Fibular hemimelia is a congenital condition characterized by a deficiency or complete absence of the fibula, which is the smaller of the two bones in the lower leg. The term “hemimelia” itself signifies a partial absence of a limb structure. This rare condition, occurring in about 1 in 40,000 to 50,000 births, is the most common congenital deficiency affecting the long bones of the body. The resulting difference in limb structure can vary significantly.
Defining Fibular Hemimelia
Fibular hemimelia is a complex limb difference that extends beyond just the missing bone, often affecting the entire lower extremity from the hip to the toes. The absence or shortening of the fibula is consistently linked to a leg length discrepancy (LLD) between the affected leg and the unaffected side. This discrepancy can range from a mild difference to a severe shortening of up to 30 centimeters at skeletal maturity.
The deficiency of the fibula disrupts the structure of the ankle joint. The fibula normally forms the lateral malleolus, a bony prominence that stabilizes the outside of the ankle. Without this support, the ankle is often unstable, leading to a malformed joint that can adapt a “ball-in-socket” configuration instead of the normal hinge joint.
Associated deformities frequently include a bowing of the tibia and significant abnormalities in the foot structure. The foot often presents with an equinovalgus deformity, meaning the foot is pointed downward and outward, and there may be a reduced number of toes or foot rays. Furthermore, tarsal coalition, where bones in the foot are abnormally fused, is a common feature that restricts normal side-to-side foot motion.
Identifying the Condition
The exact cause of fibular hemimelia remains largely unknown, with the majority of cases considered sporadic. It is understood to result from a spontaneous error during the early stages of limb development, specifically between the fifth and eighth weeks of gestation. Current research suggests this developmental disruption is related to abnormal growth patterns in the limb bud, potentially involving localized genetic mutations or environmental factors like vascular disruption.
Diagnosis can often begin before birth, as the structural abnormality may be detected during a routine prenatal ultrasound. If not identified prenatally, the condition is usually noticeable at birth due to the obvious limb length difference, bowing of the leg, or foot deformities. A physical examination by a specialist is followed by imaging tests, which are necessary to confirm the diagnosis and determine the extent of the deficiency.
X-rays are the standard tool used to visualize the bones and assess the degree of fibula absence, the integrity of the tibia, and the structure of the knee, ankle, and foot. Additional imaging, such as a magnetic resonance imaging (MRI) scan, may be employed to evaluate the soft tissues, including ligaments and cartilage, which are also often deficient or malformed.
Management and Treatment Pathways
Management of fibular hemimelia depends on the severity of the leg length discrepancy and the stability of the ankle and knee joints. The overarching goal is to achieve a stable, functional lower limb that is as close to equal in length as possible to the unaffected side by the time the child stops growing. This is accomplished through two primary pathways: limb salvage or amputation with prosthetic fitting.
Limb salvage involves a series of orthopedic procedures performed throughout childhood. For cases where the foot and ankle are considered salvageable, limb lengthening procedures are performed to correct the LLD. This often involves surgically cutting the bone and gradually pulling it apart, stimulating the body to generate new bone tissue in the gap, sometimes using specialized external frames like the Ilizarov apparatus or internal lengthening rods.
Multiple lengthening cycles may be necessary, typically spaced four to six years apart, to manage the progressive length discrepancy as the child grows. Concurrent with lengthening, reconstructive surgeries are performed to stabilize the ankle and foot, such as osteotomies (bone cuts) to correct rotational or angular deformities. Advanced techniques, such as the SUPERankle procedure, are specifically designed to address severe equinovalgus deformities and provide necessary stability to the reconstructed ankle joint.
For the most severe presentations, particularly those involving extreme LLD, significant knee instability, or a foot that cannot be functionally reconstructed, amputation may offer the best functional outcome. A Syme or Boyd amputation creates a residual limb well-suited for wearing a custom-fitted prosthesis. This approach allows the child to be fitted with a prosthesis early in life, enabling mobility and participation in activities that might be limited by an unstable or non-functional reconstructed limb.