Full backside progressive and Rx prism ..... issues?

**Trip** · 11-20-2015, 01:08 PM

Has anyone experienced any challenges with prescribed prism and full backside designs? This is a generic question vs a specific pt. issue.

thanks,

**Tallboy** · 11-20-2015, 01:19 PM

No actually they are my go to lenses in general.

**Robert Martellaro** · 11-20-2015, 02:04 PM

Originally Posted by Trip

Has anyone experienced any challenges with prescribed prism and full backside designs? This is a generic question vs a specific pt. issue.

thanks,

Yes, primarily with high values of prism, roughly over 8∆ total. The problem does not seem to be related to how the eye turns towards the prism apex requiring modification of the fitting point position.

**Tallboy** · 11-20-2015, 02:28 PM

Originally Posted by Robert Martellaro

Yes, primarily with high values of prism, roughly over 8∆ total. The problem does not seem to be related to how the eye turns towards the prism apex requiring modification of the fitting point position.

Robert do you think that this is a backside design related phenomena or is it just related to progressives with large amounts of prescribed prism? I have obvioulsy had challenges, but my success rate is pretty darn good.

**Robert Martellaro** · 11-20-2015, 02:58 PM

Originally Posted by Tallboy

Robert do you think that this is a backside design related phenomena or is it just related to progressives with large amounts of prescribed prism? I have obvioulsy had challenges, but my success rate is pretty darn good.

FBS for sure, haven't tried the hybrids. I use semi-finished for high values of prism, sometimes switching to segmented multifocals.

**Dr. Bill Stacy** · 11-20-2015, 04:57 PM

I don't encounter much prescribed prism in free form lenses, but one hell of a lot of UNPRESCRIBED prism (equithin). I think some labs even try to combine prescribed and thinning prism.

**mshimp** · 11-21-2015, 10:16 AM

Originally Posted by Dr. Bill Stacy

I don't encounter much prescribed prism in free form lenses, but one hell of a lot of UNPRESCRIBED prism (equithin). I think some labs even try to combine prescribed and thinning prism.

This is the norm for systems to combine prescribed and prism thinning together. Programs can be configured to reduce the amount of prism thinning and also the max. amount of prism thinning used.

**Dr. Bill Stacy** · 11-21-2015, 10:45 AM

Originally Posted by mshimp

This is the norm for systems to combine prescribed and prism thinning together. Programs can be configured to reduce the amount of prism thinning and also the max. amount of prism thinning used.

I'm glad to hear that; now if I can just get eyefinity to put a max amount of thinning prism field on their order form I'd be in heaven. But the first part of your answer raises a question. Consider a bilateral Rx of +2 sph +2 add, but with 2^base up on the R and 2^ base down on the L. How much and which direction would the equithin be for each eye?