Leg length discrepancy (LLD) is a common musculoskeletal variation where the two lower limbs are of unequal length, often differing by only a few millimeters. Although the body compensates for this imbalance, the resulting pain is frequently counterintuitive. Discomfort often manifests not just in the short limb, but also in the longer limb and secondary locations like the back and hips. The location and type of pain depend heavily on the body’s compensatory mechanisms, which attempt to level the pelvis and maintain balanced posture.
Defining Leg Length Discrepancy
Clinicians categorize LLD into two primary types: structural and functional. Structural LLD, also referred to as anatomical LLD, involves a true physical difference in the bone lengths of the femur or tibia. This difference results from congenital conditions, fractures that healed shorter, or issues with growth plates during development.
Functional LLD occurs when bone lengths are identical, but one leg appears shorter due to issues elsewhere in the body. This apparent difference is caused by muscle imbalances, joint contractures, or pelvic rotation that functionally alters the limb’s effective length. Both types lead to asymmetrical loading and altered gait mechanics. A difference greater than 5 to 10 millimeters is considered significant enough to cause biomechanical changes.
Pain Manifestation on the Shorter Limb
The shorter limb is subjected to biomechanical stresses primarily involving compression. Since the pelvis tilts downward on this side, the body attempts to functionally lengthen the limb during standing and walking. This often results in a compensatory foot position, such as supination, where the foot rolls outward to increase the height of the arch.
This supination places increased strain on the outer structures of the ankle and foot, potentially leading to chronic ankle instability or lateral foot pain. The entire limb is also subjected to higher compressive forces as the body’s weight is loaded unevenly to stabilize posture. Over time, this chronic joint overload can accelerate wear and tear, contributing to the earlier onset of knee or hip osteoarthritis on the shorter side.
Pain Manifestation on the Longer Limb and Compensation
Paradoxically, the longer limb is often the primary source of pain, related to the body’s attempt to functionally shorten it. To prevent tripping and maintain a level head, the body employs continuous, subtle flexion at the hip and knee joints during walking. This action effectively reduces the limb’s length during the swing phase of the gait cycle.
This constant, low-level flexion requires the sustained, abnormal firing of specific muscle groups, leading to overuse and tension injuries. The iliotibial band (IT band) is often placed under excessive tension due to altered hip mechanics, resulting in Iliotibial Band Syndrome (ITBS) and lateral knee pain. The longer leg may also exhibit excessive foot pronation, where the arch flattens to reduce leg length further, contributing to plantar fasciitis or shin splints. This chronic muscular strain makes the longer limb susceptible to overuse pathologies, including hamstring strain and patellofemoral pain syndrome.
Secondary Pain Locations (Hip and Back)
The effects of LLD extend beyond the legs, with pain frequently manifesting in the hips and spine due to kinetic chain imbalances. The discrepancy forces the pelvis to tilt, known as pelvic obliquity, making the hip joint on the longer side functionally higher. This uneven foundation transmits asymmetrical forces up the body, leading to structural and muscular adaptations in the trunk.
The spine reacts to the tilted pelvis by developing a compensatory lateral curvature, known as functional scoliosis, which helps keep the head and shoulders level. This spinal bending results in uneven muscle tension and asymmetrical loading of the vertebral discs and facet joints in the lumbar spine. Low back pain (LBP) is one of the most common secondary symptoms, often localized to the side of the longer leg. Muscles like the quadratus lumborum are chronically contracted there to stabilize the elevated hip. Long-standing LLD can contribute to degenerative changes in the lumbar spine over time due to continuous asymmetrical loading.