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After posterior stabilization of the spondylolytic lumbosacral level, mobility of the fused vertebrae could be studied before and after an additional anterior endoscopic interbody fusion using roentgen stereophotogrammetric analysis.To determine the in vivo primary lumbosacral stability of additional anterior interbody fusion after transpedicular screw fixation.In vitro studies indicate a significant decrease in segmental motion after pedicle screw fixation and additional anterior fusion. Roentgen stereophotogrammetric studies demonstrate the adequacy of transpedicular lumbar instrumentation in posterolateral fusions. There are no studies examining the effect of additional anterior interbody fusion after posterior instrumentation in vivo.In this study, 15 patients with low-grade spondylolisthesis at L5–S1 underwent a two-stage open posterior and endoscopic anterior lumbar fusion using carbon fiber (Brantigan I/F) cages. At surgery, tantalum markers were implanted into the fifth lumbar (L5) and the first sacral (S1) vertebra. All the patients were examined by roentgen stereophotogrammetric analysis after the first and second surgical procedures.After implantation of the posterior pedicle system only, the mean intervertebral mobility determined by roentgen stereophotogrammetric analysis was 0.23 mm in the transverse (x), 0.54 mm in the vertical (y), and 1.2 mm in the sagittal (z) axes. After additional anterior endoscopic fusion with carbon cages, the remaining translation between the fused segment L5/S1 decreased to 0.17 mm in the x, 0.16 mm in the y, and 0.44 mm in the z axes.Anterior endoscopic lumbosacral fusion significantly increases the primary stability of the posterior fusion with a pedicle system in two axes of motion.