Mechanical efficacy of vertebroplasty: influence of cement type, BMD, fracture severity, and disc degeneration

INTRODUCTION: Osteoporotic vertebral fractures can be treated by injecting bone cement into the damaged vertebral body. "Vertebroplasty" is becoming popular but the procedure has yet to be optimised. This study compared the ability of two different types of cement to restore the spine's mechanical properties following fracture, and it examined how the mechanical efficacy of vertebroplasty depends on bone mineral density (BMD), fracture severity, and disc degeneration. METHODS: A pair of thoracolumbar "motion-segments" (two adjacent vertebrae with intervening soft tissue) was obtained from each of 15 cadavers, aged 51-91 years. Specimens were loaded to induce vertebral fracture; then one of each pair underwent vertebroplasty with polymethylmethacrylate (PMMA) cement, the other with another composite material (Cortoss). Specimens were creep loaded for 2 h to allow consolidation. At each stage of the experiment, motion segment stiffness in bending and compression was measured, and the distribution of compressive loading on the vertebrae was investigated by pulling a miniature pressure transducer through the intervertebral disc. Pressure measurements, repeated in flexed and extended postures, indicated the intradiscal pressure (IDP) and neural arch compressive load-bearing (F(N)). BMD was measured using DXA. Fracture severity was quantified from height loss. RESULTS: Vertebral fracture reduced motion segment stiffness in bending and compression, by 31% and 43% respectively (p<0.001). IDP fell by 43-62%, depending on posture (p<0.001), whereas F(N) increased from 14% to 37% of the applied load in flexion, and from 39% to 61% in extension (p<0.001). Vertebroplasty partially reversed all these effects, and the restoration of load-sharing was usually sustained after creep-consolidation. No differences were observed between PMMA and Cortoss. Pooled results from 30 specimens showed that low BMD was associated with increased fracture severity (in terms of height loss) and with greater changes in stiffness and load-sharing following fracture. Specimens with low BMD and more severe fractures also showed the greatest mechanical changes following vertebroplasty. CONCLUSIONS: Low vertebral BMD leads to greater changes in stiffness and spinal load-sharing following fracture. Restoration of mechanical function following vertebroplasty is little influenced by cement type but may be greater in people with low BMD who suffer more severe fractures.     

Lumbar spine postures in Marines during simulated operational positions

Low back pain has a 70% higher prevalence in members of the armed forces than in the general population, possibly due to the loads and positions soldiers experience during training and combat. Although the influence of heavy load carriage on standing lumbar spine posture in this population is known, postures in other operationally relevant positions are unknown. Therefore, the purpose of this study was to characterize the effect of simulated military operational positions under relevant loading conditions on global and local lumbar spine postures in active duty male US Marines. Secondary objectives were to evaluate if intervertebral disc degeneration and low back pain affect lumbar spine postures. Magnetic resonance images were acquired on an upright scanner in the following operational positions: Natural standing with no external load, standing with body armor (11.3 kg), sitting with body armor, and prone on elbows with body armor. Custom software was used to measure global lumbar spine posture: Lumbosacral flexion, sacral slope, lordosis, local measures of intervertebral angles, and intervertebral distances. Sitting resulted in decreased lumbar lordosis at all levels of the spine except L1–L2. When subjects were prone on elbows, a significant increase in local lordosis was observed only at L5–S1 compared with all other positions. Marines with disc degeneration (77%) or history of low back pain (72%) had decreased lumbar range of motion and less lumbar extension than healthy Marines. These results indicate that a male Marine's pathology undergoes a stereotypic set of postural changes during functional tasks, which may impair performance. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2145–2153, 2017.

     

Anterior thoracic posture increases thoracolumbar disc loading

In the absence of external forces, the largest contributor to intervertebral disc (IVD) loads and stresses is trunk muscular activity. The relationship between trunk posture, spine geometry, extensor muscle activity, and the loads and stresses acting on the IVD is not well understood. The objective of this study was to characterize changes in thoracolumbar disc loads and extensor muscle forces following anterior translation of the thoracic spine in the upright posture. Vertebral body geometries (C2 to S1) and the location of the femoral head and acetabulum centroids were obtained by digitizing lateral, full-spine radiographs of 13 men and five women volunteers without previous history of back pain. Two standing, lateral, full-spine radiographic views were obtained for each subject: a neutral-posture lateral radiograph and a radiograph during anterior translation of the thorax relative to the pelvis (while keeping T1 aligned over T12). Extensor muscle loads, and compression and shear stresses acting on the IVDs, were calculated for each posture using a previously validated biomechanical model. Comparing vertebral centroids for the neutral posture to the anterior posture, subjects were able to anterior translate +101.5 mm±33.0 mm (C7–hip axis), +81.5 mm±39.2 mm (C7–S1) (vertebral centroid of C7 compared with a vertical line through the vertebral centroid of S1), and +58.9 mm±19.1 mm (T12–S1). In the anterior translated posture, disc loads and stresses were significantly increased for all levels below T9. Increases in IVD compressive loads and shear loads, and the corresponding stresses, were most marked at the L5–S1 level and L3–L4 level, respectively. The extensor muscle loads required to maintain static equilibrium in the upright posture increased from 147.2 N (mean, neutral posture) to 667.1 N (mean, translated posture) at L5–S1. Compressive loads on the anterior and posterior L5–S1 disc nearly doubled in the anterior translated posture. Anterior translation of the thorax resulted in significantly increased loads and stresses acting on the thoracolumbar spine. This posture is common in lumbar spinal disorders and could contribute to lumbar disc pathologies, progression of L5–S1 spondylolisthesis deformities, and poor outcomes after lumbar spine surgery. In conclusion, anterior trunk translation in the standing subject increases extensor muscle activity and loads and stresses acting on the intervertebral disc in the lower thoracic and lumbar regions.This study was presented, in part, at the 29th annual meeting of the International Society for the Study of the Lumbar Spine, Cleveland, OH, USA, 2002 May 14–18     

Biomechanics of vertebral compression fractures and clinical application

Local biomechanical factors in the etiology of vertebral compression fractures are reviewed. The vertebral body is particularly vulnerable to compression fracture when its bone mineral density (BMD) falls with age. However, the risk of fracture, and the type of fracture produced, does not depend simply on BMD. Equally important is the state of degeneration of the adjacent intervertebral discs, which largely determines how compressive forces are distributed over the vertebral body. Disc height also influences load-sharing between the vertebral body and neural arch, and hence by Wolff’s Law can influence regional variations in trabecular density within the vertebral body. Vertebral deformity is not entirely attributable to trauma: it can result from the gradual accumulation of fatigue damage, and can progress by a quasi-continuous process of “creep”. Cement injection techniques such as vertebroplasty and kyphoplasty are valuable in the treatment of these fractures. Both techniques can stiffen a fractured vertebral body, and kyphoplasty may contribute towards restoring its height. The presence of cement can limit endplate deformation, and thereby partially reverse the adverse changes in load-sharing which follow vertebral fracture. Cement also reduces time-dependent “creep” deformation of damaged vertebrae.     

颈脊柱姿势及预负荷史对压缩强度影响的生物力学研究

目的研究颈脊柱姿势及预负荷对颈脊柱压缩强度的影响。方法选用12具成人健康新鲜尸体颈段脊柱,解剖出C3,4、C5,6共24个运动单位(包括上下两个椎体和椎间盘)。施以压缩负荷,观察两种预负荷状态(脱水和高度水化状态)和两种姿势(中立位和屈曲位)下,颈段脊柱的极限压缩强度。通过解剖和x线检查来明确颈椎的损伤。结果标本屈曲位时极限压缩强度比中立位时小(P<0001),在中立位损伤负荷下,高度水化状态标本的极限压缩强度比脱水状态标本低29%(P<001)。结论晨起椎间盘高度水化状态下以及颈脊柱屈曲位时,受到外伤负荷颈脊柱更易损伤。     

Sudden and unexpected loading generates high forces on the lumbar spine

STUDY DESIGN: A cross-sectional study of spinal loading in healthy volunteers. OBJECTIVES: To measure the bending and compressive forces acting on the lumbar spine, in a range of postures, when unknown loads are delivered unexpectedly to the hands. SUMMARY OF BACKGROUND DATA: Epidemiologic studies suggest that sudden and unexpected loading events often lead to back injuries. Such incidents have been shown to increase back muscle activity, but their effects on the compressive force and bending moment acting on the spine have not been fully quantified. Furthermore, previous investigations have focused on the upright posture only. METHODS: In this study, 12 volunteers each stood on a force plate while weights of 0, 2, 4, and 6 kg (for men, 40% less for women) were delivered into their hands in one of three ways: 1) by the volunteer holding an empty box with handles, into which an unknown weight was dropped; 2) by the same way as in 1, but with volunteer wearing a blindfold and earphones to eliminate sensory cues; or 3) by the volunteer sliding a box of unknown weight off a smooth table. Experiments were carried out with participants standing in upright, partially flexed, and moderately flexed postures. Spinal compression resulting from muscular activity was quantified using electromyographic signals recorded from the back and abdominal muscles. The axial inertial force acting up the long axis of the spine was calculated from the vertical ground reaction force. The bending moment acting on the osteoligamentous spine was quantified by comparing measurements of lumbar curvature with the bending stiffness properties of cadaveric lumbar spines. RESULTS: The contribution from abdominal muscle contraction to overall spinal compression was small (average, 8%), as was the axial inertial force (average, 2.5%), and both were highest in the upright posture. Peak bending moments were higher in flexed postures, but did not increase much at the moment of load delivery in any posture. Peak spinal compressive forces were increased by 30% to 70% when loads were suddenly and unexpectedly dropped into the box, and by 20% to 30% when they were slid off the table, as compared with loads simply held statically in the same posture (P < 0.001). The removal of audiovisual cues had little effect. CONCLUSIONS: Sudden and alarming events associated with manual handling cause a reflex overreaction of the back muscles, which substantially increases spine compressive loading. Manual handling regulations should aim to prevent such events and limit the weight of objects to be lifted.     

Study of the innervation of the spinal ligaments at the lumbar level

The authors studied the innervation of the human lumbar spinal ligaments on cadaver or surgical specimens. In the ligaments annexed to the neural arch and in the posterior longitudinal ligament were found free-ending fibers and amyelinic perivascular fibers. In the anterior longitudinal ligament, coexisting with perivascular fibers were found encapsulated corpuscular formations on the ventrolateral aspect of the junction between the intervertebral disc and vertebral body. These findings comfort the role of the anterior longitudinal ligament in proprioception essential both in static and dynamic function of the spine.     

Basic science of spinal degeneration

This article summarizes recent advances in our understanding of spinal pathology and pain. Degeneration appears to start in the intervertebral discs, often before age 20 years, and can be distinguished from ‘normal’ ageing by the presence of physical disruption, typically in the form of annulus fissures, prolapse or endplate fracture. Disruption is ultimately mechanical, but frustrated attempts by a small population of disc cells to heal a large avascular matrix give rise to the typical biological features of disc degeneration. Genetic inheritance and ageing are important risk factors for disc degeneration because they can weaken the disc matrix, and hinder repair processes. Discogenic pain appears to arise from the disc periphery as a result of in-growing nerves being sensitized by soluble factors from activated disc and blood cells. A degenerated disc loses pressure in the nucleus and bulges radially outwards, like a flat tyre. This often leads to a transient segmental instability, which can be reversed by the growth of osteophytes around the margins of the vertebral body. Annulus collapse in severe disc degeneration transfers compressive load-bearing to the neural arch, leading to facet joint osteoarthritis, and possibly to degenerative scoliosis. The anterior vertebral body then becomes relatively unloaded, and consequent focal bone loss (exacerbated by systemic osteoporosis) increases the risk of anterior wedge deformities, and senile kyphosis. Future interventions may include physical therapy to aid disc healing, disc prostheses with no moving parts, and injection therapies to block pain pathways.     

腰椎终板骨折特点及相关因素的实验研究

目的 探讨腰椎椎体终板压迫骨折的特点及其影响因素。方法 19个尸体标本的腰椎运动节段（FSU）,根据X线检查来确定椎间盘的退变分级,在双能X线吸收仪器（DEXA）上测量椎体和椎体终板的骨密度（BMD）和骨矿含量（BMC）,然后分别承受过度负荷导致终板骨折,根据力-位移曲线来确定终板骨折和终板的极限负荷（FL）。终板骨折后,运动节段解剖为单独的椎体,观察终板骨折情况。结果 19个FSU标本中,16个发生肉眼可见的终板骨折,骨折仅仅发生在下位椎体的上终板,比率为84．2％,骨折均位于终板的中央部分或前部,表现为星形放射状、阶梯状、局部突入型等;终板的FL与椎体及终板的BMD、BMC呈正相关。同一个椎体内,上终板的BMD、BMC显著低于下终板,而同一个椎间盘周围,上终板的BMD高于下终板者,BMC却无差异。结论终板压迫骨折易发生于椎体的上终板;不同退变程度椎间盘邻近终板骨折的特点不同,但这种骨折在X线片上很难显现,终板的极限负荷与椎体终板的BMD、BMC呈正相关。     

Regional variations in trabecular architecture of the lumbar vertebra: Associations with age,disc degeneration and disc space narrowing

Previous studies suggest that age and disc degeneration are associated with variations in vertebral trabecular architecture. In particular, disc space narrowing, a severe form of disc degeneration, may predispose the anterior portion of a vertebra to fracture. We studied 150 lumbar vertebrae and 209 intervertebral discs from 48 cadaveric lumbar spines of middle-aged men to investigate regional trabecular differences in relation to age, disc degeneration and disc narrowing. The degrees of disc degeneration and narrowing were evaluated using radiography and discography. The vertebrae were dried and scanned on a μCT system. The μCT images of each vertebral body were processed to include only vertebral trabeculae, which were first divided into superior and inferior regions, and further into central and peripheral regions, and then anterior and posterior regions. Structural analyses were performed to obtain trabecular microarchitecture measurements for each vertebral region. On average, the peripheral region had 12–15% greater trabecular bone volume fraction and trabecular thickness than the central region (p < 0.01). Greater age was associated with better trabecular structure in the peripheral region relative to the central region. Moderate and severe disc degeneration were associated with higher trabecular thickness in the peripheral region of the vertebral trabeculae (p < 0.05). The anterior region was of lower bone quality than the posterior region, which was not associated with age. Slight to moderate narrowing was associated with greater trabecular bone volume fraction in the anterior region of the inferior vertebra (p < 0.05). Similarly, greater disc narrowing was associated with higher trabecular thickness in the anterior region (p < 0.05). Better architecture of peripheral trabeculae relative to central trabeculae was associated with both age and disc degeneration. In contrast to the previous view that disc narrowing stress-shields the anterior vertebra, disc narrowing tended to associate with better trabecular architecture in the anterior region, as opposed to the posterior region.     

The vertical stability of the cervical spine

The concept of the three-column cervical spine and load transmission through each column was experimentally tested. Material consisted of five cervical columns removed from cadavers. The experiment was conducted on an Instron load testing machine. Load was applied on superior articular surfaces of the axis vertebra and was recorded below from each column separately at the level of the sixth cervical vertebra. It was found that 36% of the total load applied on the top of the specimen is transmitted through the anterior column formed by bodies and intervertebral discs and 32% each through the two posterior cervical columns formed by the articular processes. The experiment very strongly supported the role of neural arch in transmission of vertebral compressive forces.     

Kinetic potential of the lumbar trunk musculature about three orthogonal orthopaedic axes in extreme postures

Many studies have examined the mechanics of the lumbar spine in various planes, but only a limited number of three-dimensional investigations have been reported. Analysis of the low back during complex, dynamic postures demands rigorous representation of the trunk musculature. The data of this study demonstrated the force and torque contributions of approximately 50 laminas of various trunk muscles to flexion-extension, lateral bending, and axial twisting torque at the L4-L5 joint. This analysis was conducted with the spine in an upright standing posture and when fully flexed (60 degrees), laterally bent (25 degrees), and axially twisted (10 degrees) together with two examples of combined postures. Maximum moment potential, muscle length excursions, and the resultant compressive, anteroposterior shear, and lateral shear forces on the joint were also computed. The results indicate that the position of the vertebrae and their orthopaedic axes, which are a function of spinal posture, are an important factor in the reasonable determination of joint compressive, lateral shear, and anteroposterior shear loads. Muscle length changes that exceeded 20% of their respective length during upright standing were not observed during a full axial twist, but were observed in portions of the abdominal obliques during lateral bending, and in some extensors during full flexion. Extreme postures tended to change the torque potential of some muscles and influence joint load. Various portions of erector spinae were observed to have appreciable potential to generate torque about all three orthopaedic axes. This observation supports the notion held by some therapists that conditioning of the erector spinae is of utmost importance.     

Laminectomy contributes to cervical spine deformity demonstrated by holographic interferometry

Multiple factors contribute to the pathogenesis of postlaminectomy deformity and instability of the cervical spine. The complex alterations in both static and dynamic biomechanics after laminectomy are incompletely defined. We sought to examine the role of the lamina in compressive load bearing across the vertebral body. Holographic interferometry was used to study the surface deformation of single axially loaded cervical vertebral bodies before and after hemilaminotomy, hemilaminectomy, and experimental acrylic laminar reconstruction. Our results showed that hemilaminotomy did not alter the surface deformation because of axial loading across the cervical vertebral body. However, gross alterations in surface deformation across the cervical vertebral body were consistently observed after hemilaminectomy. Experimental reconstruction of the laminar arch using acrylic restored the deformation pattern to the prelaminectomized baseline. Our results support a role for the lamina and the integrity of the laminar arch in axial load bearing across the cervical vertebral body. The altered axial load bearing may be a significant contributor to postlaminectomy deformity and instability. These findings offer an additional biomechanical advantage to minimal bony intervention for cervical spine pathology.     

Abnormal stress concentrations in lumbar intervertebral discs following damage to the vertebral bodies: a cause of disc failure?

Summary The purpose of this investigation was to test the hypothesis that damage to a lumbar vertebral body can lead to abnormal stress concentrations in the adjacent intervertebral discs. Twenty-three cadaveric lumbar motion segments, from persons who had died aged between 19 and 87 years, were subjected to substantial compressive loading while in the neutral, lordotic and flexed postures. During the loading period, a miniature pressure transducer was pulled through the disc along its mid-sagittal diameter and graphs of horizontal and vertical compressive stress against distance were obtained. Measurements were repeated after each motion segment had been compressed up to the point of mechanical failure: at this point the vertebral bodies suffered minor damage to the trabecular arcades, and sometimes to the end-plate, but the structure remained essentially intact and motion segment height was reduced by only 1%–2%. After damage, the stress in the nucleus and anterior annulus fell by about 30%, and high stress peaks appeared in the inner posterior annulus. These changes were more pronounced in lordotic posture and less pronounced in flexion. The youngest discs showed the smallest changes. It is concluded that minor compressive damage to the vertebral body can lead to high stress concentrations in the posterior annulus. Since the vertebral body is the weak link of the lumbar spine, this may be a frequent precipitating cause of isolated disc failure in living people.AcroMed Prize of the European Spine Society 1992     

Lumbar spine disc height and curvature responses to an axial load generated by a compression device compatible with magnetic resonance imaging.

STUDY DESIGN: Axial load-dependent changes in the lumbar spine of supine healthy volunteers were examined using a compression device compatible with magnetic resonance imaging. OBJECTIVE: To test two hypotheses: Axial loading of 50% body weight from shoulder to feet in supine posture 1) simulates the upright lumbar spine alignment and 2) decreases disc height significantly. SUMMARY OF BACKGROUND DATA: Axial compression on the lumbar spine has significantly narrowed the lumbar dural sac in patients with sciatica, neurogenic claudication or both. METHODS: Using a device compatible with magnetic resonance imaging, the lumbar spine of eight young volunteers, ages 22 to 36 years, was axially compressed with a force equivalent to 50% of body weight, approximating the normal load on the lumbar spine in upright posture. Sagittal lumbar magnetic resonance imaging was performed to measure intervertebral angle and disc height before and during compression. RESULTS: Each intervertebral angle before and during compression was as follows: T12-L1 (-0.8 degrees +/- 2.5 degrees and -1.5 degrees +/- 2.6 degrees ), L1-L2 (0.7 degrees +/- 1.4 degrees and 3.3 degrees +/- 2.9 degrees ), L2-L3 (4.7 degrees +/- 3.5 degrees and 7.3 degrees +/- 6 degrees ), L3-L4 (7.9 degrees +/- 2.4 degrees and 11.1 degrees +/- 4.6 degrees ), L4-L5 (14.3 degrees +/- 3.3 degrees and 14.9 degrees +/- 1.7 degrees ), L5-S1 (25.8 degrees +/- 5.2 degrees and 20.8 degrees +/- 6 degrees ), and L1-S1 (53.4 degrees +/- 11.9 degrees and 57.3 degrees +/- 16.7 degrees ). Negative values reflect kyphosis, and positive values reflect lordosis. A significant difference between values before and during compression was obtained at L3-L4 and L5-S1. There was a significant decrease in disc height only at L4-L5 during compression. CONCLUSIONS: The axial force of 50% body weight in supine posture simulates the upright lumbar spine morphologically. No change in intervertebral angle occurred at L4-L5. However, disc height at L4-L5 decreased significantly during compression.     

In vivo intradiscal pressure measurement in healthy individuals and in patients with ongoing back problems

STUDY DESIGN: In vivo intradiscal pressure measurement in different postures in healthy individuals and in those with ongoing back problems. OBJECTIVES: With the most recent technique, 1) to analyze the influence of degeneration on the intradiscal pressure, 2) to calculate the spinal load on the L4-L5 intervertebral discs, and 3) to assess the relation between the spinal load and movement of the intervertebral motion segment. SUMMARY OF BACKGROUND DATA: Almost all the data on intradiscal pressure are from the studies by Nachemson. The results from these pioneering studies have formed the basis for current knowledge about the in vivo loading conditions of the human spine. Although performed already during the 1960s and 1970s with the technique available at that time, virtually no other similar studies have been performed to corroborate the findings. METHODS: The intradiscal pressure (vertical and horizontal) was measured using an advanced pressure sensor in 8 healthy volunteers and 28 patients with ongoing low back pain, sciatica, or both at L4-L5. Among other calculations, the actual loading conditions in various body positions were calculated in relation to the angle between the two vertebrae of the studied motion segments. RESULTS: The effect of respiration on intradiscal pressure was shown as a continuously periodic fluctuation in the healthy prone individual. The intradiscal pressure was significantly reduced according to the degree of disc degeneration as estimated by magnetic resonance imaging. There possibly was a difference between the vertical and horizontal pressures in the degenerated and nondegenerated discs because the nucleus pulposus was pressure-tropic property. The spinal load increased in the following order of body positions: prone, 144 N; lateral, 240 N; upright standing, 800 N; and upright sitting, 996N (P < 0.0001). In the standing and sitting body positions, the spinal load increased not only with forward bending, but also with backward bending. The spinal load was highly dependent on the angulation in the motion segment. The movements of the spine from a flexed to an extended position made the load of the spine change in a curvilinear fashion, fitting a squared equation in the standing body position. There was a correlation between the spinal load and the angle of the motion segment in the standing but not in the sitting body position. CONCLUSIONS: The spinal load was highly dependent on the angle of the motion segment in normal discs in vivo. The intradiscal pressure in degenerated discs was significantly reduced compared with that of normal discs. However, further studies on the effect of respiratory movement on intradiscal pressure, the difference between vertical and the horizontal pressures, and the difference in the spinal load between standing and the sitting body positions are necessary. The data obtained from the current study are fundamental to understanding the pathomechanisms and biomechanical problems of disc disease.     

Radiographic parameters for evaluating the neurological spaces in experimental thoracolumbar burst fractures

It is important to know the condition of neural spaces during the nonoperative treatment of thoracolumbar burst fractures. The goals of the current study were to identify the correlation between the degree of deformity of a fractured vertebra and the encroachment of neural spaces, and to determine how the encroachment and the deformity can be improved by the extension posture simulating the postural reduction. Experimental burst fractures were produced in L1 vertebrae of nine human thoracolumbar spine segments (T11-L3) with neural spaces lined with tiny steel balls. Lateral radiographs were taken in neutral and extended posture before and after the trauma. Anterior vertebral height, posterior vertebral height, vertebral height ratio, vertebral kyphotic angle, posterior vertebral body angle, and the cross diagonal angle were the geometric parameters used to describe the vertebral deformity. The canal diameter and superior and inferior intervertebral foramen areas were defined as the neural spaces. All parameters were measured on the scanned images of radiographs, as seen on the computer screen. Among the vertebral body parameters, the posterior vertebral height, posterior vertebral body angle, and cross diagonal angle showed significantly higher correlations with the canal encroachment. The extended posture did not improve the canal and intervertebral foramen encroachments. The kyphotic deformity (vertebral kyphotic angle and anterior vertebral height) was improved but the deformity of the vertebral posterior wall (posterior vertebral height and posterior vertebral body angle) was not improved because of the extended posture.     

Influence of spine morphology on intervertebral disc loads and stresses in asymptomatic adults: implications for the ideal spine.

BACKGROUND CONTEXT: Sagittal profiles of the spine have been hypothesized to influence spinal coupling and loads on spinal tissues. PURPOSE: To assess the relationship between thoracolumbar spine sagittal morphology and intervertebral disc loads and stresses. STUDY DESIGN: A cross-sectional study evaluating sagittal X-ray geometry and postural loading in asymptomatic men and women. PATIENT SAMPLE: Sixty-seven young and asymptomatic subjects (chiropractic students) formed the study group. OUTCOME MEASURES: Morphological data derived from radiographs (anatomic angles and sagittal balance parameters) and biomechanical parameters (intervertebral disc loads and stresses) derived from a postural loading model. METHODS: An anatomically accurate, sagittal plane, upright posture, quadrilateral element model of the anterior spinal column (C2-S1) was created by digitizing lateral full-spine X-rays of 67 human subjects (51 males, 16 females). Morphological measurements of sagittal curvature and balance were compared with intervertebral disc loads and stresses obtained using a quadrilateral element postural loading model. RESULTS: In this young (mean 26.7, SD 4.8 years), asymptomatic male and female population, the neutral posture spine was characterized by an average thoracic angle (T1-T12) = +43.7 degrees (SD 11.4 degrees ), lumbar angle (T12-S1) = -63.2 degrees (SD 10.0 degrees ), and pelvic angle = +49.4 degrees (SD 9.9 degrees ). Sagittal curvatures exhibited relatively broad frequency distributions, with the pelvic angle showing the least variance and the thoracic angle showing the greatest variance. Sagittal balance parameters, C7-S1 and T1-T12, showed the best average vertical alignment (5.3 mm and -0.04 mm, respectively). Anterior and posterior disc postural loads were balanced at T8-T9 and showed the greatest difference at L5-S1. Disc compressive stresses were greatest in the mid-thoracic region of the spine, whereas shear stresses were highest at L5-S1. Significant linear correlations (p < .001) were found between a number of biomechanical and morphological parameters. Notably, thoracic shear stresses and compressive stresses were correlated to T1-T12 and T4-hip axis (HA) sagittal balance, respectively, but not to sagittal angles. Lumbar shear stresses and body weight (BW) normalized shear loads were correlated with T12-S1 balance, lumbar angle, and sacral angle. BW normalized lumbar compressive loads were correlated with T12-S1 balance and sacral angle. BW normalized lumbar disc shear (compressive) loads increased (decreased) significantly with decreasing lumbar lordosis. Cervical compressive stresses and loads were correlated with all sagittal balance parameters except S1-HA and T12-S1. A neutral spine sagittal model was constructed from the 67 subjects. CONCLUSIONS: The analyses suggest that sagittal spine balance and curvature are important parameters for postural load balance in healthy male and female subjects. Morphological predictors of altered disc load outcomes were sagittal balance parameters in the thoracic spine and anatomic angles in the lumbar spine.     

Radiographical and anatomical studies on the pathogenesis of degenerative spondylolisthesis (author's transl)

In order to elucidate the causes of degenerative spondylolisthesis (D.S), the functional role of lumbar intervertebral joints and also laminae of the vertebral arch was studied by means of roentgenographic and also pathological exams. The inclination of the laminae of the vertebral arch and the intervertebral joints changed with age in accordance with statokinetic loading of the lumbar spine. In the group with increased lumbar lordosis at the upper level, movement of the 3rd and 4th lumbar vertebrae was great, accompanied by horizontal inclination of the laminae of the vertebral arch and the intervertebral joints and by osteoarthritic changes (O.A). This is similar to D.S. D.S is caused by abnormal spinal curvature and structural specificity of the lower limbs vertebrae, resulting in degeneration of the laminae of the vertebral arch and the intervertebral joints, changes in the inclination, decreased control of movement, and sliding of the vertebrae. Prodromal conditions resulting in the sliding of the vertebrae and in disc degeneration appear to be caused by D.S.     

Body posture and backpack loading: an upright magnetic resonance imaging study of the adult lumbar spine

Purpose

Axial loading of the spine while supine, simulating upright posture, decreases intervertebral disc (IVD) height and lumbar length and increases lumbar lordosis. The purpose of this study is to measure the adult lumbar spine’s response to upright posture and a backpack load using upright magnetic resonance imaging (MRI). We hypothesize that higher spinal loads, while upright and with a backpack, will compress lumbar length and IVD height as well as decrease lumbar lordosis.

Methods

Six volunteers (45 ± 6 years) underwent 0.6 T MRI scans of the lumbar spine while supine, upright, and upright with a 10 % body weight (BW) backpack. Main outcomes were IVD height, lumbar spinal length (distance between anterior–superior corners of L1 and S1), and lumbar lordosis (Cobb angle between the superior endplates of L1 and S1).

Results

The 10 % BW load significantly compressed the L4–L5 and L5–S1 IVDs relative to supine (p < 0.05). The upright and upright plus 10 % BW backpack conditions significantly compressed the anterior height of L5–S1 relative to supine (p < 0.05), but did not significantly change the lumbar length or lumbar lordosis.

Conclusions

The L4–L5 and L5–S1 IVDs compress, particularly anteriorly, when transitioning from supine to upright position with a 10 % BW backpack. This study is the first radiographic analysis to describe the adult lumbar spine wearing common backpack loads. The novel upright MRI protocol described allows for functional, in vivo, loaded measurements of the spine that enables the study of spinal biomechanics and therapeutic interventions.