MECHANICS OF THE EXTERNAL FIXATION-TEST IN THE LUMBAR SPINE - A ROENTGEN STEREOPHOTOGRAMMETRIC ANALYSIS

Study Design. By implanting tantalum indicators percutaneously during application of pedicular screws, lumbosacral mobility could be studied with roentgen stereophotogrammetric analysis in seven patients having a diagnostic external fixation test. Objectives. To determine the mec hanical effects on the segmental mobility during an external fixation test of the lumbar spine. Summary of Background Data. External pedicul ar fixation test of the lumbar spine has been reported a valuable prog nostic instrument in fusion for low back pain. Methods. A Magerl exter nal fixation device was applied in seven patients with low-grade spond ylolysis-olisthesis. By using roentgen stereophotogrammetric technique , the intervertebral translations in the lumbosacral segment were dete rmined. Each patient had three separate examinations; with the frame f ixed, with the frame loosened, and without frame 6 weeks after screw r emoval. Results. With the external frame fixed, the sagittal intervert ebral translations were significantly reduced, in three cases to a lev el beneath the accuracy of the measuring method. One patient had the s ame immobilizing effect even with the frame loosened while for the oth ers loosening of the frame meant regained mobility of the segment. Con clusion. The properties of the external fixator give an adequate mecha nical basis for the prognostic external fixation test in lumbar fusion .