aarn

This is defintely an issue with stays and one of the reasons that Arcteryx packs never fit me right. Without a local outfitter that knew what they were doing I was wary of doing it myself and wrecking the stays. Arcteryx.com does have instructions on stay bending here:

http://arcteryx.com/pack-fit.aspx

Have you considered speaking with Dan Mchale (www.mchalepacks.com)? His products are not cheap but are fully custom with pre-fitting (by mail), etc, that may preclude you from needing a front pack. Instead – getting a pack that fits your specific body to begin with.

However, I am not a doctor nor specialist in your condition but it may be worth some time to contact Dan.

Isaac-
Almost regardless of the pack you get, you'll end up hunched against it to some degree. Reason? When you stick any kind of weight behind you it shifts your center of gravity. Alternatives: Keep your pack as light as possible… if anyone was ever a candidate for a UL/SUL set up, it's you. Go for a base weight of ten pounds or so, shouldn't be too hard especially if you're traveling with someone and can split shelter and cooking weights. When you pack whatever weight you have, make sure that the heaviest and densest things are as close to your back as possible. Stack things higher in the pack, rather than shoving them all into a short fat package. Lastly, I could be off base here, but I've worn some particularly rigid frames that are so good at transferring weight to my hips that all my shoulders did was stabilize the pack a little… and if felt like I could stand a little more upright. The Osprey Exos was one of these, and another was actually an external frame pack. My guess is I still leaned forward a bit, hard to escape the laws of physics. But worth considering.

Also, I wouldn't spend too much time worrying about bending frame stays. It's very hard to do effectively, and much of the time you'd be better served by getting a pack that fits you better from the get-go. Another plug for the Exos, it has a trampoline-style mesh back that would conform to the shape of pretty much any back, negating the need to shape the pack to you.

"When you stick any kind of weight behind you it shifts your center of gravity"
That is exactly why the Aarn Bodypacks are different…
http://aarnpacks.com/principles/index.html
By the look of those pockets , I guess that I still had at least 2-3 L of water in there, so around 25 lbs all up.

Franco

Aarn's website has a video that shows you how to adjust the waist belt, stays, and everything else. It's easy to get a good fit.

thanks everybody

Whoops…

Didn't realize a chest pack was standard issue with Aarn packs. That definitely helps center of gravity.

Without knowing anything about your particular pathology and/or diagnosis – regardless of the pack you use, I would offer the general advice to minimize your pack weight, and keep your back as strong as possible.

Thankfully, the type of spondylolisthesis I was diagnosed with 30 yrs ago hasn't gotten any worse, and I probably wouldn't have any problems at all if I could lose this counterweight I've acquired on the opposing side ….. ;^)

Isaac,
Having already acquired a rare form of on spond., I broke my back at L-2 a little over ten years ago. Was really lucky to be able to backpack again. My solution has been the stretching and strengthening exercises I took away from physical therapy, and using a pack which is bowed so only suspended mesh touches my back (except at the hip belt and top of shoulder straps). My pack is home made, but several Osprey models have this feature. Already mentioned were packing light and so as to move the center of gravity forward as much as possible. Good luck and good hiking.
Sam, Chocorua NH

Isaac,
I think an Aarn pack would be good for lots of people with back problems.
It takes a bit of time to fit an Aarn pack, but it is not that difficult. If you get in trouble I am sure Franco or myself would answer questions to get it sorted. I think Aarn would too.