Quantifying the centre of rotation pattern in a multi-body model of the lumbar spine.
ABSTRACT Understanding the kinematics of the spine provides paramount knowledge for many aspects of the clinical analysis of back pain. More specifically, visualisation of the instantaneous centre of rotation (ICR) enables clinicians to quantify joint laxity in the segments, avoiding a dependence on more inconclusive measurements based on the range of motion and excessive translations, which vary in every individual. Alternatively, it provides motion preserving designers with an insight into where a physiological ICR of a motion preserving prosthesis can be situated in order to restore proper load distribution across the passive and active elements of the lumbar region. Prior to the use of an unconstrained dynamic musculoskeletal model system, based on multi-body models capable of transient analysis, to estimate segmental loads, the model must be kinematically evaluated for all possible sensitivity due to ligament properties and the initial locus of intervertebral disc (IVD). A previously calibrated osseoligamentous model of lumbar spine was used to evaluate the changes in ICR under variation of the ligament stiffness and initial locus of IVD, when subjected to pure moments from 0 to 15 Nm. The ICR was quantified based on the closed solution of unit quaternion that improves accuracy and prevents coordinate singularities, which is often observed in Euler-based methods and least squares principles. The calculation of the ICR during flexion/extension revealed complexity and intrinsic nonlinearity between flexion and extension. This study revealed that, to accommodate a good agreement between in vitro data and the multi-body model predictions, in flexion more laxity is required than in extension. The results showed that the ICR location is concentrated in the posterior region of the disc, in agreement with previous experimental studies. However, the current multi-body model demonstrates a sensitivity to the initial definition of the ICR, which should be recognised as a limitation of the method. Nevertheless, the current simulations suggest that, due to the constantly evolving path of the ICR across the IVD during flexion-extension, a movable ICR is a necessary condition in multi-body modelling of the spine, in the context of whole body simulation, to accurately capture segmental kinematics and kinetics.
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ABSTRACT: The instantaneous center of rotation in a functional spinal unit is an indicator for mechanical disorders and is relevant for the development of motion preserving techniques. In addition to the intervertebral disc, the facet joints also play a major role for load transmission through the spine, providing stability to it. The relationship between the rotation center and facet joint forces is not fully understood, since previous studies have separated both; spinal motion and facet forces. A finite element model of a L4-5 lumbar spinal segment was exposed to an axial compression preload of 500 N. Pure unconstrained moments of 7.5 Nm were additionally applied in the three anatomical main planes. The instantaneous center of rotation and the facet joint forces were investigated. For small moments, the center of rotation was found to be almost in the center of the disc, no matter what motion direction. With an increasing flexion moment, the center of rotation moved anteriorly. The facet joints remained unloaded in flexion. With proceeding extension movement, the center of rotation moved posteriorly. The facet forces increased up to 50 N. In lateral bending, with increasing moment the center of rotation migrated posteriorly in the ipsilateral side of the disc. The forces in the facet joints rose to 36 N. In axial rotation, the center of rotation migrated towards the compressed facet joint with increasing moment. Axial rotation yielded the maximum facet forces with 105 N. The determination of the rotation center is highly sensible against measurement resolution obtained during in vivo and in vitro studies. This finite element method can be used to complement the knowledge of the rotation center location from former experimental findings.Clinical Biomechanics 04/2008; 23(3):270-8. · 1.87 Impact Factor
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ABSTRACT: The finite center of rotation (FCR) is often used to assess joint function. It was the purpose of this study to compare the accuracy of the procedure of Crisco et al.  for estimating the FCR with a procedure which uses least-squares principles. The procedures were evaluated using noisy data rotated about a known FCR. Both procedures demonstrated increasing accuracy of FCR estimation with increasing rotation angle. As the centroid of a pair of markers was moved further from the FCR, accuracy of its location decreased. Noise levels had a strong influence on FCR estimation accuracy, with the least-squares procedure being better able to cope with noise. Increasing the number of landmarks increased FCR estimation accuracy. The accuracy of the procedure of Crisco et al.  increased when multiple estimates of the FCR were averaged. On all of the evaluations performed, the least-squares procedure gave small improvements in the accuracy of estimating the FCR, but was not able to circumvent the inaccuracies which arise when landmarks are not appropriately positioned, numerous, or if the rotation angle is small.Medical Engineering & Physics 05/2001; 23(3):227-33. · 1.78 Impact Factor
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ABSTRACT: Data on the stiffnesses of spinal ligaments are required for analytical studies on the mechanical behavior of spinal segments. Values obtained experimentally vary widely in the literature. A finite element model of an L3/L4 functional spinal unit was used to determine the influence of ligament stiffness on intersegmental rotation and forces in the ligaments. The lowest values for ligament stiffness selected from the literature were used in one set of calculations, and the highest values were simulated in a second set. The nonlinear model was loaded with pure moments of 7.5 and 15 Nm in the three main anatomical planes. The mechanical behavior of the functional spinal unit was strongly influenced by ligament stiffness. In some cases, a ligament with low stiffness does not carry any load, while the same ligament with high stiffness has to carry a high load. This indicates that finite element models of spinal segments have to be validated and that a realistic quantitative prediction of ligament forces is extremely difficult.Journal of Biomechanics 08/2004; 37(7):1107-11. · 2.72 Impact Factor