1. Betzbone, Precise, and Fitbone comes in at least 2 sizes. Average sizes are usually between 10-13mm. Precise has a 8.5mm. If you only want 3-3.5cm, not sure why you're worried about lengthening along the anatomical axis if alignment issues are minimized at your desired distraction.
2. If you do tibia/fibula, you break 4 bones vs 2 femurs. Tibias can't tolerate the same amount of lengthening, and the achilles has proven to be especially problematic. Most people will only do one surgery in their lifetime, so my suggestion is always internal femurs unless you have freakishly long femurs vs tibias, or if you're able to fix a deformity and also lengthen your tibias at the same time.
Thanks I didn't know Precice had a nail that small and it looks reliable from the little research I've done. This is something I have to ask the doctors about though for sure.
Are alignment issues really minimized if lengthened 3-3.5cm? Dr. Paley and other doctors report that for every 1 centimeter lengthened, there's a lateral shift of the mechanical axis of 1mm. Honestly, I think 3 to 3.5mm on each leg is noticeable. When I wear shoe lifts, I notice there's a slight lateral shift as well, and it does not feel the same when walking barefoot. I actually feel more discomfort on the inner knee when I run or stay in the lifts for a long time.
Having already experienced the slight lateral shift with my shoe lifts, I can probably deal with the slightly noticeable discomfort, but I probably wouldn't be playing sports as intensely as I could. That's why if at all possible I would prefer minimizing any chance of a lateral shift. So a 3.5cm gain with an internal nail lengthened at an angle in alignment of the mechanical axis would be amazing for me. I would have to ask one of the doctors about this in due time.
For the tibia/fibula, I would again be very conservative in lengthening. I would probably do 3cm maximum. I am assuming that is a safe amount from what I have read on this forum so far. Actually I did a consultation with a doctor 2 years ago, and he recommended I do 6cm. Like you said, I want to avoid any complications with the Achilles tendon, so 3cm would be my maximum length.
I do have thoughts whether lengthening tibia/fibula 3cm comes with other subtle complications that would not even make it worth it to be 3cm taller in this segment. I wonder whether there are subtle permanent defects for people who have even done 1cm, 2cm , or 3cm. Does having an osteotomy in itself have an effect for the tibia/fibula? The femur bone I can trust that it will heal completely at only 3-3.5cm, but the lower legs seem to be a bit more delicate considering the smaller fibula and tibia bones, ankle, and feet are all connected. Does the fibula bone ever fully recover after osteotomy? I would imagine at 3cm, the fibula bone would fully consolidate, but would it still support your weight like it did before?