A method to evaluate the in vivo behaviour of lumbar spine implants

The aim of the study was to design a method for evaluating the stabilizing effect of different lumbar spine implants in vivo, and to apply this method to a comparison of plates versus rods in lumbar spine posterolateral fusion using transpedicular screw fixation. Fourteen patients, seven operated on with transpedicular plates and screws (VSP), and seven operated on with rods and screws (Diapason), matched according to number of levels fused, had tantalum markers inserted in the vertebrae at surgery, enabling roentgen stereophotogrammetric analysis (RSA). Mean patient age was 45 (range 33-56) years. In each group, two patients underwent fusion between L4 and L5, three between L5 and S1, and two from L4 to S1. In three patients, concomitant nerve root decompression was performed using a facet joint preserving technique. RSA was performed 4 weeks after surgery. This interval was chosen to allow enough time for soft tissue healing, but not fusion healing, to occur. RSA was performed in supine and standing position without any mobility provocation, in line with the postoperative regimen given. Movements between the outermost vertebrae of the fusion were calculated along the transverse, vertical and sagittal axes. The method of measurement along these three axes has previously been determined to be accurate to 0.3, 0.6 and 0.7 mm, respectively. One patient stabilized with rods and screws between L5 and S1 displayed a sagittal translation of 1.01 mm but no mobility along the transverse or vertical axes. In the remaining 13 patients, positional change from supine to standing did not provoke any intervertebral mobility above the RSA accuracy along any of the axes. With the limited provocation described, in line with the postoperative regimen for lumbar fusion patients, plates with transpedicular screws and rods with transpedicular screws both seem to give adequate intervertebral stability in posterolateral lumbar fusions.