Volumetric change of the graft bone after intertransverse fusion

Study Design. Patients in whom good intertransverse fusion had been achieve d were selected for the volumetric study of the fusion mass using sequentia l computed tomography scans. Objectives. To assess the natural volumetric change of intertransverse fusi on mass and the effect of the disease entity and spinal instrumentation on the fusion mass volume. Summary of Background Data. The magnitude of volumetric change of the graft bone after intertransverse fusion is still inconclusive. Methods. Fifteen adult patients who underwent decompression surgery with si ngle-level lumbar and lumbosacral intertransverse fusion were selected for this study. Preoperative diagnoses were degenerative spondylolisthesis in n ine patients and isthmic spondylolisthesis in six. Seven of the 15 patients received pedicle screw fixation. They were categorized into two major grou ps: 1) instrumented and noninstrumented groups and 2) isthmic and degenerat ive groups. To assess the volumetric change of the graft bone, sequential c omputed tomography scans were obtained 2 weeks after surgery and again 18 m onths after surgery. Results. The overall initial mean graft volume was 6251 mm(3), which decrea sed to 2842 mm3 by 18 months after surgery (P < 0.001). The overall mean vo lume loss between the two periods was 54.8% of the initial graft volume. Al though there was no significant difference in the mean graft volume between the groups at either 2 weeks or 18 months after surgery (P > 0.05 in all c omparisons), the mean graft volume in each group decreased significantly du ring the observation period (P < 0.01 in all comparisons). There was no sig nificant difference in the mean volume loss or in the ratio of residual vol ume to the initial graft volume between the groups during the study period (P > 0.05 in all comparisons). The initial graft volume correlated positive ly with the graft volume at 18 months after surgery (r = 0.612, P < 0.01) a nd volume loss (r = 0.949, P < 0.01), but negatively with the residual volu me ratio (r = -0.507, P < 0.01). Conclusions. These results showed that more than one half of the initial gr aft bone volume was being absorbed during the consolidation processes of th e graft bone, and that the volume loss during the period was not significan tly affected by the spinal instrumentation or by the disease entity. It was also found that the greater the amount of the initial graft bone, the larg er the fusion mass at 18 months after surgery. The volume loss, however, in creased proportionally to an increase in the initial graft bone volume. The efficiency (ratio of residual volume to the initial graft volume) of the i ntertransverse fusion also tended to decline as the initial graft volume in creased.