Load-Sharing and Kinematics of the Human Cervical Spine Under Multi-Axial Transverse Shear Loading: Combined Experimental and Computational Investigation.

The cervical spine experiences shear forces during everyday activities and injurious events yet there is a paucity of biomechanical data characterizing the cervical spine under shear loading. This study aimed to (1) characterize load transmission paths and kinematics of the subaxial cervical spine under shear loading, and (2) assess a contemporary finite element cervical spine model using this data. Subaxial functional spinal units (FSUs) were subjected to anterior, posterior, and lateral shear forces (200 N) applied with and without superimposed axial compression preload (200 N) while monitoring spine kinematics. Load transmission paths were identified using strain gauges on the anterior vertebral body and lateral masses and a disc pressure sensor. Experimental conditions were simulated with cervical spine finite element model FSUs (GHBMC M50 version 5.0). The mean kinematics, vertebral strains, and disc pressures were compared to experimental results. The shear force-displacement response typically demonstrated a toe region followed by a linear response, with higher stiffness in anterior shear relative to lateral and posterior shear. Compressive axial preload decreased posterior and lateral shear stiffness and increased initial anterior shear stiffness. Load transmission patterns and kinematics suggest the facet joints play a key role in limiting anterior shear while the disc governs motion in posterior shear. The main cervical spine shear responses and trends are faithfully predicted by the GHBMC cervical spine model. These basic cervical spine biomechanics and the computational model can provide insight into mechanisms for facet dislocation in high severity impacts, and tissue distraction in low severity impacts.

Posture analysis of lifting a load for head carriage and comparison between pregnant and non-pregnant women.

BACKGROUND: In Western Africa, women continue performing heavy physical work that includes carrying loads on their heads during pregnancy. Women may adapt to pregnancy related body changes by modifying their postures to perform such tasks. OBJECTIVE: The objectives of this biomechanical task analysis study were to 1) determine sagittal plane postures of the trunk and upper extremities at specific events during the task of lifting and lowering a load to be carried on the head, 2) compare postures of pregnant and non-pregnant participants, 3) evaluate risk for musculo-skeletal disorders (MSD) with the rapid entire body assessment (REBA) criteria. PARTICIPANTS: Twenty-six pregnant (26 ± 5 years, 159 ± 9 cm, 63 ± 15 kg, 25 ± 9 weeks of pregnancy) and 25 paired non-pregnant retail merchants were recruited in Porto-Novo (Benin). METHODS: Participants were recorded on video in a laboratory setting while they lifted a tray (20% body weight) from a stool to their head and then put it back down. Trunk inclination and knee, shoulder and elbow flexion angles were determined using Dartfish® software. RESULTS: The trunk was bent by more than 80° at pick-up and set-down and knees were moderately flexed, significantly less (< 11°) for pregnant women, possibly because it was harder to lift the trunk, or for stability. For all postures analysed, the majority of trials were classified as "high" risk or "very high risk" for MSD. CONCLUSIONS: Future research should investigate prevalence of MSDs in this population to confirm the results of this study.

Back pain during pregnancy and living conditions: a comparison between Beninese and Canadian women.

OBJECTIVE: The objective of this exploratory study was to investigate and underline the contrasts between African and Canadian pregnant women, and their living conditions. We also intended to evaluate how they compared on low back pain, a condition that seems common across all pregnant women everywhere in the world. SUBJECTS AND METHOD: Thirty Beninese and 50 Canadian women were surveyed with demographic disability questionnaires O.D.I at approximately 25 weeks of pregnancy. RESULTS: There were large differences between the two groups due to the differences between the life style. Beninese women were more likely to be self-employed or housewives, while Canadian women were more likely to be employed. Beninese women worked for 18hours more per week, and had on average one more child at home. A higher percentage of Beninese women reported back pain, 83% versus 58% for Canadian women, but the disability scores were in the "moderate disability" range for both groups. A higher percentage of Beninese women also reported at least severe disability, 33% versus 14% for Canadian women. CONCLUSION: The results suggest that the higher percentages of Beninese women affected by back pain and by severe back pain is related to the longer hours worked and more strenuous physical work performed.

Reproducibility of a task description questionnaire for working pregnant women.

The objective of this study was to evaluate the reproducibility of a Task Description Questionnaire that was designed to investigate exposures to, and influential factors for, problematic tasks experienced by working pregnant women. The questionnaire comprised questions concerning 22 task components (covering working posture, manual material handling, work pace, prolonged postures and others), eight influential factors contributing to problematic tasks, discomfort (measured using a body map) and level of effort to perform the tasks. Reproducibility of the questionnaire was assessed by interviewing participants on two occasions one week apart for interviews at both 20 and 34 weeks of pregnancy. Eleven and 13 problematic tasks were reported by 21 working pregnant women at 20 and 34 weeks of pregnancy, respectively. These tasks were surveyed using the Task Description Questionnaire. Kappa statistics and correlation coefficients (supplemented by paired t-tests) were used to examine the reproducibility of responses to the questionnaire. The results showed that most of the variables were measured with very good or satisfactory reproducibility. The reproducibility of exposure to work posture was higher than that of exposure to manual material handling. There was no significant difference between test and retest means for the discomfort scores measured on the body map, except for the maximum discomfort score for the whole body in the 34 weeks survey. The study suggests that the questionnaire can be reliably used in the study of problematic tasks experienced by pregnant women. But an initial preview of the questions by the subjects and explanation of the questions given to the subjects by the interviewer may help to produce more reliable results.

Analysis of self-reported problematic tasks for pregnant women.

The objective of this study was to identify major components of, and influential factors in, problematic tasks performed by pregnant women employed in education, health care and service areas. Seventy-two pregnant women were surveyed using specially designed questionnaires consisting of an Initial Survey, a Job Analysis Questionnaire and a Task Description Questionnaire. Forty-four subjects (60%) had difficulty performing at least one work task and reported 105 tasks that were problematic at work. Reaching above the head, bending forward, bending and twisting, pushing, repeating actions and working at a fast pace were identified as the task components requiring the greatest level of effort. Excessive effort, excessive time, getting tired, repetitive actions, stress and fear of injury were identified as factors that had strong associations with the six major task components. Findings of this study suggest that these task components and factors should be considered when designing, assigning or analysing tasks for working pregnant women.

Prediction of back strength using anthropometric and strength measurements in healthy females.

OBJECTIVES: The purpose of this study was to develop a regression equation to predict back extensor maximal voluntary contraction (back strength) for females based on several anthropometric and strength measurements using a multiple regression technique. BACKGROUND: Back strength is an important parameter in low back pain studies. However, the measurement of back strength is problematic in certain populations such as low back pain patients and pregnant women. METHODS: Back strength was measured as both moment at L4/L5 and force. Ten anthropometric or strength measurements were chosen to develop the prediction equation. The data used for developing the models were from eighty non-pregnant female subjects, age 18-42 and with no history of back pain in the past year. Backwards stepwise analysis was performed in order to choose the best fit predictors. The predictive ability of each of the models was checked using the cross-validation technique on 20 other subjects. FINDINGS: Two prediction models were developed for moment and force, respectively. The models explained 46.9% and 48.2% of the variance in back strength. No multicollinearity problem was found. The validation study showed that the observed back strength was highly correlated with the predicted back strength. INTERPRETATION: Mass, height, trunk length, grip strength and quadriceps strength are the best predictors of back strength in this study. The models developed in this study can be used for both general female low back pain patients and the pregnancy population.

Evaluation and calibration of an electromagnetic tracking device for biomechanical analysis of lifting tasks.

Electromagnetic motion tracking devices are increasingly used as a kinematic measuring tool. The aim of this study was to evaluate a long-range transmitter in an environment with a conventional force plate present in order to assess its suitability for further biomechanical applications. Using a calibration apparatus developed in our lab and Optotrack measurements, the performances of the Motion Star were evaluated. Positions and orientations were measured in a 140 x 80 x 120 cm(3) space centered on the force plate. Using a mathematical model developed at Queen's University, these data were calibrated. Errors on position and orientation were less than 150 mm and 10 degrees before calibration of the Motion Star, and less than 20mm and 2 degrees after calibration, with no differences between data collected with the force plate switched on/off. These errors did not depend on sensor orientation. Variability of the signal was small indicating minimal noise. Field distortion was the largest source of measurement error, which increased with the distance between the transmitter and the sensor and the proximity of the sensor to the force plate. Before its use for biomechanical analysis of lifting tasks and validation of dynamic models using force plate data, the data from electromagnetic motion tracking devices must be calibrated to decrease the errors due to electromagnetic field distortion.

Animation of in vitro biomechanical tests.

Interdisciplinary communication of three-dimensional kinematic data arising from in vitro biomechanical tests is challenging. Complex kinematic representations such as the helical axes of motion (HAM) add to the challenge. The difficulty increases further when other quantities (i.e. load or tissue strain data) are combined with the kinematic data. The objectives of this study were to develop a method to graphically replay and animate in vitro biomechanical tests including HAM data. This will allow intuitive interpretation of kinematic and other data independent of the viewer's area of expertise. The value of this method was verified with a biomechanical test investigating load-sharing of the cervical spine. Three 3.0 mm aluminium spheres were glued to each of the two vertebrae from a C2-3 segment of a human cervical spine. Before the biomechanical tests, CT scans were made of the specimen (slice thickness=1.0 mm and slice spacing=1.5 mm). The specimens were subjected to right axial torsion moments (2.0 Nm). Strain rosettes mounted to the anterior surface of the C3 vertebral body and bilaterally beneath the facet joints on C3 were used to estimate the force flow through the specimen. The locations of the aluminium spheres were digitised using a space pointer and the motion analysis system. Kinematics were measured using an optoelectronic motion analysis system. HAMs were calculated to describe the specimen kinematics. The digitised aluminium sphere locations were used to match the CT and biomechanical test data (RMS errors between the CT and experimental points were less than 1.0 mm). The biomechanical tests were "replayed" by animating reconstructed CT models in accordance with the recorded experimental kinematics, using custom software. The animated test replays allowed intuitive analysis of the kinematic data in relation to the strain data. This technique improves the ability of experts from disparate backgrounds to interpret and discuss this type of biomechanical data.

A longitudinal study of the development of low back pain in an industrial population.

STUDY DESIGN: This is a longitudinal study in which industrial workers without chronic low back pain (LBP) were initially assessed with a comprehensive test battery and surveyed every 6 months thereafter for 2 years. OBJECTIVE: To determine factors that may predispose industrial workers who lift over 5000 kg per shift to LBP. SUMMARY OF BACKGROUND DATA: Prospective studies are small in number and often limited in breadth or depth of the test battery, methodologic issues, or investigator expertise. There are no prospective studies that focus on a homogeneous work sample of industrial employees. METHODS: Production workers (n = 149) who volunteered for the 2-year study were assessed using physical measures (e.g., muscular strength, endurance, and flexibility), lifting kinematics (a sagittal plane box lift), and health, lifestyle, and work environment data (paper questionnaires). Follow-up questionnaires were distributed every 6 months for 2 years. RESULTS: Using self-report of LBP as the main outcome measure, eight variables predicted LBP in this sample with a 75% correct prediction rate. Predictor variables included age, thoracic acceleration during the trunk velocity test, median frequency intercept of electromyography of the right L3 erector spinae, quadriceps strength, quadriceps endurance, self-assessment of fitness, having a confidante, and number of medications currently taken. CONCLUSION: Results confirmed the multifactorial nature of low back pain and suggest that personal fitness is an important defense against low back pain, even in manual handling lifting tasks.

A minimally disruptive technique for measuring intervertebral disc pressure in vitro: application to the cervical spine.

A novel technique to measure in vitro disc pressures in human cervical spine specimens was developed. A miniature pressure transducer was used and an insertion technique was designed to minimise artefacts due to insertion. The technique was used to measure the intradiscal pressure in cervical spines loaded in pure axial compression. The resulting pressure varied linearly with the applied compressive force with coefficients of determination (r(2)) greater than 0.99 for each of the four specimens. Peak pressures between 2.4 and 3.5MPa were recorded under 800N of compression.

Calibration of position and angular data from a magnetic tracking device.

This paper describes a method for calibrating data from a magnetic tracking device. Position and orientation data were collected in a 1. 6x0.8x1.4m(3) volume using a Polhemus Fastrak((R)) in conjunction with both a long-range and standard transmitter. Position and orientation data were calibrated using a locally linear model based on the position of the measurement. After calibration, the average position and angular errors were less than 1.8cm and 1.2 degrees up to 1.8m from the transmitter for the long-range transmitter. For the standard transmitter, even after calibration, errors increased sharply when the sensor was more than 1.2m from the transmitter. Up to that distance, post-calibration errors were less than 1.2cm and 1. 2 degrees, while up to 1.8m they were below 5cm and 4 degrees. These errors could be further reduced by noise filtering. However, use of the standard transmitter is not recommended at distance greater than 1.2m due to orientation-based effects. It was concluded that for the volume investigated, tracking devices could provide similar three-dimensional accuracy to video systems.

A continuous pure moment loading apparatus for biomechanical testing of multi-segment spine specimens.

An apparatus is described that enables the application of continuous pure moment loads to multi-segment spine specimens. This loading apparatus allows continuous cycling of the spine between specified flexion and extension (or right and left lateral bending) maximum load endpoints. Using a six-degree-of-freedom load cell and three-dimensional optoelectronic stereophotogrammetry, characteristic displacement versus load hysteresis curves can be generated and analyzed for different spinal constructs of interest. Unlike quasi-static loading, the use of continuous loading permits the analysis of the spine's behaviour within the neutral zone. This information is of particular clinical significance given that the instability of a spinal segment is related to its flexibility within the neutral zone. Representative curves for the porcine lumbar spine in flexion-extension and lateral bending are presented to illustrate the capabilities of this system.

In vitro flexibility of an experimental pedicle screw and plate instrumentation system on the porcine lumbar spine.

The positive correlation between spinal construct stiffness and fusion rate has led to the use of increasingly rigid surgical spinal instrumentation systems. Unfortunately, however, these rigid systems have also been correlated with sub-optimal fusion quality measures. To date, in vivo studies to explore these relationships have involved the use of different implants and surgical procedures to influence the biomechanical environment at the fusion site. In order to avoid these confounding variables, a novel experimental instrumentation system has been developed which is capable of independently controlling spinal construct flexibility (inverse of stiffness). In the present study, this experimental pedicle screw and plate system was subjected to rigorous pure moment flexibility testing in flexion-extension and lateral bending using an in vitro porcine lumbar spine model. Analysis of the data showed that the system provided a reproducible, stepwise-modulated spinal construct flexibility as measured by neutral zone flexibility, laxity angle and range of motion parameters. Differences in flexibility were most evident using the neutral zone parameters (neutral zone flexibility and laxity angle). This is of particular interest given that the clinical instability of a spinal segment may be related to its behaviour within the neutral zone. This information will ultimately guide the design of improved human spinal instrumentation systems.

Evaluation of a long-range transmitter for use with a magnetic tracking device in motion analysis.

The suitability of a long-range transmitter was evaluated for use with a Polhemus Fastrak magnetic tracking device in kinesiologic studies. Performance was judged by comparing positional and rotational accuracy to a standard transmitter. Data were obtained at distances of up to 2.7 and 5.0 m for the standard and long-range transmitters, respectively. Use of the long-range transmitter improved system performance allowing reproducible measurements at a greater distance. However, it is necessary to calibrate the system in each new test environment as there can be significant distortion of the magnetic field.

The influence of muscle fiber size and type distribution on electromyographic measures of back muscle fatigability.

STUDY DESIGN: This was a cross-sectional study carried out on a group of 31 healthy, consenting volunteers with no history of low back pain (17 men, 14 women). OBJECTIVES: To evaluate the relationship between electromyographic measures of erector spinae fatigability and the muscle's fiber type characteristics. SUMMARY OF BACKGROUND DATA: Using electromyographic techniques, a pronounced fatigability of the muscles of patients with low back pain has been identified. It has been postulated that this is the result of an unfavorable back muscle fiber type distribution, although an association between electromyographic measures of fatigue and the muscle's fiber type characteristics has never been established. METHODS: Two tests of back extensor fatigability were performed (on separate days), each to the limit of endurance: 1) maintenance of 60% total maximum voluntary contraction of the back extensors, and 2) performance of the Biering-Sørensen test. Pairs of surface electrodes were attached to the skin overlying the belly of the erector spinae, bilaterally, at T10 and L3. The median frequency was computed from the electromyographic power spectrum, and fatigability was given by the slope of the linear regression of median frequency on time (MFgrad; %.s-1). One week later, two percutaneous erector spinae muscle biopsy samples were obtained from the same sites described for electromyography (left side only). Samples were prepared for histochemistry for the identification of muscle fiber types. Fiber sizes (cross-sectional areas) were quantified using computerized image analysis. RESULTS: The mean fiber size at each erector spinae region showed a significant correlation with maximum back extensor strength. In the thoracic region, the relative area of the muscle occupied by Type I fibers (which accounts for the relative size and distribution of the fiber types) showed a significant relationship with MFgrad recorded during each fatigue test. A similar relationship was observed for the lumbar region, but for the Biering-Sørensen test only. CONCLUSIONS: The electromyographic changes recorded in back muscles during fatigue appear to be related to the underlying muscle fiber type area distribution. This confirms the usefulness of electromyography in reflecting such muscle characteristics in a noninvasive manner, when monitoring changes in function consequent to the development of, or rehabilitation from, low back pain.

Mechanical behavior of the female sacroiliac joint and influence of the anterior and posterior sacroiliac ligaments under sagittal loads.

OBJECTIVE: The purpose of this study was to examine the mechanical behaviour of the female sacroiliac joint and the effects of its two major ligaments to joint stability. DESIGN: A cadaveric model was used to study the mechanical behaviour of the sacroiliac joints, and sequential dissection was performed to examine the contribution of the anterior and posterior sacroiliac ligaments in joint stability. BACKGROUND: Instability of the sacroiliac joints have been suspected as a possible cause of low back pain. Despite several investigations on joint anatomy and joint mobility, its stabilising mechanism is still not clear. METHODS: Four fresh cadaveric specimens of the female pelvis were tested on an Instron material testing machine. Eccentric compressive force of 60% of the subject's body weight was applied to the pelvis through the sacrum. Relative three-dimensional six-degree-of-freedom movement at the left sacroiliac joints was recorded with a specially designed motion tracking device. The device has an accuracy of 0.01 mm and is compact enough to be mounted across the joint. The test was repeated after sequential selective dissection of the bilateral anterior, and then posterior sacroiliac ligaments. RESULTS: Rotation up to 1.2 degrees and translation up to 0.9 mm were measured from the intact specimens. Lateral rotation, which tended to open the top portion of the joint, and sacral nutation were the primary rotations. On average, the rotation angles increased 10% when either the anterior or posterior ligaments were cut, and 30% when both ligaments were cut. CONCLUSIONS: Lateral rotation and nutation rotation of the sacrum were found to be the predominant motion, though the values were limited to less than 1.2 degrees. Both the anterior and posterior sacroiliac ligaments were found to play an important role in resisting rotations at the joints.

Measuring collagen fiber orientation: a two-dimensional quantitative macroscopic technique.

This paper describes the design, evaluation, and application of a new system for quantifying two-dimensional collagen fiber orientation in soft tissue. Series of transmitted polarized light images were collected using a custom-designed macroscope. Combined analysis of pixel brightness, and hue from images collected with a compensator plate, permitted the assignment of each pixel into the appropriate orientation band. Experiments were performed to quantify the linearity and noise of the system. Validation was performed on a specimen composed of strain-birefringent plastic strips at various orientations. Preliminary collagen fiber orientation data is presented from a tendon specimen. This study demonstrates the utility of this approach for studying collagen fiber orientation across large areas.

Laxity of knee cruciate ligaments during pregnancy.

Pregnancy-related increase in ligament laxity may cause joint instability. The purpose of this study was two-fold: 1) to assess knee laxity changes during pregnancy and 2) to evaluate the effect of exercise on knee laxity due to a typical prenatal fitness program. The subjects were healthy pregnant women. One group (N = 27) participated in exercise classes designed according to national guidelines. The second group (N = 38) was more sedentary. A clinical arthrometer, KT-1000, was used, and anterior and posterior drawer tests were performed. The results were added and averaged for the two knees. Laxity was constant in the second half of pregnancy and had significantly decreased by 14% 4 months after birth. No influence of parity or exercise was detected. The exercise program employing minimal to moderate weight bearing did not result in any measurable increases in knee laxity and, therefore, appears to be appropriate with regard to knee stability. These results should not, however, be extended to different types of exercise programs without additional research.

Muscle fibre size and type distribution in thoracic and lumbar regions of erector spinae in healthy subjects without low back pain: normal values and sex differences.

This study sought to investigate the normal muscle fibre size and type distribution of the human erector spinae, both in thoracic and lumbar regions, in a group of 31 young healthy male (n = 17) and female (n = 14) volunteers. Two percutaneous muscle biopsy samples were obtained under local anaesthesia, from the belly of the left erector spinae, at the levels of the 10th thoracic and 3rd lumbar vertebrae. Samples were prepared for routine histochemistry for the identification of fibre types. Fibre size (cross-sectional area (CSA) and narrow diameter (ND)) was quantified using computerised image analysis. The mean CSA/ND for each fibre type was greater in the thoracic than the lumbar region, but there was no difference between the 2 regions either for percentage type I (i.e. percentage distribution by number), percentage type I area (i.e. relative area of the muscle occupied by type I fibres) or the ratio describing the size of the type I fibre relative to that of the type II. Men had larger fibres than women, for each fibre type and at both sampling sites. In the men, each fibre type was of a similar mean size, whereas in the women the type I fibres were considerably larger than both the type IIA and type IIB fibres, with no difference between the latter two. In both regions of the erector spinae there was no difference between men and women for the proportion (%) of a given fibre type, but the percentage type I fibre area was significantly higher in the women. The erector spinae display muscle fibre characteristics which are clearly very different from those of other skeletal muscles, and which, with their predominance of relatively large type I (slow twitch) fibres, befit their function as postural muscles. Differences between thoracic and lumbar fascicles of the muscle, and between the muscles of men and women, may reflect adaptive responses to differences in function. In assessing the degree of any pathological change in the muscle of patients with low back pain, it seems clear that (1) sex cannot be disregarded and (2) 'atrophied' (using the criteria from other muscles) type II fibres are not necessarily abnormal for the erector spinae, particularly in women.

New insight into the mechanics of the lumbar interspinous ligament.

STUDY DESIGN: Repeated in vitro mechanical tests were performed on porcine and human interspinous ligament specimens with progressive disruption of the collagen fiber network to evaluate the existence of mechanical interactions between collagen fibers. OBJECTIVE: To evaluate the existence of a load pathway in the interspinous ligament whereby loads are transmitted between collagen fibers. SUMMARY OF BACKGROUND DATA: Mechanical tests demonstrate that the interspinous ligament resists spinal flexion, but the collagen fibers are not oriented to oppose separation of the spinous processes. This seeming contradiction could be explained by the existence of mechanical interactions between collagen fibers of the interspinous ligament. METHODS: In vitro mechanical tests were performed on porcine and human bone-interspinous ligament-bone specimens. The collagen network of the ligament substance was disrupted by passing a scalpel blade though the ligament substance. Repeated tests were performed with progressive disruption of the collagen fiber network. RESULTS: The stiffness of the interspinous ligament specimens was reduced as the collagen fiber network was disrupted, but approximately half of the initial stiffness was maintained when the collagen fiber network was severely disrupted. In this case, no intact collagen fibers remained. CONCLUSIONS: Mechanical interactions exist between collagen fibers in the interspinous ligament. The mechanism of the interactions is unknown.