Progressive disc degeneration at C5-C6 segment affects the mechanics between disc heights and posterior facets above and below the degenerated segment: A flexion-extension investigation using a poroelastic C3-T1 finite element model.

Disc degeneration (DD) is often accompanied by a height reduction of the anterior and posterior discs (AD and PD, respectively), and this affect the way in which articulating posterior facets (PFs) come into contact during physiological motions. Any increase in the contact between overlapping articulating facet surfaces increases PF loading. Development of adjacent segment disease is a significant clinical concern. It still is not clear how degenerative motion changes in AD and PD heights affect the mechanics of adjacent segment discs and facets. We hypothesized that changes in axial height patterns (in the AD and PD) at the degenerated C5-C6 disc-segment would affect axial height patterns (in the AD and PD) above and below the degenerated disc-segment. A previously validated poroelastic three-dimensional finite element (FE) model of a normal C3-T1 segment was used. Two additional C3-T1 models were built with moderate and severe DD at C5-C6. The three FE models were evaluated in flexion and extension. With progressive C5-C6 DD, AD and PD flexibility (axial deformation or elongation per unit load) at C5-C6 decrease with a compensatory corresponding flexibility increase in adjacent segments (normal), whereas PF loading increased at all segments only during extension. Changes in AD and PD flexibility and PF loading were higher at inferior segments than at superior segments. This study confirmed the hypothesis that the anterior and posterior discs and articulating facets of cervical spine segments are affected during flexion and extension motions when a disc-segment degenerates. Motion changes involving a higher PD height loss, both at the degenerated and adjacent segments, would further increase PF loading along the posterior spinal column - a possible mechanism for the dysfunctioning of the facet joints. The current data should be compared to other multi-segmental cervical spine experiments.