Congenital longitudinal deficiency of the fibula or fibular hemimelia, as the condition is frequently called, is also described as postaxial deficiency of the lower limb. The latter term most accurately describes the actual condition because the entire lateral (postaxial) aspect of the limb is affected to various degrees (Figures 1 and 2). It is a spectrum of disease where, at a minimum, the limb manifests a mild degree of fibular dysplasia, a ball-and-socket ankle joint, a lax (dysplastic) anterior cruciate ligament, and a mildly hypoplastic lateral femoral condyle. At the other end of the spectrum, fibular deficiency can include femoral shortening, knee valgus from a significantly dysplastic lateral femoral condyle, complete absence of the cruciate ligaments, complete absence of the fibula, a shortened bowed tibia, stiff equinovalgus deformity of the ankle, and a dysplastic foot with hindfoot coalitions and absent lateral rays. It is important to focus on the various presentations of congenital fibular deficiencies; classification systems; factors that influence the choice and types of treatment; and prosthetic consideration for patients treated with foot ablation.

Fibular deficiency is the most common hemimelia¹ with an incidence of 5.7 to 20 cases per million births² as well as the most common congenital long bone deficiency occurring in 1 to 2/100,000 births.³ The deficiency is twice as common in males as in females, and can be unilateral (usually right) or bilateral.⁴

Various mechanisms including absence of the anterior tibial artery,⁵ defects in muscle development,¹ disruption of the apical ectodermal ridge expression,⁶ and developmental field defects⁷ have been proposed as etiologic factors. However, at present, the etiology remains unknown. Most cases are sporadic, while some are associated with a syndrome and other abnormalities.³ Teratologic insults including radiation, busulfan, and retinoic acid have been identified.

The spectrum of fibular deficiency includes varying degrees of deformities of the limb including the pelvis, femur, knee, tibia, ankle, and the foot.⁸ It is important for the clinician to identify associated abnormalities as they will often significantly impact treatment decisions.^9,10,11,12 The most common abnormalities in the femur are proximal femoral focal deficiency, congenital short femur, and coxa vara.^10,11 Distal femoral valgus can lead to genu valgum which
can be associated with patellar instability.¹¹ The cruciates are often absent, with clinical laxity but rarely functional instability.¹¹ Foot abnormalities include ball and socket ankle,¹³ tarsal coalitions,¹⁴ and absent lateral rays. The tibia can be dysplastic or bowed, possible secondary to a residual fibrotic remnant of the fibula restricting normal growth.¹

In addition, the upper extremity may be involved and range from syndactyly to a hypoplastic or aplastic ulna.^9,15,16 While upper extremity abnormalities do not affect ambulation they can limit donning and doffing capability. In cases of bilateral upper extremity abnormalities, the feet may be required for prehension.

A classification system should aid the clinician in choosing the most effective interventions for a given problem, provide information useful for counseling the patient and parents regarding diagnosis, and serve as a basis for future studies of the patient cohort. Ideally, the classification system should further define the deficiency with variables that are known to affect the outcome and that help to identify effective interventions. The identification of these determinant variables must be reproducible to allow researchers to appropriately define populations for study.

The variables that have the greatest effect on outcome in fibular deficiency are limb length discrepancy, ankle function, foot function, and the presence and extent of upper extremity abnormalities.

Coventry et al¹⁷ published a classification system based on radiographic appearance of the fibula and the presence of associated abnormalities. The system did not address function. Type I involves a unilateral deficiency with a hypoplastic fibula, no foot involvement, and a substantial limb-length discrepancy. Type II is a unilateral abnormality, with nearly complete or complete absence of the fibula. Type III is a bilateral abnormality with complete absence of the fibula and a proximal femoral focal deficiency or upper limb involvement or involvement of the contralateral foot or tibia.¹⁷

Achterman and Kalamchi¹⁸ presented a simplified classification system based only on the radiographic appearance of the fibula. It is most applicable for retrospective evaluation when only radiographs are available for review. In type IA abnormalities, the entire fibula is present but dysplastic, and in type 1B there is a partial absence of the proximal fibula and the distal fibula does not support the ankle. A type II abnormality is characterized by complete absence of the fibula. The authors recommended limb length equalization for near normal fibulas (type IA) and early amputation for absence or deficiency associated with an unstable ankle (IB and II).

Birch et al¹⁹ reported on a retrospective review of 104 patients with fibular deficiencies treated at one
institution between 1971 and 2005. They noted that both the Achterman and Kalamchi and Coventry and Johnson classification scheme’s treatment-predictive value was poor. In addition, they found that femoral shortening was of a magnitude that it impacted the overall treatment plan in greater than 80% of the patients. They proposed a simplified classification system based on the clinical status of the foot and the overall limb length discrepancy expressed as a percentage. This allowed the classification system to be applied at birth and was found to more accurately predict management at their institution.

The deficiency is usually apparent at birth, manifested as variable shortening of the fibula and an equinovalgus foot with a stiff hindfoot. There may be associated shortening and distal valgus of the femur and the tibia with apex anterior angulation and an anterior skin dimple over the apex. Frequently, lateral rays of the foot are missing and there is syndactylization of the toes (Figure 4). Knee cruciate deficiency and lateral femoral condyle hypoplasia can produce clinical knee laxity and genu valgum. The patient should be fully examined to assess upper extremity abnormalities as well as contralateral deficiencies. Radiographs of bilateral lower extremities with a ruler are obtained to assess total limb length inequality as a percentage and assess angular deformities.