How limb lengthening can correct (or screw up) your leg mechanics

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In trying to figure out why none of the surgeons seem to care about people's mechanics, I wanted to do some simple experiments to see how significant an axis shift the standard approach to leg lengthening provides.

I just did a preliminary test with a random x-ray I found online. I lengthened the femur along the anatomical axis of the femur (just straight along the length of it) and realigned the leg so the foot still ends up in the same place. The teal line is the "mechanical axis" of the leg.

I only lengthened the right (normal) leg for illustration purposes.

Before:



After:



Change:

In this case, the individual seems to have had an improvement in their mechanical axis to my eyes. The teal line now runs straight through the center of their knee where before it was slightly off towards the inside.

One can see either way there is a shift in the axis that does occur. Although it is not very large, this shift pushes the axis towards the outside of the knee. Thus if you have varus knees (bow legged), it will correct them. If you have valgus knees (knock kneed) it will make them worse.

So you should probably assess if you have a bit of varus or valgus to your knees before LL. If varus, then just going with the standard approach would be fine. But if valgus or perfectly aligned to begin with, the reverse planning method would be best to prevent your mechanics from getting worse.