Dynamics of syndesmophyte growth in AS as measured by quantitative CT: Heterogeneity within and among vertebral disc spaces

Objective. Syndesmophytes in AS typically grow slowly, but it is not known whether growth is uniform among syndesmophytes in the same intervertebral disc space (IDS) or among different IDSs in the same patient or if growth is heterogeneous. We examined the dynamics of syndesmophyte growth over 24 months using CT, with the main aim of determining if syndesmophytes in the same IDS or the same patient grow at similar rates.Methods. We performed lumbar spine CT scans on 33 patients and measured syndesmophytes in four IDSs using a validated computer algorithm. Scans were done at baseline and 12 and 24 months. We compared absolute and percentage changes in volume from baseline to 12 months and to 24 months among syndesmophytes in the same IDS and among four IDSs of each patient. We also examined whether growth among all IDSs differed between study years.Results. Among 60 IDSs with at least two syndesmophytes at baseline (range 2-6), there was substantial heterogeneity in both absolute (P < 0.0001) and percentage (P = 0.0002) volume increases among syndesmophytes in the same IDS. Several IDSs had both syndesmophytes with no growth and syndesmophytes that increased by >100 mm³. Similarly there was significant heterogeneity in syndesmophyte growth among IDSs of individual patients. Increases in total syndesmophyte volume for each patient also tended to differ between study years (P = 0.07).Conclusion. Syndesmophytes in AS do not all grow continuously. Rates of growth over 24 months commonly differ between syndesmophytes in the same IDS and between different IDSs in the same patient, suggesting that local factors regulate syndesmophyte growth.