Cellular biomechanics: Fluid-structure interaction or structural simulation?

The mechanisms by which cells respond to their changing mechanical environment and how this stimulus is decoded intracellularly from the tissue to the organ level, are widely considered as fundamental for most biological processes. Despite this, the underlying phenomena of mechanotransduction, are still not very well understood. Over the last years, numerical modeling has emerged as a cohesive element in the interpretation of biophysical and biochemical assays, concerning cellular mechanotransduction. We hypothesize that the consideration of continuum mechanics (studying all cellular entities as solids) is an inherent limitation of these models, and in part, responsible for their restricted application in cellular biomechanics. To evaluate this, a (verified and validated) 3D model of osteoblast is simulated through structural analysis, employing conventional Finite Element (FE) modelling and the results compared to a Fluid-Structure Interaction (FSI) analysis. Among the trend observed, FSI systematically leads to a higher stimulation of the nucleus (by up to 200%), while FE produced a more uniform stress field, resulting in the deformation of a notably larger portion of its volume. Although FE modelling captures a seemingly correct kinematic response of the cell when subjected to the simulated loading scenario, FSI represents a more realistic alternative. The equitable consideration of both, liquid- and solid-state material characteristics, in the latter analysis, revealed intra-cellular loading patterns that were more realistic from a biomechanical perspective. In conclusion, FSI can provide refined insight as to nuclear loading, thus serving as a far more accurate framework for decoding cellular mechanotransduction.

Lumbar stability following graded unilateral and bilateral facetectomy: A finite element model study.

BACKGROUND: Excision of excessive amount of facet joint during lumbar discectomy or decompression can cause segmental instability of the lumbar spine. This study was performed to assess the segmental instability, facet joint loading and intradiscal pressure following graded lumbar facetectomy. This biomechanical study was performed using a verified and validated L3-S1 finite element model. METHODS: Nine scenarios were analysed. Intact model as control, 30%, 45%, 60% and complete facet joint excision in unilateral and bilateral setting. The effect of progressive graded facetectomy of L4-L5 on the segmental mobility, facet loading and intradiscal pressure was assessed. FINDINGS: In comparison with control 30% excision of the facet joint mainly caused increase in mediolateral mobility. With 45% excision of the facet joint there was increase in both anteroposterior and mediolateral mobility, this was worse in bilateral and unilateral models respectively. This worsened with larger facet excision scenarios. Facet load increased significantly on extension with excision of 45% & 60% unilaterally and 100% bilaterally. Flexion produced rise in intradiscal pressure in all scenarios. INTERPRETATION: The increased spinal mobility, facet loading and intradiscal pressure with more than 30% facetectomy highlights the importance of preserving the facets during decompression thereby safeguarding accelerated degeneration of these segments and iatrogenic segmental instability. The findings from this study could also potentially explain the correlation between spinal instability, disc degeneration and facet joint arthrosis as noted in clinical studies.