Mechanics of guided growth of the distal femur for correction of fixed knee flexion deformities: an extra-articular technique.

INTRODUCTION: Anterior distal femoral hemiepiphysiodesis using intra-articular plates for correction of pediatric fixed knee flexion deformities (FKFD) has two documented complications: postoperative knee pain and implant loosening. The aim of this study is to investigate the mechanical properties of a novel extra-articular technique for anterior distal femoral hemiepiphysiodesis in patients with FKFD and to compare them to the conventional technique. MATERIALS AND METHODS: Sixteen femoral sawbones were osteotomized at the level of the distal femoral physis and fixed by rail frames to allow linear distraction simulating longitudinal growth. Each sawbone was tested twice: first using the conventional technique with eight plates placed anteriorly just medial and lateral to the femoral sulcus (group A) and then with plates inserted in the proposed novel location at the most anterior part of the medial and lateral surfaces of the femoral condyles with screws in the coronal plane (group B). Gradual linear distraction was performed, and the resulting angular correction was measured. Strain gauges were attached to the plates, and the amount of strain (and equivalent stress) over the plates in response to linear distraction was recorded. The two groups were compared using the Wilcoxon signed-rank test. RESULTS: The amount of angular correction was statistically higher in group B (extra-articular plates) at 5, 10-, and 15-mm of distraction (p < 0.001). As regards stress over the plates, the maximum stress and the area under the curve (sum of all stresses measured throughout the distraction process) were significantly higher when the plates were inserted at the conventional position (group A) (p < 0.001). CONCLUSIONS: During anterior distal femoral hemiepiphysiodesis, the fixation of the eight plates in the coronal plane at the anterior part of the femoral condyles may produce a greater amount of correction and a lower degree of stress over the implants as compared to the conventional technique.