Anisomelia – Leg Length Differences

It sounds like a pretty name from an exotic dialect. It actually is the medical term for a leg length discrepancy (LLD). So, sorry, don’t name your child that! This condition is described by the difference in length of the long bones of the lower extremity – most commonly the femur (thigh bone).  LLD continues to be a controversial issue among researchers and clinicians. It has been shown that differences exist in most people. In fact, Knutson et al 2005, did a study with precise x-ray methods and found that 90% of the population have an average leg difference of 5-9mm. This means that only 10% of the population have exactly equal leg lengths!

There is however, little consensus in the research as to the clinical relevance, prevalence, reliability and validity of measuring methods for LLD.  Research and the debate continue regarding the effects of LLD in function and its role in various neuromusculoskeletal conditions (Physiopedia 2023).

There are two types of LLD, structural and functional.  Structural conditions are a physical shortening of one limb that may be a congenital developmental abnormality or an acquired condition from trauma, fractures, degenerative diseases or surgical implants, such as a joint replacement.  A systemic review in the literature evaluated the prevalence of LLD by radiographic measurements, showed that 90% of the NORMAL population have some variance in bony leg length, with 20% having a difference of >9mm (Khamis et al. Journal of Orthopaedics, 2017).

A functional LLD is a “unilateral asymmetry of the lower extremity without any shortening of the osseous (bony) components of the lower limb” (Physiopedia ’23). This functional difference may be caused by lower body mechanics, adaptive muscle shortening, or muscle imbalances. It is impossible to detect this type of difference using static radiography (x-ray, CT), as these faulty mechanics may develop over time due to abnormal movement at the hip, knee, ankle or foot in any of the three planes of motion.

Other than precise x-ray methods, which are expensive and impractical, we may conduct some observational tests to determine if LLD may be contributing to a patient’s discomfort. It may be concluded that a LLD or abnormal foot mechanics may be the cause if a patient complains of lower back pain with standing and walking, has an observational difference of >1cm or presents with bilateral or unilateral flat feet (overpronation). Inexpensive insoles may be a solution that can be effective, or for differences of greater than 2cm – gradually correcting inequality with shoe inserts or custom orthotics. The bottom line is leg length differences continue to be debated in the literature! If you are concerned with your alignment or are suffering from lower back or lower leg issues our team of therapists and our Orthotics in Motion Pedorthist are well equipped to help straighten you out!

Jennifer Gordon (BSc.PT, GunnIMS, AFCI) Physiotherapist