THE STATIC AND DYNAMIC BEHAVIOR OF TIBIAL FRACTURES DUE TO UNLOCKING EXTERNAL FIXATORS

Objective. To determine how the mechanical environment of a tibial fra cture is influenced by unlocking an external fixator frame. Design. A clinical study examined 10 consecutive patients fixed with the Orthofi x DAF. Background. It has been claimed that the healing of diaphyseal tibial fractures is assisted by unlocking external fixators to allow f ree 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 w eeks in 10 subjects. Four subjects were monitored over the first hour after unlocking to measure interfragmentary gap shortening. Results. M ean 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 torsiona l 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 0.3 9) 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. Co nclusions. Unlocking was found more often to reduce both axial and ang ular motion, but to increase shear. Overall, this may reduce maximum l ongitudinal strains in the external callus. The reduced motion may ari se from gap shortening. Relevance Reduced longitudinal strains in exte rnal callus may reduce the risk of re-fracture. Gap shortening brings the fragment ends into closer proximity, this should aid the stages of bone formation and remodelling. Therefore, strain reduction and gap s hortening may be responsible for the benefit to healing claimed for th e unlocking procedure. Copyright (C) 1996 Elsevier Science Ltd.