This study was performed to investigate the range of motion and the forces on the facet joints that are neighboured to spondylodesis on thoracolumbar spine. We used a special spine testing device for a continuous application of pure moments in each direction. For measuring the ranges of motion we used a magnetic tracking device for measuring forces on facet joints we chose a direct measuring system of quartz crystal and prepared for investigation of the spine. The biomechanical testing was done on 18 human spinal specimen. We investigated the range of motion and the forces on facet joints in T11/12 and L2/3 segment with a maximal loading of 8 Nm in each direction (flexion, extension, lateral bending and rotation). This was done before and after double level dorsal instrumentation T12-L2 with an internal fixateur. Statistical analysis was performed using the paired t-test and the Wilcoxon test (p < 0.05). After double level instrumentation there were significant larger ranges of motion in flexion and extension and significant larger forces on facet joints in left lateral bending in the T11/12 segment. No significant differences were found in the L2/3 segment. Our findings could be an indication for changing in joints loading. This could be an explanation for early degenerative changes in spinal segments adjacent to spondylodesis. The results confirm the demand of short segment instrumentation and early remove of implants to keep influence as low as possible.