What Fuses When Dynamic X-rays Document Stability Following Posterior Cervical Fusion?

How would CT-based criteria correlate with dynamic X-rays in documenting posterior cervical fusion? To study this, CT and dynamic radiographic studies were prospectively evaluated 3, 6, and up to 12 months postoperatively in 14 patients with severe myeloradiculopathy (Nurick Grade IV) undergoing dorsal decompression with fusion). “Focal” 1-2 level laminectomies with undercutting of adjacent levels were accompanied by average 6.5 level posterior fusions utilizing a rod/eyelet/braided cable system in conjunction with iliac crest autograft and demineralized bone matrix. Fusion on dynamic X-rays was defined by less than 1 mm of motion between adjacent spinous processes with less than 3 mm of translation between the vertebral bodies. Four CT-based indicators for fusion were evaluated: facet fusion, graft fusion [bone continuity overlying facet joints/ lamina], new anterior vertebral fusion, and >50% of total graft mass remaining. CT-based facet fusion most closely correlated with dynamic X-ray confirmation of fusion. At 3 months, 10 patients showed facet fusion while 8 demonstrated stability on dynamic films; all 14 patients met both fusion criteria by 6 months and continued to demonstrate these findings at one year postoperatively. At 3 and 6 months, graft fusion increased from 5 to 9 while anterior vertebral body fusion increased from 3 to 5; by 1 year, 12 demonstrated graft fusion and 7 anterior fusion. However, CT-based graft mass regressed rather than progressed over time; 7 showed graft mass of over 50% 3 months postoperatively which further diminished to 7 at 6 months and 6 by 1 year postoperatively. Together, dynamic X-rays and 4 CT-based criteria of fusion provided a more complete picture of the evolution of posterior cervical fusion.