No abstract

No abstract

Simple theoretical deflection analysis and experimental data have been used to assess the stiffness of the Oxford External Fixator for a range of fixator configurations. The loading modes comprised axial compression, bending (in two planes), and torsion. Data are also provided on the effects of clinically loose Schanz bone screws (or pins) on the fixator stiffness. In general :loosening of one bone pin significantly decreases fixator stiffness; loosening of two pins may decrease the stiffness by 50 per cent or more; loosening of one bone pin causes increased loading on the adjacent tight pin, which may lead to loosening of the second pin; the fixator stiffness is maximized by using the shortest practicable bone pin length and the widest practicable spacing between adjacent pins in the same bone segment; short pins and wide spacing result in the lowest bone/pin interface stresses and minimize the possibility of bone pins loosening.

A theoretical and experimental evaluation of the Mark II Oxford External Fixator with sliding clamps has shown that: the effective axial stiffness may readily be varied over a wide range; significant load-bearing may be achieved on the fractured bone while maintaining adequate fracture alignment and stability; the axial sliding facility is likely to be of greatest benefit in the early stages of fracture healing. Sliding decreases rapidly as fracture stiffness increases and ceases altogether at a relatively early stage in the healing process; the effects of bone pin loosening are similar to those previously reported for the fixed-clamp Oxford External Fixator; bone healing assessment using a strain-gauge transducer clamped to the fixator column may become difficult and inaccurate when sliding of the clamps occurs. Temporary locking of the clamps is recommended during the assessment procedure to ensure that sequential tests are strictly comparable.

No abstract