A multibody modelling approach to determine load sharing between passive elements of the lumbar spine

The human spinal segment is an inherently complex structure, a combination of flexible and semi-rigid articulating elements stabilised by seven principal ligaments. An understanding of how mechanical loading is shared among these passive elements of the segment is required to estimate tissue failure stresses. A 3D rigid body model of the complete lumbar spine has been developed to facilitate the prediction of load sharing across the passive elements. In contrast to previous multibody models, this model includes a non-linear, six degrees of freedom intervertebral disc, facet bony articulations and all spinal ligaments. Predictions of segmental kinematics and facet joint forces, in response to pure moment loading (flexion–extension), were compared to published in vitro data. On inclusion of detailed representation of the disc and facets, the multibody model fully captures the non-linear flexibility response of the spinal segment, i.e. coupled motions and a mobile instantaneous centre of rotation. Predicted facet joint forces corresponded well with reported values. For the loading case considered, the model predicted that the ligaments are the main stabilising elements within the physiological motion range; however, the disc resists a greater proportion of the applied load as the spine is fully flexed. In extension, the facets and capsular ligaments provide the principal resistance. Overall patterns of load distribution to the spinal ligaments are in agreement with previous predictions; however, the current model highlights the important role of the intraspinous ligament in flexion and the potentially high risk of failure. Several important refinements to the multibody modelling of the passive elements of the spine have been described, and such an enhanced passive model can be easily integrated into a full musculoskeletal model for the prediction of spinal loading for a variety of daily activities.     

A multibody modelling approach to determine load sharing between passive elements of the lumbar spine

The human spinal segment is an inherently complex structure, a combination of flexible and semi-rigid articulating elements stabilised by seven principal ligaments. An understanding of how mechanical loading is shared among these passive elements of the segment is required to estimate tissue failure stresses. A 3D rigid body model of the complete lumbar spine has been developed to facilitate the prediction of load sharing across the passive elements. In contrast to previous multibody models, this model includes a non-linear, six degrees of freedom intervertebral disc, facet bony articulations and all spinal ligaments. Predictions of segmental kinematics and facet joint forces, in response to pure moment loading (flexion-extension), were compared to published in vitro data. On inclusion of detailed representation of the disc and facets, the multibody model fully captures the non-linear flexibility response of the spinal segment, i.e. coupled motions and a mobile instantaneous centre of rotation. Predicted facet joint forces corresponded well with reported values. For the loading case considered, the model predicted that the ligaments are the main stabilising elements within the physiological motion range; however, the disc resists a greater proportion of the applied load as the spine is fully flexed. In extension, the facets and capsular ligaments provide the principal resistance. Overall patterns of load distribution to the spinal ligaments are in agreement with previous predictions; however, the current model highlights the important role of the intraspinous ligament in flexion and the potentially high risk of failure. Several important refinements to the multibody modelling of the passive elements of the spine have been described, and such an enhanced passive model can be easily integrated into a full musculoskeletal model for the prediction of spinal loading for a variety of daily activities.     

A methodological approach for the biomechanical cause analysis of golf-related lumbar spine injuries

A new methodological approach employing mechanical work (MW) determination and relative portion of its elemental analysis was applied to investigate the biomechanical causes of golf-related lumbar spine injuries. Kinematic and kinetic parameters at the lumbar and lower limb joints were measured during downswing in 18 golfers. The MW at the lumbar joint (LJ) was smaller than at the right hip but larger than the MWs at other joints. The contribution of joint angular velocity (JAV) to MW was much greater than that of net muscle moment (NMM) at the LJ, whereas the contribution of NMM to MW was greater rather than or similar to that of JAV at other joints. Thus, the contribution of JAV to MW is likely more critical in terms of the probability of golf-related injury than that of NMM. The MW-based golf-related injury index (MWGII), proposed as the ratio of the contribution of JAV to MW to that of NMM, at the LJ (1.55) was significantly greater than those at other joints ( < 1.05). This generally corresponds to the most frequent occurrence of golf-related injuries around the lumbar spine. Therefore, both MW and MWGII should be considered when investigating the biomechanical causes of lumbar spine injuries.     

Three-dimensional static modeling of the lumbar spine

This paper presents three-dimensional static modeling of the human lumbar spine to be used in the formation of anatomically-correct movement patterns for a fully cable-actuated robotic lumbar spine which can mimic in vivo human lumbar spine movements to provide better hands-on training for medical students. The mathematical model incorporates five lumbar vertebrae between the first lumbar vertebra and the sacrum, with dimensions of an average adult human spine. The vertebrae are connected to each other by elastic elements, torsional springs and a spherical joint located at the inferoposterior corner in the mid-sagittal plane of the vertebral body. Elastic elements represent the ligaments that surround the facet joints and the torsional springs represent the collective effect of intervertebral disc which plays a major role in balancing torsional load during upper body motion and the remaining ligaments that support the spinal column. The elastic elements and torsional springs are considered to be nonlinear. The nonlinear stiffness constants for six motion types were solved using a multiobjective optimization technique. The quantitative comparison between the angles of rotations predicted by the proposed model and in the experimental data confirmed that the model yields angles of rotation close to the experimental data. The main contribution is that the new model can be used for all motions while the experimental data was only obtained at discrete measurement points.     

Effect of lumbar fasciae on the stability of the lower lumbar spine

The biomechanical effect of tensioning the lumbar fasciae (LF) on the stability of the spine during sagittal plane motion was analysed using a validated finite element model of the normal lumbosacral spine (L4-S1). To apply the tension in the LF along the direction of the fibres, a local coordinate was allocated using dummy rigid beam elements that originated from the spinous process. Up to 10 Nm of flexion and 7.5 Nm of extension moment was applied with and without 20 N of lateral tension in the LF. A follower load of 400 N was additionally applied along the curvature of the spine. To identify how the magnitude of LF tension related to the stability of the spine, the tensioning on the fasciae was increased up to 40 N with an interval of 10 N under 7.5 Nm of flexion/extension moment. A fascial tension of 20 N produced a 59% decrease in angular motion at 2.5 Nm of flexion moment while there was a 12.3% decrease at 10 Nm in the L5-S1 segment. Its decrement was 53 and 9.6% at 2.5 Nm and 10 Nm, respectively, in the L4-L5 segment. Anterior translation was reduced by 12.1 and 39.0% at the L4-L5 and L5-S1 segments under 10 Nm of flexion moment, respectively. The flexion stiffness shows an almost linear increment with the increase in fascial tension. The results of this study showed that the effect of the LF on the stability of the spine is significant.     

Transfer of the heritable nonhemolytic jaundice trait from the Gunn rat to the Sprague-Dawley rat

Congenital nonhemolytic jaundice as observed in the Gunn rat was transferred successfully to the Sprague-Dawley rat. This jaundice trait occurs as the result of a deficiency of bilirubin glucuronyltransferase and appeared to transfer by simple Mendelian inheritance. A comparison of jaundiced Gunn with jaundiced Gunn-Sprague-Dawley cross rats for plasma bilirubin level, bilirubin glucuronyltransferase activity and female reproductive performance showed no significant difference between the two jaundiced rat groups. The phenotypic expression of the jaundice trait as viewed by the parameters used in this study appeared to be the same for both the Gunn and Gunn-Sprague-Dawley cross rats. The transfer of the jaundice trait to another rat strain enhances the opportunity to characterize this animal model and to determine the possible influence of a long-term closed mating system.     

An improved multi-joint EMG-assisted optimization approach to estimate joint and muscle forces in a musculoskeletal model of the lumbar spine

Muscle force partitioning methods and musculoskeletal system simplifications are key modeling issues that can alter outcomes, and thus change conclusions and recommendations addressed to health and safety professionals. A critical modeling concern is the use of single-joint equilibrium to estimate muscle forces and joint loads in a multi-joint system, an unjustified simplification made by most lumbar spine biomechanical models. In the context of common occupational tasks, an EMG-assisted optimization method (EMGAO) is modified in this study to simultaneously account for the equilibrium at all lumbar joints (M-EMGAO). The results of this improved approach were compared to those of its conventional single-joint equivalent (S-EMGAO) counterpart, the latter method being applied to the same lumbar joints but one at a time. Despite identical geometrical configurations and passive contributions used in both models, computed outcomes clearly differed between single- and multi-joint methods, especially at larger trunk flexed postures and during asymmetric lifting. Moreover, muscle forces predicted by L5-S1 single-joint analyses do not maintain mechanical equilibrium at other spine joints crossed by the same muscles. Assuming that the central nervous system does not attempt to balance the external moments one joint at a time and that a given muscle cannot exert different forces at different joints, the proposed multi-joint method represents a substantial improvement over its single-joint counterpart. This improved approach, hence, resolves trunk muscle forces with biological integrity but without compromising mechanical equilibrium at the lumbar joints.     

Vertebrae numbers of the early hominid lumbar spine

General doctrine holds that early hominids possessed a long lumbar spine with six segments. This is mainly based on Robinson's (1972) interpretation of a single partial Australopithecus africanus skeleton, Sts 14, from Sterkfontein, South Africa. As its sixth last presacral vertebra exhibits both thoracic and lumbar characteristics, current definitions of lumbar vertebrae and lumbar ribs are discussed in the present study. A re-analysis of its entire preserved vertebral column and comparison with Stw 431, another partial A. africanus skeleton from Sterkfontein, and the Homo erectus skeleton KNM-WT 15000 from Nariokotome, Kenya, did not provide strong evidence for the presence of six lumbar vertebrae in either of these early hominids. Thus, in Sts 14 the sixth last presacral vertebra has on one side a movable rib. In Stw 431, the corresponding vertebra shows indications for a rib facet. In KNM-WT, 15000 the same element is very fragmentary, but the neighbouring vertebrae do not support the view that it is L1. Although in all three fossils the transitional vertebra at which the articular facets change orientation seems to be at Th11, this is equal to a large percentage of modern humans. Indeed, a modal number of five lumbar vertebrae, as in modern humans, is more compatible with evolutionary principles. For example, six lumbar vertebrae would require repetitive shortening and lengthening not only of the lumbar, but also of the entire precaudal spine. Furthermore, six lumbar vertebrae are claimed to be biomechanically advantageous for early hominid bipedalism, yet an explanation is lacking as to why the lumbar region should have shortened in later humans. All this raises doubts about previous conclusions for the presence of six lumbar vertebrae in early hominids. The most parsimonious explanation is that they did not differ from modern humans in the segmentation of the vertebral column.     

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.     

Hypoplasia of the adenohypophysis in a Sprague-Dawley rat

Pathological examination performed on a male 70-day-old Sprague-Dawley rat revealed formation of a large cyst in the hypophysis. The cyst, which completely occupied the adenohypophyseal area, was lined by the epithelial cells. Small masses of the adenohypophyseal cells were occasionally seen to sprout from the cyst wall epithelium.     

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.     

Directional sensitivity of velocity sense in the lumbar spine

Regulating spinal motion requires proprioceptive feedback. While studies have investigated the sensing of static lumbar postures, few have investigated sensing lumbar movement speed. In this study, proprioceptive contributions to lateral trunk motion were examined during paraspinal muscle vibration. Seventeen healthy subjects performed lateral trunk flexion movements while lying prone with pelvis fixed. A 44.5-Hz vibratory stimulus was applied to the paraspinal muscles at the L3 level. Subjects attempted to match target paces of 9.5, 13.5, and 17.5 deg/s with and without paraspinal muscle vibration. Vibration of the paraspinal musculature was found to result in slower overall lateral flexion. This effect was found to have a greater influence in the difference of directional velocities with vibration applied to the left musculature. These changes reflect the sensitivity of lumbar velocity sense to applied vibration leading to the perception of faster muscle lengthening and ultimately resulting in slower movement velocities. This suggests that muscle spindle organs modulate the ability to sense velocity of motion and are important in the control of dynamic motion of the spine.     

Pathology of sternal deformity in the Sprague-Dawley rat

Pathological examination of two 12-week-old male Sprague-Dawley rats revealed bending of the sternebrae. Grossly, the sternebrae distal to the third sternebra were bent towards the underside, and the episternal extremity was turned towards the head. Microscopically, the 4th sternebra was observed to have slipped behind the third sternebra, and fractures of the episternal cortex and slanting of the episternal cartilage were also observed.     

FEBio finite element models of the human lumbar spine

Finite element analysis has proven to be a viable method for assessing many structure-function relationships in the human lumbar spine. Several validated models of the spine have been published, but they typically rely on commercial packages and are difficult to share between labs. The goal of this study is to present the development of the first open-access models of the human lumbar spine in FEBio. This modeling framework currently targets three deficient areas in the field of lumbar spine modeling: 1) open-access models, 2) accessibility for multiple meshing schemes, and 3) options to include advanced hyperelastic and biphasic constitutive models.     

Spontaneous teratoma in a Sprague-Dawley rat

An 6-week-old male Sprague-Dawley rat which had undergone no experimental treatment died from enlargement of the abdomen. At necropsy, a spherical mass measuring about 3 X 4cm in diameter was connected to the retroperitoneal region cross to the middorsal line of the body. The tumor was composed of skin, ganglion, striated and smooth muscle, cartilage, bone, pancreas, glands, lymph node, and fibrous and adipose tissue, and diagnosed as teratoma.