Quote from: KiloKAHN on April 08, 2015, 10:30:00 PMThere has to be some reason why many highly regarded limb lengthening surgeons prefer internal femurs to external femurs, though, despite the mechanical axis deviation issue present with internals. Even Drs Herzenberg and Paley won't do external femurs for cosmetic reasons, and they did the study about mechanical axis deviation with internal femur nailing: http://www.ncbi.nlm.nih.gov/pubmed/22933497
Thank you very much for that reference. That's a really good way of quantifying it. So for a 7.5 cm internal femur lengthening, you get a 7.5 mm deviation of the axis.
At the risk of further irritating those who think I talk too much, 
I'm going to speculate on the question you raise, because I think it's interesting. I think the reasons these reputable surgeons love internal femurs are the following:
1) They are fast
2) They are very reliable and give predictable results
3) Less infection
4) Less nonweightbearing time, which leads to less depression and atrophy
5) Few surgeries required in most cases
6) Less surgeon-dependent "maintenance" than an Ilizarov that requires regular adjustment and monitoring
The "cost" of having a slight misalignment of the mechanical axis is mostly paid by the patient, not the surgeon. The surgeon has already told you there is a risk of arthritis or joint aches, so it is not really his direct problem or liability if a mild genu valgum deformity leads to those mild complications for you over the next few years.
The surgeon's liability is more related to the acute recovery period - contractures, limb deformities, fractures, infections, etc. So from a surgeon's perspective, it makes sense to favor an approach with the most consistent results and fewest short term problems. Even from a patient perspective this may be ideal. Many patients will still probably be happy with internal femurs even if they get mild chronic knee pain because the surgery and recovery can go so relatively smooth, and they got their height without losing a limb.
But for those of us who really want to try to maintain our joint axes in normal alignment, I am thinking the most ideal available option may be tibial LATN. This approach involves lengthening based on an Ilizarov cage, which in the right hands should be able to let you lengthen as in my diagram above, without shifting the axes at all (though margin of error of Ilizarov is unknown to me at this time). Then with the nailing after the lengthening, you get the benefits of early weightbearing and less nonunion. Personally, I also think the long tibia looks better than the long femur, though I know this is subjective and not that important in the scheme of things.
Disadvantage of tibial LATN is primarily it's slower than femoral internals, there's still some total down time, and you have pin infections/risk. Also, I imagine it's more difficult for the surgeon, and requires more skill and monitoring to get a good result, as an Ilizarov can require periodic readjustments to maintain axis. You are at your surgeon's mercy with this approach to get it correct.
Issues with LATN I want to understand better would be: What is the margin of error with the Ilizarov distraction? ie. Can it reliably produce a straight vertical lengthening without significant axis/rotational deviations? Also how well do the talotibial, talofibular, and tibiofibular (ie. ankle) joints typically function after tibial LATN?
A femoral LATN would be another option but most seem to avoid Ilizarov cages on the upper leg due to how cumbersome they become and difficulties with moving around, wheelchairs, etc in them.
Then after a perfectly done tibial LATN, a tibial internal nail like Precise 2 or Guichet would provide potentially the next least amount of mechanical misalignment.
