Early Changes in Bone Mineral Density Above a Combined Anteroposterior L4-S1 Lumbar Spinal Fusion

Study Design Fifteen patients meeting rigid study entrance criteria for undergoing a combined anteroposterior L4-S1 spinal fusion as a first time operation were included in the study. Bone mineral density changes at adjacent segments (L3 and L2) were assessed by scanning patients in the anteroposterior and lateral projections before surgery and 3 and 6 months after surgery. Objectives To prospectively evaluate changes in bone mineral density at adjacent segments after lumbar spinal fusion and postoperative bracing. Summary of Background Data The majority of studies investigating changes in bone mineral density after spinal fusion have been performed in canines. Although such work has provided valuable insight, its application to humans is questionable because the loading mechanics are so different. Methods Bone mineral density was assessed noninvasively by dual energy x-ray absorptiometry in the lateral and anteroposterior projections. Data were analyzed with special edge detection software. Results In the lateral projection at L3, one level above the fusion, the preoperative bone mineral density was 0.91 ± 0.24 (mean ± standard deviation in g/cm2). This decreased significantly at 3 months to 0.82 ± 0.24 and remained reduced at 6 months (0.80 ± 0.21). This reflected a loss of primarily trabecular bone. In the anteroposterior projection at L2, there was a significant loss at 3− and 6-month follow-up (1.24 ± 0.15 to 1.19 ± 0.15 and 1.19 ± 0.14). This may reflect changes at either or both of the enterior and posterior columns. Conclusion Significant decreases in bone mineral density were noted above fused lumbar segments. This may have been related to immobilization or altered mechanics associated with arthrodesis. Trabecular bone of the axial skeleton is most responsive to change, compared with cortical bone, because of the higher metabolic activity. This may explain the reduction of bone mineral density at L3. Changes at L2 may have been related to altered loads at the anterior or posterior columns and should be further investigated.