Interaction between finite helical axes and facet joint forces under combined loading

Finite element study.

To investigate the interaction between the finite helical axis and facet joint loads under combined loading.

Finite helical axes (FHA) in a functional spinal unit can indicate mechanical disorders and are relevant for the development of new arthoplasty techniques. The facet joints protect the intervertebral discs from excessive movements. The relationship between the FHAs and facet joint forces is not well-understood, because previous studies have separated both, spinal motion and facet forces.

A finite element model of a lumbar spinal segment L4-L5 was used to simulate axial compression load of 500 N together with moments starting from 0 to 7.5 Nm in single anatomic main planes. Load combinations of 7.5 Nm were generated by changing the load direction in steps of 15 degrees between each pair of the 3 anatomic mainplanes.

For single axes loading, the FHAs were found to be in the center of the disc under small moments, independently from load directions. The facet joints were only slightly loaded. Higher moments increased the forces in facet joints up to 105 N in axial rotation, followed by extension (50 N) and lateral bending (36 N). Combined moments did not essentially increase the facet forces compared with the same moment applied in an anatomic main direction. High facet forces might have directed the FHAs to migrate posteriorly, especially for axial rotation. This situation resulted in FHAs outside the disc toward the compressed facet joint.

For clinical practice, patients immediately after the operation, or patients with facet joint arthritis should reduce or avoid axial rotation alone or in combination with other load applications, especially axial rotation plus lateral bending or flexion.