Fitting Support > Frequently Asked Questions
Frequently Asked Questions
How much does the base curve (BC), overall diameter (OAD), prism or segment height have to change in order to be effective?
To make a significant fitting difference, the base curve should be changed a minimum of 0.50D or 0.05mm and the power adjusted accordingly. OAD should be changed a minimum of 0.3 mm. A general rule of thumb is a 0.5mm change in OAD results in a 0.25D diopter change in sagittal depth. Prism should be changed a minimum of 0.50D . Segment height should be raised or lowered a minimum of 0.3 mm.
Which solution system should be used for plasma treated lenses?
Plasma treated lenses should only be cleaned with non-abrasive cleaning systems such as Menicare, Unique pH, Optimum, or Boston Simplus.
How often should GP lenses be replaced?
Lenses should be replaced whenever the contact lens professional determines the current lenses no longer provide acceptable acuity and/or a comfortable, healthy fit. A patient's current lens can often be sent to the lab for reasonable power modification as well as a clean and polish.
Which soft lens works well for a piggyback system?
A plus power silicone hydrogel lens usually provides the best centration of a GP lens in a piggyback system.
What’s the difference between front and back vertex power?
Front vertex power is measured with the concave side up (cup side up toward the viewer). Back vertex power is measured with the convex side up (cup side down toward the eyepiece). Higher powered plus lenses will measure significantly different depending on the method used.
Are the GP lenses warranted for breakage?
Breakage warranties depend on the particular lens material. Lens pieces must be returned for breakage credit. Please contact a customer service representative for details.
Which topography maps should be used to design lenses?
The axial map and numeric/keratometric maps are preferred if they are available. Absolute and normalized versions of corneal maps will also provide the right information.
Can segmented multifocals be made in a toric design?
Yes, the Solitaire Thin, Llevations Thin, and Triune are all available as back, front, and bitoric designs.
What’s the difference between the Triune and Llevations Thin?
Triune is a trifocal that has an aspheric intermediate segment. Llevations Thin is a true trifocal with three distinct spherical powers.
Can the Prefix II be made with more add power?
If there is an acceptable fit that also results in good distance vision, additional add power can be placed on the front surface in 0.25D increments.