Measurement of a spinal motion segment stiffness matrix

A Pure Moment Based Tester for Spinal Biomechanics

“…Hence, these methods are incapable of reproducing physiological loading of the specimen. Stokes et al [11] used a series of six linear actuators to enable the six degrees of freedom to be independently controlled. Six linear encoders were utilized to measure and control the displacement of the testing machine.…”

Spine stability: The six blind men and the elephant

“…The contribution of each component changes depending on the size of the perturbation, as well as on the initial state of the spine system. For instance, feedforward input from the CNS can be used to increase trunk muscle co-activation prior to a perturbation, thus increasing muscle stiffness (and intervertebral stiffness as well (Stokes et al, 2002)) allowing the intrinsic properties of the system to contribute more to the perturbation than responses with delays. Indeed, several studies have shown that voluntarily pre-tensioning of trunk muscles obviates the need for a reflex response, which is less likely to occur following a perturbation (Stokes et al, 2000;Granata et al, 2004a).…”

On the modeling of the intervertebral joint in multibody models for the spine

“…1 Subsequent works by Gardner-Morse, Stokes and their coworkers [11,33,34,35] have measured the 12 parameters of K P . However, over twenty years elapsed since Panjabi's initial work before detailed spinal models featuring a complete stiffness matrix were introduced.…”

“…Prior to elaborating upon this however, it is appropriate to first describe how the intervertebral joint stiffness matrix elements are determined experimentally. Most commonly, the lower vertebra is fixed, the upper vertebra subjected to an infinitesimal motion, and the forces and moments due to the ensuing deformation measured [10,11,34,35]. Alternatively, a force or moment is applied and the resulting deformations measured [4,27] (Fig.…”

“…One of the more common choices of the landmark points X K A are the vertebral centers of geometry but one can also opt to use other landmarks such as points on the upper and lower vertebral surfaces [4,11,27,34,35]. Note that, if these, or other, experimental data are used to populate bushing stiffness matrices it is crucial to identify the landmark material points X Further details with regards to the derivation of the stiffness matrix parameterization of the intervertebral joint can be found in Sections 4-6 of the Online Resource.…”