This study seems interesting:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5376249/
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1.1.1. There are basically two definitions of LLD
True leg length discrepancy (TLLD) is defined as the anatomical difference between the lengths of the two limbs between the proximal edge of the femoral head to the distal edge of the tibia which can be congenital or acquired. Congenital conditions include mild developmental abnormalities found at birth or childhood, whereas acquired conditions include trauma, fractures, orthopedic degenerative diseases and surgical disorders such as joint replacement. A systemic review evaluating the prevalence of LLD by radiographic measurements revealed that 90% of the normal population had some type of variance in bony leg length, with 20% exhibiting a difference of >9 mm.
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Functional leg length discrepancy (FLLD) is defined as a condition of asymmetrical leg length, not necessarily a result or compensation of a true bony length difference. FLLD may be caused by an alteration of lower limb mechanics, such as joint contracture, static or dynamic mechanical axis malalignment, muscle weakness or shortening. It is impossible to detect these faulty mechanics using a non-functional evaluation, such as radiography. FLLD can develop due to an abnormal motion of the hip, knee, ankle or foot in any of the three planes of motion."