea. |
| 2. | Select the lens diameter 2.5 mm smaller than the example. |
Example: |
HVID = 11.5 mm |
| Diameter Rule - 2.5 mm | |
| Overall Lens Diameter 9.0 mm |
| 3. | Measure the central corneal curvature and identify the Flat "K". |
HVID |
Lens Diameter |
| Less than 11.3 mm |
8.5 mm - 11.0 - 10.5 |
| 11.3 to 11.7 mm |
9.0 mm - 11.5 - 12 |
| Greater than 11.7 | 9.5 mm - 12.5 or larger |
| Step Two - Base Curve Selection | |
| 1. | Measure the central corneal curvature and identify the Flat "K". |
Example: |
"K" = 43.00 @ 180 / 44.75 @ 90 |
| Flat "K" = 43.00 D |
| 2. | Calculate the corneal astigmatism ( difference between the flat and steep "K". |
Example: |
"K's" = 43.00 @ 180 / 44.75 @ 90 |
| Corneal Astigmatism = 1.75 D |
| 3. | Select the Base Curve based on Flat "K" and the amount of Corneal Cylinder. |
Example: |
Flat "K" = 43.00 D |
| Corneal Astigmatism = 1.75 D | |
| Lens Diameter = 9.0 mm | |
| Base Curve, .05 steeper than | |
| "K" = 43.50 D |
| Base Curve Selection Nomogram |
| 9.0 mm Lens Diameter |
Corneal Cylinder |
Base Curve Radius |
| 0.00 to 1.00 |
On Flat "K" |
| 1.12 to 1.50 |
0.25 D steeper than Flat "K" |
| 1.62 to 2.00 | 0.50 D steeper than Flat "K" |
| 2.12 to 2.50 | 0.75 steeper than Flat "K" |
| Tip: |
| For smaller lens diameters (8.5 mm) select a base curve 0.25 D steeper than the 9.0 mm nomogram. |
| For larger lens diameter (9.5 mm) select a base curve 0.25 flatter than the 9.0 mm nomogram. |
| Step Three - Power Calculation | |
| 1. | Perform a spectacle refraction and place prescription in minus cylinder form. |
Example: |
Rx Plus cylinder = -7.75 +1.75 x 90 |
| Rx Minus cylinder = -6.00 -1.25 x 180 |
| Tip: |
| If fitting steeper than Flat "K", add minus power to the lens. "SAM" (Steeper Add Minus) |
| If fitting flatter than flat "K", add plus power by the same amount as the tear lens power. "FAP" (Flatter Add Plus) |
| 2. | If the spherical component of the spectacle refraction is greater than + or - 4.00 D, correct for vertex distance (see vertex chart) |
Example: |
-6.00 D spectacle power = -5.50 at the corneal plane |
| 3. | Determine tear lens power by calculating the difference between the basic curve and the Flat "K". |
Example: |
Base Curve = 43.50 D |
| Flat "K" = 43.00 D | |
| Tear lens power = 0.50 D |
| 4. | Combine the spherical component of the spectacle refraction (corrected for vertex distance if appropriate) with the tear lens power to obtain the final contact lens power. |
Vertexed spectacle refraction |
= -5.50 D |
Base Curve (see chart) |
= -43.50 D |
Flat "K" |
= -43.00 D |
Tear lens power (FAP) |
= 0.50 D |
Final lens power |
= -6.00 |
Steve
President
"Valley Contax feels proud to be one of the select laboratories who have earned this prestigious award..."
"...as a reflection of the importance we place on quality."
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