A method to simulate in vivo cervical spine kinematics using in vitro compressive preload

Study Design. An in vitro flexibility study of C2-T1 specimens under compressive preload. Objective. To determine three-dimensional flexibility test moments needed to obtain spinal kinematics representative of the in vivo spine studies. Summary of Background Data. Most previous three-dimensional in vitro cervical spine studies have used equal moments in all three planes to evaluate spinal flexibilities. Recent advances have made it possible to apply physiologic compressive preload. It is unclear what moments should be applied to these preloaded spine segments to simulate in vivo kinematics. Methods. Six fresh human cadaveric cervical spine specimens (C2-T1) were used. The preload (100 N) was applied by flexible cables, which passed through guides attached to each vertebra. Flexibility tests of flexion-extension and bilateral axial torsion and lateral bending were performed. Two protocols were compared, 1:1:1 with equal pure moments of 1 Nm for each direction and 2:4:2 with pure moments of 2 Nm for flexion-extension and lateral bending and 4 Nm for axial torsion. Ranges of motion were calculated from the flexibility tests. Results. The 2:4:2 protocol resulted in significantly better agreement with in vivo data than did the 1:1:1 protocol. In flexion-extension, the 2 Nm value was within 17% of the average in vivo value. In axial torsion, the 4 Nm value was within 22% of the in vivo average. In lateral bending, the 2 Nm value was within 15% of the in vivo average. Conclusions. To obtain human in vivo-like kinematics using 100 N preload, the 2:4:2 protocol is to be recommended.