Mechanics of the External Fixation Test in the Lumbar Spine

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Abstract
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. To determine the mechanical effects on the segmental mobility during an external fixation test of the lumbar spine. External pedicular fixation test of the lumbar spine has been reported a valuable prognostic instrument in fusion for low back pain. A Magerl external fixation device was applied in seven patients with low-grade spondylolysis-olisthesis. By using roentgen stereophotogrammetric technique, the intervertebral translations in the lumbosacral segment were determined. Each patient had three separate examinations; with the frame fixed, with the frame loosened, and without frame 6 weeks after screw removal. With the external frame fixed, the sagittal intervertebral translations were significantly reduced, in three cases to a level beneath the accuracy of the measuring method. One patient had the same immobilizing effect even with the frame loosened while for the others loosening of the frame meant regained mobility of the segment. The properties of the external fixator give an adequate mechanical basis for the prognostic external fixation test in lumbar fusion.