title: Selecting boundary conditions in physiological strain analysis of the femur: Balanced loads, inertia relief method and follower load.
abstract: Selection of boundary constraints may influence amount and distribution of loads. The purpose of this study is to analyze the potential of inertia relief and follower load to maintain the effects of musculoskeletal loads even under large deflections in patient specific finite element models of intact or fractured bone compared to empiric boundary constraints which have been shown to lead to physiological displacements and surface strains. The goal is to elucidate the use of boundary conditions in strain analyses of bones. Finite element models of the intact femur and a model of clinically relevant fracture stabilization by locking plate fixation were analyzed with normal walking loading conditions for different boundary conditions, specifically re-balanced loading, inertia relief and follower load. Peak principal cortex surface strains for different boundary conditions are consistent (maximum deviation 13.7%) except for inertia relief without force balancing (maximum deviation 108.4%). Influence of follower load on displacements increases with higher deflection in fracture model (from 3% to 7% for force balanced model). For load balanced models, follower load had only minor influence, though the effect increases strongly with higher deflection. Conventional constraints of fixed nodes in space should be carefully reconsidered because their type and position are challenging to justify and for their potential to introduce relevant non-physiological reaction forces. Inertia relief provides an alternative method which yields physiological strain results.
authors.affiliation: Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Electronic address: mark.heyland@charite.de.
authors.forename: Mark
authors.initials: M
authors.lastname: Heyland
authors.affiliation: Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany.
authors.forename: Adam
authors.initials: A
authors.lastname: Trepczynski
authors.affiliation: Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany.
authors.forename: Georg N
authors.initials: GN
authors.lastname: Duda
authors.affiliation: Department of structural mechanics (Strukturmechanik und Strukturberechnung), Institut für Mechanik, Technische Universität Berlin, Sekr. C8-3, Str. d. 17. Juni 135, 10623 Berlin, Germany.
authors.forename: Manfred
authors.initials: M
authors.lastname: Zehn
authors.affiliation: Center for Musculoskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany.
authors.forename: Klaus-Dieter
authors.initials: KD
authors.lastname: Schaser
authors.affiliation: Center for Musculoskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany.
authors.forename: Sven
authors.initials: S
authors.lastname: Märdian
mesh_terms: D005269:Femur / Q000000981:diagnostic imaging / Q000502:physiology*; D020342:Finite Element Analysis; D006801:Humans; D066230:Patient-Specific Modeling; D013314:Stress, Mechanical; D014057:Tomography, X-Ray Computed; D016474:Weight-Bearing
keywords: Balanced load; Biomechanics; Boundary condition; Boundary constraint; Displacement constraint; Finite element analysis; Follower load; Inertia relief; Joint loads; Load equilibrium; Muscle loads