Recent advances in lumbar spinal mechanics and their clinical significance

Effects of backward bending on lumbar intervertebral discs. Relevance to physical therapy treatments for low back pain

STUDY DESIGN

Mechanical testing of cadaveric motion segments.

OBJECTIVES

To test the hypothesis that backward bending of the lumbar spine can reduce compressive stresses within lumbar intervertebral discs.

SUMMARY OF BACKGROUND DATA

Lumbar extension affects the distribution of compressive stress inside normal cadaveric discs, but little is known about its effect on mechanically disrupted and degenerated discs.

METHODS

Nineteen lumbar motion segments (mean donor age, 48 years) were subjected to complex mechanical loading to simulate the following postures: moderate lumbar flexion, 2 degrees of extension, 4 degrees of extension, and the neutral position (no bending). The distribution of compressive stress within the disc matrix was measured in each posture by pulling a miniature pressure transducer along the midsagittal diameter of the disc. Stress profiles were repeated after a mechanical treatment that was intended to simulate severe disc degeneration in vivo.

RESULTS

The "degeneration" treatment reduced pressure in the nucleus pulposus and generated stress concentrations within the anulus, in a manner similar to that found in severely degenerated discs in vivo. When all discs were considered together, 2 degrees of extension increased the maximum compressive stress within the posterior anulus by an average of 16%, compared with the neutral posture. The size of localized stress peaks within the posterior anulus was increased by 43% (P = 0.02). In 4 degrees of extension, changes observed between 0 degree and 2 degrees were usually exaggerated. In contrast, moderate flexion tended to equalize the distribution of compressive stress. In 7 of the 19 discs, 2 degrees of lumbar extension decreased maximum compressive stress in the posterior anulus relative to the neutral posture by up to 40%. Linear regression showed that lumbar extension tended to reduce stresses in the posterior anulus in those discs that exhibited the lowest compressive stresses in the neutral posture (P = 0.003; R2 = 41%).

CONCLUSIONS

The posterior anulus can be stress shielded by the neural arch in extended postures, but the effect is variable. This may explain why extension exercises can relieve low back pain in some patients.

Viscoelastic finite-element analysis of a lumbar motion segment in combined compression and sagittal flexion. Effect of loading rate

STUDY DESIGN

A study using a validated viscoelastic finite-element model of a L2-L3 motion segment to identify the load sharing among the passive elements at different loading rates.

OBJECTIVE

To enhance understanding concerning the role of the loading rate (i.e., speed of lifting and lowering during manual material handling tasks) on the load sharing and safety margin of spinal structures.

SUMMARY OF BACKGROUND DATA

Industrial epidemiologic studies have shown that jobs requiring a higher speed of trunk motion contribute to a higher risk of industrial low back disorders. Consideration of the dynamic loading characteristics, such as lifting at different speeds, requires modeling of the viscoelastic behavior of passive tissues. Detailed systematic analysis of loading rate effects has been lacking in the literature.

METHODS

Complex flexion movement was simulated by applying compression and shear loads at the top of the upper vertebra while its sagittal flexion angle was prescribed without constraining any translations. The lower vertebra was fixed at the bottom. The load reached its maximum values of 2000 N compression and 200 N anterior shear while L2 was flexed to 10 degrees of flexion in the three different durations of 0.3, 1, and 3 seconds to represent fast, medium, and slow movements, respectively. The resisted bending moment, gross load-displacement response of the motion segment, forces in facet joints and ligaments, stresses and strains in anulus fibrosus, and intradiscal pressure were compared across different rates.

RESULTS

The distribution of stress and strain was markedly affected by the loading rate. The higher loading rate increased the peak intradiscal pressure (12.4%), bending moment (20.7%), total ligament forces (11.4%), posterior longitudinal ligament stress (15.7%), and anulus fiber stress at the posterolateral innermost region (17.9%), despite the 15.4% reduction in their strain.

CONCLUSIONS

Consideration of the time-dependent material properties of passive elements is essential to improving understanding of motion segment responses to dynamic loading conditions. Higher loading rate markedly reduces the safety margin of passive spinal elements. When the dynamic tolerance limits of tissues are available, the results provide bases for the guidelines of safe dynamic activities in clinics or industry.

Maximum forces sustained during various methods of exiting commercial tractors, trailers and trucks

Many commercial vehicles have steps and grab-rails to assist the driver in safely entering/exiting the vehicle. However, many drivers do not use these aids. The purpose of this study was to compare impact forces experienced during various exit methods from commercial equipment. The study investigated impact forces of ten male subjects while exiting two tractors, a step-van, a box-trailer, and a cube-van. The results showed that exiting from cab-level or trailer-level resulted in impact forces as high as 12 times the subject's body weight; whereas, fully utilizing the steps and grab-rails resulted in impact forces less than two times body weight. An approach that emphasizes optimal design of entry/exit aids coupled with driver training and education is expected to minimize exit-related injuries.

Continuous assessment of back stress (CABS): a new method to quantify low-back stress in jobs with variable biomechanical demands

Jobs with a high degree of variability in manual materials handling requirements expose limitations in current low-back injury risk assessment tools and emphasize the need for a probabilistic representation of the biomechanical stress in order to quantify both acute and cumulative trauma risk. We developed a hybrid assessment methodology that employs established assessment tools and then represents their evaluations in a way that emphasizes the distributions of biomechanical stress. Construction work activities in the home building industry were evaluated because of the high degree of variability in the manual material handling requirements. Each task was evaluated using the Revised NIOSH Lifting Equation, The University of Michigan Three-Dimensional Static Strength Prediction Program, and the Ohio State University Lumbar Motion Monitor Model. The output from each model was presented as time-weighted histograms of low-back stress, and the assessments were compared. The results showed considerable differences in what were considered high-risk activities, indicating that these 3 assessment tools consider the risk of low-back injury from different perspectives. The time-weighted distribution aspect of this methodology also contributed vital information toward the identification of high-risk activities. These results illustrate the necessity for more advanced low-back injury risk assessment techniques for jobs with highly variable manual materials handling requirements.

Biomechanical analysis of materials handling manipulators in short distance transfers of moderate mass objects: joint strength, spine forces and muscular antagonism

Although often suggested as a control measure to alleviate musculoskeletal stresses, the use of mechanical assistance devices (i.e. manipulators) in load transfers has not been extensively studied. Without data describing the biomechanical effects of such devices, justification for decisions regarding implementation of such tools is difficult. An experimental study of two types of mechanical manipulators (articulated arm and overhead hoist) was conducted to determine whether biomechanical stresses, and hence injury risk, would be alleviated. Short distance transfers of loads with moderate mass were performed both manually and with manipulator assistance under a variety of task conditions. Using analysis and output from new dynamic torso models, strength demands at the shoulders and low back, lumbar spine forces, and lumbar muscle antagonism were determined. Strength requirements decreased significantly at both the shoulders and low back when using either manipulator in comparison with similar transfers performed manually. Peak spine compression and anterior-posterior (a-p) shear forces were reduced by about 40% on average, and these reductions were shown to be primarily caused by decreases in hand forces and resultant spinal moments. Two metrics of muscular antagonism were defined, and analysis showed that torso muscle antagonism was largest overall when using the hoist. The results overall suggest that hoist-assisted transfers, although better in reducing spine compression forces, may impose relatively higher demands on coordination and/or stability at extreme heights or with torso twisting motions. The relatively higher strength requirements and spine compression associated with the articulated arm may be a result of the high inertia of the system. Potential benefits of practice and training are discussed, and conclusions regarding implementation of mechanical manipulators are given.

Sitting biomechanics part I: review of the literature

OBJECTIVE

To develop a new sitting spinal model and an optimal driver's seat by using review of the literature of seated positions of the head. spine, pelvis, and lower extremities.

DATA SELECTION

Searches included MEDLINE for scientific journals, engineering standards, and textbooks. Key terms included sitting ergonomics, sitting posture, spine model, seat design, sitting lordosis, sitting electromyography, seated vibration, and sitting and biomechanics.

DATA SYNTHESIS

In part I, papers were selected if (1) they contained a first occurrence of a sitting topic, (2) were reviews of the literature, (3) corrected errors in previous studies, or (4) had improved study designs compared with previous papers. In part II, we separated information pertaining to sitting dynamics and drivers of automobiles from part 1.

RESULTS

Sitting causes the pelvis to rotate backward and causes reduction in lumbar lordosis, trunk-thigh angle, and knee angle and an increase in muscle effort and disc pressure. Seated posture is affected by seat-back angle, seat-bottom angle and foam density, height above floor, and presence of armrests.

CONCLUSION

The configuration of the spine, postural position, and weight transfer is different in the 3 types of sitting: anterior, middle, and posterior. Lumbar lordosis is affected by the trunk-thigh angle and the knee angle. Subjects in seats with backrest inclinations of 110 to 130 degrees, with concomitant lumbar support, have the lowest disc pressures and lowest electromyography recordings from spinal muscles. A seat-bottom posterior inclination of 5 degrees and armrests can further reduce lumbar disc pressures and electromyography readings while seated. To reduce forward translated head postures, a seat-back inclination of 110 degrees is preferable over higher inclinations. Work objects, such as video monitors, are optimum at eye level. Forward-tilting, seat-bottom inclines can increase lordosis, but subjects give high comfort ratings to adjustable chairs, which allow changes in position.

A biomechanical model on muscle forces in the transfer of spinal load to the pelvis and legs

Based on musculoskeletal anatomy of the lower back, abdominal wall, pelvis and upper legs, a biomechanical model has been developed on forces in the load transfer through the pelvis. The aim of this model is to obtain a tool for analyzing the relations between forces in muscles, ligaments and joints in the transfer of gravitational and external load from the upper body via the sacroiliac joints to the legs in normal situations and pathology. The study of the relation between muscle coordination patterns and forces in pelvic structures, in particular the sacroiliac joints, is relevant for a better understanding of the aetiology of low back pain and pelvic pain. The model comprises 94 muscle parts, 6 ligaments and 6 joints. It enables the calculation of forces in pelvic structures in various postures. The calculations are based on a linear/non-linear optimization scheme. To gain a better understanding of the function of individual muscles and ligaments, deviant properties of these structures can be preset. The model is validated by comparing calculations with EMG data from the literature. For agonistic muscles, good agreement is found between model calculations and EMG data. Antagonistic muscle activity is underestimated by the model. Imposed activity of modelled antagonistic muscles has a minor effect on the mutual proportions of agonistic muscle activities. Simulation of asymmetric muscle weakness shows higher activity of especially abdominal muscles.

Effects of pacing when using material handling manipulators

Common manipulator-assisted materials handling tasks were performed in a laboratory simulation at self-selected and faster (paced) speeds. The effects of pacing on peak hand forces, torso kinematics, spine moments and forces, and muscle antagonism were determined, along with any influences of several task variables on these effects. The faster trials were performed 20% more rapidly than the self-paced trials. It was found that (a) achieving this level of performance required approximately 10% higher hand forces and 5%-10% higher torso moments, (b) consistent torso postures and motions were used for both speed conditions, and (c) the faster trials resulted in approximately 10% higher spine forces and approximately 15% higher levels of lumbar muscle antagonism. On whole, these results suggest a higher risk of musculoskeletal injury associated with performance of object transfers at faster than self-selected speeds with and without a manipulator. Further analysis provided evidence that the use of manipulators involves higher levels of motor coordination than do manual tasks. Several implications regarding the use of material handling manipulators in paced operations are discussed. Results from this investigation can be used in the design, evaluation, and selection of material handling manipulators.

Management of acute lumbar disk herniation initially presenting as mechanical low back pain

OBJECTIVE

To describe the clinical management with spinal manipulation of a male patient with risk factors for lumbar disk herniation initially suffering from what appeared to be mechanical low back pain that evolved into radiculopathy; also to review issues pertinent to chiropractic/manipulative management of disk herniation.

CLINICAL FEATURES

The patient initially suffered from unilateral low back pain and nonradicular/nonlancinating referral to the ipsilateral lower extremity.

INTERVENTION AND OUTCOME

Disk herniation-in-evolution was included in the differential diagnosis, which was discussed with the patient, who then gave verbal informed consent for manipulative management. A day or so after the initial manipulation the presentation evolved to include S1 radiculopathy. Computed tomography, just after onset of radiculopathy, confirmed the clinical diagnosis of lumbosacral disk herniation. The patient continued with manipulative management and repeat computed tomography examination after clinical resolution about 2 months later revealed reduction in size of the apparently clinically significant herniation.

CONCLUSION

Risk factors for the development of disk herniation should be considered when assessing patients suffering from what appears to be mechanical low back pain. The role played by manipulation in the development of disk herniation in this case was believed to be circumstantial rather than causal. Manipulation was used in the treatment of this patient over a period of approximately 2 months; after this time, clinical and partial computed tomography imaging resolution was evident. Ongoing clinical (neurologic) evaluation of patients with manifest or suspected disk herniation is an important aspect of management. Good-quality trials of manipulation for patients with disk herniation are imperative for the chiropractic profession.

Correlation and quantification of projected 2-dimensional radiographic images with actual 3-dimensional Y-axis vertebral rotations

BACKGROUND

Historically, measurement of 2-dimensional (2-D) radiographic images on the anteroposterior radiograph has been made to assess 3-dimensional (3-D) y-axis vertebral rotations.

OBJECTIVES

To correlate and quantify measurements of the projected 2-D radiographic image with the degree of 3-D y-axis rotation.

STUDY DESIGN

A computer model was positioned in a simulated x-ray beam. Points of model contact with the simulated beam were projected onto a line in the neutral position and the first 7 degrees of both positive and negative y-axis rotation using two different axes of rotation. A larger model, a shape-altered model, and a decreased source-object-distance model were also studied.

RESULTS

3-D y-axis rotation of vertebrae causes an off-center displacement of the 2-D projected lamina junction in relation to the projected vertebral body. The magnitude of displacement increases with increasing degrees of rotation. In our model, no clinically significant difference was found in the amount of the projected off-center displacement of the lamina junction between either of our two chosen axes of rotation. However, significant differences in the projected offset were found between vertebrae with the same degree of rotation as a result of changes in vertebral shape, size, and positioning. The projected lamina junction off-centering at a given rotation is quantified for our model.

CONCLUSION

Use of millimetric measurement of the projected lamina offset on the anteroposterior radiograph is an inaccurate method for the assessment of the degree of 3-D y-axis vertebral rotation.

Three-dimensional measurement of lumbar spine kinematics for fast bowlers in cricket

OBJECTIVE: To determine whether the three-dimensional (3-D) lumbar spine kinematics for the mixed fast bowling technique differed to those of the side-on and front-on fast bowling techniques. BACKGROUND: It has been previously shown that bowlers who utilise a mixed bowling technique are more likely to show lumbar spine pathology than those who bowl with either the side-on or front-on techniques. METHODS: An electromagnetic device (3-Space(R)Fastrak(TM)) operating at 120 Hz captured range of motion and 3-D lumbar spine kinematics during the delivery stride of 20 young high performance subjects. The trajectory of shoulder and pelvic girdle markers were simultaneously captured and these data were used to classify bowlers into either a side-on, front-on or mixed technique group. RESULTS: No significant differences (P<0.004) existed between the side-on/front-on and mixed groups for 12 selected variables derived from the lumbar spine kinematic data. However, an examination of effect sizes revealed evidence that the mixed group showed: a greater amount of left lateral bend and an extended lumbar spine at front foot impact; a body position further from a neutral orientation at lease; and a greater range of motion and angular velocity of the trunk in the lateral bending and flexion/extension axes. CONCLUSIONS: Selected lumbar range of motion and velocity measures tended to be higher for mixed bowlers than side-on/front-on bowlers. RELEVANCE: Overuse injuries to fast bowlers in bricket are common. To better understand the mechanics of injury it is necessary to understand the 3-D rotations of the lumbar spine during this activity.

An assessment of complex spinal loads during dynamic lifting tasks

STUDY DESIGN

An electromyogram-assisted free-dynamic lifting model was used to quantify the patterns of complex spinal loads in subjects performing various lifting tasks.

OBJECTIVES

To assess in vivo the three-dimensional complex spinal loading patterns associated with high and low risk lifting conditions that matched those observed in industrial settings.

SUMMARY OF BACKGROUND DATA

Combined loading on the spine has been implicated as a major risk factor in occupational low back disorders. However, there is a void in the literature regarding the role of these simultaneously occurring complex spinal loads during manual lifting.

METHODS

Eleven male subjects performed symmetric and asymmetric lifting tasks with varying speed and weight. Reactive forces and moments at L5-S1 were determined through the use of electrogoniometers and a force plate. An electromyogram-assisted model provided the continuous patterns of three-dimensional spinal loads under these complex lifting tasks.

RESULTS

The results showed that complex dynamic motions similar to those observed in risky industrial tasks generated substantial levels of combined compressive and shear loads. In addition, higher loading rates were observed under these conditions. Unlike loading magnitudes, loading rate was a better indicator of dynamic loading because it incorporated both the duration and magnitude of net muscle forces contributing to total spinal loading during the lifting conditions.

CONCLUSIONS

Quantification of spinal combined motions and loading in vivo has not been undertaken. This study provided a unified assessment of the effects of combined or coupled motions and moments in the internal loading of the spine. Dynamic lifting conditions similar to those observed in risky industrial situations generated unique complex patterns of spinal loading, which have been implicated to pose a higher risk to the spinal structure. The higher predicted loading and loading rate during asymmetric lifting conditions can be avoided by appropriate ergonomic workplace modifications.

Back injury and work loss. Biomechanical and psychosocial influences

The exponential increase in occupational low back pain disability is a problem that is not being addressed adequately in clinical practice. The notion of achieving primary control through ergonomic intervention, based on biomechanics principles, has so far been unhelpful. The traditional secondary prevention strategies of rest and return to restricted work duties are seemingly suboptimal. Biomechanics/ergonomic considerations may be related to the first onset of low back pain, but there is little evidence that secondary control based solely on these principles will influence the risk of recurrence or progression to chronic disability. More promising in this respect are programs that take account of the psychosocial influences surrounding disability. Work organizational issues are clearly important, but so also is the behavior of clinicians. The balance of the available evidence suggests that clinicians generally should adopt a proactive approach to rehabilitation by recommending, whenever possible, early return to normal rather than restricted duties as well as complementary psychosocial advice if the issue of chronic disability is to be successfully tackled.

Finite element models in tissue mechanics and orthopaedic implant design

This article attempts to review the literature on finite element modelling in three areas of biomechanics: (i) analysis of the skeleton, (ii) analysis and design of orthopaedic devices and (iii) analysis of tissue growth, remodelling and degeneration. It is shown that the method applied to bone and soft tissue has allowed researchers to predict the deformations of musculoskeletal structures and to explore biophysical stimuli within tissues at the cellular level. Next, the contribution of finite element modelling to the scientific understanding of joint replacement is reviewed. Finally, it is shown that, by incorporating finite element models into iterative computer procedures, adaptive biological processes can be simulated opening an exciting field of research by allowing scientists to test proposed 'rules' or 'algorithms' for tissue growth, adaptation and degeneration. These algorithms have been used to explore the mechanical basis of processes such as bone remodelling, fracture healing and osteoporosis. RELEVANCE: With faster computers and more reliable software, computer simulation is becoming an important tool of orthopaedic research. Future research programmes will use computer simulation to reduce the reliance on animal experimentation, and to complement clinical trials.

The biomechanics of low back injury: implications on current practice in industry and the clinic

The purpose of this paper is to introduce some concepts of low back injury for use towards developing better injury risk reduction strategies and advancing rehabilitation of the injured spine. Selected issues in low back injury are briefly reviewed and discussed, specifically, the types of tissue loads that cause low back injury, methods to investigate tissue loading, and issues which are important considerations when formulating injury avoidance strategies such as spine posture, and prolonged loading of tissues over time. Finally, some thoughts on current practice are expressed to stimulate discussion on directions for injury reduction efforts in the future, particularly, the way in which injuries are reported, the use of simple indices of risk such as load magnitude, assessment of the injury and development of injury avoidance strategies. This paper was written for a general biomechanics audience and not specifically for those who are spine specialists.

Occupational risk factors for the first-onset and subsequent course of low back trouble. A study of serving police officers

STUDY DESIGN

A survey of occupational risks for low back trouble in two police forces discordant for one known physical stressor (wearing body armor weighing approximately 8.5 kg.

OBJECTIVES

To determine the hazard for first-onset and subsequent course of low back trouble associated with occupational physical and psychosocial stressors.

SUMMARY OF BACKGROUND DATA

Various occupational physical stressor have been associated with the prevalence of back pain, but their relationship with first-onset low back trouble is uncertain. Psychosocial factors reportedly are important determinants of chronicity.

METHODS

Anamnestic data on low back trouble were collected from representative random samples of "exposed" and control forces, along with variables describing exposure to occupational physical stressors and sports results. Psychometric tests were administered.

RESULTS

Occupational risk factors for first-onset low back trouble were determined from lifetables based on officers with no previous back pain history. Survival time to first onset was affected adversely by wearing body armor and, less so, by vehicular exposure and sports participation. The proportion with persistent (chronic) trouble did not depend on length of exposure since onset, but longer service was associated with recurrent episodes. Chronicity was related to distress and blaming police work. Work loss was associated with blaming work and wearing body armor. Changing to lighter duties after development of low back trouble occurred rarely.

CONCLUSIONS

Exposure to occupational physical stress seems detrimental; It reduced survival time to first-onset of low back trouble. Recurrence was associated with time since onset, but persistent trouble was not. Sports participation was a risk if occupational hazards were high.

Psychological questionnaires: do "abnormal" scores precede or follow first-time low back pain?

STUDY DESIGN

A prospective study of psychological risk factors for first-time low back pain with repeated use of psychological questionnaires.

OBJECTIVES

To measure the reproducibility of scores from psychological questionnaires, and to compare this with changes that follow an individual's first attack of back pain. Secondly, to determine which scores predict first-time back pain.

SUMMARY OF BACKGROUND DATA

"Abnormal" psychometric scores are associated with several aspects of back pain behavior. Little is known, however, about their reproducibility or long-term stability, and there has been no definitive answer to the question: which comes first, "abnormal" scores or low back pain?

METHODS

403 volunteers with no history of "serious" low back pain (defined as pain requiring medical attention or absence from work) participated in a functional spinal assessment. At the time of initial assessment and at 6-month intervals thereafter, the volunteers completed the following questionnaires: the Health Locus of Control, which was subdivided into three sections labelled "Internal," "Powerful others," and "Chance"; the Modified Somatic Perception Questionnaire; and the Zung depression scale. Scores from the Modified Somatic Perception Questionnaire and from the Zung depression scale were added to form a measure of psychological distress. Additional questionnaires inquired about any back pain experienced in the previous 6 months. Only three volunteers had left the study at the 18-month follow-up. At that time 162 participants had reported "any" low back pain, of which 79 were "serious."

RESULTS

Intraclass correlation coefficients for scores repeated after 6 months ranged from 0.67-0.80, and reproducibility of scores was equally high between the 0-, 6-, 12- and 18-month assessments. None of the scores were affected by "any" low back pain, and only the Modified Somatic Perception Questionnaire scores changed after "serious" back pain was reported. In a multivariate analysis, the most significant predictor of first time "serious" or "any" back pain was a history of non-"serious" back pain (P < 0.001). Of the psychological factors, the sum of Modified Somatic Perception Questionnaire scores and Zung questionnaire scores was the best predictor of "serious" back pain (P = 0.037), and the Modified Somatic Perception Questionnaire score was the best predictor of "any" back pain (P = 0.002). The 25% of participants with the highest sum of scores from the Modified Somatic Perception Questionnaire and Zung questionnaire was 2.7 times more likely to develop "serious" back pain than the 25% with the lowest sum of these scores. Nevertheless, after accounting for the affects of a history of non-"serious" back pain, psychometric scores predicted less than an additional 3% of reported back pain.

CONCLUSIONS

The scores from the Modified Somatic Perception Questionnaire and Zung questionnaire were reproducible over 18 months and were affected little by first episodes of back pain; yet these scores were significant predictors of it. "Abnormal" scores from these questionnaires precede back pain in a small number of people.