Variation of lumbar spine stiffness with load

Mechanical studies of the Functional Spinal Unit (FSU) in-vitro have shown that the slopes of the load-displacement curves increase with load. This nonlinearity implies that the stiffness of the FSU is not constant over the range of physiologic loads, and that measurements obtained for FSU specimens through the application of individual loads cannot be summed to predict the response of the specimens to combined loads. Both experimental and analytical methods were developed in the present study to better quantify the nonlinear FSU load-displacement response and to calculate the coupled stiffness of FSU specimens at combined states of load reflecting in-vivo conditions. Results referenced to the center of the vertebral body indicate that lumbar FSU specimens are stiffer in flexion than in extension, and that FSU specimens loaded in flexion are stiffer at high loads than at low loads. The importance of combined load testing and a nonlinear interpretation of load-displacement data is demonstrated.     

Cervical and axioscapular muscle stiffness measured with shear wave elastography: A comparison between different levels of work-related neck disability

Assessing muscle mechanical properties in terms of stiffness may provide important insights into mechanisms underlying work-related neck pain. This study compared stiffness of cervical and axioscapular muscles between 92 participants (sonographers) with no (n = 31), mild (n = 43) or moderate/severe (n = 18) neck disability. It was hypothesized that participants with more severe neck pain and disability would present with altered distribution of stiffness in cervical and axioscapular muscles than those with no disability. Using shear wave elastography, the shear modulus (kPa) of five cervical and six axioscapular muscles or muscle segments were measured in a relaxed seated upright or side-lying position. Muscle activity was measured simultaneously using surface electromyography during the elastography measurements and scapular depression was measured using a measurement tape and inclinometer before the elastography measurements to evaluate their potential confounding influences on shear modulus. Increased shear modulus was found in deeper than superficial cervical muscles and more cranial than caudal axioscapular muscles. However, no differences in shear modulus of the cervical or axioscapular muscles were found between sonographers with varying levels of disability. This study suggests no alterations in stiffness of cervical and axioscapular muscles were associated with work-related neck pain and disability.     

Intra-abdominal pressure increases stiffness of the lumbar spine

Intra-abdominal pressure (IAP) increases during many tasks and has been argued to increase stability and stiffness of the spine. Although several studies have shown a relationship between the IAP increase and spinal stability, it has been impossible to determine whether this augmentation of mechanical support for the spine is due to the increase in IAP or the abdominal muscle activity which contributes to it. The present study determined whether spinal stiffness increased when IAP increased without concurrent activity of the abdominal and back extensor muscles. A sustained increase in IAP was evoked by tetanic stimulation of the phrenic nerves either unilaterally or bilaterally at 20 Hz (for 5 s) via percutaneous electrodes in three subjects. Spinal stiffness was measured as the force required to displace an indentor over the L4 or L2 spinous process with the subjects lying prone. Stiffness was measured as the slope of the regression line fitted to the linear region of the force-displacement curve. Tetanic stimulation of the diaphragm increased IAP by 27-61% of a maximal voluntary pressure increase and increased the stiffness of the spine by 8-31% of resting levels. The increase in spinal stiffness was positively correlated with the size of the IAP increase. IAP increased stiffness at L2 and L4 level. The results of this study provide evidence that the stiffness of the lumbar spine is increased when IAP is elevated.     

Cervical spine injuries in rugby players.

Nine patients with serious cervical spine injuries that occurred while they were playing rugby were seen in a British Columbia acute spinal cord injury unit during the period 1975-82. All the injuries had occurred during the "scrum" or the "tackle". Two of the patients were rendered permanently quadriplegic, and one patient died. There is a need for a central registry that would record all cervical spine injuries in rugby players as well as for changes in the rules of the game.     

Dynamic dorsoventral stiffness assessment of the ovine lumbar spine

Posteroanterior spinal stiffness assessments are common in the evaluating patients with low back pain. The purpose of this study was to determine the effects of mechanical excitation frequency on dynamic lumbar spine stiffness. A computer-controlled voice coil actuator equipped with a load cell and LVDT was used to deliver an oscillatory dorsoventral (DV) mechanical force to the L3 spinous process of 15 adolescent Merino sheep. DV forces (48 N peak, approximately 10% body weight) were randomly applied at periodic excitation frequencies of 2.0, 6.0, 11.7 and a 0.5-19.7 Hz sweep. Force and displacement were recorded over a 13-22 s time interval. The in vivo DV stiffness of the ovine spine was frequency dependent and varied 3.7-fold over the 0.5-19.7 Hz mechanical excitation frequency range. Minimum and maximum DV stiffness (force/displacement) were 3.86+/-0.38 and 14.1+/-9.95 N/mm at 4.0 and 19.7 Hz, respectively. Stiffness values based on the swept-sine measurements were not significantly different from corresponding periodic oscillations (2.0 and 6.0 Hz). The mean coefficient of variation in the swept-sine DV dynamic stiffness assessment method was 15%, which was similar to the periodic oscillation method (10-16%). The results indicate that changes in mechanical excitation frequency and animal body mass modulate DV spinal stiffness.     

A direct comparison of spine rotational stiffness and dynamic spine stability during repetitive lifting tasks

Stability of the spinal column is critical to bear loads, allow movement, and at the same time avoid injury and pain. However, there has been a debate in recent years as to how best to define and quantify spine stability, with the outcome being that different methods are used without a clear understanding of how they relate to one another. Therefore, the goal of the present study was to directly compare lumbar spine rotational stiffness, calculated with an EMG-driven biomechanical model, to local dynamic spine stability calculated using Lyapunov analyses of kinematic data, during a series of continuous dynamic lifting challenges. Twelve healthy male subjects performed 30 repetitive lifts under three varying load and three varying rate conditions. With an increase in the load lifted (constant rate) there was a significant increase in mean, maximum, and minimum spine rotational stiffness (p<0.001) and a significant increase in local dynamic stability (p<0.05); both stability measures were moderately to strongly related to one another (r=-0.55 to -0.71). With an increase in lifting rate (constant load), there was also a significant increase in mean and maximum spine rotational stiffness (p<0.01); however, there was a non-significant decrease in the minimum rotational stiffness and a non-significant decrease in local dynamic stability (p>0.05). Weak linear relationships were found for the varying rate conditions (r=-0.02 to -0.27). The results suggest that spine rotational stiffness and local dynamic stability are closely related to one another, as they provided similar information when movement rate was controlled. However, based on the results from the changing lifting rate conditions, it is evident that both models provide unique information and that future research is required to completely understand the relationship between the two models. Using both techniques concurrently may provide the best information regarding the true effects of (in) stability under different loading and movement scenarios, and in comparing healthy and clinical populations.     

Variations of stiffness and strength along the human cervical spine

The load-displacement response and strength of the mid (C2-C5) and lower (C5-T1) cervical regions were determined for combinations of sagittal loads, in vitro. In unpaired t-test comparisons, the mid cervical region was significantly stiffer in compression and extension than the lower region. In tests to failure, failure in six out of seven mid cervical specimens resulted from flexion alone, while combined compression-flexion was required to fail five of the eight lower cervical specimens. Post-test dissections revealed no regional differences in the pattern of failure. In addition to sagittal tests, the load-displacement responses of three-vertebrae cervical specimens were measured with the upper body axially rotated with respect to the lower body. The effect of this pre-torsion was to diminish the zone of low slope near zero load for axial, shear, and flexion motion. Three of the four axially rotated specimens failed in flexion without added compression. These controlled load-displacement measurements of cervical spine specimens describe for the first time the continuous flexion-compression response up to failure, and suggest that consideration of the biomechanics of three apparently distinct mobile regions of the cervical spine (C1-C2, C2-C5, C5-T1) may facilitate the interpretation of hazardous conditions and the diagnosis of injury. These data also provide basic information for the in vitro investigation of passive cervical spine protection such as helmets and head-rests, suggesting that the head should be kept in a non-rotated position to reduce risk of injury.     

Cervical spine vertebral and facet joint kinematics under whiplash.

Whiplash injuries sustained during a rear-end automobile collision have significant societal impact. The scientific literature on whiplash loading is both diverse and confusing. Definitive studies are lacking to describe the local mechanisms of injury that induce either acute or chronic pain symptoms. A methodology has been presented to quantify the kinematics of the cervical spine components by inducing controlled whiplash-type forces to intact human head-neck complexes. The localized facet joint kinematics and the overall segmental motions of the cervical spine are presented. It is anticipated that the use of this methodology will assist in a better delineation of the localized mechanisms of injury leading to whiplash pain.     

The effect of unstable loading versus unstable support conditions on spine rotational stiffness and spine stability during repetitive lifting

Lumbar spine stability has been extensively researched due to its necessity to facilitate load-bearing human movements and prevent structural injury. The nature of certain human movement tasks are such that they are not equivalent in levels of task-stability (i.e. the stability of the external environment). The goal of the current study was to compare the effects of dynamic lift instability, administered through both the load and base of support, on the dynamic stability (maximal Lyapunov exponents) and stiffness (EMG-driven model) of the lumbar spine during repeated sagittal lifts. Fifteen healthy males performed 23 repetitive lifts with varying conditions of instability at the loading and support interfaces. An increase in spine rotational stiffness occurred during unstable support scenarios resulting in an observed increase in mean and maximum Euclidean norm spine rotational stiffness (p=0.0011). Significant stiffening effects were observed in unstable support conditions about all lumbar spine axes with the exception of lateral bend. Relative to a stable control lifting trial, the addition of both an unstable load as well as an unstable support did not result in a significant change in the local dynamic stability of the lumbar spine (p=0.5592). The results suggest that local dynamic stability of the lumbar spine represents a conserved measure actively controlled, at least in part, by trunk muscle stiffening effects. It is evident therefore that local dynamic stability of the lumbar spine can be modulated effectively within a young-healthy population; however this may not be the case in a patient population.     

Cervical spine arthrodesis in rheumatoid arthritis: a long-term follow-up.

Forty-one patients with rheumatoid arthritis involving the cervical spine had a posterior cervical arthrodesis. They were followed for a minimum period of seven years. The diagnoses prior to surgery included cranial settling, atlantoaxial subluxation, subaxial subluxation, and any combination of these three. All patients had posterior arthrodesis, with or without methylmethacrylate, and iliac crest autogenous bone graft. In addition, one patient had an anterior vertebrectomy, and two had transoral resection of the odontoid. Follow-up consisted of a subjective questionnaire, standard radiographs, and physical examination, including a neurologic exam. This information was compared to preoperative data available in the patient''s medical record, postoperative data, and the information obtained in a similar study undertaken in 1987. At the time of follow-up, thirteen patients were known to be dead. One patient could not be located. Of the remaining twenty-six patients, eighteen underwent the full examination, including physical exam and radiographs. The remaining nine patients were contacted and interviewed, but were unavailable for exam and radiographs. All patients considered the operation a success. Only one patient at follow-up had a non-union. This was stable over time. No patient had a deterioration in neurologic function. There was no significant degeneration or instability seen at levels adjacent to the fused segments as compared to the rest of the cervical spine. Posterior cervical spine arthrodesis for rheumatoid involvement of the neck is a safe, efficacious procedure with no significant deterioration of effects over time.     

Reduced scapular muscle control and impaired shoulder joint position sense in subjects with chronic shoulder stiffness

The purpose of the present study was to determine (1) if joint position sense (JPS) in subjects with shoulder stiffness (SS) differs from that in controls; (2) if, when JPS is reduced in SS, it is related to scapular muscular activities in the mid/end ranges of motion; and (3) if a person’s function is associated with his or her level of JPS. Eighteen subjects with unilateral SS and 18 controls were included. Each subject performed abduction by self-selecting an end/mid range position. The electromagnetic motion-capturing system collected kinematic data while surface electromyography collected muscle activities (upper trapezius, lower trapezius, and serratus anterior muscles). Subjects were asked to move the upper limb to the target position (end/mid range) accurately without visual guidance. Reduced JPS was observed in subjects with SS (2.7 degrees in mid range, p < 0.05). The JPS was enhanced by an increased scapula muscular activation level in the end range of motion (R = ?0.61 for SS and ?0.41 for controls) and by coordination among muscles’ activation in the mid-range of motion (R = ?0.87 for SS and R = ?0.53 for controls). Impaired JPS was also related to self-reported functional status (R = ?0.56) in subjects with SS. Shoulder JPS in subjects with chronic SS is impaired in comparison with controls. In the mid-range motion, the coordination of scapula muscular activation is related to shoulder JPS. Impaired JPS is also function-related in subjects with SS. These findings suggest that the coordination among scapula muscles’ activation were important to consider in the rehabilitation of patients with chronic SS.