Thoracic Volume Progressively Decreases in a Virtual Osteoporotic Vertebral Wedging Model with Increasing Severity of Kyphotic Deformity

Abstract

Introduction Osteoporotic vertebral fractures commonly result in increased kyphotic angle. Several studies have shown that compared with controls increasing severity of osteoporotic kyphosis significantly decreases volumetric parameters of pulmonary function such as vital capacity. However the effect on the thoracic volume has been less fully studied. With the ability to accurately model thoracic volumes, we are now capable of virtually modeling the effect of increasing kyphosis on thoracic volume through the computer graphical software, Blender. We hypothesize the computed thoracic volume measured from a 3D virtual model decreases with progression of kyphosis from 50 to 100 degrees and the wedging fracture vertebrae only occur in sagittal plane. Methods Free opensource Blender software was used to create threedimensional thorax models with normal coronal and sagittal balance. By virtually wedging the vertebral bodies, the virtual osteoporotic models were progressively deformed by 10 degrees from 50100 degrees of kyphosis. Thoracic volume was simulated and measured via shrinkwrap modifier (Blender built-in function) applied to a sphere-shaped object within the virtual thoracic cavity. Results Virtual wedging of vertebral bodies was necessary to achieve virtual progression of sagittal curve. As kyphotic angle increased, more vertebral segments had to be virtually wedged. Thoracic volume in the virtual adult thorax with a 50 degrees kyphosis was 3643cc. The thoracic volumes approximately decreased in a linear fashion by means of 6.4% for every 10 degrees more of kyphosis. The final thoracic volume was 2472cc at 100 degrees of kyphosis, which was a 32.2% decrease in volume from baseline. Conclusion As the deformity progresses from 50 to 100 degrees, there is an overall 32.2% decrease in thoracic volumes. The decrease appears approximately linear. Although there is not a direct relationship between thoracic volume and pulmonary functions, adequate thoracic volume is a necessary condition for adequate pulmonary function. This study helps establish the relative magnitude of thoracicvolume decrease based on the sagittal Cobb angle.