OTA 1997 Posters - Spine Fractures

*In vivo Stiffness of Spinal Segments after Healing of Unstable Thoracolumbar Burst Fractures

Lothar L. J. Rudig, MD, Jürgen Degreif, MD, Hans-Joachim Wilke, PhD, Lutz E. Claes, PhD, Pol Maria Rommens, MD

University of Mainz Medical School, Mainz, Germany, University of Ulm, Ulm, Germany

Purpose: To investigate whether an instability is obtained if unstable thoracolumbar fractures are not treated by a bony fusion of the traumatized segment.

Materials and Methods : 15 unstable thoracolumbar burst fractures [10x without, 5x with disruption of the posterior spinal elements (DPSE)] stabilized by bisegmental AO internal fixator without interbody fusion. Measuring of the stiffness of the treated spinal segments under general anaesthesia at the time of implant removal 6 months after fracture stabilization: after removal of the longitudinal rods we introduced a defined moment of 10 Nm simultaneously to the right Schanz screws, one in the upper, one in the lower fracture-adjacent vertebra, creating right and left axial rotation, flexion and extension (three cycles per direction). The three-dimensional motion of the segments was registrated by a three-dimensional electrogoniometric linkage system fixed to the left Schanz screws. A coordinate transformation respecting the orientation of the Schanz screws known by lateral roentgenograms and CT scan had to be carried out in order to describe the motion of center of the two treated vertebrae. After mathematical transformation the three-dimensional deformations at the maximum moment were evaluated and represented the maximum rotational angles in degrees of two spinal segments around the three axis of rotation. The mean stiffness of one spinal segment was calculated by the formula:

Mean segmental stiffness [Nm/°] = (10 Nm / max. rotational angle °) x 2

Results: Mean segmental stiffness [Nm/°] ± standard deviation (Neutral Zone (°) in parentheses):

	without DPSE (n = 10)	with DPSE (n = 5)
right axial rotation	6,7 ± 1,9 (0,3)	6,9 ± 1,4 (0,1)
left axial rotation	7,5 ± 2,3 (0,2)	6,3 ± 0,5 (0,3)
flexion	5,0 ± 1,9 (0,1)	7,3 ± 3,6 (0,3)
extension	6,5 ± 3,9 (0,2)	8,3 ± 4,8 (0,1)

Discussion : Concerning the remaining segmental stability there was no significant difference between unstable thoracolumbar burst fractures with and without disruption of the posterior spinal elements. We found fairly good agreement between our in vivo results and the values of in vitro studies performed on intact cadaveric spines by other authors.

Conclusion: Unstable thoracolumbar burst fractures can heal with sufficient stability of the traumatized segments. An obligatory interbody fusion doesn't seem required to prevent instability.