Fibular hemimelia is a birth defect where part or all of the fibular bone is missing, as well as associated limb length discrepancy, foot deformities, and knee deformities. Fibular hemimelia (FH) is a very rare disorder, occurring in only 1 in 40,000 births. Bilateral fibular hemimelia (affecting both legs) is even rarer.
It is currently unknown why fibular hemimelia occurs. Research has demonstrated that if the genes guiding the formation of the limb are activated in an abnormal order, fibular hemimelia can occur. Other studies have demonstrated that isolated mutations of genes in the forming limb bud can lead to fibular hemimelia. Although genetic abnormalities are linked to FH, the condition is not heritable. The gene mutations and abnormalities are occurring only in the forming limb and not anywhere else, and thus cannot be transmitted to the next generation.
Furthermore, the vast majority of children born with this condition have no family history of other birth defects. Neither the parents of the child with FH nor the child themselves have any increased risk of producing additional children with this or other birth defects.
Children with fibular hemimelia present with three major complaints:
- Limb length discrepancy
- Foot and ankle deformities
- Knee deformity
For more information on the etiology and treatment of fibular hemimelia, please see our Fibular Hemimelia FAQ.
Limb Length Discrepancy
Fibular hemimelia leads to limb length discrepancy because the tibia on the affected side grows at a slower rate than the tibia on the opposite side. In addition, many patients with FH have a slower growing femur as well. This combination of slower tibia and femur growth leads to a limb length discrepancy. Furthermore, children with FH have associated foot deformities that result in a shorter foot which also contributes to the limb length discrepancy.
For more information, see Lengthening for Fibular Hemimelia.
The foot deformity is one of the biggest issues with fibular hemimelia. The foot deformity is related to the abnormal ankle joint as well as missing parts of the foot. The extent of ankle joint deficiency may range from a relatively normal ankle to a very unstable, abnormally-shaped ankle with limited mobility. In normal anatomy, the fibula contributes to the stability of the ankle. The end of the fibula can be felt as a large bump on the lateral sides of our ankles, called the lateral malleolus. Children with fibular hemimelia are missing part or all of their fibula and this bump may be completely missing. When the lateral malleolus is present, it buttresses the talus (ankle bone) and prevents it from coming out of the joint. When it is missing, this stabilizing effect is absent. The ankle joint is primarily made up of the lower end of the tibia, which is often severely deformed as well in patients with fibular hemimelia. The deformity comes from a bend in the main shaft of the tibia and forms a knuckle-like appearance of the bone (often with a skin dimple over the knuckle).
A more subtle deformity of the ankle is one that is often not visible on the x-ray: malorientation of the joint itself. This malorientation points the foot towards the outside (lateral) and down (posterior), creating what is called an equinovalgus deformity. This deformity was thought to be due to tight soft tissues (such as the Achilles tendon) as well as the presence of a fibrous remnant of the fibular bone, known as an anlage. In 1996, Dr. Paley was the first to identify that the equinovalgus deformity was not caused by tight muscles or the anlage, but rather, it is due to malorientation of the joint itself, which is invisible to x-ray since the joint is mostly made of cartilage at a young age. His findings have since been confirmed by both MRI and open surgical examination. This discovery led to the development of the SUPERankle procedure, a comprehensive surgical treatment for reconstructing the foot and ankle of patients with fibular hemimelia (SUPER is an acronym for Systematic Utilitarian Procedure for Extremity Reconstruction).
For more information, see SUPERankle.
In addition to ankle deformities, the foot in patients with FH may also have a deformity between the talus (ankle) and calcaneus (heel bone). Normally, these two bones are connected through the subtalar joint. The ankle joint moves the foot up and down and the subtalar joint moves the foot side-to-side, which is important for walking on uneven ground. The subtalar joint in fibular hemimelia is usually absent because the two bones are fused. Despite fusion of the bones, side-to-side motion is present in FH due to an abnormal, ball-and-socket shaping of the ankle joint. Therefore, the ankle joint functions for both the ankle and subtalar joints. This fusion of function is called a subtalar coalition. If the subtalar coalition connects the talus and calcaneus in a normal position such that the heel is in line with the ankle bone, then it does not contribute to additional deformity of the foot. If, however, this coalition is joined in an abnormal fashion, so that it is tilted outwards (valgus) or inwards (varus) then it leads to additional deformity of the foot and ankle. Dr. Paley was one of the first surgeons to recognize the contribution of subtalar coalition deformities to fibular hemimelia which helped form the basis of the SUPERankle procedure, which aims to correct these deformities.
The third element of the foot deformity is the absence of some of the toes, including the foot metatarsal bones (the long bones that lead down to the toes). Normally, there are five metatarsals and five toes, but in fibular hemimelia there may be more or less. Some of the toes may be joined together (syndactyly) or separated. The big toe may also be pointing inwards, away from the rest of the foot. This particular deformity is called a delta metatarsal and requires a specialized operation to correct.
For more information, see Toe Reconstruction.
In patients with fibular hemimelia, the knee joint frequently has a valgus deformity (knocked knee). This alignment can be related to the lower end of the femur or the upper end of the tibia, or both. It is important to realign the knee during treatment of fibular hemimelia. Most patients with FH will also have absent or deformed knee ligaments. In particular, the anterior cruciate ligament (ACL) is often under-developed (hypoplastic) or absent. No initial treatment for this is provided but ligament reconstruction should be considered if the patient begins to develop problems of knee instability. Children with deficient or absent cruciate ligaments often do very well and can engage actively in various sports that other children their age can participate in.
For more information, see Knee Reconstruction