Bone Mineral Densities of the Vertebral Body and Intertransverse Fusion Mass After Instrumented Intertransverse Process Fusion

STUDY DESIGN: We used dual-energy x-ray absorptiometry to examine the bone mineral densities (BMDs) of the vertebral bodies at the fused level (the fused vertebral BMDs), at the unfused level (the unfused vertebral BMDs), and the intertransverse fusion mass (the fusion mass BMD) after instrumented intertransverse process fusion. OBJECTIVE: We wanted to determine whether there are any relationships among the unfused vertebral BMDs, the fused vertebral BMD, and the fusion mass BMD after successful solid union. SUMMARY OF BACKGROUND DATA: Device-related vertebral osteoporosis is a well-known phenomenon that occurs in an early adaptive phase after instrumented spinal fusion. However, any relationships among the unfused vertebral BMDs, the fused vertebral BMD, and the fusion mass BMD in a later phase after obtaining successful spinal union are unknown. METHODS: This study included 17 female patients, who had a solid union after instrumented intertransverse process fusion at the L4-L5 level. The average patient age at the time of this study was 54 years (range: 46-69 years). The vertebral BMDs at the fused level (L4), the vertebral BMDs at the unfused levels (L1-L3), and the fusion mass BMD were measured at a mean follow-up period of 42 months (range: 24-131 months) after surgery. The reciprocal correlations among the vertebral BMDs and the fusion mass BMD were analyzed. RESULTS.: The vertebral BMDs (g/cm) at the fused level (L4), the unfused level (L1-L3), and the fusion mass BMD were 0.84 ± 0.27, 0.72 ± 0.23, 0.56 ± 0.19, 0.58 ± 0.21, and 0.54 ± 0.16, respectively. There were significant reciprocal correlations among the vertebral BMDs at the fused and unfused level. However, the fusion mass BMD had no significant correlations with any of the vertebral BMDs. CONCLUSION: The vertebral bodies at the fused level may undergo osteoporosis in a pattern similar to what naturally occurs in the vertebral bodies at the unfused level. However, the fusion mass BMD seems to be influenced by bone remodeling mechanisms that are different from those that influence the vertebral bodies.