Function of the spine

In spite of the considerable effort which has been invested in attempts to understand the mechanism of human spines, substantial controversy remains, particularly in connection with assumptions which have to be made by those engaged in biological modelling.The hypothesis presented here is that the living joint has stress sensors driving a feedback mechanism, an arrangement which could react to imposed loads by modifying muscular action in such a way as to minimize stress at the joints and therefore the risk of injury. A theory of this kind gives an image of the spine not in terms of a spatial picture, as would a CAT scan, but in terms of stresses, forces and moments acting at the intervertebral joints. Calculations show that the erectores spinae alone cannot support more than about 50 kg; there must be some other mechanism to explain man's ability substantially to exceed that load.It is suggested that the interaction between the erectores spinae and the abdominals are of fundamental importance in the function of the spine; how they are co-ordinated during the lifting of weights is examined in detail. The theory resulting from this hypothesis is used to relate spinal injury and an injured subject's posture and behaviour.A mathematical formulation permits an objective evaluation of the spine, and a procedure for determining an automatic diagnosis of lumbar spine disabilities is proposed.     

Distributed body weight over the whole spine for improved inference in spine modelling

Numerous models used to investigate the causes of low back pain are based upon the concept of the lever model that considers equilibrium of forces and moments about a single intervertebral joint. Consideration of forces and moments at each intervertebral joint is essential if a more realistic idea of the loading on the spine is to be obtained. This will also allow the role of the curvature of the spine to be investigated. A distributed loading pattern for forces due to body weight for the whole spine has not been investigated before. In this paper, a distributed loading pattern for the whole spine for various postures is investigated and the potential impact on the calculations is discussed.     

Viscoelastic creep induced by repetitive spine flexion and its relationship to dynamic spine stability

Repetitive trunk flexion elicits passive tissue creep, which has been hypothesized to compromise spine stability. The current investigation determined if increased spine flexion angle at the onset of flexion relaxation (FR) in the lumbar extensor musculature was associated with altered dynamic stability of spine kinematics. Twelve male participants performed 125 consecutive cycles of full forward trunk flexion. Spine kinematics and lumbar erector spinae (LES) electromyographic (EMG) activity were obtained throughout the repetitive trunk flexion trial. Dynamic stability was evaluated with maximum finite-time Lyapunov exponents over five sequential blocks of 25 cycles. Spine flexion angle at FR onset, and peak LES EMG activity were determined at baseline and every 25th cycle. Spine flexion angle at FR increased on average by 1.7° after baseline with significant increases of 1.7° and 2.4° at the 50th and 100th cycles. Maximum finite-time Lyapunov exponents demonstrated a transient, non-statistically significant, increase between cycles 26 and 50 followed by a recovery to baseline over the remainder of the repetitive trunk flexion cycles. Recovery of dynamic stability may be the consequence of increased active spine stiffness demonstrated by the non-significant increase in peak LES EMG that occurred as the repetitive trunk flexion progressed.     

Induction of spine growth and synapse formation by regulation of the spine actin cytoskeleton

We explored the relationship between regulation of the spine actin cytoskeleton, spine morphogenesis, and synapse formation by manipulating expression of the actin binding protein NrbI and its deletion mutants. In pyramidal neurons of cultured rat hippocampal slices, NrbI is concentrated in dendritic spines by binding to the actin cytoskeleton. Expression of one NrbI deletion mutant, containing the actin binding domain, dramatically increased the density and length of dendritic spines with synapses. This hyperspinogenesis was accompanied by enhanced actin polymerization and spine motility. Synaptic strengths were reduced to compensate for extra synapses, keeping total synaptic input per neuron constant. Our data support a model in which synapse formation is promoted by actin-powered motility.     

Dendritic spine morphogenesis and plasticity

Dendritic spines are small protrusions off the dendrite that receive excitatory synaptic input. Spines vary in size, likely correlating with the strength of the synapses they form. In the developing brain, spines show highly dynamic behavior thought to facilitate the formation of new synaptic contacts. Recent studies have illuminated the numerous molecules regulating spine development, many of which converge on the regulation of actin filaments. In addition, interactions with glial cells are emerging as important regulators of spine morphology. In many cases, spine morphogenesis, plasticity, and maintenance also depend on synaptic activity, as shown by recent studies demonstrating changes in spine dynamics and maintenance with altered sensory experience.     

Eph receptors tingle the spine

During the development of excitatory synapses, molecules cluster on dendrites to form postsynaptic densities, and specialized structures known as spines appear. EphB2 is demonstrated to control this process by associating with and phosphorylating a key postsynaptic molecule, syndecan-2, thereby initiating the maturation of dendritic spines.     

Stem cells for spine surgery

In the past few years, stem cells have become the focus of research by regenerative medicine professionals and tissue engineers. Embryonic stem cells, although capable of differentiating into cell lineages of all three germ layers, are limited in their utilization due to ethical issues. In contrast, the autologous harvest and subsequent transplantation of adult stem cells from bone marrow, adipose tissue or blood have been experimentally utilized in the treatment of a wide variety of diseases ranging from myocardial infarction to Alzheimer’s disease. The physiologic consequences of stem cell transplantation and its impact on functional recovery have been studied in countless animal models and select clinical trials. Unfortunately, the bench to bedside translation of this research has been slow. Nonetheless, stem cell therapy has received the attention of spinal surgeons due to its potential benefits in the treatment of neural damage, muscle trauma, disk degeneration and its potential contribution to bone fusion.     

Cadherin regulates dendritic spine morphogenesis

Synaptic remodeling has been postulated as a mechanism underlying synaptic plasticity, and cadherin adhesion molecules are thought to be a regulator of such a process. We examined the effects of cadherin blockage on synaptogenesis in cultured hippocampal neurons. This blockade resulted in alterations of dendritic spine morphology, such as filopodia-like elongation of the spine and bifurcation of its head structure, along with concomitant disruption of the distribution of postsynaptic proteins. The accumulation of synapsin at presynaptic sites and synaptic vesicle recycling were also perturbed, although these synaptic responses to the cadherin blockade became less evident upon the maturation of the synapses. These findings suggest that cadherin regulates dendritic spine morphogenesis and related synaptic functions, presumably cooperating with cadherin-independent adhesive mechanisms to maintain spine-axon contacts.     

Biomechanics of snowmobile spine injuries

A combined experimental and clinical study relative to the production of spinal injuries common to the winter sport of snowmobiling indicates that snowmobile spinal injuries are a repeatable phenomenon with seats of the current design. The number of injuries appears to be growing rapidly with the increased popularity of the sport. However, the redesign of snowmobile and snowmobile suspension systems should allow the elimination of these injuries for most practical conditions.     

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.     

Validation of lumbar spine loading from a musculoskeletal model including the lower limbs and lumbar spine

Low back mechanics are important to quantify to study injury, pain and disability. As in vivo forces are difficult to measure directly, modeling approaches are commonly used to estimate these forces. Validation of model estimates is critical to gain confidence in modeling results across populations of interest, such as people with lower-limb amputation. Motion capture, ground reaction force and electromyographic data were collected from ten participants without an amputation (five male/five female) and five participants with a unilateral transtibial amputation (four male/one female) during trunk-pelvis range of motion trials in flexion/extension, lateral bending and axial rotation. A musculoskeletal model with a detailed lumbar spine and the legs including 294 muscles was used to predict L4-L5 loading and muscle activations using static optimization. Model estimates of L4-L5 intervertebral joint loading were compared to measured intradiscal pressures from the literature and muscle activations were compared to electromyographic signals. Model loading estimates were only significantly different from experimental measurements during trunk extension for males without an amputation and for people with an amputation, which may suggest a greater portion of L4-L5 axial load transfer through the facet joints, as facet loads are not captured by intradiscal pressure transducers. Pressure estimates between the model and previous work were not significantly different for flexion, lateral bending or axial rotation. Timing of model-estimated muscle activations compared well with electromyographic activity of the lumbar paraspinals and upper erector spinae. Validated estimates of low back loading can increase the applicability of musculoskeletal models to clinical diagnosis and treatment.