Geometric Induction in Chiral Superconductors.

We consider a number of effects due to the interplay of superconductivity, electromagnetism, and elasticity, which are unique for thin membranes of layered chiral superconductors. Some of them should be within the reach of present technology, and could be useful for characterizing materials. More speculatively, the enriched control of Josephson junctions they afford might find useful applications.

Proposed Spontaneous Generation of Magnetic Fields by Curved Layers of a Chiral Superconductor.

We demonstrate that two-dimensional chiral superconductors on curved surfaces spontaneously develop magnetic flux. This geometric Meissner effect provides an unequivocal signature of chiral superconductivity, which could be observed in layered materials under stress. We also employ the effect to explain some puzzling questions related to the location of zero-energy Majorana modes.

Strong and parallel salinity-induced phenotypic plasticity in one generation of threespine stickleback.

Phenotypic plasticity is a major factor contributing to variation of organisms in nature, yet its evolutionary significance is insufficiently understood. One example system where plasticity might have played an important role in an adaptive radiation is the threespine stickleback (Gasterosteus aculeatus), a fish that has diversified after invading freshwater lakes repeatedly from the marine habitat. The parallel phenotypic changes that occurred in this radiation were extremely rapid. This study evaluates phenotypic plasticity in stickleback body shape in response to salinity in fish stemming from a wild freshwater population. Using a split-clutch design, we detected surprisingly large phenotypically plastic changes in body shape after one generation. Fish raised in salt water developed shallower bodies and longer jaws, and these changes were consistent and parallel across families. Although this work highlights the effect of phenotypic plasticity, we also find indications that constraints may play a role in biasing the direction of possible phenotypic change. The slopes of the allometric relationship of individual linear traits did not change across treatments, indicating that plastic change does not affect the covariation of traits with overall size. We conclude that stickleback have a large capacity for plastic phenotypic change in response to salinity and that plasticity and evolutionary constraints have likely contributed to the phenotypic diversification of these fish.

Reconstructing nonlinearities with intermodulation spectroscopy.

We describe a method of analysis which allows for reconstructing the nonlinear disturbance of a high Q harmonic oscillator. When the oscillator is driven with two or more frequencies, the nonlinearity causes intermodulation of the drives, resulting in a complicated spectral response. Analysis of this spectrum allows one to approximate the nonlinearity. The method, which is generally applicable to measurements based on resonant detection, increases the information content of the measurement without requiring a large detection bandwidth, and optimally uses the enhanced sensitivity near resonance to extract information and minimize error due to detector noise.

Topological aspects of π phase winding junctions in superconducting wires.

We theoretically investigate Josephson junctions with a phase shift of π in various proximity induced one-dimensional superconductor models. One of the salient experimental signatures of topological superconductors, namely the fractionalized 4π periodic Josephson effect, is closely related to the occurrence of a characteristic zero energy bound state in such junctions. We make a detailed analysis of a more general type of π-junctions coined 'phase winding junctions' where the phase of the order parameter rotates by an angle π while its absolute value is kept finite. Such junctions have different properties, also from a topological viewpoint, and there are no protected zero energy modes. We compare the phenomenology of such junctions in topological (p-wave) and trivial (s-wave) superconducting wires, and briefly discuss possible experimental probes. Furthermore, we propose a topological field theory that gives a minimal description of a wire with defects corresponding to π-junctions. This effective theory is a one-dimensional version of similar theories describing Majorana bound states in half-vortices of two-dimensional topological superconductors.

Mechanical behavior of the human lumbar spine. I. Creep analysis during static compressive loading.

The in vitro viscoelastic "creep" behavior was examined in 18 cadaveric human lumbar motion segments subjected to static axial compressive loads. Axial deformation was followed for 30 min under constant applied load. Compressive material constants (moduli and viscosity coefficients) were then determined for each intervertebral disc using a linearization method based on a Taylor series expansion of experimental data for the "three parameter" viscoelastic creep model. The degree of disc degeneration and bone mineral content (BMC) were also assessed. Good correlation between the experimentally determined and model predicted strain values were found, with the average error less than 1%. We found that motion segments from older and more degenerated lumbar discs were less stable and had lower material constants than segments from younger and less degenerated discs. Material constants and BMC correlated closely, suggesting that an interdependency of disc and vertebral body properties exists. No correlation between the creep characteristics and disc height, disc area, segment level, or sex were noted.

Mechanical behavior of the human lumbar spine. II. Fatigue strength during dynamic compressive loading.

Seventeen cadaveric human lumbar motion segments from eight spines were cyclically loaded in vitro under axial compression. Loading frequency and magnitude were chosen to simulate rigorous activity within an in vivo physiological level. The load magnitude was determined as a percentage of the ultimate compressive load, the latter estimated from the bone mineral content (BMC) of lumbar vertebrae determined by dual-photon absorptiometry. Following testing, the degree of macroscopic disc degeneration was assessed and the type of fracture in each specimen was determined from serial sagittal sections. Fractures were found in all but one specimen. Three types of fractures were formed: the node of Schmorl and Junghanns (type I), central endplate fracture (type II), and a crush or burst fracture (type III). The results suggested that type I fractures were predominantly associated with segments with normal discs, type II fractures were found primarily in segments with moderately degenerated discs, and type III fractures were associated with segments that failed on the first cycle. Segment stiffness and fatigue strength (cycles to failure) were correlated with disc degeneration, age, and segment BMC, the latter an in vivo measure of bone density. Fatigue strength also decreased in proportion to a power coefficient with increasing relative stress (cyclic stress range/ultimate stress).

Aging, vertebral density, and disc degeneration alter the tensile stress-strain characteristics of the human anterior longitudinal ligament.

The mechanical properties of the human lumbar anterior longitudinal ligament were investigated, and the influence of aging, disc degeneration, and vertebral bone density on these properties was determined. Tensile mechanical properties of the vertebra-anterior longitudinal ligament-vertebra complex were determined for 16 segments from cadavera of individuals who had been 21-79 years old (mean, 52.1 years) at the time of death. Regional strain patterns associated with three sites across the width and three sites along the length of the anterior longitudinal ligament were measured with use of a video-based motion analysis system. In the young, normal anterior longitudinal ligament, the elastic moduli of the insertion and substance regions of the ligament were similar (approximately 500 MPa). During aging (21-79 years), the elastic modulus of the substance region increased 2-fold, whereas the elastic modulus of the insertion decreased 3-fold; this resulted in an approximately 5-fold difference in elastic modulus between these regions in the older spine. The strength of the bone-ligament complex decreased approximately 2-fold (from 29 to 13 MPa) over this same age range. The outer portion of the anterior longitudinal ligament consistently had the highest peak tensile strains (11.8 +/- 2.7%) in all of the specimens examined. Preparations with nondegenerated discs and high bone density were significantly stronger (66%) and failed in the ligament substance; in contrast, segments from older individuals with degenerated discs and lower bone density failed in the ligament insertion regions.

Mechanical properties of the human lumbar anterior longitudinal ligament.

A new technique incorporating a motion analysis system and a materials testing machine was used to investigate regional differences in the tensile mechanical properties of the lumbar spine anterior longitudinal ligament (ALL). Bone-ALL-bone specimens were prepared from young human cadaveric motion segments with no disc or bony pathology. Each specimen was distracted until failure at a constant crosshead displacement rate of 2.5 mm s-1 (approximately 1.0% strain per second). Strains were evaluated from digitized video recordings of markers attached to the ALL at 12 sites along its length and width, including the ligament substance and insertions. The 'overall' strain in the ligament was calculated from the outermost pairs of markers along the ligament length. The average tensile strength, the 'overall' tensile modulus and the 'overall' strain of the ALL at failure were 27.4 MPa (S.D. 5.9), 759 MPa (S.D. 336) and 4.95% (S.D. 1.51), respectively. Large and significant variations in the strains were present along the width and length of the ALL. Peak substance strains were over twofold greater than peak strains at the ligament insertion sites, whereas across the ligament width, peak strains in the outer portion of the ligament were over 40% greater than in the central region. Failure consistently occurred in the ligament mid-substance and ultimate strains at the ligament failure site averaged 12.1% (S.D. 2.3). These results indicate that the strains are highly nonuniform in the normal ALL.

The effects of common medical interventions on pain, back function, and work resumption in patients with chronic low back pain: A prospective 2-year cohort study in six countries.

STUDY DESIGN: A prospective cohort study with identical questionnaires and inclusion criteria was performed. OBJECTIVES: To compare in six different countries the frequencies and effects of the common medical interventions used for patients with low back pain who are work incapacitated. SUMMARY OF BACKGROUND DATA: Low back pain is a huge problem with increasing costs for health care, industry, and society. METHODS: Cohorts of employed men and women ages 18 to 59 years who had been sick-listed (100%) for a minimum of 90 days because of low back pain were recruited in Denmark, Germany, Israel, Sweden, the Netherlands, and the United States. The subjects received three separate questionnaires with identical questions after 90 days, 1 year, and 2 years. The questionnaires included separate questions about background factors, treatment, and the like, as well as validated scales such as the Hannover Activities of Daily Living, von Korff pain score, Short Form-36, and Karasek-Theorell. Working status was obtained from registers. Main outcome measures were working/not working, back function, and pain. RESULTS: All three questionnaires were completed by 2080 subjects in the six countries. With few exceptions, there were great similarities in the appointments, examinations, and treatments in the different countries. Considerable differences were found between the back surgery rates, which ranged from 6% in Sweden to 32% in the United States during the first 90 days of the study. Very few of the interventions had any noticeable positive effects on work resumption, pain, or back function. Back surgery in Sweden was a striking exception, positively affecting all three outcome measures. The frequencies of work resumption within the first year ranged from 73% in the Netherlands to 32% in Denmark. CONCLUSIONS: Almost none of the commonly occurring and frequently practiced medical interventions for patients who are sick-listed because of low back pain had any positive effects on either the recorded health measures or work resumption.

A study of the compressive properties of lumbar vertebral trabeculae: effects of tissue characteristics.

The compressive material properties of human lumbar vertebral trabecular bone were examined and compared to in vivo and in vitro measures of bone density and to the degree of macroscopic disc degeneration. No significant differences in the material properties were found for segment level (L1-L4) or for degeneration grade (I-IV), although trabecular bone specimens from segments with slightly degenerated disks (I-II) were stronger (24%) and stiffer (16%) on the average than specimens from segments with severely degenerated discs. Significant positive logarithmic regressions were obtained, however, between the in vivo and in vitro measures of trabecular bone density and trabecular bone mechanical properties. The results indicate that in vivo mineral measurements can accurately estimate the compressive material properties of lumbar vertebral trabeculae.

Experimental instability in the lumbar spine.

STUDY DESIGN: An in vivo animal model of lumbar segmental instability, involving both passive and active stabilizing components of the spine, was developed. OBJECTIVE: The aim of this investigation was to dynamically study the alterations in segmental kinematics as a result of interventions to the passive stabilizing components and to the lumbar musculature. SUMMARY OF BACKGROUND DATA: Segmental instability in the lumbar spine is associated with abnormal intervertebral motion. The majority of biomechanical studies have examined the in vitro effects of transecting individual stabilizing structures (i.e., intervertebral disc, facet joints, and ligaments), and have not simultaneously considered the effects of active musculature on spinal kinematics, which exist in the in vivo environment. Also, few studies have evaluated the kinematic behavior in the neutral region, for example, the transition phase between flexion and extension. METHODS: Four experimental groups comprised 33 pigs, each of which followed different surgical injury sequences to the L3-L4 motion segment. An instrumented linkage attached to the L3-L4 motion segment was used to measure the sagittal kinematics during dynamic flexion-extension after each surgical injury and after bilateral stimulation of the lumbar paraspinal musculature. RESULTS: Injuries to the disc resulted in greater overall axial translation. Graded injuries to the facet joint mainly caused changes in sagittal rotation and shear translation. When the facet injuries were compounded by removal of the transverse processes, there was significantly greater coupled motion and increased hysteresis in the neutral region for rotation. Extensive muscular stimulation after each of the injuries caused significantly greater rotation and shear translation, along with a tendency toward reduced axial translation, when compared to the unstimulated case. Although increasing the range of motion, increased muscular activity stabilized the injured motion segment by smoothing the erratic rotation pattern of motion, particularly in the neutral region. CONCLUSIONS: Because of the direct attachment to the vertebrae, both passive and active strain from the musculature influence the spinal kinematics in normal or destabilized motion segments. Although increasing the range of motion, stimulation of the musculature surrounding the injured motion segment has a stabilizing effect by reducing abrupt kinematic behavior, particularly in the neutral region where the muscles are under reduced tension. A facetectomy produces a paradoxical kinematic behavior, which enhances the unstable condition of the motion segment. Surgical and rehabilitative treatments for patients with segmental instability need to consider the physiologic influences of the spinal musculature.

1997 Volvo Award winner in biomechanical studies. Kinematic behavior of the porcine lumbar spine: a chronic lesion model.

STUDY DESIGN: Experimental models of intervertebral disc and facet joint degeneration were created in vivo in the porcine lumbar spine for studying spinal kinematics, using a dynamic technique. OBJECTIVES: To quantify the changes in spinal kinematics and the stabilizing capacity of the lumbar musculature caused by chronic lesions in the intervertebral disc and facet joints. SUMMARY OF BACKGROUND DATA: Segmental kinematics are detrimentally altered by acute injury to passive structures of the motion segment. However, stimulation of the surrounding musculature adds stability to the motion segment. The in vivo kinematics of a degenerated lumbar motion segment and the stabilizing function of the surrounding musculature have not been quantified dynamically. METHODS: Forty-four pigs were used in six chronic lesions models: sham, disc anulus, disc nucleus, facet capsule, facet joint slit, and facet joint wedge. Three months after injury, an instrumented linkage was used to measure continuously the sagittal kinematics of the L3-L4 motion segment during flexion-extension, with and without stimulation of the lumbar paraspinal musculature. Flexion-extension end point and maximum ranges of motion, and hysteresis were analyzed. RESULTS: Significant alterations in the kinematics caused by chronic lesions were observed, particularly when using the maximum range of motion and when comparing changes in axial translation. Muscular stimulation reduced the hysteresis in the sham, facet capsule, and disc nucleus groups; however, increased hysteresis was observed in the remaining lesion groups. CONCLUSIONS: The kinematic behavior of motion segments with chronic lesions was established. The maximum range of motion, which must be measured using a dynamic technique, was a more sensitive parameter for identifying changes in segmental kinematics caused by chronic lesions than was the end range of motion. The lumbar musculature was less efficient overall in stabilizing the motion segment, possibly because of altered mechanisms in the neuromuscular feedback system.

Pressure changes within the cauda equina following constriction of the dural sac. An in vitro experimental study.

In order to register pressure changes within the cauda equina, a highly sensitive pressure measuring catheter was inserted through a hole in the dural sac. Its tip was placed among the nerve roots of the cauda equina at levels L2, L3, and L4 in seven freshly removed human specimens. The cross-sectional area of the dural sac then was decreased gradually by constriction of a circular clamp. The inside diameter of the clamp and the corresponding area was determined when further reduction of the circumference of the clamp caused the first signs of a pressure increase, the critical size, among the nerve roots. The average critical size was 76.9 mm2 at the L2 level, corresponding to a diameter of 11.4 mm At the L3 level, the corresponding figures were 71.5 mm2 and 11.1 mm, and at the L4 level 64.8 mm2 and 10.6 mm, respectively. It seemed reasonable to assume that the critical sizes determined in this way could be threshold values below which a further constriction of the size of the dural sac would cause an impairment of the circulatory and/or the nerve function of the cauda equina.

The load on the lumbar spine during isometric strength testing.

Reports of back discomfort and even back injuries during isometric strength testing in specific lifting positions indicated an analysis of the loads on the lumbar spine during this type of testing. A biomechanical analysis, which has been validated against EMG and intravital disc pressure measurements, was used for the calculations of the loads in four test persons. The calculations indicated compressive loads on L3 ranging from 5000-11,000 N during squat and torso lifting. Such loads in vitro have been found to cause structural failures of the vertebral endplates. Similar loads also may result in damage to the spine in vivo.

1990 Volvo Award in experimental studies. The dependence of intervertebral disc mechanical properties on physiologic conditions.

In vivo creep-recovery and disc pressure measurements were performed on the lumbar spine of immature and mature swine. The creep-recovery measurements were performed using a custom materials testing apparatus designed to apply static or dynamic loads to the spine of anesthetized animals. A series of three separate experiments were performed to assess the effects of: (I) animal death, (II) graded injury to the disc anulus, and (III) respiratory mechanics on the biomechanical response of the porcine L1-L3 vertebral unit (VU). In Experiments I and II, creep rate, modulus, and viscosity parameters were computed using a three-parameter solid rheological analysis of the displacement-time response recorded during the application of a 300-N load. In Experiment III, the effects of respiratory volume and frequency changes on disc pressure were assessed in the unloaded, statically loaded, and immobilized porcine VU. Our results indicated that the adult VU tended to be stiffer, deform or creep more slowly, and had a significantly higher viscosity than the VU of immature pigs. The results of Experiment I demonstrated that the biomechanical response for the VU was significantly altered by the death of the animal; the VU of the living animal (adolescent or mature) was more compliant and deformed at a faster rate than the VU of the same animal after death. Disc injury produced changes in stiffness, viscosity, and creep rate analogous to that of aging, and on the basis of the graded injuries created in this study, it appears that a small defect in the annulus is just as deleterious as removing a large section of anular material. The results of Experiment III indicated that respiration plays an important role in the normal, in vivo mechanical and nutritional behavior of the porcine VU. Altogether, these results demonstrate that, in the absence of normal physiologic conditions, one may not be able to reliably predict the mechanical response of the lumbar spine, and suggest that standards for the testing, handling, and storage of biologic tissue should be established.

Regional variations in the compressive properties of lumbar vertebral trabeculae. Effects of disc degeneration.

The compressive mechanical properties of human lumbar vertebral trabeculae were examined on the basis of anatomic origin, bone density, and intervertebral disc properties. Trabecular bone compressive strength and stiffness increased with increasing bone density, the latter proportional to strength and stiffness to the one-half power. Regional variations within each segment were found, the most prevalent differences occurring in regions of bone overlying the disc nucleus in comparison with bone overlying the disc anulus. For normal discs, the ratio of strength of bone overlying the disc nucleus to bone overlying the disc anulus was 1.25, decreasing to 1.0 for moderately degenerated discs. These results suggest that an interdependency of trabecular bone properties and intervertebral disc properties may exist.

A prospective study of work perceptions and psychosocial factors affecting the report of back injury.

A longitudinal, prospective study was conducted on 3,020 aircraft employees to identify risk factors for reporting acute back pain at work. The premorbid data included individual physical, psychosocial, and workplace factors. During slightly more than 4 years of follow-up, 279 subjects reported back problems. Other than a history of current or recent back problems, the factors found to be most predictive of subsequent reports in a multivariate model were work perceptions and certain psychosocial responses identified on the Minnesota Multiphasic Personality Inventory (MMPI). Subjects who stated that they "hardly ever" enjoyed their job tasks were 2.5 times more likely to report a back injury (P = 0.0001) than subjects who "almost always" enjoyed their job tasks. The quintile of subjects scoring highest on Scale-3 (Hy) of the MMPI were 2.0 times more likely to report a back injury (P = 0.0001) than subjects with the lowest scores. The multivariate model, including job task enjoyment, MMPI Scale-3, and history of back treatment, revealed that subjects in the highest risk group had 3.3 times the number of reports in the lowest risk group. These findings emphasize the importance of adopting a broader approach to the multifaceted problem of back complaints in industry and help explain why past prevention efforts focusing on purely physical factors have been unsuccessful.

A prospective study of the role of cardiovascular risk factors and fitness in industrial back pain complaints.

The authors conducted a prospective study of risk factors for industrial back pain complaints among 3,020 aircraft manufacturing employees. The study subjects completed a cardiovascular risk questionnaire, and were asked about their smoking status and past medical history, including previous back problems. Premorbid submaximal treadmill testing to predict maximum oxygen uptake (Vo2max) was completed in 2,434 subjects who were not excluded from testing due to cardiovascular risk screening. During several years of subsequent follow-up, 279 subjects reported back problems. Those who reported smoking at the time of the premorbid examination were significantly more likely to report a subsequent back problem than nonsmokers (P = 0.002). When controlling for sex and age, cardiovascular fitness, as measured through VO2max, was not predictive of future back injury reports (P = 0.26).

The reliability of measurements of the lumbar spine using ultrasound B-scan.

Diagnostic ultrasound is used in research and clinical settings to determine the size of the lumbar spinal canal. When the technique was first introduced, measurements were obtained directly from an A-scan while concurrently viewing a B-scan. However, measurements obtained directly from a B-scan are now commonly used despite the undetermined reliability of the technique. To determine the reliability, 50 randomly selected ultrasound B-scan examinations were read on two separate occasions by three investigators. For each spinal level, the mean intraobserver error (same investigator), from the first to the second reading was determined, as was the interobserver error (between investigators) and the error due to variability in obtaining the ultrasound images. The resultant errors were approximately double those reported previously for the technique using A-scan. Interobserver variations were the major source of measurement error.