Early Union of Grafted Bone in Ankylosing Spondylitis: Comparative Study with Degenerative Spinal Disease

BACKGROUND Patients with ankylosing spondylitis (AS) achieve early bone union compared to those with other spinal diseases. This study compared the time to bone union after surgery between AS patients and degenerative spinal disease patients. METHODS Patients with degenerative spinal diseases (control group) and AS (experimental group) underwent pedicle subtraction osteotomy followed by posterolateral fusion, and decompression and posterolateral fusion, respectively. There were 10 patients in the experimental group. The control group included 26 patients who were less than 50 years of age and underwent two-level autogenous grafting after decompression and spinal fusion. Autogenous grafts and a range of bone substitutes were used in the experimental group, whereas only autogenous grafts were used in the control group. Bone union was determined on the radiographs and 3-dimensional CT scan images. The level of union was assessed using the Lenke's and Christensen's classification systems. RESULTS In the experimental group, the mean age was 41.3 years (range, 30 to 67 years), the mean follow-up period was 21.7 months (range, 12 to 43 months), and bone union was confirmed at an average of 3.5 months (range, 3 to 5 months) after surgery. In the control group, the mean age was 43.1 years (range, 35 to 50 years), the mean follow-up period was 21.8 months (range, 12 to 74 months), and bone union was observed at an average of 5.6 months (range, 4 to 12 months) after surgery. The difference in the time to bone union between the two groups was significant (p = 0.023). CONCLUSIONS The union of grafted bone was obtained earlier in patients with AS than in those with degenerative spinal diseases. Therefore, future studies should examine the factors affecting the early union in AS patients.