Patients with chronic low back pain are sometimes treated with lumbar fusion. Two or more vertebrae are held together with bone graft. Metal plates and screws, called instrumentation, may be applied to hold the problem area still and help the graft heal. Fusion limits motion at that level of the spine. However, it may cause increased movement at the levels just above and below. Some researchers think that the fused spine causes degenerative changes in the joints just above and below the fusion.
Engineers, physicians, and scientists worked together in this study to measure forces on the fused spine. They used human cadavers (bodies preserved for study). Special technology was used to apply pressure to the spine at various levels. Range of motion was measured at each level during this testing. Pressure within the disc was also measured. They found there is increased tension on the disc at the level of the fusion in some (not all) motions. Movement at the level of the fusion was reduced, but not eliminated.
The authors also reported some motions are affected at the fused level. The spine is stiffer, but not necessarily blocked from moving. Fusion appears to cause increased motion in the segments above and below. However, these segments don’t move any more than other areas of the spine.
Current research shows that all spinal levels in the low back are affected by fusion in this part of the spine. Increased loading and motion can be measured as proof. According to the authors, the spine degenerates in sections. It is likely that the segments just above and below the fused level are prone to damage from the added motion. Parts of the spine that are already degenerated seem to affected the most, whether they are close to the fusion or farther above or below.