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Biomedical subjects

I A Stokes

Publications and source records attributed to I A Stokes.

At least 19 recordsLinked to original sources
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

Changes in shape of the adolescent idiopathic scoliosis curve after surgical correction.

STUDY DESIGN: The effect of spinal instrumentation in idiopathic scoliosis was studied in 21 patients who had Harrington instrumentation and 15 who had Wisconsin-Drummond instrumentation. OBJECTIVE: Radiographs were analyzed to determine if the frontal and transverse plane shape of the scoliosis curve was changed by surgery, with and without segmental fixation. SUMMARY OF BACKGROUND DATA: Previous reports were based on frontal plane measurements of the curve (Cobb angle). The study reports correction in the frontal plane (Cobb angle) and transverse plane (apical vertebral rotation), as well as the regional distribution of the correction. METHODS: Radiographs before surgery, soon after, and between 5 and 48 months after surgery were marked and digitized to measure the regional distribution of the frontal plane shape and transverse plane vertebral rotation. RESULTS: Despite improvement in the magnitude of the deformity, the scoliosis curve shape remained almost constant postoperatively. There was minimal correction of the apical vertebra axial rotation in either group. CONCLUSION: This study documents that although the Harrington and Wisconsin-Drummond instrumentation systems decrease the Cobb angle, they do not change the shape of the curve or correct apical vertebra axial rotation. Newer instrumentation designs need to look beyond the Cobb angle as the only measure of outcome.

Adolescent↗
PubMed1994

Three-dimensional terminology of spinal deformity. A report presented to the Scoliosis Research Society by the Scoliosis Research Society Working Group on 3-D terminology of spinal deformity.

Conventional terminology of three-dimensional description of spinal deformity is ambiguous and mostly tied to either a frontal or sagittal plane view of the spine. The article proposes a rationalized system for describing the shape of the spine. The spine is viewed as a line in space ('vertebral body line') with three 'angulations' specifying the orientation of each vertebra. Four axis systems are defined for the whole body, the spine, curve regions, and individual vertebrae, respectively. These in turn define the principal planes of the body, spine, curve regions, and vertebrae. Curvature can be defined as a local measure at a point on the vertebral body line, or as a regional measure between specified end vertebrae. Torsion is defined both as a local geometric property of the vertebral body line, and as measure of the relative axial plane angulations between specified vertebrae. Linear distance measures define the deviations of specified vertebrae from the local, regional, spinal, and global axis systems. Practical recommendations for positioning patients are made. This new system of terminology recognizes the 3-dimensional nature of scoliosis and other spinal deformities and is intended to rationalize communication in both research and clinical practice.

Humans↗
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↗
PubMed1994

Comparison of curve shape between children with cerebral palsy, Friedreich's ataxia, and adolescent idiopathic scoliosis.

Fourteen patients with cerebral palsy (CP), 12 with Friedreich's ataxia (FA) and 26 with adolescent idiopathic scoliosis (AIS) were studied to determine whether the shape of the scoliosis curve differs between these categories. The slope of the regression relationship between vertebral rotation and lateral deviation was greater for the CP group compared with the FA and AIS groups. The authors conclude that the scoliosis curve pattern of children with Friedreich's ataxia and adolescent idiopathic scoliosis is similar. In contrast, the scoliosis curve of children with CP was distinctly different, with more rotation of the apical vertebrae into the convexity of the scoliosis curve (transverse plane deformity) in relation to the amount of lateral deviation of the apical vertebrae from the spinal axis (coronal plane deformity).

Adolescent↗
PubMed1994

[Intraoperative three-dimensional evaluation of Cotrel-Dubousset's procedure for the treatment of idiopathic scoliosis].

In order to evaluate with accuracy the tridimensional (3D) vertebral correction induced by the surgical correction of idiopathic scoliosis with Cotrel-Dubousset instrumentation and technique, we have developed a new per operative measurement system based on 3D digitization with magnetic fields. This method has been used on 23 adolescent patients treated with surgical correction. A statistically significant change in vertebral 3D orientation of 11.4 degrees +/- 5.6 degrees has been found. The measurement repeatability was +/- 2.5 degrees. We conclude that the Cotrel-Dubousset technique truly realizes a 3D correction of the thoracic and lumbar spine of adolescent idiopathic scoliosis.

Adolescent↗
PubMed1993

Reexamination of the Cobb and Ferguson angles: bigger is not always better.

In scoliosis, the Cobb measure of curve severity has been recommended over the Ferguson method because it had greater magnitude and appeared more sensitive to changes during progression and after treatment. This study made comparisons between the Cobb and Ferguson measures in radiographs of patients with idiopathic scoliosis to test whether the methods were really different, and to compare their precision. In 138 observations of 77 untreated patients there was a very high correlation (R2 = 0.98) between Cobb and Ferguson angle, with Cobb angle averaging 1.35 times greater. For sequential measures (mean interval 10 months), the percent changes agreed closely (R2 = 0.5). The relationship between Cobb and Ferguson angles remained the same in measurements of 24 patients wearing a brace compared with the unbraced condition and in 18 patients measured before and after Harrington rod surgery. Repeated measurements were made by three observers with the apex and end vertebrae pre-marked and held constant. For Cobb angle, the greatest range of measurements on any film was 8 degrees (pooled SD = 1.3 degrees). For Ferguson angle the greatest range was also 8 degrees (pooled SD = 1.8 degrees). Ferguson angle was slightly more sensitive to incorrect selection of end vertebrae. It was concluded that both methods can be useful for measuring curve magnitude. Ferguson angle should be measured and then adjusted by multiplying it by 1.35 in situations where Cobb angle measurement is technically difficult or invalid. Ferguson angle is better suited to automated measurement.

Adolescent↗
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↗
PubMed1991

Biomechanical testing and scoliosis. In vivo methods.

Patients with scoliosis deformity can be evaluated from a biomechanical viewpoint by measurement of spinal morphology, back surface topography, and changes in these measurements both acutely and over time. In addition, the forces acting on the spine can be estimated in vivo. Many ethical and practical challenges have been overcome, and these measurement techniques have provided information on spinal deformity and its treatment. Further insights have come from biomechanical analyses of these measurements, and complementary modeling studies. This study reviews the available methodologies and the progress that has been made toward using these methods in assessing outcomes in treatment. Possible future directions for in vivo measurements are suggested for studies of etiology, growth regulation, progression of deformity, three-dimensional assessment of treatment outcome, and monitoring of individual patients.

Biomechanical Phenomena↗
PubMed1990

Three-dimensional osseo-ligamentous model of the thorax representing initiation of scoliosis by asymmetric growth.

A biomechanical model of the human thorax was constructed to investigate how asymmetric growth of the thorax might initiate spinal lateral curvature and axial rotation as seen in scoliosis deformities. Geometric data specifying nodal points of the model were taken from stereo-radiographs of an adolescent subject. An initially symmetrical geometry was created by 'mirroring' measurements of a hemi-thorax and spine. Published data provided cross-sectional measurements of the ribs, material properties of tissues and global flexibilities of the intervertebral motion segments. The ribs, sternum, intervertebral motion segments and intercostal ligaments were represented by elastic elements. Model deformations were calculated by the direct stiffness finite element method, with growth represented by an initial strain term in the constitutive law. Non-linear behavior was accommodated by running the model recursively, with updated node locations at each step. Both stress relaxation and stress modulation of growth in the component tissues were simulated. Thoracic growth of 20% with asymmetric growth of the ribs was simulated to give rib length asymmetries of 11%. similar to that observed in a previous study of patients with idiopathic scoliosis. This resulted in the model having a small thoracic scoliosis curvature convex toward the side of the longer ribs. Variations of the model which permitted free motion at the costo-vertebral joints or produced changes in the curvature of the posterior parts of the ribs resulted in axial rotation of the vertebrae similar to that observed clinically. The model supports the idea that growth asymmetry could initiate a small scoliosis during adolescence.

Adolescent↗
PubMed1990

Intersegmental spinal flexibility with lumbosacral instrumentation. An in vitro biomechanical investigation.

Flexibility of the porcine lumbosacral spine was measured after application of six different types of surgical instrumentation, and in a control state. Fifteen adult pig spines were tested with flexion, extension, lateral bending, and axial rotation torques applied to the upper end with the pelvis fixed. Instrumentation was applied across two lumbar segments and the lumbosacral level (L5-6, L6-7, and L7-S1). Stereophotogrammetry was used to track markers applied to each vertebra. Intersegmental motion was measured as three angles and as the relative linear translation of adjacent transverse processes and spinous processes. Results showed that all instrumentation systems reduced intersegmental motion compared with the control state, except for minimal reduction at L5-6 by Harrington instrumentation in all loading directions, especially axial rotation. The pedicle screw systems were always the most rigid. After applying instrumentation, there were differences in the motion occurring at different anatomic levels, most commonly with the least motion occurring in the middle of the instrumented segment (L6-7). When intervertebral motion was expressed as the linear motion between adjacent spinous and transverse processes, the usual site of posterolateral fusion, it was 0.6 to 1.8 mm per degree of angular motion at the transverse processes and 1.3 to 2.1 mm per degree at spinous processes.

Animals↗
PubMed1990

Incorporation of spinal flexibility measurements into finite element analysis.

This technical note demonstrates two methods of incorporating the experimental stiffness of spinal motion segments into a finite element analysis of the spine. The first method is to incorporate the experimental data directly as a stiffness matrix. The second method approximates the experimental data as a beam element.

Elasticity↗
PubMed1990

Effects of axis placement on measurement of isokinetic flexion and extension torque in the lumbar spine.

We wanted to ascertain whether changing the machine axis placement relative to the anatomic landmarks of a tested subject in isokinetic flexion and extension trunk strength would influence the measurements obtained. Twenty healthy volunteers were tested with the machine axis intentionally displaced from a reference position at the lumbosacral junction. Displacing the axis 50 mm vertically produced on average 15% increase in the torque produced, and displacing it 10 mm horizontally produced approximately 5% reduction in torque, independent of the direction of displacement from the reference position. The angle at which maximum torque occurred and the ratio of extension to flexion torque were unaffected by axis placement. Intertester reliability in axis placement was found experimentally to produce variations of usually less than 10 mm. We conclude that isokinetic trunk strength measurement is affected by testing axis placement; this should be controlled, especially in repetitive testing of the same individual.

Adult↗
PubMed1989

Rib cage asymmetry in idiopathic scoliosis.

Seventy-one patients attending a scoliosis clinic and 10 control subjects were studied by a stereoradiographic three-dimensional reconstruction of the spine and rib cage. The symmetry of each rib pair (at each anatomic level) was described by measurements of rib arc length, chord length, enclosed area, maximum curvature, and frontal and lateral angulations. Patients were divided into four groups: 19 with a single right thoracic curve, 15 with a single left lumbar or thoracolumbar curve, 22 with double curves, and 15 with a curve with less than 10 degrees Cobb angle. In the control group and the group with minimal scoliosis, there was no statistically significant rib asymmetry. Among the patients with scoliosis, 11 of 19 patients with right single thoracic curves had rib arc lengths greater on the right side at the curve apex, and nine of 15 patients with left lumbar scoliosis had longer ribs on the left side in the corresponding region of the thoracic spine. Eleven of 22 patients with double curves had symmetrical rib lengths (within +/- 3%), the other 11 had ribs longer on the left. These proportions should not have occurred by chance (p less than 0.001). The mean rib length difference in patients with single thoracic curves was 1.39% (right longer than left), in single lumbar curves it was 3.57% (left longer than right), and in double curves 3.18% (left longer than right). These differences between the groups of patients and control subjects were statistically significant (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1989

Axial rotation component of thoracic scoliosis.

The axial rotation (rotation about a vertical axis) of the vertebrae, of the ribs, and of the back surface are components of the deformity recognized clinically as the "rib hump" in thoracic scoliosis. Relationships of these rotations to the lateral deviation and lateral curvature of the spine were studied in 40 patients with idiopathic scoliosis. Stereoradiographs of the spine and rib cage were used to measure three components of axial rotation: rotation of the vertebrae, of the rib cage, and of the plane of maximum curvature of the spine. Stereotopographs of the back surface were digitized to measure the axial rotation of the back surface. In individual patients, there were high correlations of all components of axial rotation at each spinal level with the corresponding vertebral lateral deviation from the spinal axis. By regression analyses of the maximum values of each rotation in each curve, the rotation of the apex vertebra was found to be generally of lesser magnitude than the rotation of the plane of maximum curvature of the spine and in an opposite sense in kyphotic curves. The rib cage rotation was generally of lesser magnitude than the vertebra rotation, and the back surface rotation was less than both of these skeletal rotations. Vertebra rotation correlated most closely with lateral deviation of the spine. Simple segmental coupling of axial rotation and lateral bending could not be responsible for this axial rotation.

Humans↗