Thoracolumbar spine mechanics contrasted under compression and shear loading (original) (raw)
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Effects of Dorsal Versus Ventral Shear Loads on the Rotational Stability of the Thoracic Spine
Spine, 2007
A biomechanical in vitro study on porcine and human spinal segments. To investigate axial rotational stability of the thoracic spine under dorsal and ventral shear loads. Idiopathic scoliosis is a condition restricted exclusively to humans. An important difference between humans and other vertebrates is the fact that humans ambulate in a fully erect position. It has been demonstrated that certain parts of the human spine, more specifically the dorsally inclined lower thoracic and high lumbar parts, are subject to dorsally directed shear loads. It has been hypothesized that these dorsal shear loads reduce the rotational stability of the spine, thereby increasing the risk to initiate idiopathic scoliosis. Fourteen porcine and 14 human thoracic functional spinal units (FSUs) with intact costotransverse and costovertebral articulations were used for biomechanical testing. In both dorsal and ventral directions, shear loads were applied to the upper vertebra of the FSU in the midsagittal plane (centrally), and at 1 cm to the right and to the left (eccentrically), resulting in a rotary moment. Vertebral rotation was measured at 3 incremental loads by an automated optoelectronic 3-dimensional (3D) movement registration system. The results of this study showed that eccentrically applied shear loads induce vertebral rotation in human as well as in porcine spinal segments. At the mid-thoracic and lower thoracic levels, significantly more vertebral rotation occurred under dorsal shear loads than under ventral shear loads. These data show that, in humans and in quadrupeds, the thoracic spine is less rotationally stable under dorsal shear loads than under ventral shear loads.
Shear strength of the human lumbar spine
Clinical Biomechanics, 2012
Background: Shear loading is recognised as a risk factor for lower back pain. Previous studies of shear loading have either not addressed the influence of age, bone mineral density, axial height loss due to creep or were performed on animal specimens. Methods: Intact human lumbar motion segments (L2-3) were tested in shear using a modified materials testing machine, while immersed in a Ringer bath at 37°C. Vertebrae were rigidly embedded in neutral posture (0°flexion) and subjected to a constant axial compression load of 500 N. Shear was applied to three groups: 'Young-No-Creep' (20-42 years), 'Young-Creep' (22-38 years, creep 1000 N for 1 h) and 'Old-No-Creep' (44-64 years). Failure was induced by up to 15 mm of anterior shear displacement at a rate of 0.5 mm/s. The trabecular and apophyseal joint bone mineral densities were evaluated from computed tomography images of the intact lumbar spines. Findings: Peak shear force correlated positively with trabecular bone mineral density for specimens tested without axial creep. No significant differences were observed with respect to age. During shear overload specimens increased in height in the axial direction. Interpretation: Trabecular bone mineral density can be used to predict the peak force of lumbar spine in shear in neutral posture.
2011
Our objective was to examine the load-bearing capacity of the transverse processes of human cadaveric thoracic vertebrae to vertical loads and axial rotation moments (i.e., moment applied in the transverse plane). A secondary objective was to examine the effect of the attached rib stumps. We wanted to demonstrate that the transverse process is durable enough to support the CAB hook-a complementary hook to the CD system-and can handle the vertical load or axial rotation moment during correction of scoliosis. We used 107 thoracic vertebrae removed from 10 cadavers. They were prepared in vertebral pairs, and were fixed into a material testing apparatus. Superoinferior vertical loads and axial rotation moments were applied to the transverse process using the CAB hooks at a rate of 30 mm/min and 8.58/s respectively until it fractured. We recorded 142 measurements, 99 were for vertical load and 43 for axial rotation moment. The average ultimate vertical load was 338 (SD ¼ 128) N and the average ultimate axial rotation moment was 14.4 (SD ¼ 4.52) Nm. The ultimate axial rotation moment for specimens with rib stumps attached was significantly greater than for specimens without rib stumps 15.9 (SD ¼ 4.1) Nm versus 12.5 (SD ¼ 4.4) Nm. Our results showed that both the vertical and axial rotation loading capability of the transverse process are large enough to withstand significant correctional forces, without fracture, through the CAB hooks.
The Spine Journal, 2005
BACKGROUND CONTEXT: Axial back pain affects a large percentage of the population. Often aggravated by weight-bearing activity, these patients frequently have associated degenerative or post-traumatic lumbar disc disease. Aquatherapy is frequently used to transition patients from less activity limited by pain to greater activity by reducing weight-bearing load of the lumbar spine. Development of a means to permit patients similar spinal unloading while active during normal daily living would have the potential to promote similar effects. PURPOSE: The purpose of this study is to measure internal disc pressure at L4/L5 in response to forces exerted by an external vest. The study hypothesis anticipated an unloading of the lumbar spine during upright posture, as measured by intradicsal pressure at the L4/5 disc, correlating with external forces provided to the trunk by the device. STUDY DESIGN: A controlled experimental study of spine biomechanical loading was undertaken using isolated cadaver torsos obtained from an approved tissue source. Ages ranged at death with a mean of 65Ϯ6 years. METHODS: The distractive force created by inflating a set of pneumatic lifters within vests for treatment of low back pain were calibrated in a materials testing machine. Effects of inflation on the disc pressures within the lumbar spine then were tested. A microscopic pressure sensor (Samba, Gothenburg, Sweden) was placed into the nucleus of the L4/L5 disc of six isolated cadaver torsos (1 female, 5 male) using a 15-gauge spinal needle under direct fluoroscopic visualization. The pressure sensor was 0.42 mm in diameter, and had a calibrated response range of 0-7500 mm Hg. A pneumatically actuated lumbar vest was fit snugly to the torso. Each torso was supported in an upright, weight-bearing position for testing. The vest was inflated while the internal disc pressure was monitored and recorded. The data were analyzed to test for correlation between the amount of external unloading force provided by the vest and the intradiscal pressure measured in vitro. RESULTS: Application of external loads between the pelvis and ribcage by the vest demonstrated a maximum mean reduction of internal disc pressure at L4/L5 of 25% when the vest was inflated to a level producing approximately 400 N of effective load. The reduction in disc pressure was significantly different compared with baseline (upright, weight-bearing disc pressure without the vest) for all distraction settings (pϽ.01) except for the very lowest setting which was significant only at pϭ.025. CONCLUSIONS: Spinal unloading with an externally applied vest with adequate surface interface is effective in reducing intradiscal pressures. Ambulatory reduction of pressure would permit beneficial reduction of loads and permit patients with weight-bearing intolerance a better quality of life.
Journal of biomechanics, 2016
The clinical relevance of mechanical testing studies of cadaveric human thoracic spines could be enhanced by using follower preload techniques, by including the intact rib cage, and by measuring thoracic intervertebral disc pressures, but studies to date have not incorporated all of these components simultaneously. Thus, this study aimed to implement a follower preload in the thoracic spine with intact rib cage, and examine the effects of follower load, rib cage stiffening and rib cage removal on intervertebral disc pressures and sagittal plane curvatures in unconstrained static conditions. Intervertebral disc pressures increased linearly with follower load magnitude. The effect of the rib cage on disc pressures in static conditions remains unclear because testing order likely confounded the results. Disc pressures compared well with previous reports in vitro, and comparison with in vivo values suggests the use of a follower load of about 400N to approximate loading in upright stand...
The Influence of Muscle Forces on the Stress Distribution in the Lumbar Spine
The Open Spine Journal, 2011
Introduction: Previous studies of bone stresses in the human lumbar spine have relied on simplified models when modeling the spinal musculature, even though muscle forces are likely major contributors to the stresses in the vertebral bones. Detailed musculoskeletal spine models have recently become available and show good correlation with experimental findings. A combined inverse dynamics and finite element analysis study was conducted in the lumbar spine to investigate the effects of muscle forces on a detailed musculoskeletal finite element model of the 4 th lumbar vertebral body. Materials and Methodology: The muscle forces were computed with a detailed and validated inverse dynamics musculoskeletal spine model in a lifting situation, and were then applied to an orthotropic finite element model of the 4 th lumbar vertebra. The results were compared with those from a simplified load case without muscles. Results: In general the von Mises stress was larger by 30 %, and even higher when looking at the von Mises stress distribution in the superio-anterior and central part of the vertebral body and in the pedicles. Conclusion: The application of spine muscles to a finite element model showed markedly larger von Mises stress responses in the central and anterior part of the vertebral body, which can be tolerated in the young and healthy spine, but it would increase the risk of compression fractures in the elderly, osteoporotic spine.
Cervical spine functional anatomy and the biomechanics of injury due to compressive loading
Journal of athletic training
To provide a foundation of knowledge concerning the functional anatomy, kinematic response, and mechanisms involved in axial-compression cervical spine injury as they relate to sport injury. We conducted literature searches through the Index Medicus, SPORT Discus, and PubMed databases and the Library of Congress from 1975-2003 using the key phrases cervical spine injury, biomechanics of cervical spine, football spinal injuries, kinematics of the cervical spine, and axial load. Research on normal kinematics and minor and major injury mechanisms to the cervical spine reveals the complex nature of movement in this segment. The movement into a single plane is not the product of equal and summative movement between and among all cervical vertebrae. Instead, individual vertebrae may experience a reversal of motion while traveling through a single plane of movement. Furthermore, vertebral movement in 1 plane often requires contributed movement in 1 or 2 other planes. Injury mechanisms are ...