Prism Reference + Oblique Astigmatism

[Oscar]

Just looking for some insight on PRP and induced prism. Let's say I have an Rx at -5.00 and my lens has a drop of 5mm to the PRP. That would be 0.5 x 5 = 2.5 base up induced prism at the pupil center. It seems adaptation isn't much of an issue if both eyes have relativley similar rx's and the induced prism is yoked.

However if the patient has oblique astigmatism, horizontal prism is also induced as you move up. I've had a lot of oddball adaptation issues and most of them are oblique.... could this be one of those factors? Is the only solution to find a product with 0 drop from the fitting cross?

[Lensman11]

The lab can put the oc anywhere that you request but will that solve your problem. If you put the oc at the fitting cross you will encounter more prism issues at the reading level.

[Robert Martellaro]

Oscar,

The in and out induced prism is not so much of a concern compared to up and down vertical imbalance (VI).

Most astigmatism, excluding corneal disorders and surgery, have mirror symmetries in their cylinder axes, where the axes sum to 180 or 90 degrees, minimizing VI. For example,

OD pl -2.00 x 60
OS pl -2.00 x 120

induces 0.00 VI at the PRP or reading depth.

When the axes do not mirror each other,

OD pl -2.00 x 60
OS pl -2.00 x 60

more than 1∆ VI is induced at a 12mm reading level and less than half that at a PRP with a 4mm drop. Likely tolerated. Separates might be best for higher degrees of VI.

Hope this helps,

Robert Martellaro

[Oscar]

Thank you everyone this makes a lot more sense. So will this actually be an issue for people who do not have symmetrical axes? What do we do in those cases?

[Robert Martellaro]

Oscar,

I suggested separates (SVNO), but there are many other possible solutions with varying configurations and tweaks. Try this search for more details. Hint: In most cases, if it's not broken (asymptomatic), don't fix it.

site:optiboard.com "multifocals" "anisometropia"

Best regards,

Robert Martellaro

[Prentice Pro 9000]

Oscar,

The in and out induced prism is not so much of a concern compared to up and down vertical imbalance (VI).

Most astigmatism, excluding corneal disorders and surgery, have mirror symmetries in their cylinder axes, where the axes sum to 180 or 90 degrees, minimizing VI. For example,

OD pl -2.00 x 60
OS pl -2.00 x 120

induces 0.00 VI at the PRP or reading depth.

When the axes do not mirror each other,

OD pl -2.00 x 60
OS pl -2.00 x 60

more than 1∆ VI is induced at a 12mm reading level and less than half that at a PRP with a 4mm drop. Likely tolerated. Separates might be best for higher degrees of VI.

Hope this helps,

Robert Martellaro

huh... learn something new everyday. I was always looking of cylinder causing VI, but never thought of whether or not they have symmetry between the two lenses. You just upped my game!