Three-dimensional biomechanical properties of the human cervical spine in vitro

RésuméL'objectif de notre étude est de déterminer le comportement mécanique du rachis cervical humain soumis á des charges physiologiques statiques. Les déplacements tridimensionnels dus à trois moments de couple purs (flexion-extension, inflexion latérale gauche-droite et torsion axiale gauche-droite), sont mesurés sur 56 unités fonctionnelles rachidiennes intactes (UF) de C2 à C7 prélevées sur 29 sujets. Les courbes effort-déplacement sont tracées pour chaque sollicitation. Nous calculons ensuite la zone neutre (ZN), la mobilité maximale (MM), le rapport de ZN à MM, le rapport du déplacement couplé au déplacement principal (RDC), le moment limite et la rigidité sécante. L'influence de la dégénérescence du disque intervertébral et du niveau d'UF sont aussi étudiées avec une analyse de variance (ANOVA). Nos résultats montrent bien la non linéarité des courbes effort-déplacement et la ZN du rachis cervical dans les trois plants de l'espace. Nous trouvons des différences significatives de rigidité entre trois sollicitations appliquées. Lorsque nous sollicitons en inflexion latérale nous observons des différences significatives de rigidité d'un niveau vertébral à l'autre. Mais la différence de rigidité concernant différents états de dégénérescence de disque n'est significative qu'en inclinaison latérale droite. Le RDC sous inflexion latérale et torsion axiale est significativement différent entre différents niveaux d'UF. L'influence du cycle d'effort et la réponse mécanique de C1-C2 en déplacement principal sont aussi présentées.SummaryOur aim was to determine the biomechanical properties of the normal human cervical spine under physiological static loads. The three-dimensional displacements under three pure moments: flexion-extension, left-right lateral bending and left-right axial torsion — were measured in 56 intact functional spinal units (FSUs) taken from between C2 and C7 in 29 human cadavers. For each mode of loading, load-displacement curves were plotted. Then we calculated each neutral zone, range of motion, neutral zone ratio, ratio of coupled motion, limit moment and secant stiffness. The effects of intervertebral disc degeneration and the disc level were also taken into account by the analysis of variance. Our results adequately demonstrated both the non-linearity of load-displacement curves and the neutral zone of the cervical spine in three-dimensional space. At the same time, we found statistically that the stiffness in the three planes are significantly different, as are the stiffnesses in lateral bending of successive different FSUs. However, significant differences of stiffness in different states of disc degeneration were only found in right lateral bending. There were significant differences between levels in ratio of coupled motion under both lateral bending and axial torsion. The loading cycle conditions and the biomechanical responses of principal motion of C1-2 are also reported.

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