RGP Bitoric Prescription Reversed?

**jeffw** · 06-02-2005, 09:32 AM

Left eye:
Spectacle correction -1.50 sphere -2.00x140 cylinder
K's 43.25@150 44.5@60

so .75 D of residual astigmatism

Doc dispensed Boston XO RGP
7.70 -2.00, 7.63 -1.50 bitoric

Is this backward? Why less power for the steeper part of my cornea? I left the office with 20/30 vision and was told to let it settle down before retesting. Lens comfort is alright but vision has not improved. I have classic bowtie regular astigmatism.

Using the Mandell-Moore Bitoric Lens Guide I come up with 7.80 -1.50, 7.67 -3.0. Who's right?

Thanks for any comment... Jeff

**Stopper** · 06-02-2005, 10:31 AM

I would start with with a 7.8/-150 in the 150 meridian and a 7.67/-3.00 in the 060 meridian.

**OPTIDONN** · 06-02-2005, 10:48 AM

It looks like he fit the lens steeper than K and has to compensate for the plus lacrimal lens by adding extra minus. As for the cyl. power a rigid lens 'can' correct astigmatism up to 3.00 diopters without a toric design. So there may have been no need to have that much cyl. correction in the lens. It's not necessarry to analyze that much. Your doctor went to school and I am sure that he knows what he is doing.

I'm very tired so if any one notices an error I made above please be forgiving!:o

**jeffw** · 06-02-2005, 11:57 AM

My doc went with bitoric because of the -.75, (-2.00 spectacle - -1.25 corneal cylinder) on cylinder, "residual" astigmatism. He said this can't be corrected with a spherical lens. We tried one and got about 20/30 BCVA which isn't acceptable to me, I get an easy 20/20 with glasses in that eye.

As far as tear film effects from a steeper K, I agree this would mean more minus. But that is not what he prescribed, he went steeper with less minus.

**OPTIDONN** · 06-02-2005, 12:14 PM

Spectacle rx: -1.50 - 2.00 x 140

Contact rx: -2.00 -1.50

He did increase you sphere power. It was -1.50 and is now -2.00 if thats not increasing I don't know what it is.

Residual astigmatism is best corrected with a bitoric of front toric design because the curvature is in your crystaline lens not the cornea. (In your case you have both).

Just keep working with your doc I'm sure that something will end up working!

**OPTIDONN** · 06-02-2005, 12:15 PM

It seems that your doctor gave you quite a bit of info!

**jeffw** · 06-02-2005, 12:31 PM

On K correction would be:
-1.50 and -150 + -2.00 = -3.50

Cylinder is added to sphere when prescribing bitoric or back toric lenses.

As far as my doc giving me info, no..., I've got the rgp case from the lab, the internet, some curiosity, a little healthy self interest, a bit of sceptsism that others will always look out for me, and 20/30 vision with this lens.

**OPTIDONN** · 06-02-2005, 12:37 PM

You are talking about calculating total power I think. This just gives you the total power in 90 degrees away from the sphere. It is this way for everything.

If you are really interested in learning how to fit rigid lenses there are several books @ alibris.com They have them for pretty cheap. Just remeber rigid lenses are not that easy that is why we have DOCTORS fit them here.

**drk** · 06-02-2005, 12:51 PM

You are unusually astute.

On face value, the power in that 7.7 meridian should be -4.00.

Have a trip back to the Rx'ing OD, and an overrefraction should elucidate the problem.

Bitorics, especially obliques, can be easy to make simple math errors on, especially at 6:00 pm after a long day. No worries.

**jeffw** · 06-02-2005, 03:51 PM

I think 7.63 and 7.70 are too steep, (see my K's above which were taken with topography). But accepting these are correct for the minute, the plus they add would end up with something close to:
7.63 at -4.00 and 7.70 at -2.00

drk, do you mean -4.00 at 7.63?

Doesn't the small radius meridian sit on the steep part of my cornea? That's the whole crux of my question, I think my OD got the powers reversed. Is a refraction even possible with a lens that screwed up? He tried refracting but couldn't improve on 20/30 so sent me home and told me to let it settle down. That's when/why I started digging into this. I do have an appt next week... I just wanted some second opinions and confirmation of my limited understanding before I ask him about this.

**OPTIDONN** · 06-02-2005, 04:04 PM

Generally lenses are fit on the flattest K reading. But you may find this usefull. To find out the base curve of a lens take the K reading and divide it by 337.5. Or if you want the radius take 337.5 and divide it by the base curve.

**drk** · 06-03-2005, 12:31 PM

Well, it depends on where the lens is orienting, and that's the problem, here.

For the lens to sit in a stable fashion, there has to be enough corneal curvature difference for a "spoon on spoon" effect to occur, without misrotation.

Your cornea is about 2D toric, which is the usual threshold for a toric back surface, but the Dr. reduced the toricity on the back surface of the lens he ordered to about 1/2 D, if your post is correct. That will essentially perform as a spherical back surface, will not "lock in place", and will rotate on the eye. I don't know why this was done.

It's difficult to know which meridian he wanted to line up with which meridian, so there are really two "correct" answers. And neither are "practically" correct.

The solution:
1.) Go with a back surface sphere, if there is not inferior-nasal decentration or "harsh bearing" with a sphere, and prism ballast the front surface and put the cylinder on the front (front surface toric design). I wouldn't try this first.

2.) Much better would be to make a "saddle fit" bitoric design, where the back of the lens is made to fit your eye exactly. I think it would stabilize the lens greatly. Then, the powers would be straightforward as well. Really, saddle fitting is greatly simplified. As long as there is sufficient lens movement with the blink, it's the way to go.

**jeffw** · 06-03-2005, 02:58 PM

drk,
Thanks for the thoughful reply. You are correct, there is only about .5 D(.07mm) of toricity on the back of this lens. I think my OD requested this flattening of the steep meridean for tear exchange. That's what the Mandell-Moore guide I mention recommends anyway.

I know the lens rotates on my eye because my vision, especially close, is variable. What's really inexplicable is the lab put a dot on my -2.25X7.71 spherical right lens! I have blue eyes and can watch the dot rotate when I blink but its not telling me anything! Why didn't they put it on the left lens, then it would be of some rotational diagnostic benefit? I guess my OD did not request this. I'll ask that it be placed on my next lens.

I've tried the right lens in my left eye (yes I'm being careful, movement, centration, and comfort are very good, I can even watch the dot rotate when I blink). Vision is no better than 20/30. I'm 43 and my reading glasses don't help much with this, I think the residual astigmatism is what's causing the poor acuity.

BTW, the right lens gives me spectacular 20/15 vision so I'm very motivated to get the left lens correct and will pursue your sugestions.

**OPTIDONN** · 06-03-2005, 03:44 PM

Originally Posted by drk

Well, it depends on where the lens is orienting, and that's the problem, here.

For the lens to sit in a stable fashion, there has to be enough corneal curvature difference for a "spoon on spoon" effect to occur, without misrotation.

Your cornea is about 2D toric, which is the usual threshold for a toric back surface, but the Dr. reduced the toricity on the back surface of the lens he ordered to about 1/2 D, if your post is correct. That will essentially perform as a spherical back surface, will not "lock in place", and will rotate on the eye. I don't know why this was done.

It's difficult to know which meridian he wanted to line up with which meridian, so there are really two "correct" answers. And neither are "practically" correct.

The solution:
1.) Go with a back surface sphere, if there is not inferior-nasal decentration or "harsh bearing" with a sphere, and prism ballast the front surface and put the cylinder on the front (front surface toric design). I wouldn't try this first.

2.) Much better would be to make a "saddle fit" bitoric design, where the back of the lens is made to fit your eye exactly. I think it would stabilize the lens greatly. Then, the powers would be straightforward as well. Really, saddle fitting is greatly simplified. As long as there is sufficient lens movement with the blink, it's the way to go.

Hey that sums it up really well! thanks doc!

**drk** · 06-03-2005, 04:08 PM

The dot is only to distinguish L from R. I find it better to color code the lenses, when available:

bLue
gReen

Get it?

**jeffw** · 06-03-2005, 08:01 PM

Right, just in this case the dot could have done double duty, that is, distinguishing left from right and helping me let my doc know if the lens was rotating during wear. I know you can see rotation with your instruments, but patient input about rotation over longer periods of wear would seem valuable too.

Actually I would prefer clear lenses, the blue tint I have in my current RGP lenses is noticable when compared to my Accuvue 2 (right lens) and Encore Toric (left lens) soft contacts. The blue RGPs are not as "bright" as the soft lenses because of the tint, imho.

**jeffw** · 06-08-2005, 01:52 PM

Just got back from follow up appt. I asked my OD to explain the parameters on the bitoric lens case I got from the lab. (see attachment) He said the toricity was 1D. The -2.00 was the sphre power and the -1.50 was the cylinder.

Hmmmm. I told him the .07 difference was only .5D. He checked a chart and agreed. I asked that if it was true that the -1.50 was a cylinder parameter then what meridian was it applied to. He wouldn't/couldn't answer.

I gave him copy of the Mandel-Moore guide at:

http://www.rgpli.org/mandell.htm

I told him what I had calculated, (basically the same as what drk and Stoppper suggested) to which he replied "that would way overcorrect you".

He did say he'd look at it, but had to go because "patients were piling up and don't worry, we'll get something ordered". Overrefraction with the screwed up lens in didn't seem to help much, and I don't know how he'll come up with the new order.

He also said that because this is the third attempt the lab is going to start charging for any more attempts. So I have to pay for his mistakes? Doesn't the lab do a sanity check on an order from ODs? If they have the K's and spectacle correction this should be possible. Am I expecting too much?

I'm in a very small town and my OD options are very limitied.

**chip anderson** · 06-08-2005, 02:39 PM

If you O.D. or Optician or whomever you buy your lenses from was worth while, they would have checked the lens specifications out before they ever called you and told you it was ready. This is why we are entitled to a mark-up over cost.

Chip

**drk** · 06-08-2005, 02:43 PM

Hey, this is getting ugly. I'd go along with him for now, but the relationship is deteriorating. Maybe it's our fault, and I feel a little responsible.

Bottom line is that he's the one in control of the situation. If he wants to change the parameters, he will, if he doesn't, he won't. It may not work out to your satisfaction.

You may need to talk to him one-on-one when the meter's not running to see what you can do. It'd intimidate any Dr. to have a patient with so much information while you were still trying to get the optimum fit. Maybe you can reach some type of agreement.

I think I learned a lesson here, board.

**jeffw** · 06-08-2005, 03:29 PM

drk, I didn't tell him I had asked any other OD their opinion, just that I had done some research. If I were in in his shoes I would welcome patient input and learn from it when possible. I am always tactful, so no, the relationship is not deteriorating, imo.

Please don't stop giving advice, opinions, or sharing your experience.

I pay the bills writing software. I know 1/1000 of what their is about computer science, yet I consider myself in the top 5% of my profession. The wise man knows he's a fool, right?

Chip, I buy my glasses, soft contacts, and now RGPs from the OD who prescribed them. I am loyal to those loyal to me. What I'm asking is whether the lab does or should do a sanity check on what the OD is asking for to avoid mistakes up front.

I'm asserting that the parameters I've shown in the attachment are based on an optical cross calculation in which each meridian is thought of as a separate, spherical lens. If that's the case then what he prescribed makes no sense for a regular astigmatism patient with the K's and spectacle prescription I gave earlier and should raise some kind of red flag at the lab.

**chip anderson** · 06-08-2005, 07:53 PM

Lab's sanity check would depend on the relationship between the two. If they are on good terms they may question. Some doc's are such egomaniacs that any question of Rx impugnes the relationship and drives the account off.

Have worked in the lab where we had a sign in the locker room that said: Rule #1 "The doctor is always right!" Then is smaller print it said: "No matter what that fool said." Quite often the best policy is to fill the order as written without question. Of course if the doc is a nice guy, you try to make him look good by catching his mistakes before he gets egg on his face.

Can't criticize the lab people either way, unless the doc was one that just takes measurements and leaves all calculation to the lab. Then it's the lab's mistake but also the doc's for being too ignorant or lazy to catch same before it got to the patient.

Chip
"Mean ain't I?"

The responsiblity ultimately lies with the one who gets the money and hands you the product. If the lab makes a mistake, it should be caught before it gets to the patient, the possible exception being soft contact lenses where pre-dispensing inspection is not a practical application.

**OPTIDONN** · 06-08-2005, 08:30 PM

Hey Jeff thanks for the link to the Mandel-Moore guide. I didn't know that it was there.:cheers:

**Stopper** · 06-08-2005, 09:58 PM

Originally Posted by chip anderson

Lab's sanity check would depend on the relationship between the two. If they are on good terms they may question. Some doc's are such egomaniacs that any question of Rx impugnes the relationship and drives the account off.

Have worked in the lab where we had a sign in the locker room that said: Rule #1 "The doctor is always right!" Then is smaller print it said: "No matter what that fool said." Quite often the best policy is to fill the order as written without question. Of course if the doc is a nice guy, you try to make him look good by catching his mistakes before he gets egg on his face.

Can't criticize the lab people either way, unless the doc was one that just takes measurements and leaves all calculation to the lab. Then it's the lab's mistake but also the doc's for being too ignorant or lazy to catch same before it got to the patient.

Chip
"Mean ain't I?"

The responsiblity ultimately lies with the one who gets the money and hands you the product. If the lab makes a mistake, it should be caught before it gets to the patient, the possible exception being soft contact lenses where pre-dispensing inspection is not a practical application.

Chip,
Not mean at all. The truth just hurts sometimes.

Jeff,
The lab is not to blame. they are just making what the Dr ordered. The only way the can know what is going on is if they know your Ks and refraction. Some docs will just send that info to the lab and let the lab calculate the lens parameters but that is the lazy way out. Either way the lens shoud be verified before it is dispensed. Hopefully your doc will start over and figure it out. But he shouldn't be charging you extra.