Biomechanics of flail chest injuries: tidal volume and respiratory work changes in multiple segmental rib fractures

Flail chest (FC) injuries are segmental osseous injuries of the thorax that typically result from high-energy blunt trauma and regularly occur in multiple trauma (MT) patients. FC injuries are associated with paradoxical chest wall movements and, thus, have a high risk of respiratory insufficiency or even death. An increasing number of studies recommend an early surgical stabilization of FC injuries, but a definite trigger that would indicate surgery has, thus far, not been identified.

Based on real-world injury computed tomography (CT) data, this study aimed to establish a finite elements (FE) model of a thorax simulating spontaneous breathing. The model is based on a 0.625 mm slice thickness CT data set. In this FE model, various FC injury patterns were implemented to examine the impact of an increasingly large flail segment on tidal volume and respiratory work. The impact of the segmental defect sizes on the outcome measures mentioned above was examined using correlation analyses.

The FE model in this study reliably simulated the spontaneous breathing patterns of an actively breathing patient in an uninjured setting as a reference and showed clinically realistic movements of the flail segments for various injury settings. Correlation analysis showed a significant negative correlation between the FC size and tidal volume (R² = 0.852, p = 0.003), while absolute (R² = 0.845, p = 0.0096) and relative loss (R² = 0.844, p = 0.0096) of tidal volume concerning the intact model and the compensatory respiratory work required (R² = 0.816, p = 0.0136) were positively correlated with FC size.

This study presents an FE model of the thorax of a patient who presented to our clinic as an MT patient with an FC injury. The FE model fulfills physiologic active breathing patterns and simulates an FC injury's paradoxical movement, realistically depicting clinical observations. The FE model showed that the number of consecutive ribs involved in the flail segment and the length of the flail segment significantly impacted active breathing concerning tidal volumes and respiratory work. With this, we have made the first step to define a trigger for surgery.