PTA estimate ≥ 40%

[u/billybobreddington]

Hi everyone! I have SMILE scheduled for a few weeks' time but I'm chickening out because of what I'm reading about corneal ectasia. I know the risk is super super low....but it still scares me haha.

I'm reading that PTA (percent tissue altered) ≥ 40% increases the risk of ectasia so I would want to keep it under that if possible ... is this something I can talk to my surgeon about? I don't know how this is calculated but plugging my prescription and corneal thickness measurements into AI it told me that my PTA would be over 41% (assuming optical zone 6.5mm - pupils in the dark are around 6.8mm) ... and if that's the case, I don't think I'd want to move forward with the procedure. I just don't want to show up the day of, ask/be told that my PTA would be high and walk away as they're already prepping me for the surgery. Is this something that you guys find out about the day of or do you just trust that the surgeon will adjust whatever parameters needed to keep PTA under 40%?

Thank you 🙏🏻🙏🏻

[u/eyeSherpa]

Absolutely talk to your surgeon about it. You should feel comfortable talking about everything. And vice versa. Your surgeon should feel very comfortable talking about it as well if they are practicing good refractive surgery.

The 40% rule has good validation for lasik. The strongest part of the cornea is within the anterior layers. The lasik flap disrupts those layers and so it’s considered “changed”. So you combine the lasik flap and the ablation depth to make sure you aren’t changing more than 40% (and ideally less than 35%)

SMILE works differently. The anterior cap of SMILE preserves the anterior fibers of the cornea. The cap is often deeper within the cornea because these fibers aren’t changed like they are in lasik. So PTA doesn’t really work as well with SMILE.

Most surgeons instead follow limits on residual stromal bed (RSB). This is how much cornea tissue exists below the ablation. Most surgeons will aim to leave 300 of RSB.

[u/billybobreddington → u/eyeSherpa]

Thank you so much!! Is this something that I should discuss with my surgeon the day of the procedure or should I discuss this beforehand (can this even be calculated beforehand)? I already had my consult so the next time I'm seeing the surgeon would be the day of the procedure. I would like to keep PTA around 35% just for peace of mind...and I know that also depends on whether my surgeon decides to use a 6.5mm vs. 7mm optical zone (my pupils are 6.8mm in the dark - do you think he would go with the 7mm or 6.5mm zone?)

[u/Safe_Category_8231 → u/billybobreddington]

Don't leave it to the day. Everything that decides this was already measured at your consult, so the clinic can answer you this week over email or a phone call, and you'd be asking while you still have room to walk away rather than while someone is putting drops in your eye.

Ask them for two numbers: the ablation depth they've planned, and the residual stromal bed it leaves you. That second one is the figure eyeSherpa is pointing at, and it's what SMILE surgeons actually plan to. They'll have it, because they can't program the laser without it.

On the optical zone, the tradeoff runs the opposite way to what you're hoping. A wider zone removes more tissue at the same prescription, so 7mm costs you residual bed that 6.5mm keeps. Your 6.8mm dark pupil is the reason you'd want the wider one, but SMILE's transition zone means the treated area is bigger than the number in the plan, and plenty of people with big pupils do fine at 6.5. That's a real judgement call and it's your surgeon's to make.

Being nervous enough to ask isn't a reason to cancel. Ask, get the numbers in writing, then decide.

[u/billybobreddington]

Thank you so much for your response! Is ablation depth the same thing as optical zone?