Biomechanical and CT analysis of triplanar facial strut trauma following steering wheel impact

The probability of facial bone fracture following dynamic contact with steering assembly impacts was determined. Energy absorbing and standard steering wheels were used. Using a specially designed vertical-drop test system, either zygoma was impacted once onto the junction of the left lower spoke and rim with velocities ranging from 6.93 - 2.01 m/s. A six-axis load cell was placed underneath the hub of the steering wheel which was oriented at 30° to the horizontal. A combination of linear and rotary potentiometers recorded the steering wheel deformations. Dynamic forces at the zygoma were computed using these steering wheel deformations (translations and rotations) and six-axis load cell data by transformation principles. A triaxial accelerometer was placed on the posterior parietal region of the skull opposite to the impact site to record acceleration histories. Two-dimensional and 3-D CT, x-rays and defleshing techniques were used to assess injury. Fractures were classified into solitary strut, complex strut and transfacial fractures. These included fractures of the zygomatic area, orbit, nasal and maxillary regions. High speed photography was used to document the kinematics. Fractures correlated well with dynamic impact forces but poorly with accelerations, steering wheel deformations, head injury criteria, and contact areas at the impact site. These findings suggest that a criterion to assess the probability of facial fructure should be based on the impact force.