induced prism

**sumarez** · 03-08-2007, 09:56 AM

what exactly causes induced prism in a progreesive lens

**lensgrinder** · 03-08-2007, 11:10 AM

If you are referring to prism at the PRP (Prism Reference Point), there should be non unless there is prism thinning. If you are referring to prism at the DRP(Distance Reference Point), then this point is located about 8mm above the PRP and has prism due to the fact that this is not the "OC" of the lens.

**sumarez** · 03-08-2007, 11:49 AM

I'm speaking more of prism due to extreme situations of anisometropia. Have you had pts with similar problems I usually recomend a flat top or in extreme situations slab off.

**sumarez** · 03-08-2007, 11:56 AM

I"m speaking more of prism caused in cases of anesometropia. Usually I suggest a flat top bifocal or in extreme cases slab off.

**OPTIDONN** · 03-08-2007, 11:59 AM

Looking away from the optical center will cause prism in any lens. If the powers differ by more than a few diopters the wearer may not be able to fuse the two images. In the case of a multifocal the wearer has to look away from the optical center toward the bottom of the lens for reading, as a result presbyopes are more sensitive to this. As for slab off I'm pretty sure that only a Dr. can prescribe that.

**HarryChiling** · 03-08-2007, 12:36 PM

Originally Posted by OPTIDONN

As for slab off I'm pretty sure that only a Dr. can prescribe that.

Really?

**OPTIDONN** · 03-08-2007, 12:40 PM

Yeah. Thats seems to be the word on the street.

**Audiyoda** · 03-08-2007, 12:45 PM

Originally Posted by OPTIDONN

Yeah. Thats seems to be the word on the street.

Word on the street is wrong. Any optician worth his/her salt should specific a slab off when the Rx warrants. Professionally it's a courtesy to inform the prescribing OD/MD that you're doing it, but it's the optician's job to correct for near field imbalance.

**HarryChiling** · 03-08-2007, 12:48 PM

Originally Posted by Audiyoda

but it's the optician's job to correct for near field imbalance.

Whew, that's how I have been doing it. Thanks for the heads up.

**OPTIDONN** · 03-08-2007, 12:54 PM

Originally Posted by Audiyoda

Word on the street is wrong. Any optician worth his/her salt should specific a slab off when the Rx warrants. Professionally it's a courtesy to inform the prescribing OD/MD that you're doing it, but it's the optician's job to correct for near field imbalance.

I'm not so sure the word on the street is wrong. I feel that its well within the scope of the optician to correct for this. But I'm not so sure if this technically should be done with out a prescription.

**Audiyoda** · 03-08-2007, 12:54 PM

Originally Posted by HarryChiling

Whew, that's how I have been doing it. Thanks for the heads up.

That's how I'd been doing it as well - needed to check that ol' ABO reference when that came up but it's in there :bbg:

Honestly, 99 times out of 100 the prescribing doc will indicate a slab off. But if it's not there, I'll order it.

**HarryChiling** · 03-08-2007, 12:57 PM

In Donns defense, might his state be different.

**Audiyoda** · 03-08-2007, 01:00 PM

Originally Posted by OPTIDONN

I'm not so sure the word on the street is wrong. I feel that its well within the scope of the optician to correct for this. But I'm not so sure if this technically should be done with out a prescription.

There's nothing to prescribe. The prescribing doctor has given you a Rx:

OD +2.50 -1.50 x 180
OS -3.00 -1.50 x 90
+2.75 OU add

The opticians job is to fill this Rx with slab off (in this case OS) to correct for the imbalance vertically in the near field.

I think to often opticians are afraid to tread on the toes of the prescriber - but in this case slab off is indicated and should be ordered. Most prescriber's I know simply expect it to be ordered.

**Bezza** · 03-08-2007, 01:02 PM

Originally Posted by OPTIDONN

As for slab off I'm pretty sure that only a Dr. can prescribe that.

Hrm that is not the case here in the UK, I recommend a fair few slab offs for presbyopic anisometropes. From my experience i have found that although most pxs will get on ok with a flat top with slab off they generally prefer a slab off vari although they can be tricky to fit, the trick is adjusting the position of the slab line so as it offers the prism eqaulizing as high in the lens as possible without interfering with the patients distance vision, usually just below the lower pupil margin, just above lower limbus, i have found to be the sweet spot.
If you are considering anisometropia in single vision specs is to use an aspheric lens for the highest powered lens and a spherical lens for the other, this helps to negate the effects of unequal image sizes due to the difference in power.

**OPTIDONN** · 03-08-2007, 01:04 PM

I really don't know what the reality is. When I was looking stuff up for my Demystifying Slab Off article I came upon several references saying that it was up to the prescriber to determine if slab off were needed but it was up to the Optician to calculate it. Funny thing is the sister article I did on iseikonic lenses there was no mention that this needed to be done by prescription only. Funny how one may need to be done by prescription but not the other when both are results from the same refractive issue.

**OPTIDONN** · 03-08-2007, 01:06 PM

Originally Posted by Bezza

Hrm that is not the case here in the UK, I recommend a fair few slab offs for presbyopic anisometropes. From my experience i have found that although most pxs will get on ok with a flat top with slab off they generally prefer a slab off vari although they can be tricky to fit, the trick is adjusting the position of the slab line so as it offers the prism eqaulizing as high in the lens as possible without interfering with the patients distance vision, usually just below the lower pupil margin, just above lower limbus, i have found to be the sweet spot.
If you are considering anisometropia in single vision specs is to use an aspheric lens for the highest powered lens and a spherical lens for the other, this helps to negate the effects of unequal image sizes due to the difference in power.

Here's the big difference. In the UK Opticians are trained very, very well. In Illinois you just need to fill out a job application

**Bezza** · 03-08-2007, 01:08 PM

Originally Posted by OPTIDONN

Here's the big difference. In the UK Opticians are trained very, very well. In Illinois you just need to fill out a job application

yeah i just found out a couple weeks back that our ABDO course has been granted degree accreditation. So when i finish it ill have a BSc :)

**HarryChiling** · 03-08-2007, 01:09 PM

Donn, the issue may lie with the state and not having a definition for an optician. If the optometry law says you can't then you can't, but somehow I don't think they can tell you not to do it. Afterall it is a part of your job. And technically you are not prescribing anything, you are getting rid of an error that is induced by the prescription. Kinda like picking the correct base curve.

**OPTIDONN** · 03-08-2007, 01:10 PM

Originally Posted by Bezza

yeah i just found out a couple weeks back that our ABDO course has been granted degree accreditation. So when i finish it ill have a BSc :)

I LOVE the ABDO book store!! The shipping is a pretty penny but well worth it!

**OPTIDONN** · 03-08-2007, 01:14 PM

Originally Posted by HarryChiling

Donn, the issue may lie with the state and not having a definition for an optician. If the optometry law says you can't then you can't, but somehow I don't think they can tell you not to do it. Afterall it is a part of your job. And technically you are not prescribing anything, you are getting rid of an error that is induced by the prescription. Kinda like picking the correct base curve.

I agree I'm just saying what I read. For the life of me I wish I could remeber where I read this info. There are several factors here: 1) the book where I got the info may have been written in a state where slab off needs a prescription 2) the book may be very out dated 3) the person may have been full of crap. All I know is what I read but I do not know it to be fact. I feel that it is well within the range of the optician to compensate for any power errors.

**Uncle Fester** · 03-11-2007, 12:39 AM

Don't fit progressives to any patient with prescribed vertical prism unless you have in detail explained the risks, which are considerable, if there is any variation in the right and left power (be sure to calculate the "vertical" meridians power!). Otherwise the prescribed prism will only be correct at the lens manufacturers OC (the 180 line of the laser etchings).

Everything away from that is subject to Prentisses Rule of determining prism!

I reluctantly fit the insistant patient with progressives only after they understand that they will probably see double when reading if the reading prism is greater than 1 prism diopter beyond prescribed. Some patients can tolerate up to 3 prism diopters of vertical OK but I cover my behind with 1! I will redo the lenses in a bifocal at "cost" with slab off. I have gained many faithful patients who were fit by opticians who are ignorant of my first sentence.

**EyeFitWell** · 03-11-2007, 01:01 PM

Here in NC, I know it's legal for the optician to add the slab-off, and also to present the numerous (faster and cheaper) alternatives. I do think it's important to notify the doctor so it can be added to the pt's records. You never know when VA of that eye could be so bad it doesn't matter, or some other reason it's not necessary.

**tmorse** · 03-11-2007, 03:40 PM

[quote=EyeFitWell;180782]Here in NC, I know it's legal for the optician to add the slab-off,quote]

If State law required a Dr's Rx for slab-off, and an OD/MD failed to put it in the Rx, and the optician put it in for the Dr., wouldn't that Dr be grateful that the optician covered up his/her negligence?

**harry a saake** · 03-11-2007, 07:24 PM

this also goes along with using a distometer, if an od prescribes -15.00 and the frame is sitting 12 mm out and the refraction was done at 8 mm, would you give them -15.00 and expect them to see correct?. i,m afraid too many opticians do not understand what they are to do with a prescription once they recieve it as oppposed to just typing it into a computer.

**HarryChiling** · 03-12-2007, 10:43 AM

Originally Posted by harry a saake

this also goes along with using a distometer, if an od prescribes -15.00 and the frame is sitting 12 mm out and the refraction was done at 8 mm, would you give them -15.00 and expect them to see correct?. i,m afraid too many opticians do not understand what they are to do with a prescription once they recieve it as oppposed to just typing it into a computer.

You are right most opticians don't understand what to do with Rx's although I don't think this is the case with Donn, the issues in this particular thread as well as with the more educated opticians out there is:

What are we allowed to do?

It varies state by state and I have often seen comments posted about opticians having to watch themselves (refering to what they do or are allowed to do). I am from the school of thought where I do what ever needs to be done for the patient to see correctly and screw any one who thinks I shouldn't do it (with exception of course). If it relates to the lens thats my domain and in my opinion all opticians.