A Biomechanical Study Of Vertebroplasty And Kyphoplasty For Osteoporotic Vertebral Compression Fractures

Chapter 1. Investigation of Muscle Forces for Flexion and Extension by Using a Finite Element Model of the Thoracolumbar SpineStudy Design. The muscle forces, intradiscal pressure, and von Mises stresses in the cortical bone were calculated using a finite element model of the thoracolumbar spine. Objectives. To investigate the trunk muscle forces during flexion and extension in the thoracolumbar spine and to determine their influence on intradiscal pressure and stresses in the cortical bone.Summary of Background Data. The spine is primarily stabilized by muscle forces. However, little is known about the influence of trunk muscle forces on the deformation of, and stresses in, the spine. In most studies, muscle forces are neglected.Methods. A three-dimensional finite element model of the ligamentous thoracolumbar spine was created. It was validated by use of experimental data from in vitro measurements on cadaver specimens. Good agreement between analytical and experimental results proved achievable. The model was loaded with the upper body weight (260N), a compressive follower load (0N, 100N, and 200N, respectively) to account for the local muscle forces, and a force in the m. erector spinae. The force in the m. erector spinae, intradiscal pressure, and stresses in the cortical bone were estimated during extension and flexion in the thoracolumbar spine.Results. The force in the m. erector spinae increased with the flexion angle. The estimated forces in the erector spinae were 0N for 10°extension, 150N for standing, and 410N for 20°flexion in the thoracolumbar spine. Intradiscal pressure and von Mises stresse in the cortical bone increased with the flexion angle, but were slightly influenced by extension. The force in the m. erector spinae was decreased, and intradiscal pressure was increased with increasing the follower load. Intradiscal pressure and stresses in the cortical bone will be changed considerably when the muscle forces were neglected and only a pure moment was applied.Conclusions. This study confirms that the muscle forces should not be neglected for biomechanical studies at the spine. The muscle forces have a strong influence on intradiscal pressure and stresses in the cortical bone during extension and flexion in the thoracolumbar spine. Applying a follower load instead of a great number of small forces simulating the local muscles makes realistic loading in vitro studies feasible.Chapter 2. Biomechanical determination of bone cement augmentation of osteoporotic vertebral body in the thoracolumbar spine Study Design. The effect of prophylactic bone cement augmentation on osteoporotic thoracolumbar spinal units was determined using finite element models.Objectives. To estimate the stress levels in the bone of treated and adjacent vertebral bodies following prophylactic bone cement augmentation.Summary of Background Data. Osteoporosis is the most frequent skeletal disease of the elderly, leading to weakness of the bony structures. Cement injection into vertebral bodies has been used to treat osteoporotic compression fractures of the spine. The clinical results are encouraging. However, it remains unclear whether adjacent vertebral body fractures are related to the natural progression of osteoporosis or if adjacent fractures are a consequence of bone cement augmentation. Experimental biomechanical studies showed significant increase in stiffness and strength of treated bodies.Methods. In finite element models of the ligamentous thoracolumbar spine different levels of bone tissue loss (moderate osteoporosis and severe osteoporosis) were simulated by changing the elastic modulus of the cortical and trabecular bone. The elastic modulus of bone cement inserted in the L1 vertebral body was varied between 1.0GPa and 3.0GPa. In order to simulate'standing', the models were loaded with the upper body weight (260N), a follower load (200N), and a case-dependent force in the m. erector spinae. The intradiscal pressure and stresses in the cortical and trabecular bone were investigated during standing. Results. L1 vertebral body compressive stiffness decreased when moderate osteoporosis and severe osteoporosis were simulated. The intradiscal pressure and von Mises stresses in the cortical and trabecular bone depended strongly on the grade of osteoporosis in the vertebral body. Intradiscal pressure increased by 22.6% for moderate osteoporosis, and by 36.2% for severe osteoporosis compared intact model. Maximum von Mises stress in the cortical bone increased by 50.2% for moderate osteoporosis, and by 77.7% for severe osteoporosis compared intact model. The effects of bone cement augmentation and elastic modulus of bone cement on intradiscal pressure and stresses in the adjacent cortical and trabecular bone were smaller. For the treated vertebral body stresses in the cortical and trabecular bone were increased.Conclusions. Osteoporosis may lead to decreased compressive stiffness and reduced spinal stability. The grade of osteoporosis has stronger effects on the biomechanical behavior of thoracolumbar spine. The effects of bone cement augmentation are much smaller.Chapter 3. Biomechanical Effects of Vertebroplasty and Kyphoplasty on Adjacent Vertebral Body and Intervertebral Disc Study Design. The effects of vertebroplasty and kyphoplasty on the adjacent vertebral body and intervertebral disc were investigated using finite element models of the thoracolumbar spine.Objectives. To estimate the force in the m. erector spinae, intradiscal pressure, and von Mises stresses in the endplates during standing for a severe osteoporotic thoracolumbar spine, as well as after vertebroplasty and kyphoplasty treatment.Summary of Background Data. Vertebroplasty and kyphoplasty are frequently used for internal stabilization of a fractured vertebral body. Fractures in the adjacent vertebral body after vertebroplasty or kyphoplasty do occur occasionally. A wedge-shaped compression fracture shifts the centre of gravity of the upper body anteriorly and generally. However, it is unclear how a wedge-shaped compression fracture of a vertebral body increases muscle forces and intradiscal pressure.Methods. A wedge-shaped L1 vertebral body was simulated in finite element models of the severe osteoporotic thoracolumbar spine. For the vertebroplasty and kyphoplasty model an anterior height reduction of 35% and 10%, respectively, related to the severe osteoporotic model were assumed. In order to simulate'standing', the models were loaded with the upper body weight (260N), a follower load (200N), and a case-dependent force in the m. erector spinae. The force in the m. erector spinae, intradiscal pressure, and von Mises stresses in the endplates were investigated during standing.Results. A wedge-shaped vertebral body shifted the centre of gravity of the upper body anteriorly. This increased the flexion bending moment and thus the force in the m. erector spinae necessary for balancing the spine. Without compensation of upper body shift, the force in the m. erector spine increased by 183% for vertebroplasty and by only 56% for kyphoplasty compared to the severe osteoporotic spine. Intradiscal pressure increased by 60% and 22% for vertebroplasty and kyphoplasty, respectively. Maximum von Mises stress in the endplates increased by 60% and 21% for vertebroplasty and kyphoplasty, respectively. In contrast, with shift compensation of the upper body, the increase in the muscle forces was much lower, increase in intradiscal pressure was only 27% and 6.5%, and increase in maximum von Mises stress in the endplates was only 39% and 10.1% for for vertebroplasty and kyphoplasty, respectively. Bone cement augmentation had a much smaller effect on intradiscal pressure and maximum von Mises stress in the endplates. Moreover the effect of upper body shift after a wedge-shaped vertebral body fracture on spinal load was more pronounced than that of stiffness due to cement infiltration.Conclusions. The advantages of kyphoplasty found in this study will be apparent only if nearly full fracture reduction is achieved. Our results do not suggest that the adjacent vertebral body fractures after vertebroplasty or kyphoplasty are caused by the higher stiffness of the treated vertebral body but by the anterior shift of the upper body.