Leg length discrepancy (LLD) is a condition where the lower limbs differ in length, ranging from a few millimeters to several centimeters. Minor variations are common and often cause no symptoms. When the difference exceeds 5 millimeters or 1 centimeter, however, it can begin to affect the body’s mechanics and posture. Understanding the origin, effects, and measurement of LLD is necessary for determining the appropriate management strategy.
How the Discrepancy Develops
Leg length discrepancy arises from two main categories: structural and functional causes. Structural LLD, or true LLD, involves an actual difference in the length of the bones, specifically the femur or tibia. This means the skeletal structure of one limb is physically shorter or longer than the other.
Structural causes are categorized by onset. Congenital LLD is present at birth due to developmental issues where the bones did not form fully or grew at uneven rates. Developmental causes occur during childhood growth, resulting from conditions like Blount’s disease or damage to the growth plates (physes) from trauma or infection.
Acquired LLD develops later in life, most commonly following a severe fracture that heals shortened or misaligned. Prior orthopedic surgeries, such as hip or knee replacements, can also result in LLD. These structural differences create a fixed skeletal inequality.
Functional LLD is an apparent discrepancy where bone lengths are equal, but the leg appears shorter due to issues elsewhere in the body. This type is often caused by muscle imbalances, joint contractures, or asymmetrical posture, such as a pelvic tilt or rotation. Functional discrepancies stem from alignment issues in the joints or soft tissues.
Symptoms and Physical Effects
The body attempts to compensate for unequal leg lengths, leading to a cascade of physical effects. One noticeable manifestation is an altered gait, which may present as a limp or a shift in weight distribution. Individuals may also exhibit toe-walking on the shorter leg to make up the difference in length.
The mechanical stress travels up the body, leading to changes in posture. The pelvis often tilts down on the side of the shorter leg, known as pelvic obliquity. The spine compensates for this uneven foundation by developing a sideways curvature, termed functional scoliosis.
This uneven loading places abnormal strain on the musculoskeletal system, often resulting in chronic pain in the lower back, hips, and knees. The joints on the longer side may experience increased compressive forces, potentially accelerating wear and tear or leading to conditions like hip bursitis. The severity of these symptoms generally correlates with the magnitude of the discrepancy.
Diagnosing and Measuring the Difference
Accurate diagnosis begins with a thorough physical examination, which includes a visual assessment of posture and gait. Clinicians often use the standing block test, placing measured blocks beneath the foot of the shorter leg until the pelvis appears level. While clinical methods provide a useful initial screening, they are not precise enough for definitive treatment planning.
To accurately quantify structural LLD, imaging modalities are necessary. The radiographic scanogram, also called a full-length standing anteroposterior radiograph, is considered the gold standard for precise measurement. This technique involves three focused X-ray exposures—of the hip, knee, and ankle—stitched together to capture the entire length of both lower limbs while the patient is standing. A ruler or grid is included in the image to provide a calibration scale, allowing for measurement of the actual bone lengths, often down to the millimeter.
The scanogram helps differentiate between structural and functional discrepancies by showing the true bone lengths. For complex cases, such as those involving significant rotational deformities or hardware, a computed tomography (CT) scanogram may be used to obtain additional cross-sectional data. Precise measurement is necessary because the size of the discrepancy guides the choice between non-surgical and surgical management strategies.
Treatment and Management Strategies
Treatment strategies for LLD are highly individualized, depending on the patient’s age, the underlying cause, and the magnitude of the difference. For small discrepancies, typically less than 2 centimeters, non-surgical management is the first approach. This conservative treatment involves using shoe lifts or heel inserts placed under the shorter limb to artificially equalize the leg lengths. Physical therapy is also employed to address associated functional components, such as muscle imbalances or gait abnormalities.
For discrepancies between 2 and 5 centimeters, or if conservative methods fail, surgical intervention may be considered. The choice of surgical procedure is heavily influenced by whether the patient is skeletally mature.
In children who still have growth remaining, a procedure called epiphysiodesis is often preferred. This minimally invasive surgery involves slowing or stopping the growth of the longer leg’s growth plates (physes). The goal is to allow the shorter leg to “catch up” during the remaining growth period, resulting in leg length equality by the time the child reaches skeletal maturity.
For adults or discrepancies over 5 centimeters, limb lengthening is the primary surgical option. This process, known as distraction osteogenesis, involves surgically cutting the bone (osteotomy) and then gradually pulling the two segments apart using an internal rod or external fixator. New bone tissue forms in the gap created by the slow separation, incrementally increasing the limb’s length. In rare instances of moderate discrepancy in adults, a shortening procedure may be performed on the longer leg by removing a segment of bone.