Internal load-sharing in the human passive lumbar spine: Review of in vitro and finite element model studies.
Human lumbar motion segment is composed of various components with distinct contributions to its gross mechanical response. 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. 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. 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. 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). Structural transection order affects both findings and conclusions not only in force-control protocols but also in displacement-control loading regimes. Disc degeneration, endplate fracture and surgical resections significantly alter load transmission in the lumbar spine. 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.