A Study of Effects of Age and Hyperkyphosis on Spine Motion and Loading

The purpose of this study is to obtain unique measures of thoracic spinal motion in young, older, and hyperkyphotic older adults. We will then develop unique subject-specific musculoskeletal models of these individuals to estimate loads applied to the vertebrae in vivo, and examine how spinal motion and loading vary with age and increased kyphosis. Vertebral fractures (VFs) are the most common type of fracture in older adults, occurring in 20-35% of women and 15-25% of men over the age of 50, and are associated with significant morbidity, increased mortality, and annual costs exceeding $1 billion in the United States. However, limited understanding of the mechanisms (beyond low vertebral bone mineral density and strength) that lead to VFs hinders our ability to predict and prevent these injuries. Similarly, hyperkyphosis, defined as excess forward curvature of the thoracic spine, is suffered by 20-40% of older adults, but its causes are poorly understood and it has no standard clinical treatment. Hyperkyphosis and VFs are inter-related, as individuals with VFs often have worse kyphosis, while hyperkyphosis is an independent risk factor for future VFs. Hyperkyphosis may increase VF risk through increased vertebral loading, but better understanding is needed of the biomechanics of this common spine condition. VFs occur more often at mid-thoracic (T7-T8) and thoraco-lumbar (T12-L1) vertebrae than elsewhere in the spine, and it has been suggested that biomechanical factors predispose these areas to fracture by increasing vertebral loading. In the first phase of this project, a novel musculoskeletal model was developed that uniquely predicts peaks in vertebral loading around the T12-L1 region of the spine, but this was not observed in the mid-thoracic region. Our preliminary data suggested that increased thoracic stiffness causes greater vertebral loading at mid-thoracic levels (T7-T9), while increased thoracic kyphosis increases vertebral loading, particularly in the thoraco-lumbar (T12-L1) region. Further advances in musculoskeletal modeling will are needed to fully evaluate these possibilities, but a particular knowledge gap remains regarding the in vivo kinematics of the thoracic spine and ribcage in both healthy and hyperkyphotic individuals. This project aims to fill that gap by producing novel in vivo measurements of thoracic spine motion in young, older, and hyperkyphotic older adults.