How does free rod-sliding affect the posterior instrumentation for a dynamic stabilization using a bovine calf model?

A biomechanical cadaveric study in lumbar calf spine.

Evaluation of the effects of selected degrees of freedom (df) on the dynamic stabilization of the spine in terms of segmental range of motion (RoM), center of rotation (CoR), and implant loadings.

For dorsal stabilization, rigid implant systems are becoming increasingly complemented by numerous dynamic systems based on pedicle screws and varying df. However, it is still unclear which df is most suitable to accomplish a physiologically related dynamic stabilization, and which loadings are induced to the implants. Human and calf specimens are reported to show certain similarities in their biomechanics. Young healthy calf specimens are not degenerated and show less interindividual differences than elderly human specimens. However, the existing differences between species limit the conclusions in a preclinical setting.

Six calf specimens from level L3-L4 were analyzed in flexion and extension with a 6-df robotic spine simulator. A clinical functional radiological examination tool was used and parameters such as RoM, CoR, and implant loadings were determined for 6 configurations: (1) intact, (2) defect, (3) rigid fixation, (4) free craniocaudal (CC) rod-sliding, (5) free polyaxiality, and (6) combined free rod-sliding and free polyaxiality. The location of the CoR was determined relative to vertebral body dimensions. A CoR repositioning was defined as sufficient when its median differed less than 5% of the vertebral body dimensions.

Free rod-sliding in the CC direction restored the CoR from the defect back to the intact condition. The RoM could be significantly reduced to approximately 1/2 of the intact condition. Compared with the rigid condition, the implant bending moments increased from 0.3/-0.8 Nm (flexion/extension) to 1.3/-1.2 Nm for the free CC rod-sliding condition.

Free CC rod-sliding restores the intact conditions of the tested kinematic parameters most suitably and at the same time reduces the RoM. Stabilization toward the intact condition could decrease the risk of stress shielding and the progress of segment degeneration.

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