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A spinal rod for thoraco-lumbar spinal injuries: Biomechanical evaluation
343
Abstracts
The results showed : (1) the stiffness increased with increased loading rate ; (2) the stiffness in compression was considerably higher than in tension; (3) in lateral, A-P and P-A shear loading directions, the stiffnesses were almost symmetrical; (4) the data on four intervertebral joints showed fewer differences between the quasistatic and dynamic tests than the variability between specimens. A SrMPLlFlED
INTERVERTEBRAL DISC FINITE-ELEMENT
MODEL WITH A FIBER-COMPOSITE
ANNIJLUS R. L. SPILKER
and D. M. DAUGIRDA(Department of Materials Engineering, University of Illinois at Chicago Circle, Box 4348, Chicago, IL 60680. U.S.A.)
A simplified finite element model of the intervertebral disc and adjacent vertebral bodies is constructed. The model geometry is axisymmetric, but through a semi-analytic approach complex (non-axisymmetric) loads can be applied. Four regions are modeled; the inferior and superior vertebral body-endplate regions are treated as linear, isotropic, nearly-rigid materials, and the nucleus is assumed to be an incompressible, inviscid fluid. The annulus is modeled as a fiber-reinforced composite material with layers of alternating ( f 6) fiber direction. It is shown that a single set of fiber-composite material constants can lead to model predictions which are in reasonable agteement with mean experimental results for motion segments subject to compression, torsion. shear, and moment loading. Next the model is used to study the effects of geometry, material properties, and enucleation for compression and torsion loading. MECHANICAL
PROPERTIES OF SPINAL LIGAMENTS
DAVID L. HYLER ROBERTWM. LITTLEand ROBERTP. HUBBARD(Department of Biomechanics, Michigan State University, East Lansing, MI 48864, U.S.A.) Four spinal ligaments, anterior and posterior longitudinal ligaments, ligamenta flava and supraspinous ligaments, from chimpanzee cadavers were tested in a uniaxial mode to determine basic viscoelastic tissue properties. Histological studies in conjunction with mechanical testing established correlations between composition and structure of the tissue with mechanical properties. Strain rate effects, hysteresis, cyclic response, relaxation and preconditioning effects were examined for each ligament and comparisons were made between digerent ligaments and at different vertebral levels. A SPINAL ROD FOR THORACO-LUMBAR
SPINAL INJURIES: BIOMECHANICAL
EVALUATION
R. R. JACOBS(University of Kansas Medical Center, Department of Orthopedic Surgery, 39th & Rainbow Blvd, Kansas City, Kansas 66103, U.S.A.) F. SCHLAEPFER, R. MATHY~,JR., A. NACHEM~ON and S. M. PERREN(Laboratory for Experimental Surgery, Davos, Switzerland, and Department of Orthopaedic Surgery I. University of Gothenburg, Sweden) Upper and lower hooks are rigidly fixed to an 8 mm stainless steel rod by meshing radial grooves with washers and nuts. The top hook has a sliding cover which locks the lamina within the hook. Four point bending tests gave a yield point of 33 + 2.3 Nm for the experimental device compared with 20.0 + 0.5 Nm for the Harrington rod. In intact human cadaver spines Harrington rods applied with 40 kp of distraction had a transverse top hook pullout strength of 81.1 f 11.4 kp. In contrast the experimental design applied with only 12.5 kp of distraction failed at 124 f 13.2 kp. With simulated fracture dislocations under four point bending the reduction and stability were similar for both methods. The bending moment at failure was 125 + 17 Nm for the experimental device and 44.1 f 2.1 for the Harrington rods. The flexion deformity was only 2.5 + 2.9” compared to 9.3 + 1.9’ and energy absorption 1319 + 3.7 J compared with 5.7 f 1.3 J. MECHANICAL
CHARACTERISTICS OF HUMAN BONE-CEMENT
COMPOSITES
MICHAELH. JOFE, SUSAN S. SCHEIN, AUGUSTUS A. WHITE III and WILSONC. HAYES (Orthopaedics Biomechanics Laboratory, Beth Israel Hospital and Harvard Medical School, 330 Brookline Ave., Boston, MA 02215, U.S.A.) A leading cause of prosthesis loosening in total joint replacement is failure of the bone-cement interface. Improved cementing techniques and pressurized injection of low viscosity cements have been advocated to improve the strength of the interface. This experiment was undertaken to explore the compressive mechanical properties ofmthe bone-cement composite formed using these new methods. Cylindrical specimens of human bone from the proximal tibia, approximately 5 mm thick and 9.2 mm in diameter were prepared and tested. Following cement application, the specimens were tested in uniaxial strain at 0.01 s- 1.
Abstracts
The results showed : (1) the stiffness increased with increased loading rate ; (2) the stiffness in compression was considerably higher than in tension; (3) in lateral, A-P and P-A shear loading directions, the stiffnesses were almost symmetrical; (4) the data on four intervertebral joints showed fewer differences between the quasistatic and dynamic tests than the variability between specimens. A SrMPLlFlED
INTERVERTEBRAL DISC FINITE-ELEMENT
MODEL WITH A FIBER-COMPOSITE
ANNIJLUS R. L. SPILKER
and D. M. DAUGIRDA(Department of Materials Engineering, University of Illinois at Chicago Circle, Box 4348, Chicago, IL 60680. U.S.A.)
A simplified finite element model of the intervertebral disc and adjacent vertebral bodies is constructed. The model geometry is axisymmetric, but through a semi-analytic approach complex (non-axisymmetric) loads can be applied. Four regions are modeled; the inferior and superior vertebral body-endplate regions are treated as linear, isotropic, nearly-rigid materials, and the nucleus is assumed to be an incompressible, inviscid fluid. The annulus is modeled as a fiber-reinforced composite material with layers of alternating ( f 6) fiber direction. It is shown that a single set of fiber-composite material constants can lead to model predictions which are in reasonable agteement with mean experimental results for motion segments subject to compression, torsion. shear, and moment loading. Next the model is used to study the effects of geometry, material properties, and enucleation for compression and torsion loading. MECHANICAL
PROPERTIES OF SPINAL LIGAMENTS
DAVID L. HYLER ROBERTWM. LITTLEand ROBERTP. HUBBARD(Department of Biomechanics, Michigan State University, East Lansing, MI 48864, U.S.A.) Four spinal ligaments, anterior and posterior longitudinal ligaments, ligamenta flava and supraspinous ligaments, from chimpanzee cadavers were tested in a uniaxial mode to determine basic viscoelastic tissue properties. Histological studies in conjunction with mechanical testing established correlations between composition and structure of the tissue with mechanical properties. Strain rate effects, hysteresis, cyclic response, relaxation and preconditioning effects were examined for each ligament and comparisons were made between digerent ligaments and at different vertebral levels. A SPINAL ROD FOR THORACO-LUMBAR
SPINAL INJURIES: BIOMECHANICAL
EVALUATION
R. R. JACOBS(University of Kansas Medical Center, Department of Orthopedic Surgery, 39th & Rainbow Blvd, Kansas City, Kansas 66103, U.S.A.) F. SCHLAEPFER, R. MATHY~,JR., A. NACHEM~ON and S. M. PERREN(Laboratory for Experimental Surgery, Davos, Switzerland, and Department of Orthopaedic Surgery I. University of Gothenburg, Sweden) Upper and lower hooks are rigidly fixed to an 8 mm stainless steel rod by meshing radial grooves with washers and nuts. The top hook has a sliding cover which locks the lamina within the hook. Four point bending tests gave a yield point of 33 + 2.3 Nm for the experimental device compared with 20.0 + 0.5 Nm for the Harrington rod. In intact human cadaver spines Harrington rods applied with 40 kp of distraction had a transverse top hook pullout strength of 81.1 f 11.4 kp. In contrast the experimental design applied with only 12.5 kp of distraction failed at 124 f 13.2 kp. With simulated fracture dislocations under four point bending the reduction and stability were similar for both methods. The bending moment at failure was 125 + 17 Nm for the experimental device and 44.1 f 2.1 for the Harrington rods. The flexion deformity was only 2.5 + 2.9” compared to 9.3 + 1.9’ and energy absorption 1319 + 3.7 J compared with 5.7 f 1.3 J. MECHANICAL
CHARACTERISTICS OF HUMAN BONE-CEMENT
COMPOSITES
MICHAELH. JOFE, SUSAN S. SCHEIN, AUGUSTUS A. WHITE III and WILSONC. HAYES (Orthopaedics Biomechanics Laboratory, Beth Israel Hospital and Harvard Medical School, 330 Brookline Ave., Boston, MA 02215, U.S.A.) A leading cause of prosthesis loosening in total joint replacement is failure of the bone-cement interface. Improved cementing techniques and pressurized injection of low viscosity cements have been advocated to improve the strength of the interface. This experiment was undertaken to explore the compressive mechanical properties ofmthe bone-cement composite formed using these new methods. Cylindrical specimens of human bone from the proximal tibia, approximately 5 mm thick and 9.2 mm in diameter were prepared and tested. Following cement application, the specimens were tested in uniaxial strain at 0.01 s- 1.