Study Design. Multidirectional flexibility tests were conducted on 10 human thoracic spines with intact rib cage.
Objectives. To determine the amount of stability the rib cage imparts to the thoracic spine and to show the amount of stability lost by a sternal fracture.
Summary of Background Data. There is no published study of biomechanical testing of human cadaveric specimens with the rib cage intact.
Methods. In this study, 10 human cadaveric thoracic spines with the rib cage intact were tested using a biaxial material testing machine and an opto-electronic three-dimensional motion measuring device (Opto-trak 3020). The specimens were tested in axial compression, axial rotation, lateral bending, and flexion/extension. First, the specimens were tested through all four loading types with the sternum and rib cage intact. Next, the sternum was fractured at the sternomanubrial junction displacing the proximal fragment posteriorly. Lastly, the entire rib cage was removed.
Results. The rib cage increased the stability of the thoracic spine by 40% in flexion/extension (P = 0.012), 35% in lateral bending (P = 0.008), and 31% in axial rotation (P = 0.008). An indirect flexion-compression type of sternal fracture decreased the stability of the thoracic spine by 42% in flexion/extension (P = 0.036), 22% in lateral bending (P = 0.038), and 15% in axial rotation (P = 0.011).
Conclusion. The rib cage significantly increases the stability of the thoracic spine in flexion/extension, lateral bending, and axial rotation. A sternal fracture significantly decreases the stability of the thorax.