A difference in length between the two lower limbs is known as Leg Length Discrepancy (LLD) or anisomelia. While slight variations are generally unnoticeable and harmless, greater differences can affect posture and movement, potentially leading to discomfort. LLD is broadly categorized into two types: structural and functional. Understanding this distinction is the starting point for determining the underlying cause and the most appropriate course of action.
Structural vs. Functional Discrepancy
A Structural Leg Length Discrepancy represents a true anatomical difference in the length of the bones between the hip and the ankle. This LLD is caused by one of the long bones, typically the femur or the tibia, being physically shorter than its counterpart. The discrepancy can be precisely measured using diagnostic imaging, such as a full-length standing X-ray. This difference creates a permanent inequality that the body must compensate for.
In contrast, a Functional Leg Length Discrepancy occurs when the actual bone lengths are equal, but the leg appears shorter due to alignment issues elsewhere in the body. The apparent shortening is often a result of mechanical problems, such as a pelvic tilt or a deformity in the foot or knee. This discrepancy is dynamic and subject to change based on posture and muscular tension, rather than fixed by bone structure. Determining the type dictates whether treatment focuses on physically leveling the limbs or correcting mechanical imbalances.
Developmental and Congenital Causes
Structural LLD can originate from abnormal development before birth, leading to congenital conditions that affect the growth of the long bones. These conditions involve inherent problems with the bone-forming tissues or the growth plates.
One example is Congenital Short Femur, which involves deformities ranging from a slightly shortened thigh bone to a complete absence of the proximal femur. This condition results in a lifelong and progressively increasing LLD as the child grows.
Another congenital cause is Fibular Hemimelia, characterized by the partial or total absence of the fibula. This malformation also causes shortening of the tibia and often leads to foot and ankle deformities, contributing to a substantial LLD. The severity of these congenital cases often requires complex, long-term management.
Genetic syndromes can also cause one side of the body to grow faster than the other, a condition called Hemihypertrophy. This results in the long bones on the larger side being longer than those on the opposite side. The continuous nature of these growth differences means the LLD may worsen until skeletal maturity is reached.
Acquired Causes from Trauma and Disease
Structural LLD can be acquired later in life due to external factors that disrupt normal bone growth or integrity. In children, a common cause is injury to the physeal plate, or growth plate. A severe fracture can cause premature closure of the plate on one side of the leg. If the growth plate closes early, the bone’s subsequent growth is stunted, leading to an LLD over time.
Infections of the bone, such as osteomyelitis, can also lead to acquired LLD, especially if the infection infiltrates the growth plate. The resulting damage can destroy the cartilage cells responsible for longitudinal growth, causing the affected bone to stop growing. Bone tumors, such as enchondromas or osteochondromas, can also interfere with the growth plate or cause bone loss, physically shortening the limb.
For adults, a poorly healed bone fracture of the femur or tibia can result in measurable shortening if the bone heals in an overlapping position. Severe osteoarthritis of the hip can also lead to LLD. The collapse of the joint space and destruction of the articular cartilage causes the affected leg to be drawn up, creating an acquired structural discrepancy.
Alignment and Postural Causes (Functional LLD)
Functional LLD results from issues in the mechanics of the body’s alignment rather than a difference in bone length. A frequent cause is pelvic obliquity, where one side of the pelvis is higher than the other, simulating a shorter leg on the lower side. This tilt often stems from chronic muscle imbalances, such as tightness in the hip flexors or the quadratus lumborum muscle, which pull the hip upward. This imbalance creates an apparent, rather than a true, leg length difference.
Curvature of the spine, or scoliosis, can also contribute to a functional LLD by causing a secondary pelvic tilt. The spine’s lateral curve introduces uneven forces that pull the trunk and pelvis out of alignment, making one leg appear shorter. Soft tissue contractures, such as persistent tightness in the hip abductors or adductors, can also restrict joint movement and force the limb into a position that mimics a shorter length.
Issues originating in the foot and ankle can also produce a functional discrepancy. Excessive pronation (arch collapse) or supination (foot rolling outward) affects the height of the entire limb. These deviations alter the foundation of the leg, changing the effective length of the limb relative to the ground. These alignment-based causes are manageable through non-surgical interventions like physical therapy, stretching, or corrective shoe inserts.