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I find no reference to tillaux in my Taber's or the Index to ICD-9. Using Dogpile (metasearch engine), I find that it is an eponym describing an avulsion fracture in which "[t]he anterior tibio fibular ligament avulses piece of epiphysis resulting in a Salter-Harris type-III fracture of the anterolateral portion of the distal tibia."
(http://www.rcsed.ac.uk/fellows/lvanrensburg/classification/paediatric/p ankle/tillaux.htm)
So, I would use Fx of the tibia, distal end, 824.8 (closed) or 824.9 (open).