INTRODUCTION: Previous studies have shown that the posterior elements/facet joints provide strength to the overall functional spine unit by taking 3-25% of vertical compressive load off the intervertebral disc [1]. However, little is known regarding whether this offloading has a protective effect against endplate fracture. Interestingly, studies which used spinal units with the posterior elements removed tended to report failure after a lower number of loading cycles when compared to studies that used intact FSUs [2-4]. Therefore, the purpose of this study was to investigate if the posterior elements provide a protective role to the endplate in porcine cervical spines under fracture-inducing conditions.

METHODS: Twenty-two cervical porcine functional spine units (C5/6 level) were randomized into two groups: 1) a control group which had their posterior elements left intact (n=11); 2) an experimental group which had the posterior elements removed (n=11). Each spinal unit underwent a previously reported rapid disc pressurization protocol in order to create endplate fractures [5]. Briefly, hydraulic fluid was rapidly injected into the disc via a standard inflation needle inserted through the anterior annulus which was connected in series to a hydraulic pump and pressure transducer. From the pressure-time tracing, rate of pressurization and peak pressure were determined for each specimen. Following rapid pressurization, each spinal unit was dissected axially though the disc to determine the presence and size of endplate fracture. Fracture area was quantified from axial images of the transected disc using ImageJ.

RESULTS: Peak pressurization and rate of pressurization were not found to differ between intact and cut specimens (p=0.313 and 0.101, respectively). Despite this, significantly more cut spinal units sustained an endplate fracture (11/11) compared to intact spinal units (5/11); p=0.012. Further, in the specimens which sustained a fracture, cut spinal units resulted in a fracture area 1.91 times greater in size compared to the fractures observed in the intact FSUs (p=0.011).

DISCUSSION: The current study found that a significantly greater number of specimens sustained an endplate fracture following rapid intradiscal pressurization when the posterior elements were removed compared to when the posterior elements were intact. This difference was not due to greater rates of pressurization or higher peak pressures reached within the disc as these were not found to differ between the two groups. Further, in these fractured specimens, the overall size of the fracture was greater in cut specimens compared to intact specimens. This study provides evidence to support the notion that the posterior elements lend mechanical strength and support to the endplate and subsequently the entire spinal unit likely as a result of a redistribution of force across the whole endplate/vertebra.