Description:
Also known as congenital absence of the fibula, congenital fibular deficiency, paraxial fibular hemimelia and aplasia/hypoplasia of the fibula, fibular hemimelia has an incidence of 7.4 per 20 million live births, making it the most common long bone deficiency. It represents a broad spectrum of deformity, ranging from mild limb length inequality to gross shortening of the affected side with accompanying foot and ankle deformities. The femur may be affected as well. It is bilateral in 9-52% of cases.
Epidemiology:
The etiology of fibular hemimelia remains unclear. Sporadic appearance is most common, but chromosomal anomalies with autosomal dominant, autosomal recessive and x-linked transmission have been reported. Theories exist that intimate an exogenous vascular or mechanical interference with limb bud function in utero. (Graham, 1983) The so-called fibular developmental field is thought to influence the development of lateral foot rays, the ACL, patella, proximal femur, acetabulum and pubic bone, which would explain some of the spectrum of deformities observed in patients with fibular hemimelia. (Lewin, 1986)
Clinical Findings:
The appearance of a child with fibular hemimelia depends on the severity of the deformity. There is always a component of limb length discrepancy that may range in severity from mild to distinctive shortening. The shortened tibia often includes an anterior or anteromedial bow, thought to arise due to the tethering effect of the rudimentary fibular anlage which clinically presents as a taut posterior band in the fibular area. There may be a skin dimple at the apex of the bow. Distally, foot and ankle deformities are common and include equinovalgus, absence of the lateral rays, tarsal coalition (talocalcaneal most common) and ankle instability. Sometimes, the ankle assumes a spherical articulation, giving rise to the term ‘ball-and-socket ankle deformity’, which may be congenital or develop in response to the commonly found tarsal coalitions.
Proximally, shortening of the femur is common. The severity does not correlate with the severity of the fibular hemimelia. There is often a hypoplastic lateral femoral condyle, giving rise to genu valgum. The ACL and PCL are frequently absent but clinical instability requiring intervention is rare.
Imaging Studies:
The radiographic appearance is variable and depends on the severity of deformity. Plain radiographs will demonstrate the shortening and bowing of the tibia. The fibular growth plates are displaced from their natural levels, establishing a position distal to the tibial growth plate proximally and proximal to the talar dome distally. The tibial spine may be absent, indicating a lack of the ACL. Plain radiography may also be used in determination of deformities affecting the foot although tarsal coalition may not be readily apparent due to its cartilaginous nature. MRI should be reserved for patients undergoing soft tissue reconstruction of the knee or ankle. Full-length lower extremity studies to determine leg length discrepancy are recommended as well. CT scanographs are a reliable alternative to these.
Etiology:
Numerous classification systems for fibular hemimelia have been proposed. (Coventry, 1952; Letts, 1993; Stanitski, 2003) The most significant among these are the Achterman-Kalamchi and Birch classifications. (Achterman, 1979; Birch, 2011) Over time, the significance of the reconstructability of the foot has gained importance, leading into the classification system by Paley which makes treatment recommendations based on foot deformity alone.
Achterman-Kalamchi Classification
This classification takes into account the morphology of the fibula. In Type 1, the fibula is present but hypoplastic, whereas in Type 2, it is completely absent. (Achterman, 1979) Type 1 is further subdivided into 1A, where the proximal growth plate is distal to the tibial one, and the distal fibular growth plate is proximal to the talar dome, and 1B, where there is 30-50% of length missing proximally and there is no distal support for the ankle joint. Bowing of the tibia is more severe in Type 2 deformities.
Birch classification
The Birch classification divides fibular hemimelia into two categories based on the potential of the foot to be reconstructed. In Type 1, the foot is reconstructable, whereas in Type 2, it is not. The determination of reconstructability is made by the number of foot rays, where a foot slated to be reconstructed should have at least three rays present. Amputation is considered in patients where the foot is not reconstructable, in the presence of functional upper extremities.
Type 1 hemimelia is further subdivided into four groups according to LLD. In Type 1A, LLD is less than 6%, Type 1B it is 6-10%, Type 1C is 11-30% and Type 1D is more than 30%. Treatment recommendations by the authors are made based on the classification of limbs, and will be discussed in detail below.
Paley Classification
The extent of the fibular deficiency is not the main point of focus in the Paley classification. It is instead concerned with the extent of tibial deformity and the reconstruction potential of the foot and ankle. The Paley classification is as follows:
Type 1: stable normal ankle
Type 2: dynamic valgus ankle
Type 3: fixed equino-valgus ankle; it is further subdivided according to the orientation of the ankle and subtalar joints
Type 4: fixed equino-varus ankle (clubfoot type)
Treatment:
The treatment of fibular hemimelia aims to achieve a functional limb nearly equal in length to the normal limb at maturity, with a plantigrade, stable and flexible foot. The treatment of fibular hemimelia is highly dependent on the potential of the foot to be reconstructed, and should be held primarily in mind when deciding on amputation versus limb salvage. Treatment of fibular hemimelia should be highly individualized to the patient and undertaken in experienced centers with access to a multidisciplinary management team including a pediatrician, physical therapists and social workers.
Patients should be screened for associated anomalies of the cardiac and renal systems before surgical treatment is initiated. The knee must be assessed carefully before initiation of limb lengthening treatment in order to prevent joint subluxation or dislocation. In cases where the femur also requires lengthening there should be an evaluation for acetabular hypoplasia for similar reasons. The percentage of LLD in fibular hemimelia has been shown to remain constant in a majority of patients, allowing for prediction of LLD at maturity utilizing classical methods. (Birch, 2011) Amputation should be avoided in children with non-functional upper extremities. Regardless of type, if the fibular anlage provides a taut posterior tether, it should be excised during the first surgical intervention to prevent worsening of tibial bowing.
Birch Types 1A and 1B
In patients with a plantigrade foot and an LLD less than 6%, surgical treatment may not be necessary and orthotic treatment, such as shoe lifts may be sufficient. Contralateral epiphysiodesis may be an option in patients who are reluctant to consider life-long orthotic treatment. LLD prediction should be performed with care, and the family should be informed that this prediction is not completely accurate.
Birch Types 1C, 1D and 2: Amputation vs. Limb Salvage
The decision to pursue amputation versus limb salvage should focus on the possibility of foot reconstruction and expected LLD. Limbs with a non-reconstructable foot or large discrepancy in excess of 30% will require multiple procedures. A detailed and prolonged discussion of amputation and subsequent prosthetic treatment is warranted in this situation. Families often struggle with making treatment decisions in this scenario and often find it helpful to speak with families who have undergone either treatment.
Both Syme and Boyd amputations have been utilized successfully in the reconstruction of fibular hemimelia. Amputation should be timed well with the child’s attempts at walking. If the residual stump is too short or bowed for prosthetic fitting, lengthening or corrective osteotomy may be considered. Distal tibial and fibular epiphysiodesis should be considered to create a discrepancy in the tibial segment length as the child grows. This will allow for more choice in prosthetic fitting and componentry as an adolescent and adult. Genu valgum continues to be a concern after amputation and may affect prosthetic fitting. Guided growth methods have been utilized effectively to treat this.
Reconstructive procedures for ankle instability have been previously described, but larger series do not exist. Free tissue transfer has been utilized to give the talus a lateral buttress. For this purpose, a variety of free and vascularized graft options and reconstructive techniques have been described. Overcorrecting the distal tibia into varus using guided growth versus acute correction may help with valgus instability. Arthrodesis remains a last resort in these patients.
Distraction osteogenesis for the equalization of LLD and correction of deformities has been successfully utilized in fibular hemimelia. Often, more than one session of lengthening is required, and surprisingly, complication rates between the first and second rounds of lengthening are not excessively different. However, once lengthening surpasses 15-20%, complication rates increase significantly. Contralateral epiphysiodesis may be considered as well in cases where excessive lengthening is required.
Knee deformities, most commonly genu valgum, and instability may arise during the treatment process. If there is indication of knee subluxation during the course of treatment, the knee should be spanned with the external fixation frame. Genu valgum in fibular hemimelia is thought to be related to the hypoplasia of the lateral femoral condyle, the tibial angulation and the resultant increase in compressive forces across the lateral half of the physis, culminating in stunted growth in this area.
Hemiepiphysiodesis of the medial femoral condyle may be sufficient in subtle deformities, whereas more severe instances may require correction by osteotomy. Clinically relevant knee instability in fibular hemimelia is rare, despite the high incidence of congenital absence of the ACL and PCL and reconstruction should be considered only in symptomatic cases.
Paley has described essentially all fibular hemimelia cases as reconstructable. A more detailed description of his treatment protocol is available. (Paley, 2016) A two-center comparative study of patients with severe, unilateral fibular deficiency who underwent either reconstruction or primary amputation was performed. The average patient age was approximately 10 years and the majority of salvage patients had future reconstructions planned. This interim study identified essentially equivalent functional outcomes and patient/parent satisfaction with the outcome/treatment course chosen. (Birch, 2009; Paley, 2011)
Complications:
As the most common long-bone deficiency, fibular hemimelia is a spectrum of deformity whose landmarks are shortening of the affected limb, abnormalities of the foot and ankle, genu valgum and an absence of the ligamentous structures of the knee. Most cases are sporadic and the etiology of fibular hemimelia remains unclear. Treatment often includes orthotics and prosthetics, with or without surgical intervention. The decision to amputate versus reconstruct the extremity depends on the extent of foot deformity, the LLD, and the desires of the individual patient and family. When reconstruction is chosen, multiple sessions of lengthening are often required, with or without contralateral epiphysiodesis. Other angular deformities of the lower extremity and knee instability may require surgical intervention as well.
