Why LON with external fixators (femur) is bad?

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Hello all,
Sorry for my English.

I can't find any information about lon on femur.
But I always see messages "don't do that!"

Also, anyway I did LON on femur with monorail
Right now I have second month of distraction. 
I could walk without crutches after surgery (after 6 days)
In some cases I feel a crackling/clicking  ( i not sure how it correct), so I use crutches for support.
I can walk more then hour, I can stay without any problems.
I feel a slight pain in left leg in some cases, but it's about 0-3/10.
I don't use a painkillers (maybe is some times, about 1-2 times per week)
I can sleep/seat/etc with my monorail mechanism.
I don't feel any pain around pins. (Only a discomfort in some positions).
I did even distraction speed 2mm (3 days) and anyway it's not a problem. I have fast consolidation =/
Right now I increase ~4 cm, and still don't understand  why this is bad?

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Where did you do it ?
How many cms you gained ?

How are you overall
Any issues ??