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External fixator-augmented flexible intramedullary nailing of an unstable pediatric femoral shaft fracture model: a biomechanical study

The objective of this study was to test the compressive strength and torsional stiffness provided by the addition of a two-pin external fixator to an unstable pediatric femoral shaft fracture model after being instrumented with flexible intramedullary nailing (FIMN), and to compare this to bridge plating and FIMN alone. A length-unstable oblique diaphyseal fracture was created in 15 pediatric sized small femur models. Fracture stabilization was achieved by three constructs: standard retrograde FIMN with two 3.5-mm titanium (Ti) nails (Group 1), FIMN augmented with a two-pin external fixator (Group 2), and a 4.5-mm bridge plate (Group 3). Groups I and II were tested in 10 cycles of axial rotation to 10° in both directions at 0.1 Hz under 36 kg of compression. Torsional stiffness was calculated. Compressive strength was calculated by applying an axial load of 5 mm/min until failure was encountered. Failure was defined as the force required to achieve 10° varus at the fracture site or shortening of 2 cm. Group II demonstrated a greater compressive strength compared to Group I (1067.32 N vs 453.49 N, P < 0.001). No significant difference in torsional stiffness was found between Groups I and II (0.45 vs 0.38 Nm/deg, P = 0.18). Group III showed superior compressive strength and rotational stiffness compared to Groups I and II. In an unstable pediatric femoral shaft fracture model, augmenting FIMN with a two-pin external fixator increased the compressive strength by 147%, but did not increase torsional stiffness. Bridge plating with a 4.5-mm plate provided superior compressive strength and torsional stiffness.