Single and incremental trauma models: a biomechanical assessment of spinal instability

Biomechanical analysis of spinal injury in the laboratory requires the development of trauma models that simulate spinal instability. Current experimental trauma protocols consist of two types: single or incremental impacts. The incremental protocol has several advantages. However, the equivalence of the spinal instabilities produced by the two trauma protocols is currently unproven. The purpose of this study was to investigate whether the single and incremental trauma models produce equivalent soft tissue instabilities in the lumbar spine. Ten freshly frozen porcine lumbar spines were divided into two functional spinal units (FSUs), L2-L3 and L4-L5. FSUs were then randomized to either the single trauma (ST) or incremental trauma (IT) protocol. The IT protocol consisted of four sequentially increasing high-speed axial compression traumas, while the ST protocol was a single impact of the same magnitude as the final trauma of the IT. Before and after the final trauma, each FSU underwent flexibility testing under flexion/extension, lateral bending, and axial torsion pure moments. No significant differences were found in neutral zone or range of motion between IT and ST specimens in any of the three axes of motion, either before or after the trauma. In addition, no differences were found between the normalized motions of the IT and ST groups. The FSUs subjected to incremental trauma do not suffer greater injury than those subjected to a single impact. The data support the equivalency of the subfailure soft tissue injuries of the spine caused by the incremental and single trauma protocols respectively. This finding is important, because only with the incremental trauma protocol is one able to obtain injury threshold, study injury progression in the same specimen, produce a defined injury more accurately, and efficiently utilize scarce human cadaveric specimens.