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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
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↗
PubMed1989

Experimental instability in the rabbit lumbar spine.

The authors performed mechanical, biochemical, and histologic analyses of changes in the rabbit lumbar spine occurring after instability had been induced by facet removal to find whether this intervention produced an experimental model for intervertebral disc degeneration. Sham operated animals and an unoperated control group were used for comparison. Half of the operated animals were housed under conditions to promote higher physical activity than the other animals housed individually in small cages. Acutely, the removal of facet joints increased the flexibility of intervertebral joints. Over the following year, this increase in flexibility was reduced to close to control levels in all groups of animals. Within the intervertebral discs, there was no significant change in proportions or solubility of collagen or proteoglycans after surgery, nor was there microscopic or macroscopic evidence of disc degeneration. The surgical procedure produced hypermobility of the spine, but there was a subsequent restabilization, and the intended disc degeneration was not produced. These findings indicate that some as yet unidentified soft tissue repair process, facilitated by activity, overcame the hypermobility created at surgery, so degenerative changes in the intervertebral discs did not result. We suggest that other animal models of disc degeneration may represent a failure of reparative response to acute injury.

Animals↗
PubMed1989

Concordance of back surface asymmetry and spine shape in idiopathic scoliosis.

In order to determine why topographic methods have shown a poor correlation with radiographically measured scoliosis in clinical studies, the accuracy of detection of the presence, side, apex, and magnitude of a scoliosis curve was determined topographically (by moiré fringe photography and by projected raster photography) in 104 patients attending a scoliosis clinic. The presence or absence of thoracic curves was correctly shown by the topograms in 77% of cases, and in the lower region (lumbar and thoracolumbar curves) in 79% of cases. For correctly identified curves, the greatest back surface rotation was, on average, 1.0 vertebral levels below the skeletal curve apex in the thoracic region and 0.5 levels below the apex in the lower region. The moiré fringe with the greatest asymmetry occurred on average at 1.5 and 1.8 vertebral levels above the spinal apex in upper and lower regions, respectively. The magnitude of the Cobb angle was determined to within +/- 5 degrees in 24% of cases by moiré measurements, and in 27% by the raster technique. The side of the scoliosis was incorrectly diagnosed by topography in ten patients with minimal or 'nonstandard' vertebral rotation. It was concluded that the presence, level, and side of a scoliosis curvature is well demonstrated by back surface topography in patients with 'standard' rotation, but the magnitude of the scoliosis cannot be determined from topograms sufficiently accurately for most clinical purposes.

Adolescent↗
PubMed1989

EMG to torque relationship in rectus abdominis muscle. Results with repeated testing.

Rectus abdominis muscles of young healthy female volunteers were studied to determine both the maximum strength in isometric flexion and the relation between the surface electromyogram (RMS EMG) and torque. Both the upper and lower portions of the abdominal muscle were studied during graded increase and decrease of torque. Repeated testing was performed over 6 weeks. The form of the relation between torque and EMG was better described by a quadratic than by a linear regression relationship, but with considerable variability about the best-fit line. The torque-increasing and torque-decreasing parts of each test were different and were analyzed separately. After 6 weeks of repeated testing, maximum voluntary isometric flexion torque increased 16.8% (P less than 0.01). There was a decrease in the ratio of electromyographic activity to torque production, which was statistically significant in the torque-increasing recordings. These changes in maximum torque and in the EMG-torque relation were attributed to learning through a test-retest effect, rather than to a true change in muscle characteristics. These findings, which show changes in normal subjects undergoing repeated testing, do not support the reliability of isometric strength measurements, or measurements based on RMS EMG recordings, for quantifying abdominal muscle function in patients with back pain, or those undergoing strengthening exercises.

Abdominal Muscles↗
PubMed1988

Spinal deformity and back surface asymmetry in idiopathic scoliosis.

Stereo radiography and stereotopography were used to record simultaneously the three-dimensional spinal and back surface shape of 141 patients with a clinical diagnosis of adolescent onset idiopathic scoliosis. Radiography confirmed 80 patients with single lateral curves (Cobb angle 5-87 degrees); 59 with double curves (Cobb angle 5-61 degrees), and two patients with triple curves. Orientation of each vertebra was measured by the lateral deviation from the spinal axis and the axial rotation. For each curve detected, the Cobb angle, the maximum deviation, the maximum axial rotation, and the sagittal plane curvature was measured. Topographic data were used to generate a profile of a cross-section through the back of each vertebral level. This section was characterized by its axial rotation, deviation of its midpoint from the spinal axis, and area symmetry about the midpoint. Maximum values of these measurements in the region of each radiographic scoliosis curve were noted. Regression and correlation analysis showed (for each curve) a high correlation of apical vertebral lateral deviation with back surface axial rotation (r = 0.79) and with vertebral axial rotation (r = 0.79). The back surface rotation was less than vertebral rotation in magnitude, however, by a mean factor of 0.55. The measurement of the back surface asymmetry that gave the highest correlation with the skeletal deformity was the axial rotation. Relationships between skeletal and surface measures were similar for curves in thoracic and lumbar regions and for patients with single and double curves. There is potential for clinical application of surface measurement techniques to prescription of orthoses and monitoring of changes due to progression or treatment.

Adolescent↗
PubMed1988

Comparison of acoustic and electrical signals from erectores spinae muscles.

Comparison was made between simultaneous recordings of the electromyogram (EMG) and acoustic myogram (AMG) signals and the torque produced by the lumbar erectores spinae muscles of three healthy subjects while they performed isometric contractions. Repeatability of each signal was studied. The AMG had a relationship to the torque which was more quadratic than linear, whereas that of the EMG was more linear. The coefficient of variability of residuals about a "best-fit" line was 8.55% for torque-increasing EMG recordings and 19.2% for torque-increasing AMG recordings. At the same torque, the EMG was on average about 10% greater for increasing-torque recordings than for decreasing-torque parts of the recordings, but the magnitude of the AMG signal was about 10% less. With repeated testing on the same day, the coefficient of variability of the signal/torque relationship (torque-increasing) was 9% for EMG and 23% for AMG signals. With long-term testing over 2 weeks these coefficients were 23 and 39%, respectively. Correlation analysis showed that the two signal/torque relationships varied independently of each other.

Electromyography↗
PubMed1988

Measurements of the three-dimensional shape of the rib cage.

A stereoradiographic method was developed to measure the three-dimensional shape of the rib cage in vivo in order to provide descriptive data and to study symmetry in the normal population. The method is also intended for use in description of rib asymmetry in scoliosis. Rib midlines were reconstructed from digitized points on lines drawn through the middle of each rib image in stereo-radiographs of the rib cage. The method relied on pairing points in one image with a corresponding point found in the second image. The error term obtained from a Direct Linear Transformation (DLT) reconstruction program was used for optimizing this correspondence. The method was used on test objects of known shape where the standard deviation of measurement errors was found to be 0.72 mm. Studies in which different sets of points on the rib midlines were selected for digitizing showed that the precision of the method was 1.04 mm (S.D.). Rib shape was described by intrinsic measurements (arc and chord length, maximum curvature and enclosed area) and rib orientation by extrinsic measurements (frontal and lateral angulations and posterior rib cage rotation). No statistically significant rib shape asymmetry was found among ten subjects without spinal or thoracic abnormalities, although a trend of inequality of rib angulation at all anatomical levels was observed.

Biomechanical Phenomena↗
PubMed1988

Bulging of lumbar intervertebral discs: non-contacting measurements of anatomical specimens.

A noncontacting photogrammetric method was used to measure intervertebral disc bulge (deformation perpendicular to the disc surface) at an anterior site and a posterolateral site of 17 human lumbar discs. Specimens were tested in constructs consisting of two lumbar vertebrae and the intervening disc. The posterior elements were removed to give unobstructed visibility of the posterolateral aspects. Stereophotogrammetry was used to record the motion of target points attached to the disc and vertebral body surfaces. In axial loading, the bulging increased linearly with disc compression, which in turn had a nonlinear relationship with the applied load. Axial loads and offset compression loads of physiologic magnitude caused disc bulge that was generally less than 1 mm. Axial torsion of an otherwise unloaded specimen produced minimal change in disc bulge. The disc bulging measured in this study is generally less than that previously reported, especially for angular motions between the vertebrae. This is probably because the photogrammetric method used here is independent of motion of the vertebrae, which can corrupt mechanical, contacting methods of measurement.

Biomechanical Phenomena↗
PubMed1988

Dimensional changes of the feet in pregnancy.

Serial measurements of the volume, length, and width of the feet of seventeen pregnant women were made at, or close to, the thirteenth and thirty-fifth weeks of pregnancy and eight weeks postpartum. The same measurements were made twice on a control group of sixteen nulliparous women at intervals that ranged from sixteen to twenty weeks. There was no change in the length or width of the feet in either group. The mean volume of the feet increased 57.2 milliliters between early and late pregnancy (p less than 0.001) and decreased by only 8.42 milliliters between late pregnancy and eight weeks postpartum. These changes were attributed to retention of fluid or to an increase in soft tissue and not to stretching or relaxation of the ligaments.

Adult↗