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Biomedical subjects

M Gardner-Morse

Publications and source records attributed to M Gardner-Morse.

11 recordsLinked to original sources
PubMed2003

Biomechanical modeling of posterior instrumentation of the scoliotic spine.

Scoliosis is a three-dimensional deformation of the spine that can be treated by vertebral fusion using surgical instrumentation. However, the optimal configuration of instrumentation remains controversial. Simulating the surgical maneuvers with personalized biomechanical models may provide an analytical tool to determine instrumentation configuration during the pre-operative planning. Finite element models used in surgical simulations display convergence difficulties as a result of discontinuities and stiffness differences between elements. A kinetic model using flexible mechanisms has been developed to address this problem, and this study presents its use in the simulation of Cotrel-Dubousset Horizon surgical maneuvers. The model of the spine is composed of rigid bodies corresponding to the thoracic and lumbar vertebrae, and flexible elements representing the intervertebral structures. The model was personalized to the geometry of three scoliotic patients (with a thoracic Cobb angle of 45 degrees, 49 degrees and 39 degrees ). Binary joints and kinematic constraints were used to represent the rod-implant-vertebra joints. The correction procedure was simulated using three steps: (1) Translation of hooks and screws on the first rod; (2) 90 degrees rod rotation; (3) Hooks and screws look-up on the rod. After the simulation, slight differences of 0-6 degrees were found for the thoracic spine scoliosis and the kyphosis, and of 1-8 degrees for the axial rotation of the apical vertebra and for the orientation of the plane of maximum deformity, compared to the real post-operative shape of the patient. Reaction loads at the vertebra-implant link were mostly below 1000 N, while reaction loads at the boundary conditions (representing the overall action of the surgeon) were in the range 7-470 N and maximum torque applied to the rod was 1.8 Nm. This kinetic modeling approach using flexible mechanisms provided a realistic representation of the surgical maneuvers. It may offer a tool to predict spinal geometry correction and assist in the pre-operative planning of surgical instrumentation of the scoliotic spine.

Adolescent↗
PubMed2001

Lumbar spinal muscle activation synergies predicted by multi-criteria cost function.

The hypothesis that control of lumbar spinal muscle synergies is biomechanically optimized was studied by comparing EMG data with an analytical model with a multi-component cost function that could include (1) trunk displacements, (2) intervertebral displacements, (3) intervertebral forces; (4) sum of cubed muscle stresses, and (5) eigenvalues for the first two spinal buckling modes. The model's independent variables were 180 muscle forces. The 36 displacements of 6 vertebrae were calculated from muscle forces and the spinal stiffness. Calculated muscle activation was compared with EMG data from 14 healthy human subjects who performed isometric voluntary ramped maximum efforts at angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees and 180 degrees to the right from the anterior direction. Muscle activation at each angle was quantified as the linear regression slope of the RMS EMG versus external force relationship, normalized by the maximum observed EMG.There was good agreement between the analytical model and EMG data for the dorsal muscles when the model included either minimization of intervertebral displacements or minimization of intervertebral forces in its cost function, but the model did not predict a realistic level of abdominal muscles activation. Agreement with EMG data was improved with the sum of the cubed muscle stresses added to the cost function. Addition of a cost function component to maximize the trunk stability produced higher levels of antagonistic muscle activation at low efforts than at greater efforts. It was concluded that the muscle activation strategy efficiently limits intervertebral forces and displacements, and that costs of higher muscle stresses are taken into account, but stability does not appear to be maximized. Trunk muscles are apparently not controlled solely to optimize any one of the biomechanical costs considered here.

Biomechanical Phenomena↗
PubMed2000

Decrease in trunk muscular response to perturbation with preactivation of lumbar spinal musculature.

STUDY DESIGN: An experimental study of healthy subjects' trunk muscle responses to force perturbations at differing angles and steady state efforts. OBJECTIVES: To determine whether increased preactivation of muscles was associated with decreased likelihood of muscular activation in response to a transient force perturbation. SUMMARY OF BACKGROUND DATA: Trunk stability (ability to return to equilibrium position after a perturbation) requires the stiffness of appropriately activated muscles to prevent buckling and consequent "self-injury." Therefore, greater trunk muscle preactivation might decrease the likelihood of reflex muscle responses to small perturbations. METHODS: Each of 13 subjects stood in an apparatus with the pelvis immobilized. A harness around the thorax provided a preload and a force perturbation by a horizontal cable and a movable pulley attached to one of five anchorage points on a wall track surrounding the subject at angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees to the forward direction. Subjects first equilibrated with a preload effort of nominally 20% or 40% of their maximum extension effort. Then a single full sine-wave force perturbation pulse of nominal amplitude, 7.5% or 15% of maximum effort, duration 80 milliseconds or 300 milliseconds, was applied at a random time, with three repeated trials of each test condition. The applied force (via a load cell) and the electromyographic activity of six right and left pairs of trunk muscles were recorded. Muscle responses were detected by two methods. 1) Shewhart method: electromyographic signal greater than "baseline" values by more than three standard deviations, and 2) Mean Electromyographic Difference method: mean electromyographic signal in a time window 25 to 150 milliseconds after the force perturbation greater than that in a 25- to 150-millisecond window before the perturbation. RESULTS: Lower preload efforts were associated with more muscle responses (overall mean response detection rate = 33% at low preload and 25% at high preload). Using the Shewhart method, there were significant differences by effort (P<0.05) for all abdominal muscles and for all left dorsal muscles except multifidus. Using the Mean Electromyographic Difference method, there were significant differences by effort (P<0.05) for the same dorsal muscles, but only for one of the abdominal muscles. CONCLUSIONS: Findings are consistent with the hypothesis that the spine can be stabilized by the stiffness of activated muscles, obviating the need for active muscle responses to perturbations.

Adult↗
PubMed1999

Quantitative anatomy of the lumbar musculature.

This paper describes the anatomy of the musculature crossing the lumbar spine in a standardized form to provide data generally suitable for static biomechanical analyses of muscle and spinal forces. The muscular anatomy from several sources was quantified and transformed to the mean bony anatomy of four young healthy adults measured from standing stereo-radiographs. The origins, insertions and physiological cross-sectional area (PCSA) of 180 muscle slips which act on the lumbar spine are given relative to the bony anatomy defined by the locations of 12 thoracic and five lumbar vertebrae, and the sacrum, and the shape and positions of the 24 ribs. The broad oblique abdominal muscles are each represented by six vectors and an appropriate proportion of the total PCSA was assigned to each to represent the muscle biomechanics.

Abdominal Muscles↗
PubMed1998

[Biomechanical modeling of instrumentation for the scoliotic spine using flexible elements: a feasibility study].

Surgical instrumentation of the scoliotic spine is a complex procedure with many parameters, such as the spinal segment to operate on, the number and position of the hooks and screws, etc. Biomechanical modeling is a tool which can be used to determine the influence of these parameters. However, technical difficulties due to the large stiffness range of involved components and the large deformations associated with surgical maneuvers are encountered when using the finite elements method. Thus, the objective of this study is to adapt a modeling approach using analysis of flexible mechanisms and evaluate its feasibility. The model combines rigid bodies for the vertebrae and flexible elements representing intervertebral structures. The mechanical properties were calculated from published data and the geometry was personalized with intraoperative measurements. Following the installation of the hooks and screws on the modeled spine, two steps were used to simulate the surgical maneuvers: 1) translation and attachment of the hooks/screws on the first rod; 2) rod rotation. The feasibility of this modeling approach was evaluated by simulating the surgical maneuvers on 2 cases: 1) a physical model; 2) a clinical case. The agreement between intraoperative measurements and simulation results (frontal curvatures are reproduced with over 80% accuracy) shows the feasibility of the modeling approach. This approach also reduces computational convergence problems because of its limited sensitivity to stiffness differences between elements, which demonstrates the advantage of flexible mechanism modeling relative to finite element modeling. Long term goals of subsequent refinements are the development of a tool for surgical correction predictions and for the design of more efficient instrumentation.

Adolescent↗
PubMed1995

Role of muscles in lumbar spine stability in maximum extension efforts.

Many problems of the lumbar spine that cause pain are attributed to instability. The ligamentous spine (without muscles) is unstable at very low compressive loads. This study examined the hypothesis that instability of the lumbar spine is prevented under normal circumstances by the stiffness of spinal musculature, without active responses from the neuromuscular control system. The effect of muscle activity (force and stiffness) on the stability of the lumbar spine was analyzed for maximum voluntary extension efforts with different spinal postures in the sagittal plane. The analysis included realistic three-dimensional representation of the muscular anatomy with muscles crossing several motion segments. The stiffness of motion segments was represented using in vitro measured properties. Under a range of conditions with maximum extension effort, active muscle stiffness was required to prevent the lumbar spine from buckling. The dimensionless value of the muscle stiffness parameter q as a function of activation and length had to be greater than a critical value in the range of 3.7-4.7 in order to stabilize the spine. Experimentally determined values of q ranged from 0.5 to 42. These analyses demonstrate how changes in motion segment stiffness, muscle activation strategy, or muscle stiffness (due to degenerative changes, injuries, fatigue, and so on) might lead to spinal instability and "self-injury."

Elasticity↗
PubMed1995

Lumbar spine maximum efforts and muscle recruitment patterns predicted by a model with multijoint muscles and joints with stiffness.

The transmission of load through the lumbar spine was analyzed in a model of the five lumbar vertebrae, the sacrum/pelvis and the thorax, and 66 symmetric pairs of multijoint muscles. The model was used to test the hypotheses that (1) the need to maintain equilibrium simultaneously at all vertebral levels precludes simultaneous maximum activation of synergistic muscles and (2) that the maximum loads which could be carried by the spine and the degree of muscle activation increases with increasing motion segment stiffness. Maximum moments applied to T12 were calculated for moments in three principal directions, subject to equilibrium at all six joints and to constraints on the maximum muscle stress and intervertebral displacements. A model with realistic motion segment stiffness predicted maximum efforts between 1.4 and 3.3 times greater than a model with 'ball-and-socket' joints, and in better agreement with published results from maximum effort experiments. The differences in maximal effort were greater than the moments transmitted through the joints. While muscle activation levels were greater, many synergistic muscles were still submaximally activated. Antagonistic muscles were recruited to maintain multijoint equilibrium. We concluded that (1) muscle activations permitted in single anatomic level analyses are generally not compatible with equilibrium at other levels; (2) the effect of moment transmission in the joints gives a more realistic representation of the lumbar spine.

Biomechanical Phenomena↗
PubMed1994

Three-dimensional simulations of the scoliosis derotation maneuver with Cotrel-Dubousset instrumentation.

The derotation maneuver using Cotrel-Dubousset instrumentation (CDI) is intended to correct the counterdirectional transverse plane rotations of the spine and of the vertebrae in thoracic scoliosis. This was simulated in a finite element model of an idealized thoracic scoliosis with an initial 65 degrees scoliosis angle and 0 degree kyphosis angle. After 90 degrees of rod rotation the apical vertebra derotated 50 degrees towards the sagittal plane but the apical vertebra axial rotation worsened by 8 degrees. The scoliosis angle corrected to 29 degrees and a 54 degrees kyphosis was created. If the initial rod curvature was reduced by 9 degrees, the model predicted only small changes in spinal curvature resulting from the forces required to connect the vertebrae to the hooks. Decreased kyphosis and scoliosis curvatures but increased vertebra axial rotation were produced by the derotation maneuver. The increase in apical vertebra axial rotation was reversed by modifying the representation of the motion segments by repositioning their effective axes 30 mm posteriorly.

Biomechanical Phenomena↗
PubMed1993

Three-dimensional simulation of Harrington distraction instrumentation for surgical correction of scoliosis.

Harrington distraction rod surgery on six female patients with idiopathic scoliosis was simulated in three-dimensional osseoligamentous finite element models with individual geometry taken from preoperative stereo roentgenographic reconstructions of the spine and ribcage and compared with the measured outcome. Boundary conditions at the ends of the spine were used to maintain pelvis and head alignment. Published material and flexibility properties were used. The amount of hook distraction was calculated from measured changes in the distance between the hook sites (range, 13-27 mm). Initial simulations underestimated the Cobb angle correction by an average 6%. They underestimated the spinal elongation by 36% and predicted an average 12 degrees increase in kyphosis angle compared with an actual 10 degrees average decrease. Agreement for sagittal plane changes improved in five cases when the beams representing the motion segments were displaced posteriorly. In the sixth case (with the rod applied over a lordotic spinal region), agreement was improved with the motion segment beams displaced anteriorly. The amount of the beam displacement that gave the best agreement was variable, and we were not able to predict it for each individual. Both measured and simulated changes in vertebral transverse plane rotations and in rib angulations were small. The greatest source of errors in these simulations appeared to be inadequate representation of in vivo motion segment behavior by in vitro measured stiffness properties.

Adolescent↗
PubMed1991

Analysis of the interaction between vertebral lateral deviation and axial rotation in scoliosis.

There is a lack of clear biomechanical analyses to explain the interaction of the lateral and axial deformity of the spine in idiopathic scoliosis. A finite element model which represented an isolated ligamentous spine with realistic elastic properties and idealized geometry was used to analyse this interaction. Three variations of this model were used to investigate two different hypotheses about the etiology of scoliosis and to define the forces required to produce a scoliosis deformity. The first hypothesis is that coupling within a motion segment produces the interaction between lateral deviation and axial rotation. The second hypothesis is that posterior tethering by soft tissues in the growing spine produces the observed interaction. Modeling of both hypotheses failed to produce the clinically observed pattern of interaction. Therefore, to find which biomechanical forces were required to produce an idealized scoliosis, prescribed displacements were applied to the model. Production of a double curve scoliosis of 10 degrees Cobb angles required lateral forces on the order of 20 N acting 40 mm anterior to the vertebral body centers. There do not appear to be any anatomic structures capable of producing such forces. Therefore, it seems unlikely that scoliosis deformity can be explained in terms of forces acting on the spine, and understanding of its origins may come from examination of other mechanisms such as asymmetric thoracic growth, or asymmetric vertebral development.

Biomechanical Phenomena↗