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[chsn]

Can someone please explain what it means when doctors refer to lengthening over the anatomical axis and mechanical axis, I havent fully understand these two terms.
What I understand is that lengthening along the anatomic axis with internal nails causes malalignment(femurs), which may lead to arthritis in the knee in the future and also the gain in height won`t correspond to gain in distraction. However lengthening along the mechanic axis will not lead to malalignment...So what do these 2 terms mean? And which nail lengthens in which axis? G-nail, precise2, fitbone? 

[JConnor]

This image visually explains the two terms quite well.  As you can see, if your mechanical axis naturally goes through the middle of your knee before lengthening, lengthening along the anatomical axis will shift the mechanical axis toward the outside of your knee at approximately 1 mm per 1 cm lengthened. Since force would now be applied unevenly across your knee there is increased concern for arthritis over time. 

Keep in mind that one's mechanical axis may not naturally go through the center of the knee.  It would be more concerning if it went through the outside part of the knee initially as lengthening would push it outward even farther. In contrast, if it deviated 3 mm toward the inside of the knee initially, lengthening 6 cm would thus make it deviate 3 mm to the outside of the knee and not raise any concern.

All of the doctors I've seen who use G-nail and precice nails lengthen along the anatomical axis.  These are some of the best doctors in the world, and it doesn't seem like they think creating mechanical axis deviation of 1 mm per 1 cm of lengthening would cause any significant problems for people with normal legs to begin with.  It looks like doctors who use fitbone may lengthen along either axis.

[chsn]

Tnx for the answer Jconner now I understand perfectly^^