Variations are common in the human body, and the length of our legs is no exception. A difference in the length of the lower limbs, known as leg length discrepancy (LLD), occurs when one leg is shorter or longer than the other. A minor difference is frequently present in individuals and typically does not cause symptoms or health concerns.
What is Leg Length Discrepancy?
Leg length discrepancy can manifest in two primary forms: structural LLD and functional LLD. Structural LLD, also referred to as true LLD, involves a genuine anatomical disparity in the bone length of the lower limb, such as the femur (thigh bone) or tibia (shin bone). Functional LLD, in contrast, is not due to a difference in bone length, but rather an apparent discrepancy. This type of LLD can arise from issues like muscle imbalances, joint contractures, pelvic asymmetry, or spinal curvature, which make one leg appear shorter than the other.
How Much Discrepancy is Considered Normal?
A small amount of leg length discrepancy is very common across the general population and often goes unnoticed because it does not cause problems. Studies indicate that 70% to 90% of individuals have some variance in bony leg length. A difference of less than 1 centimeter (or 10 millimeters) is frequently considered normal and is typically asymptomatic.
Many people can compensate for these minor differences without experiencing issues. Even discrepancies up to 1.5 centimeters may not affect gait or the condition of adjacent joints. “Normal” in this context often refers to an asymptomatic state where the body effectively adapts to the slight difference in limb length.
How is Leg Length Discrepancy Measured?
Healthcare professionals employ both clinical and imaging techniques for this purpose. Clinical measurements can involve using a tape measure, typically from the anterior superior iliac spine (ASIS) of the pelvis to the medial malleolus (inner ankle bone). This direct method provides an estimate, though it can be prone to human error and less precise due to factors like soft tissue thickness or difficulty in identifying bony landmarks.
Another clinical approach is the block test, where blocks of known thickness are placed under the shorter leg until the pelvis appears level. This indirect method helps assess the functional impact of the discrepancy and how much lift might be needed to achieve balance. For more precise and objective measurements, imaging techniques are often used. Scanograms, which are full-length standing X-rays, provide accurate bone length measurements by imaging the entire lower extremity with minimal magnification error. Computed tomography (CT) scanograms are also utilized and are considered a very accurate method, often with lower radiation doses compared to conventional X-rays.
When is Leg Length Discrepancy a Concern?
Leg length discrepancy typically becomes a concern when it reaches a certain threshold, often greater than 1 to 2 centimeters (10-20 mm), as this can lead to symptoms and affect body mechanics. Common symptoms include back pain, hip pain, knee pain, and ankle pain, often resulting from the body compensating for the unevenness. Individuals might also develop gait abnormalities, such as limping, or exhibit compensatory postures like one hip appearing higher than the other. Significant LLD can arise from various causes, including congenital conditions, growth plate injuries (which can affect bone growth), trauma such as fractures that heal in a shortened position, or infections. If these symptoms are present or a significant discrepancy is suspected, seeking evaluation from a healthcare professional is advisable.