Aphakia and aspherics! Could you help?

**lazzap** · 05-04-2007, 10:55 PM

Hi,

I am a DO student and I would like to ask for your advice regarding a topic about aphakia. I have a case of a patient with aphakia for which i have to mention the best possible spec solutions. I have done my research and I have decided what to propose, however there is a thing that troubles me and for which I couldn't find a solution everywhere I looked. That is a difference in the prescription of 1.50DS between the two eyes (main portion). The position of the NVP below the optical centre of the main portion is not given. If I propose aspheric bifocals (e.g Essilor Omega - Orma 1.5 or the aspheric lenticular) how would any differential prism be treated on these bifocals? Is slab-off a valid technique for aspheric surfaces? What happens if aphakia is related with anisometropia in general? (I have to provide only spec solutions).
Sorry if my question is stupid (perhaps that is why I could find nothing in the books) but I would appreciate an answer even if that happens to be the case.

(very).

**lensgrinder** · 05-05-2007, 08:06 AM

Speaking strictly from a textbook point of view.

If this is not a bifocal then you really do not need a slab-off.

The NVP is about 10mm from the DVP so if this power difference is in the 90^th meridian then you could use a slab-off, but since the lens is aspheric you need to put it on the back.

**C-10** · 05-05-2007, 09:44 AM

The one think you have to consider if this person is wearing aphakia eyewear now and they do not have a slab off now I would be hesitate about doing one you may just open a hole new can of worms. Fill the new Rx as close to the old one if possible.

**lazzap** · 05-05-2007, 04:29 PM

Thank you very much for your kind reply,

Both advice is very useful to me and will definitely use.

You have been very helpful to me.

Thank you again.

**HarryChiling** · 05-05-2007, 04:40 PM

1.50 D of difference in power will only equate to a 1.5 D of prism at 10mm which is an acceptable level of prism, I wouldn't worry about it unless they did mention double images.

**lazzap** · 05-05-2007, 11:20 PM

Thnx HarryChiling for helping. I accept what you say and, indeed, the difference in the prescription is 1.5DS. However, according to the textbooks "... vertical differential prism > than 1 prism dioptre will not be tolerated by patients for long term use of the spectacles ..." ... the textbooks mention about the exceptions (people that can tolerate even more prism) but I will have to say something in that case ... my main problem was that I did not know how could this be handled in the case of aspheric surfaces (mainly aspheric bifocals) - if slab off or any other technique could be used to reduce the prism. Has it ever happened to u to have an aphakic patient being anisometropic as well and needing specs?

**Robert Martellaro** · 05-07-2007, 03:09 PM

Originally Posted by lazzap

Thnx HarryChiling for helping. I accept what you say and, indeed, the difference in the prescription is 1.5DS. However, according to the textbooks "... vertical differential prism > than 1 prism dioptre will not be tolerated by patients for long term use of the spectacles ..." ... the textbooks mention about the exceptions (people that can tolerate even more prism) but I will have to say something in that case ... my main problem was that I did not know how could this be handled in the case of aspheric surfaces (mainly aspheric bifocals) - if slab off or any other technique could be used to reduce the prism. Has it ever happened to u to have an aphakic patient being anisometropic as well and needing specs?

Hyper aspherics can be slabbed, although one could argue that 1.5^ is not enough aniso to warrant it. You could drop the distance OC level with the seg, which will decrease the VI at near, although VI at far may increase, and if doing so doesn't interfere with- Martin's rule: lower the distance OC 1mm per two degrees of panto, making sure that the optical axis of the lens passes through the eyes center of rotation. Failure to do so will result in power error and astigmatism.

Regards

**drk** · 05-07-2007, 05:15 PM

One diopter of "anisometropia" does not fit most accepted criteria (2.0 D spherical or 1.0 D of cylindrical). No slab off for you!

Secondly, I wouldn't split the prismatic imbalance (my regards, Robert) but I WOULD consider fitting the segment slightly higher by 1-2 mm (about 6-7 mm below pupil centre vs 8 mm below.)

1.) Probably specify distance MRP (along with vertex distance) after adjusting spectacles and knowing pantoscopic tilt (1 mm OC drop per 2 degrees pantoscopic tilt, like Robert mentions)

2.) Take a seg height and add 1 mm

Does this fly, opticians?

**HarryChiling** · 05-07-2007, 08:36 PM

Originally Posted by drk

Does this fly, opticians?

Not pretty, but it has wings. ou could also use different seg styles to decrease, but again not enough difference regardless of what the books says. You may get a patient or two that may be this sensitive, but 9 out of 10 you are OK. Slabs cost a good deal of money to just haphazardly throw at patients on a whim as well. Only use it when you know for sure you need it.