The relative contribution of discs and vertebral bodies to thoracic kyphosis in healthy volunteers.

Introduction: Understanding the normal anatomy of thoracic kyphosis (TK) in healthy subjects is essential for evaluating sagittal malalignment and planning the surgery accordingly. The aim of this study was to identify the proportion of thoracic kyphosis originating from disc versus vertebral body shape and to describe its variation according to age and thoracic kyphosis magnitude.

Methods: This study was a retrospective review of a prospective multicenter database of healthy volunteers aged 18 years or older. Vertebral body and disc sagittal Cobb angles were measured at each level and summed within each of the 3 TK regions (Upper, Middle and Lower TK). Relative contributions of discs and vertebral bodies to Upper, Middle, Lower, and total TK were assessed in the whole cohort, and according to age and TK groups, after stratification. Finally, a multivariate analysis including age and TK magnitude was conducted.

Results: Among these 645 subjects, the mean age was 37.6 ± 16.3 years with 51% of females. Intervertebral discs were kyphotic in Upper and Middle TK with respective discs contribution to total TK of 4.2% and 9.6%, for a total of 13.8% of total kyphosis. Lower TK discs were lordotic, with a participation of -13.2% of total TK, leading to an overall discs contribution to TK of 0.6%. Vertebral bodies were all kyphotic with a contribution of 99.4% of total kyphosis. Vertebral bodies kyphosis increased across age groups for Middle TK (p = 0.004), Lower TK (p < 0.001), and Total TK (p < 0.001). Discs contributions to total TK increased significantly with increasing TK (-13.8% for Low TK, -1.5% for Average-Low TK, 5.7% for Average-High TK and 9.1% for High TK), (p < 0.001). Finally, discs contribution was significantly greater in males than in females, with respective values of 2.6% and -1.8% (p = 0.01).

Conclusion: This study highlights the predominant role of vertebral bodies contribution to thoracic kyphosis, 99.4% on average. The contribution of disc to thoracic kyphosis (values ranging from -13.8% to 9.1%) increases significantly with increasing thoracic kyphosis magnitude. The association of age with thoracic kyphosis was greater for vertebral bodies than discs, particularly in Middle and Lower TK.