An unbalanced metabolism could be induced by many different factors, including a harsh microenvironment, biomechanical cues, genetics, and infection. Remarkably, though, catabolic ECM disruption also occurs in the absence of immune cell infiltration, largely due to native disc cell production of catabolic enzymes and cytokines.

However, during degeneration immune cells and inflammatory factors can infiltrate through defects in the cartilage endplate and annulus fibrosus fissures, further accelerating the catabolic environment. IDD is often caused by structural and biochemical changes in intervertebral discs (IVD) that prompt a pathologic shift from an anabolic to catabolic state, affecting extracellular matrix (ECM) production, enzyme generation, cytokine and chemokine production, neurotrophic and angiogenic factor production. Low back pain is a highly prevalent, chronic, and costly medical condition predominantly triggered by intervertebral disc degeneration (IDD). In summary, in vitro and finite element model studies have together substantially improved our understanding of functional biomechanics (load-sharing) of human lumbar spine in normal and perturbed conditions acting as invaluable complementary tools in clinical applications. Disc degeneration, endplate fracture and surgical resections significantly alter load transmission in the lumbar spine. Structural transection order affects both findings and conclusions not only in force-control protocols but also in displacement-control loading regimes. The segment overall response as well as the relative role of its constituents are markedly influenced by alterations in resection sequence, boundary conditions, geometry, loading characteristics (rate, magnitude, combinations and preloads), disc hydration, bone quality, posture and time (creep and cyclic). Two systematic searches were performed in PubMed (October 2018 - March 2019) using three sets of concepts ('lumbar spine', 'load-sharing' and 'motion segment components') followed by a complementary generic search. This paper reviews and critically discusses the relevant findings of in vitro and finite element model studies on load-sharing in healthy, aged, degenerate and damaged human lumbar motion segments. By employing experimental and computational approaches, many studies have investigated the relative role of each component as well as effects of various factors such as boundary-initial conditions, load magnitude-combination-direction, load temporal regime, preload, posture, degeneration, failures and surgical interventions on load-sharing. Human lumbar motion segment is composed of various components with distinct contributions to its gross mechanical response.

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