The trabecular architecture of the superior articular process of the lumbar spine (L2–S1)

The role of the facet joint in low back pain has gained public attention lately. The objective of our study was to investigate whether there is any difference in the adaptation of the cancellous bone in the superior articular process depending on the specific stress condition in different levels of the spine. Therefore, the trabecular structure of the superior articular processes of L2 and S1 of 15 cadavers (aged 63–100 years) were studied using μCT (micro-computer tomography). Each sample was divided into five sections, each of which containing 20% of the slices. The following structure parameters were compared between L2 and S1 as well as within each process; bone-volume-fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), structure-model-index (SMI) and degree of anisotropy (DA). Statistically significant differences were observed between L2 and S1 for the BV/TV, SMI, Tb.Th and Tb.N in superior 2 sections. BV/TV, Tb.Th and Tb.N were higher in S1 than in L2. The SMI is lower, and even negative in S1 compared to L2, showing a more plate-like structure. Within the articular process all structure parameters show a similar distribution in L2 and S1. BV/TV, Tb.N and DA decreased from cranial to caudal while Tb.Th was highest in the most cranial and caudal sections, with the lowest value in the middle. The SMI, on the other hand, increased from cranial to caudal displaying more rod-like structures. These results can be explained by the different stress the processes of the different spinal levels are exposed to as well as the specific motion patterns of the facet joint. The processes of the os sacrum are exposed to a higher axial and ventral load due to their location and the lumbosacral flexion. In addition the upper sections of each process experience higher stress peaks than the lower ones. Therefore, this study shows the material distribution within the cancellous bone adapts to these specific stress conditions the facet joints are exposed to.