Limb Length Discrepancy, or leg length inequality, is defined as a condition in which the length of the lower extremities is remarkably unequal.  Studies have shown that LLD of 2 cm or more affects at least one in every 1000 people. 

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Limb Length Discrepancy can be subdivided into two groups: structural LLD (SLLD) and functional LLD (FLLD). SLLD, also known as anatomic LLD, is defined as structural deformities resulting from differences in true bony leg length. It is an anatomical difference between the two lower extremities measured from the femoral head to the distal tibia and can be congenital or acquired.

Congenital LLD can be induced by various abnormal developmental disorders. Causes of acquired LLD include paralysis, tumors, fractures, and surgical procedures, such as joint replacement. FLLD is defined as an asymmetrical leg length caused by altered mechanics of the lower extremities, including muscle tightness or weakness, unilateral foot pronation or supination, knee hyperextension, and hip abduction/adduction tightness/contracture. Significant lower extremity mal-alignment may result from Leg Shortening. Because abnormal bio-mechanical factors are observed in a number of orthopedic disorders, it is thought that many pathological symptoms might be related to Leg length Discrepancy. However, because several clinical factors found in orthopedic disorders can cause asymmetrical functional ability, it would be difficult to isolate and determine the effect of LLD on their function.