Osteoporosis is a systemic skeletal disease, its occurrence and development is the result of the comprehensive action of many systems, such as nerve, endocrine, immunity, reproduction, musculoskeletal and so on, it is characterized by decreased bone mass, bone microstructure degeneration, increased bone brittleness, pain, lumbar and knee soreness, spinal deformation and other symptoms, resulting in bone brittleness and fracture susceptibility, in clinical treatment, it is found that osteoporosis is one of the main causes of fracture in the elderly.According to the results of an epidemiological survey conducted by the China National Health Commission, about 19.2% of people over the age of 50 suffer from osteoporosis, with an incidence of 32.0% among people over 65, and the incidence in postmenopausal women is much higher than that in men.Osteoporotic thoracolumbar fracture is one of the common complications of osteoporosis, and which is an aging disease characterized by low back pain and limited movement, it is estimated that the prevalence rate of vertebral fractures in different parts of the world is about 16–21%, of which 30–50% of patients will have back pain, kyphosis and acute vertebral dyskinesia, neurological dysfunction caused by spinal cord injury.The risk factors of osteoporotic vertebral fracture mainly include rheumatoid arthritis, type 2 diabetes mellitus (T2DM), the use of glucocorticoid and immunosuppressant, low body mass index and so on, and the strongest risk factors for osteoporotic vertebral fracture are advanced age and bone mineral density (BMD)[19–21] .Although osteoporotic vertebral fractures usually occur in the thoracolumbar transitional zone, osteoporotic lumbar fractures are also common in the elderly in recent years, most often at the L4/L5 level, the prevalence of lumbar fractures in osteoporotic vertebral fractures is about 4–8%, osteoporotic lumbar fractures are also a common indication for spinal surgery worldwide.At present, osteoporotic fracture is considered to be an important public health problem, which not only increases the morbidity and mortality of elderly patients, but also produces very important economic costs. In addition, osteoporotic lumbar fracture often causes spinal cord injury, which is the main cause of disability, quality of life and even death in the elderly, seriously affecting the quality of life of patients and their families.However, there is a lack of early diagnosis of osteoporotic lumbar fracture, and there is no unified standard to systematically evaluate the severity of osteoporotic lumbar fracture, therefore, its serious morbidity and potential mortality are increasingly recognized as an important medical problem, and seeking prevention, diagnosis and treatment of osteoporotic lumbar fracture is still a major clinical problem.
Thoracolumbar vertebrae refers to T11-L2, a special segment of the spine, because it is located between the fixed thoracic vertebrae and the active lumbar vertebrae, the anatomical structure is relatively special, and it is the stage of transition from the physiological kyphosis of the thoracic vertebrae to the physiological protrusion of the lumbar vertebrae, so it is easy to cause injury in stress, so it is easy to cause injury.With the increase of age, the thoracolumbar vertebrae tend to show progressive degenerative changes, which causes them to become more fibrotic and not uniformly distribute compressive stress, thus exposing some parts of the vertebral body to high stress concentration, the trabecular bone adjacent to the endplate can then adapt to the changed force distribution according to Wolf's law, giving the observed changes in bone architecture and density. Osteoporosis affects the spine in another way, and severe osteoporosis affects the shape of the bone, resulting in a change in the height of the lumbar vertebrae, causing the arch of the adjacent vertebrae to move closer.According to the spinal three-column theory, the lumbar vertebrae mainly play the role of buffering and dispersing stress in spinal mechanics, when osteoporosis occurs in the lumbar vertebrae, the way in which the vertebrae bear the transmission force will also be changed. Osteoporotic vertebral compression fracture reduces the strength of the vertebral body and loses the stability of the spine, at the same time, due to the fretting of the fracture site, it may stimulate the peripheral nerves of bone marrow and periosteum and produce a sense of pain. Therefore, it is very important to study the biomechanics of this section. From the biomechanical point of view, the purpose of the treatment of thoracolumbar fractures is to restore the strength, stiffness and stability of the thoracolumbar spine and achieve the bony healing of the thoracolumbar vertebrae, and the biomechanical experimental results show that PKP and PVP can meet this requirement for osteoporotic thoracolumbar fractures. The experimental results show that the biomechanical properties of the model are significantly improved after PKP and PVP treatment, which can not only meet the requirements of normal functional kinematics of thoracolumbar vertebrae, but also restore the stability of thoracolumbar vertebrae.
At present, there are many research methods about biomechanics, among which finite element analysis, as a new biomechanical research method, has highlighted its unique superiority since its application, however, this method is not very mature at present, and there are still many defects, such as time-consuming, data loss, not all lifelike appearance and so on. With the unremitting efforts of many scholars in the past years, the clinical value of the finite element model of thoracolumbar spine is increasing, its shape is becoming more and more realistic, and its biological characteristics are getting closer and closer to the real law of motion of the human body.Under the environment of the above-mentioned advantages, this study takes the bone structure data of a patient with osteoporotic thoracolumbar fracture as the research object, uses the ultra-thin CT scanning technology to obtain the sectional image of T11-L2, and the authenticity of the original data needed for modeling is high, the CT thin layer scanning image is read into the Mimics software to establish the geometric model, and the Mimics software can automatically assign materials according to the CT value of the original data of the model, and it makes the assignment accurate and fast, so as to ensure the high accuracy of the model seen in this study.
For fracture reduction surgery, percutaneous kyphoplasty has been widely used in the treatment of thoracolumbar fractures, this method can not only restore vertebral body height and correct vertebral kyphosis deformity, but also relieve pain,it can also reduce the leakage rate of bone cement and complications such as nerve injury and pulmonary embolism. It has been reported that PKP in the treatment of thoracolumbar fracture has the characteristics of short operation time, less trauma, low cost, short radiotherapy time and so on, it has clinical application value.In the results of this study, there was no significant difference in the peak stress and displacement of the model after PKP and PVP treatment, but the difference was basically similar, and the peak difference was small.
In this study, the commonly used static fixation simulation method is used to simulate the biomechanics of different treatment methods of osteoporotic thoracolumbar fracture, in real life, the thoracolumbar spine is mainly subjected to the dynamic force in the process of action life, at the same time, muscles and other soft tissues will also have a certain impact on the force of the thoracolumbar vertebrae. Therefore, this study only reflects the stress of the thoracolumbar vertebrae under a certain action, and the later stage needs to be combined with the dynamic analysis of the musculoskeletal system to provide more accurate and practical simulation results for the clinic.
To sum up, the results of finite element analysis show that PVP and PKP in the treatment of osteoporotic thoracolumbar fracture can effectively improve the condition of patients, alleviate their clinical symptoms, have a good prognosis, have a good long-term effect, and have good biomechanical properties and remarkable application effect.