- Email: [email protected]
A model to investigate the role of spinal curvature in lifting activities
A MODEL TO INVESTIGATE THE ROLE CURVATURE IN LIFTING AS. L. Grilli and B. Serpil Acar Loughborough University
OF SPINAL
INTRODUCTION: Numerous mathematical models which investigate the high incidence of low back pain associated with lifting activities have been based upon the concept of the ‘lever’ model. This considers the muscle force required for equilibrium about a single intervertebral joint. When representing this muscle force collectively, the predicted compression at the LX?.1 intervertebral joint indicates failure of the vertebral end plates. In comparison for the same loading conditions when representing the spine as an arch, the amount of muscle force required for equilibrium and stability in a given posture has been predicted to be significantly less (Aspden, 1987). The consequent reduction in compression at the W/S1 intervertebral joint implies the curvature of the spine has a role in load bearing. METHODS: For lever models with greater anatomic detail, a reduction in compression at the IX31 joint is observed (McGill and Norman, 1987). For comparison, this paper describes the development of a model which included greater anatomic detail based upon arch theory. This included consideration of the curvature of the whole spine, a distributed loading pattern for body weight, and the activity of individual spinal muscle groups. RESULTS AND CONCLUSION: When accounting for the variation in force magnitude, direction and region of application for each individual muscle group, the model predicted the total amount of force required was greater than for a single collective muscle force. Consequently compression at the L5/S1 joint was greater, and closer to existing detailed lever model predictions. The curvature of the spine does therefore not reduce the level of loading significantly as initially proposed. However for stability of the spine, muscle activity is necessary to ensure transmission of force between vertebrae at every level. The amount of force required depends upon their configuration. Application of muscle forces for stability at a certain level can affect the loading at other levels. Consideration of the curvature of the whole spine is therefore necessary to determine more realistic loading conditions for a given posture. By considering the configuration of the spine, and the strength of the individual muscle groups, the demands on the muscles and intervertebral joints at each level of the spine can be investigated using this model. For a given posture or lifting activity possible areas of tissue overload and sources of back pain may therefore be identified REFERENCES: 1. Aspden, R.,ClinicaJ Biomechanics, Vol. 2, pp 168-174, 1987. 2. Mci;ill S. and Norman, R., J. Biomech., V20,. 6, pp 591-600, 1987. CORRJW’ONDENCE: Dr B Serpil Acar, and MS S L Grilli Dept. of Computer Studies, Loughbomugh University, Loughborough, Leics. LB1 1 3TU, UK Tel: +44 1509 222 879, Fax: +44 1509 211 586, [email protected], [email protected]
11”’ Conference
LUMBAR SPINAL FlXATION : THE INFLUENCE OF S ON ANCHOR LOADING FLE-STtFFNES A. Templier’, W. Skalli’, L. Denmnger’, C. Maxe12, F. Lava.& ‘Laboratoire de BioM&anique, ENSAM Paris *Jnstitut Mutualiste Montsouris, Pte de Choisy, Paris INTRODUCTION: The present study Finite-Element Analysis (FEA) of two devices when implanted. This implants Screw/Rod (06mm) Concept, 2) the 02~2.5~). METHODS: A narameterimd 3D FBA
is a numerical comparison using different kinds of spinal fixation are 1) the widely used dgida Twinflex@ dynamics system ( model of a W-sacrum
segment,
developed by Lavaste et al’, was used (figl). Geometrical and mechanical models of each implant were then constructed (fig.2). before being inserted in the spinal segment model. Then, for validation, these two L3S2 instrumented segment models were submitted to similar boundary conditions as used in a previous in vitro comparison of the same implants2. FJexion load-displacement curves were then controlled using experimental results. Loads acting on screws and Longitudinal Elements (LE) were calculated and analysed for a better understanding of the intrinsic diffe-rences between both constructs RESULTS: Load-displacement responses of both constructs were quite similar (L3 sagittal rotation at 10 N.m -1.5” ), while the loads in the implant were not. For example, the axial push-in forces at the Sl screws (see fig.3) were equal to 30N for the Twinflex, and 150N for the SRC. The pull-out forces at the S2 screws were respectively 1OON and 200N for the Twinflex and the SRC. At other levels, axial forces were all lower than 6ON, the Twinflex vahtes being higher than the SRC ones. Bending moments acting on screws were respectively 0.7N.m and 1.4N.m at the L3 level for the Twinflex and the SRC. At lower levels, vahres were all below 0.6N.m. again with a reversed pmportion. Bending moments calculated along LE were always lower than 0.3Nm for the Twinflex, and up to 2N.m for the SRC. Axial forces in the Twinflex LE were about 160 N, and about IOON in the other construct. CONCLUSION: Although numerical approach mainly provides tendencies, it clearly seems that reducing flexuml stiftiress of lumbar fixation induces more homogenous load transmission along the construct, and highly reduces axial push-in/pull-out forces at the SllS2 levels, and alJ this without reducing the rigidity of the whole construct. Conversely, it has been shown that &gid, longitudinal elements may concentrate stresses at the construct extremities, relieving loads at intermediate levels in the same time, which may be the sign of a stress-shielding-like phenomenon. REFERENCES: 1. F. Lavaste et al. (1992). J. of Biomechanics 25, (lo),1 153-1164. 2. A.G. Graftiaux et al. (1995) .Eump. J. of Orthopaedic Surg. 5.(4),265-269. ACKNOWLEDGEMENTS: The authors thank the Eurosurgical company for its support. CORRESPONDENCE: A.Templier, W.SkaJli 151 Bld. de I’hSpital, 75013 Paris. [email protected]
of the ESB, July 8-1 I 98. Toulouse, France
139
OF SPINAL
INTRODUCTION: Numerous mathematical models which investigate the high incidence of low back pain associated with lifting activities have been based upon the concept of the ‘lever’ model. This considers the muscle force required for equilibrium about a single intervertebral joint. When representing this muscle force collectively, the predicted compression at the LX?.1 intervertebral joint indicates failure of the vertebral end plates. In comparison for the same loading conditions when representing the spine as an arch, the amount of muscle force required for equilibrium and stability in a given posture has been predicted to be significantly less (Aspden, 1987). The consequent reduction in compression at the W/S1 intervertebral joint implies the curvature of the spine has a role in load bearing. METHODS: For lever models with greater anatomic detail, a reduction in compression at the IX31 joint is observed (McGill and Norman, 1987). For comparison, this paper describes the development of a model which included greater anatomic detail based upon arch theory. This included consideration of the curvature of the whole spine, a distributed loading pattern for body weight, and the activity of individual spinal muscle groups. RESULTS AND CONCLUSION: When accounting for the variation in force magnitude, direction and region of application for each individual muscle group, the model predicted the total amount of force required was greater than for a single collective muscle force. Consequently compression at the L5/S1 joint was greater, and closer to existing detailed lever model predictions. The curvature of the spine does therefore not reduce the level of loading significantly as initially proposed. However for stability of the spine, muscle activity is necessary to ensure transmission of force between vertebrae at every level. The amount of force required depends upon their configuration. Application of muscle forces for stability at a certain level can affect the loading at other levels. Consideration of the curvature of the whole spine is therefore necessary to determine more realistic loading conditions for a given posture. By considering the configuration of the spine, and the strength of the individual muscle groups, the demands on the muscles and intervertebral joints at each level of the spine can be investigated using this model. For a given posture or lifting activity possible areas of tissue overload and sources of back pain may therefore be identified REFERENCES: 1. Aspden, R.,ClinicaJ Biomechanics, Vol. 2, pp 168-174, 1987. 2. Mci;ill S. and Norman, R., J. Biomech., V20,. 6, pp 591-600, 1987. CORRJW’ONDENCE: Dr B Serpil Acar, and MS S L Grilli Dept. of Computer Studies, Loughbomugh University, Loughborough, Leics. LB1 1 3TU, UK Tel: +44 1509 222 879, Fax: +44 1509 211 586, [email protected], [email protected]
11”’ Conference
LUMBAR SPINAL FlXATION : THE INFLUENCE OF S ON ANCHOR LOADING FLE-STtFFNES A. Templier’, W. Skalli’, L. Denmnger’, C. Maxe12, F. Lava.& ‘Laboratoire de BioM&anique, ENSAM Paris *Jnstitut Mutualiste Montsouris, Pte de Choisy, Paris INTRODUCTION: The present study Finite-Element Analysis (FEA) of two devices when implanted. This implants Screw/Rod (06mm) Concept, 2) the 02~2.5~). METHODS: A narameterimd 3D FBA
is a numerical comparison using different kinds of spinal fixation are 1) the widely used dgida Twinflex@ dynamics system ( model of a W-sacrum
segment,
developed by Lavaste et al’, was used (figl). Geometrical and mechanical models of each implant were then constructed (fig.2). before being inserted in the spinal segment model. Then, for validation, these two L3S2 instrumented segment models were submitted to similar boundary conditions as used in a previous in vitro comparison of the same implants2. FJexion load-displacement curves were then controlled using experimental results. Loads acting on screws and Longitudinal Elements (LE) were calculated and analysed for a better understanding of the intrinsic diffe-rences between both constructs RESULTS: Load-displacement responses of both constructs were quite similar (L3 sagittal rotation at 10 N.m -1.5” ), while the loads in the implant were not. For example, the axial push-in forces at the Sl screws (see fig.3) were equal to 30N for the Twinflex, and 150N for the SRC. The pull-out forces at the S2 screws were respectively 1OON and 200N for the Twinflex and the SRC. At other levels, axial forces were all lower than 6ON, the Twinflex vahtes being higher than the SRC ones. Bending moments acting on screws were respectively 0.7N.m and 1.4N.m at the L3 level for the Twinflex and the SRC. At lower levels, vahres were all below 0.6N.m. again with a reversed pmportion. Bending moments calculated along LE were always lower than 0.3Nm for the Twinflex, and up to 2N.m for the SRC. Axial forces in the Twinflex LE were about 160 N, and about IOON in the other construct. CONCLUSION: Although numerical approach mainly provides tendencies, it clearly seems that reducing flexuml stiftiress of lumbar fixation induces more homogenous load transmission along the construct, and highly reduces axial push-in/pull-out forces at the SllS2 levels, and alJ this without reducing the rigidity of the whole construct. Conversely, it has been shown that &gid, longitudinal elements may concentrate stresses at the construct extremities, relieving loads at intermediate levels in the same time, which may be the sign of a stress-shielding-like phenomenon. REFERENCES: 1. F. Lavaste et al. (1992). J. of Biomechanics 25, (lo),1 153-1164. 2. A.G. Graftiaux et al. (1995) .Eump. J. of Orthopaedic Surg. 5.(4),265-269. ACKNOWLEDGEMENTS: The authors thank the Eurosurgical company for its support. CORRESPONDENCE: A.Templier, W.SkaJli 151 Bld. de I’hSpital, 75013 Paris. [email protected]
of the ESB, July 8-1 I 98. Toulouse, France
139