MECHANICAL TESTING OF THE TENSION BAND WIRE FIXATION IN THE PROXIMAL FEMUR

The mechanical stability of proximal femoral osteotomies fixed by the tension band wire technique was studied in flexion-compression and tor sion tests. The fixation consisted in crossing the section with two Ki rschner wires and with a wire cerclage applied to the tension surface. The study was conducted in three steps. First, cyclinders of wood wer e cut either transversely or at 30 degrees of inclination in relation to the long axis of the specimen, and fixed with two Kirschner wires a nd a wire cerclage. We concluded that the inclination of the plane of section significantly increased the stability of fixation. No signific ant difference was observed when oblique sections were made in the rev erse orientation. Second, 30 degrees subtrochanteric varus osteotomies were performed in dog femurs, so that the section plane was transvers e in one group and oblique in another, after closing the osteotomy. In both groups the fixation was achieved by two Kirschner wires that cro ssed the osteotomy and a wire cerclage placed on the lateral cortex (t ension surface). We concluded that inclination of the osteotomy plane increased the stability of osteosynthesis in bone specimens, as alread y seen with the wood pieces. Third, the stability of tension band wire fixation was compared with that provided by the AO/ASIF paediatric an gled plate. Varus osteotomies (30 degrees) were created at the subtroc hanteric level of paired dog femurs. On one side, the femur was fixed with Kirschner wires and a wire cerclage as described previously. For the other femur, the osteotomy was fixed with the angled plate. We fou nd that both types of fixation presented the same stability in flexion -compression tests. However, under torsion the tension band wire fixat ion was 30%-50% less stable than the plate fixation.