Anterior column reconstruction of the thoracolumbar spine by structural allograft has an increased potential for biological fusion when compared to synthetic reconstructive options. Estimation of cortical union and trabecular in-growth is, however, traditionally based on plain radiography, a technique lacking in sensitivity. A new assessment method of bony union using high-speed spiral CT imaging is proposed which reflects the gradually increasing biological stability of the construct. Grade I (complete fusion) implies cortical union of the allograft and central trabecular continuity. Grade II (partial fusion) implies cortical union of the structural allograft with partial trabecular incorporation. Grade III (unipolar pseudarthrosis) denotes superior or inferior cortical non-union of the central allograft with partial trabecular discontinuity centrally and Grade IV (bipolar pseudarthrosis) suggests both superior and inferior cortical non-union with a complete lack of central trabecular continuity. Twenty-five patients underwent anterior spinal reconstruction for a single level burst fracture between T4 and L5. At a minimum of two years follow up the subjects underwent high-speed spiral CT scanning through the reconstructed region of the thoracolumbar spine. The classification showed satisfactory interobserver (kappa score = 0.91) and intraobserver (kappa score = 0.95) reliability. The use of high-speed CT imaging in the assessment of structural allograft union may allow a more accurate assessment of union. The classification system presented allows a reproducible categorization of allograft incorporation with implications for treatment.