Leg Length Discrepancy (LLD), also known as anisomelia, is a condition where one lower limb is shorter than the other. This difference can range from barely noticeable to significant, affecting a person’s gait and posture. A small discrepancy is common in the general population and often causes no discomfort. Understanding the threshold where this difference moves from a normal variation to a clinical concern is essential for determining if a medical evaluation is necessary.
Defining the Types of Leg Length Discrepancy
LLD is broadly categorized into structural and functional types. Structural LLD, often called true LLD, involves an actual physical difference in the length of the bones (femur or tibia). This fixed, measurable difference is confirmed through imaging like X-rays.
Functional LLD, or apparent LLD, occurs when bone lengths are equal, but the leg appears shorter due to issues originating outside the limb. This apparent difference results from alignment problems in the pelvis, spine, or surrounding soft tissues. Causes include muscle contractures, ligament laxity, or a pelvic tilt. Individuals with functional LLD should avoid using a heel lift, as this may worsen the underlying postural issue.
The Measurement of Normal Variation
The human body is not perfectly symmetrical, and minor leg length differences are normal population variations. Up to 90% of people have some degree of LLD, with the average difference being approximately 5 millimeters (mm). This small difference is generally well-tolerated and rarely causes pain or walking difficulty.
Orthopedic professionals agree that a discrepancy of up to 5 mm is physiologically insignificant and does not require treatment. Differences between 5 mm and 20 mm are considered mild to moderate. Symptoms in this range depend on the individual’s activity level and biomechanics, though even 5 mm may increase the risk of knee or hip pathology in some cases.
Discrepancies from 10 mm up to 20 mm are more likely to create noticeable asymmetry and cause symptoms like lower back or hip pain. However, many individuals remain asymptomatic due to the body’s natural compensatory mechanisms. Once the difference exceeds 20 mm, the likelihood of gait asymmetry and long-term musculoskeletal issues significantly increases.
Common Causes of Leg Length Differences
Leg length differences are either congenital (present from birth) or acquired later in life.
Structural Causes
Structural LLD often stems from developmental conditions, such as congenital short femur or fibular hemimelia, where the bone does not fully develop. Acquired causes frequently involve trauma, such as a severe fracture that heals in a shortened position or damage to a growth plate during childhood. An infection in the bone, like osteomyelitis, or certain bone tumors can also disrupt the growth process in one limb. In adults, a poorly healed bone fracture or a complication from joint replacement surgery can result in a new structural LLD.
Functional Causes
Functional LLD is rooted in non-bony issues that affect the alignment of the pelvis and spine, creating the illusion of a shorter leg. Common causes include muscle imbalances, such as tightness in the hip flexors or weakness in the glutes, which can cause the pelvis to rotate or tilt. Spinal conditions like scoliosis, which is a sideways curvature of the spine, can also lead to pelvic obliquity and an apparent leg length difference. Foot mechanics, such as a severe over-pronation in one foot, can also lower the height of the entire limb, contributing to a functional LLD.
When Leg Length Discrepancy Requires Medical Intervention
The need for medical intervention typically arises when the LLD causes chronic symptoms or when the magnitude of the difference exceeds certain thresholds. While the body can often compensate for smaller differences, discrepancies greater than 20 mm (2 centimeters) are frequently associated with significant gait abnormalities and are the common threshold for surgical consideration.
Symptoms that warrant a visit to a doctor include chronic pain in the lower back, hip, knee, or ankle that is not relieved by rest or standard over-the-counter medication. Other indicators are a noticeable limp or an abnormal walking pattern, especially if the person tends to walk on the toes of the shorter leg to compensate for the height difference. Pain is a key factor, as even a mild discrepancy may require attention if the patient is highly active.
The initial diagnostic process often begins with a physical examination, including a block test where blocks are placed under the foot of the shorter leg until the pelvis is level. For an accurate, definitive measurement, a physician will typically order an imaging study, such as a scanogram, which is a full-length X-ray of the legs. This imaging is used to precisely measure the bone lengths and differentiate between structural and functional LLD, guiding the next steps for management.