Causes / typical injury mechanism: Leg length discrepancy (LLD) is a common condition characterized by lower limbs that are unequal in length. There are numerous potential causes for LLD that can be subdivided into 2 etiologic groups: structural (SLLD) and functional (FLLD). SLLD results from inequalities in bony structure, while FLLD results from muscle tightening, muscle weakness, or joint contractures in the lower limbs or spine.

Classic history and presentation: Patients with LLD are often asymptomatic. On physical examination, LLD can be determined with either block testing or tape measurements. LLD can also be detected using various imaging modalities (eg, x-ray or CT scanogram). LLD has been associated with altered posture, inefficient gait, lower back pain, and osteoarthritis. However, the role that LLD plays in musculoskeletal disorders and the severity of LLD that necessitates treatment is not fully understood. Age of onset and activity level have been shown to influence an individual's tolerance for LLD. Children better tolerate LLD than adults who may acquire the condition later in life. Also, individuals with higher activity levels may be more affected by LLD than those whose activity levels are lower.

Prevalence: LLD is a relatively common condition with a prevalence reported between 40%-70% in the population.

• Age – LLD usually appears during childhood, but it can also be acquired during adulthood.
• Sex / gender – LLD does not appear to have a predilection for one sex over another.

Risk factors: Risk factors include genetic predispositions, trauma, and infections; however, the majority of cases have no etiology.

Pathophysiology: SLLD can be congenital or acquired. The most common congenital causes of SLLD are congenital hip dislocation, hemiatrophy, and hemihypertrophy. Acquired causes include trauma, tumor, infections, neuromuscular disorders (eg, cerebral palsy and polio), and surgical procedures. Causes of FLLD include excessive pronation / supination of one foot, unilateral hip abduction / adduction contracture, and lumbar scoliosis leading to pelvic obliquity.

Grade / classification system: Defining a significant LLD that warrants treatment is a major source of controversy. Some people define a significant LLD according to functional characteristics, while others quantify LLD. A discrepancy of greater than 20 mm is often considered significant at skeletal maturity. Multiple authors have proposed various numerical breakdowns of LLD. For instance, Mosey proposed the following:

• 0-20 mm – Requires no treatment.
• 20-60 mm – Requires shoe lift on the affected side, epiphysiodesis / shortening of longer extremity.
• 60-200 mm – Lengthening with or without other procedures.
• Greater than 200 mm – Prosthetic fitting.