The static and dynamic behaviour of tibial fractures due to unlocking external fixators

OBJECTIVE: To determine how the mechanical environment of a tibial fracture is influenced by unlocking an external fixator frame. DESIGN: A clinical study examined 10 consecutive patients fixed with the Orthofix DAF. BACKGROUND: It has been claimed that the healing of diaphyseal tibial fractures is assisted by unlocking external fixators to allow free axial movement, but the influence on the mechanical environment at the fracture has not been established. METHODS: A transducer attached to bone screws measured dynamic interfragmentary displacement during walking both before and immediately after unlocking the fixator at 6 weeks in 10 subjects. Four subjects were monitored over the first hour after unlocking to measure interfragmentary gap shortening. RESULTS: Mean peak amplitudes of cyclical axial and angular displacement before unlocking were 0.46 mm (SD 0.27) and 0.37 degrees (SD 0.30), and after were 0.42 mm (SD 0.19) and 0.34 degrees (SD 0.28). Mean peak torsional and transverse shear displacements were 0.21 degrees (SD 0.11) and 0.30 mm (SD 0.17) before unlocking, and after were 0.42 degrees (SD 0.39) and 0.51 mm (SD 0.60). Gaps shortened permanently by unrecoverable axial translations of between 0.2 and 1.4 mm; the mean was 0.69 mm. CONCLUSIONS: Unlocking was found more often to reduce both axial and angular motion, but to increase shear. Overall, this may reduce maximum longitudinal strains in the external callus. The reduced motion may arise from gap shortening.