Leg Length Discrepancy (LLD, a.k.a. anisomelia) is defined as a measurable difference between the length of the lower extremities, and is often blamed for numerous issues related to pain or dysfunction due to the asymmetry.
It’s important to point out the underlying flawed premise here: that a structural “problem” necessarily results in symptoms. We’ve discussed at length the flaws of such a biomedical approach in the past, and will continue to do so in the future. Pain is a complex experience that cannot be reduced to simple anatomical observations that might superficially seem problematic (see Lederman 2011).
A guiding principle to follow when discussing potential correlates to pain is “does it matter?” And if “it” matters, we need to figure out 1) when it matters, and 2) what can / should we actually do about it?
Leg Length Discrepancy (LLD) has a high prevalence rate, with ~90% of the population displaying a <1.0 cm difference.
There is very weak evidence of poor quality demonstrating correlation between a particular leg length discrepancy and symptoms; broad generalizations should not be drawn from the available data at this time.
It is important to recognize that we adapt to such deviations from textbook “norm”.
Two variations of LLD have been described: “Anatomical” and “Functional”. An “anatomical” LLD describes structural differences in femur and/or tibia length, either naturally occurring in the course of development or acquired at some point during life (e.g., due to fracture, bony disease, or joint replacement). “Functional” LLD isn’t as straightforward — and although frequently discussed, lacks supporting research evidence. Typically the discussion of functional LLD is centered around pseudoscientific, poorly defined ideas such as “pelvic torsion”, “tight” vs “loose” muscles, or “subluxations”.
How-To Measure a Leg Length Discrepancy
Many perform assessments of LLD in clinics/gyms via palpation, tape measures, or blocks (to level the pelvis). Overall, the available research shows that such approaches are ineffective. A more accurate assessment can be obtained via radiological imaging, such as X-ray or CT scanogram. Sabharwal 2008, Gibbons 2002, Cooperstein 2017
However, even though we can measure LLDs, we should ask few questions before we pathologize it and turn the finding into an issue. The questions include:
How common are leg length discrepancies in the general population?
How reliable are measurements of leg length discrepancies?
Do leg length discrepancies correlate well with specific signs or symptoms?