Gradual Deformity Correction with a Computer-assisted Hexapod External Fixator in Blount's Disease

Aim:To evaluate the results in terms of correction and complications from gradual correction with a computer-assisted hexapod circular external fixator in a mixed cohort of children with Blount's disease. Materials and methods: A retrospective review was performed of the correction and complications of 19 children (25 limbs) with recurrent infantile (IBD) and late-onset Blount's disease (LOBD) treated by gradual correction with a hexapod external fixator. The correction was measured by the medial proximal tibial angle (MPTA), anatomic posterior proximal tibial angle (aPPTA) and anatomic tibio-femoral angle (TFA). Obesity was present in 76% (19/25) of cases. Fifteen limbs were classified as infantile Blount's disease and 10 limbs as late-onset Blount's disease. The mean age was 12.5 years (range 7-17 years). Results:The mean pre-operative MPTA of 59 degrees (SD 13 degrees, range 33-79 degrees) was corrected to a mean of 86 degrees (SD 5 degrees, range 77-93 degrees). The mean pre-operative aPPTA of 64 degrees (SD 14 degrees, range 33-84 degrees) was corrected to 79 degrees (SD 6 degrees, range 70-90 degrees). The median pre-operative rotation of 15 degrees internal rotation was corrected to normal (0-15 degrees of external rotation). Eight out of 25 limbs had severe deformities with varus or procurvatum greater than 40 degrees or both. The mean pre-operative TFA of 28 degrees varus (SD 13 degrees, range 4-53 degrees) was corrected to 1.8 degrees valgus (SD 6 degrees, range 14 degrees varus to 13 degrees valgus). The median follow-up was 19 months (range 6-67 months). The alignment after correction was "good" in 55% (11/20), "acceptable" in 35% (7/20) and "poor" in 10% (2/20). The median duration for correction was 16 days (IQR 11-31 days, range 7-71 days). The median number of prescribed correction programmes was 1 (IQR 1-2, range 1-5). The mean total time in the frame was 136 days (SD 34 days, range 85-201 days). All patients developed minor pin track infections that resolved with oral antibiotics (Category 1 complications). Four patients developed complications that necessitated modification of the treatment plan (Category 2 complications). In two cases, treatment objectives could not be achieved (Category 3 complications). Two patients treated before skeletal maturity developed recurrent genu varum. Conclusion: Gradual correction with a computer-assisted hexapod external fixator may be a useful technique for correcting recurrent IBD or LOBD even in children with severe deformities. The results of gradual correction were similar in the two groups. While complications occur, most can be mitigated by timely intervention during the correction phase of treatment. Recurrence remains a concern if correction is performed before skeletal maturity. Level of evidence