Abstract

It is generally considered that mechanical stress is concentrated on unfixed adjacent intervertebral discs and that degenerative change of the discs is accelerated after intervertebral fixation. However, when assessing the influence of intervertebral fixation on adjacent discs clinically, it should be considered that degenerative changes occur with aging and may also be related to various individual predisposing factors. Lumbar fusion is mainly done posteriorly, PLIF and TLIF, and both these operative methods have resembled character compared with anterior lumbar spinal fusion. We analyzed pre- and postoperative X-ray films after PLIF, and will mention about better intervertebral fusion to adjacent segments.Materials and MethodsThe subjects comprised 48 patients who had undergone PLIF of one or two discs more than 5 years previously, for a mean of 8.6 years. Measurement was performed using lateral radiographs taken in the standing position before surgery and at follow-up. Narrowing of the disc space, instability, and traction spur were checked.ResultsNarrowing of the disc space either proximal or distal to the fusion level was noted in 15 of the 48 patients (31%). Among these 15 patients, narrowing was seen at the disc just proximal to the fusion in 11 cases, at the disc distal to the fusion in 2 cases, and both proximally and distally in 2 cases. Narrowing of the disc spaces that were not adjacent to the fusion level was noted in 6 cases. New development of traction spurs was seen in two cases and the spurs increased over the preoperative size in seven cases at adjacent and nonadjacent segments, but almost always at the disc levels proximal to the fused disk. Discussion and ConclusionTo assess the incidence of narrowing at adjacent disks after intervertebral fusion, we focused on patients with fusion at L4/5. Among our 48 patients, 37 had single fusion at L4/5. Among these 37 patients, narrowing or progressive narrowing of adjacent disk spaces was seen at 3/4 in 11 cases (30%), at L2/3 in 5 cases (14%), at L1/2 in 2 cases (5%), and at L5/S in 3 cases (8%). These findings suggested that the mechanical stress was mainly concentrated at L3/4 and L2/3 after fusion at L4/5. Theoretically, L5/S would be subjected to the greatest mechanical stress because it abuts the pelvis, as well as the greatest sagittal shearing force. When intervertebral fusion at L4/5 is necessary, the L5/S level has already been stabilized, and in consequence of the concentration of mechanical stress on the proximal segments. In the Japanese population, it was reported that intervertebral narrowing or progressive narrowing occurs at L3/4 in 24% and at l2/3 in 24% after follow-up of more than 10 years. In conclusion PLIF causes increased mechanical stress on the proximal segments, but degenerative changes of the adjacent segments are not marked. When we perform posterior intervertebral fusion in the lumbar area, we have to pay attention to the result of this clinical study.