title: Compressive strength of elderly vertebrae is reduced by disc degeneration and additional flexion.
abstract: Computer tomography (CT)-based finite element (FE) models assess vertebral strength better than dual energy X-ray absorptiometry. Osteoporotic vertebrae are usually loaded via degenerated intervertebral discs (IVD) and potentially at higher risk under forward bending, but the influences of the IVD and loading conditions are generally overlooked. Accordingly, magnetic resonance imaging was performed on 14 lumbar discs to generate FE models for the healthiest and most degenerated specimens. Compression, torsion, bending, flexion and extension conducted experimentally were used to calibrate both models. They were combined with CT-based FE models of 12 lumbar vertebral bodies to evaluate the effect of disc degeneration compared to a loading via endplates embedded in a stiff resin, the usual experimental paradigm. Compression and lifting were simulated, load and damage pattern were evaluated at failure. Adding flexion to the compression (lifting) and higher disc degeneration reduces the failure load (8-14%, 5-7%) and increases damage in the vertebrae. Under both loading scenarios, decreasing the disc height slightly increases the failure load; embedding and degenerated IVD provides respectively the highest and lowest failure load. Embedded vertebrae are more brittle, but failure loads induced via IVDs correlate highly with vertebral strength. In conclusion, osteoporotic vertebrae with degenerated IVDs are consistently weaker-especially under lifting, but clinical assessment of their strength is possible via FE analysis without extensive disc modelling, by extrapolating measures from the embedded situation.
authors.affiliation: Institute of Surgical Technology and Biomechanics, University of Bern, Stauffacherstrasse 78, CH-3014 Bern, Switzerland.
authors.forename: Ghislain
authors.initials: G
authors.lastname: Maquer
authors.affiliation: Institute of Surgical Technology and Biomechanics, University of Bern, Stauffacherstrasse 78, CH-3014 Bern, Switzerland.
authors.forename: Jakob
authors.initials: J
authors.lastname: Schwiedrzik
authors.affiliation: Institute of Biomechanics, Hamburg University of Technology, Denickestraße 15, 21073 Hamburg, Germany.
authors.forename: Gerd
authors.initials: G
authors.lastname: Huber
authors.affiliation: Institute of Biomechanics, Hamburg University of Technology, Denickestraße 15, 21073 Hamburg, Germany.
authors.forename: Michael M
authors.initials: MM
authors.lastname: Morlock
authors.affiliation: Institute of Surgical Technology and Biomechanics, University of Bern, Stauffacherstrasse 78, CH-3014 Bern, Switzerland. Electronic address: philippe.zysset@istb.unibe.ch.
authors.forename: Philippe K
authors.initials: PK
authors.lastname: Zysset
mesh_terms: D000368:Aged; D016022:Case-Control Studies; D019245:Compressive Strength; D020342:Finite Element Analysis; D006801:Humans; D055959:Intervertebral Disc Degeneration / Q000000981:diagnostic imaging / Q000503:physiopathology*; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D014057:Tomography, X-Ray Computed; D016474:Weight-Bearing
keywords: Calibration; Finite element analysis; Intervertebral disc degeneration; Osteoporosis; Vertebral strength