Ankle Valgus Deformity Secondary to Proximal Migration of the Fibula in Tibial Lengthening with Use of the Ilizarov External Fixator

Abstract

Ankle valgus deformity secondary to proximal migration of the fibula following an Ilizarov tibial lengthening has not been discussed in detail in the literature. The purposes of this study were to determine the underlying mechanism of and to identify factors associated with proximal migration of the fibula that caused ankle valgus deformity after an Ilizarov tibial lengthening. We reviewed the outcome of seventy-four bilateral Ilizarov tibial lengthenings for short stature in thirty-seven patients. The mean age at the time of surgery was 21.7 years (range, thirteen to thirty-one years), and the mean duration of follow-up was forty-five months. Proximal migration of the fibula was assessed with changes in the malleolar tip distance. A valgus change of ≥ 5° in the tibiotalar angle was regarded as ankle valgus deformity following tibial lengthening. The average length gain was 6.9 cm (range, 4.7 to 11.5 cm), and the average lengthening index was 1.5 mo/cm. Valgus deformity developed in six ankles (8%) and fibular nonunion developed in ten (14%). Proximal migration of the lateral malleolus of ≥ 5 mm was related to valgus talar tilting. Bifocal tibial lengthening, rapid distraction rate of the fibula (>1 mm per day), and development of a fibular nonunion were factors associated with proximal migration of the distal end of the fibula of ≥ 5 mm, which suggests that regenerated bone of poor quality in the distraction gap may cause proximal migration of the distal end of the fibula following tibial lengthening. Proximal migration of the distal end of the fibula following tibial lengthening may occur even with the use of an Ilizarov ring fixator. This migration seems to be caused by collapse of regenerated bone of poor quality or fibular nonunion. Proximal migration of ≥ 5 mm is associated with the risk of valgus talar tilting. Surgeons should consider earlier intervention with bone-grafting if there are conditions that compromise regenerated bone formation in the fibular distraction gap, such as can occur with extensive tibial lengthening by bifocal corticotomy.