| BackgroundMetastatic tumors is most common in the spine, which is often accompanied by pathologic fracture and unbearable pain. Most patients are not suitable for conventional surgical treatment, but only for palliative treatment. In order to reduce the unbearable pain and improve the quality of life, medical workers keep exploring new minimally invasive treatment methods and techniques, with the purposes of killing metastatic as much as possible and avoiding pathologic fracture, spinal cord compression and other related complications.Real-time imaging-guided radiofrequency ablation (RFA) is a new minimally invasive treatment of tumors being carried out in recent years. Under the imaging-guided percutaneous radiofrequency, the electrode is inserted into the lesion precisely and killing tumor cells maximally with minimal trauma to the patient. It alleviates patients'intractable pain caused by the cancer and improves quality of their lives, which can be seen as an effective local treatment technique. RFA has been widely adopted to treat benign lesions such as osteoid osteoma, osteoblastoma etc, which also been performed well in the treatment of spinal metastatic tumors.During the treatment of spinal metastatic lesions, we can select appropriate radiofrequency electrode and adjust the angle, length and frequency time at the meanwhile, achieving the maximum coagulation necrosis scope and reducing pain in patients correspondingly. However, there are congenital defects in the application of RFA, such as it can not guarantee the prevention of pathologic fracture vertebral lesions and the consolidation of vertebral body duing patients'early ambulation period. With the development of radiofrequency ablation technology, some questions need immediate solution. These questions are mainly as follows:firstly, how to enhance the range of ablation in a single treatment; 2ndly, the organization non-destructive temperature research in human body; 3rdly, the real-time distinction of necrosis and residual cancer tissue during the treatment; finally, the variation of biological characteristics of cancer cells in the residual lesions. When we approaching these topics, the treatment of vertebral metastases combined with other ways is required for urgent technological innovation, in order to compensate for their own shortcomings.In the mid-eighties of twentieth century, percutaneous vertebroplasty (PVP) appeared in France, and had developped rapidly in European and American countries. It has often been used for the treatment of vertebral hemangioma, metastatic tumors, and compression vertebral fractures caused by osteoporosis. On the basic study of PVP, Garfin and others first proposed the design concept of the percutaneous kyphoplasty (PKP), which had been approved in clinical application by the U.S. FDA in 1998 and carried out gradually in North America and other developed countries with encouraging results. The applications of PKP can compensate for the lack of PVP in a certain extent, which can be used to restore the vertebral body height, and increase the mechanical stability of the vertebral body, and also reduce the rate of bone cement leakage accordingly.However, no matter PVP or PKP derived from PVP, they all have their own limitations. First of all, because the spinal metastatic lesion is irregular itself, and the tumor occupies a certain volume, it is difficult to ensure that bone cement will be evenly distributed to every corner according to the tumor lesions. Especially, when the lesions of the posterior vertebral cortex is incomplete, bone cement has more limited distribution. Secondly, because the heat production capacity of bone cement is limited, and its heat conduct is through direct contact with surface of tumor. Due to both the irregular shape of the tumor and the thermal resistance characteristics of bone tissue, it is difficult to guarantee killing the tumor tissue as much as possible. Thirdly, in the condition of a certain tumor, when the load pressure achieves a certain state, the tumor cells or the tumor fasten will prolapse and lead to distant metastasis and infiltration of the surrounding normal tissue through a rich vein vascular network around the lesions.Therefore, during the process of balloon expanding, the bone cement is being injected into the lesion. In theory, the spread of tumor cells may occur resulting from the increased pressure within the tumor lesions. Fourthly, because the wide distribution of tumor blood vessel network, before the heating period and after the injection, the bone cement will leak through the vascular network, which would be even more prone to toxic reactions and damage the surrounding normal organs and so on. In order to solve the above problems, assuming that if we undertake PVP prior to RFA, which can have the following effects:1. RFA can kill the tumor cells in larger scale through its own heat theory and technique and by adjusting the angle of the needle.2. After RFA, the tumor volume becomes smaller, which can provide the greater space for the fill of bone cement.3. Because RFA reduces the tumor load and tumor vascular network around embolism at high temperature, the possibility of leakage of bone cement and the proliferation of tumor through the vascular network will be reduced through the implementation of the PVP later.In view of the certain complementary of RFA with PVP in the treatment of spinal tumor, medical workers who are assumed to do the clinical application research work are encouraged to combine these two methods. This suggestion of combination has been proved internationally theoretically with satisfactory results. Allowing for the visible advantages of PKP, compared with PVP, we consider the possibility of combining the application of RFA with PKP in the treatment of spinal metastatic. As vertebral bodies, in front of which is important organs and blood vessels, the rear is the spinal cord and nerves, have a certain degree of complexity for anatomical structure. The treatment of spinal metastatic tumors using RFA combined with PKP, not only has significant effectiveness but also has adequate security. However, how to control the temperature distribution around the electrodes and enable to cover the total lesion which would been heated without damaging the normal tissue, is a key component of the study.The three-dimensional thermodynamic finite element analysis can be adopted to study the thermal field distribution and heat transfer of bone tumors, and also to reform complex organizational thermal analysis and simulation reconstruction. This analysis can also provides a new method in area of the radio-frequency thermal field. There is a need for in-depth development in research areas, for example the problem of lacking the related data need to be solved. We measured the normal bone tissue radiofrequency hyperthermia temperature distribution using fresh porcine spine specimens, and with the basis of heat conduction theory, Laplace equation and the Pennes bio-heat transfer equation. In the ANSYS 11.0 environment, we formed the heat transfer calculation and the simulation reconstruction of bone thermodynamic thermal field finite element model after RFA. It supports the further study of RFA combined with PKP in the treatment of spinal metastases with the experimental evidence and useful reference for clinical applications.Objective1. To use the finite element method on the thermal theoretical analysis of the bone ablation and the simulation reconstruction.2. To study the thermal field distribution and melting range of bone tissue by using Radio Frequency Ablation (RFA) Single-Electrode, and do the calculation of the Specific Absorption Rate (SAR) of bone tissue at different positions.3. To compare the difference between the measured thermal field and the simulated thermal field; also to discuss the feasibility and future prospects of the finite element method for purpose of thermal field analysis.4. To evaluate the feasibility and clinical efficacy analysis on the application of using Imaging-Guid Radio Frequency Ablation (RFA) combined with Percutaneous Kyphoplasty (PKP) in the minimally invasive treatment of vertebral metastases.Methods1. With reference to the method of bio-mechanics finite element modeling, we established the three-dimensional thermodynamics finite element model of human lumbar vertebra, together with the SAR results obtained in the Experiment 2, and with the basis of Heat Conduction Theory, Laplace Equation and the Pennes Bio-Heat Transfer Equation. Furthermore, in the ANSYS 11.0 environment, we formed the heat transfer calculation and the simulation reconstruction of bone thermodynamic thermal field finite element model after RFA. The analog computation of the thermal field of RFA and the bone cement exothermic thermal field of PKP in the simulation were done. Finally, the generated results were compared with the simulated thermal field with the analysis of the differences.2. Using the MSI S-1500 RF devices and RF single-electrode produced by U.S. Medsphere Medical Technology Co. Ltd. to ablate 15 cases of fresh porcine vertebral column specimens, we set the center temperature of single-electrode to be 95±5℃for 5minutes. To measure the temperature changes around 2cm scope of ablation electrode in the bone cortex and bone marrow, and observe the coagulation zone shape and scope, we performed the analysis of electrode temperature distribution in different parts of the surrounding, and calculating the organization's specific absorption rate (SAR) at different locations in accordance with the initial moment temperature changes.3. Total of 11 lesions on 8 cases of spinal metastases, patients aged 32 to 75 years old, with mean of 56.6 years old. Lesion in the T11 on 1 case, T12 on 1 cases, L1 on 2 cases, L2 on 2 cases, L4 on 3 cases, L5 on 2 cases. All patients were performed by the treatment of imaging-guided RFA combined with PKP on the condition of anesthesia. The intraoperative radiofrequency ablation parameter was set by 4 to 6 minutes, the treatment temperature was during 95±5℃, the effective treatment diameter of radio-frequency electrode was between 1.5cm～2.0cm. All electrodes were accurately inserted into the nidus center, adjusting electrode angle and the direction to perform multiple ablation for tumor lesions with irregular shape. The average follow-up is 8.4 months, as a preoperative and postoperative clinical evaluation parameters,the visual analogue scale (VAS) was statistically analyzed.Results1. The human lumbar thermodynamic finite element model was successfully established. The model includes the following elements:vertebral metastatic, L2-L4 vertebral body (including the trabecular bone, cortical bone); annulus fibrosus；nucleus pulposus; spinal cord; abdominal aorta; inferior vena cava and the surrounding connective tissue. The simulation results are as follows:(1) In a unipolar radiofrequency ablation field, when reaching a steady state, the closer the nodes to the ablation electrode, the higher the temperature will be. In the sagittal plane, the isotherms showed an oval-shaped longitudinal distribution around the electrode center, and the temperature gradually decreased ento-ectad. The long diameter of 42℃isotherm was about 44.8mm, the long diameter of 50℃isotherm was about 28.1mm. In the coronal plane, the isotherms showed type circular distribution, the maximum diameter of 42℃isotherm was about 38.1mm, the maximum diameter of 50℃isotherm was about 20.6mm. (2) In a multi-polar radiofrequency ablation field, the thermal field distribution pattern is similar with the pole, but the distribution is broader. In the sagittal plane, the thermal field showed an oval-shaped longitudinal distribution around the electrode center, and the temperature gradually decreased from inside to outside; the long diameter of 42℃isotherm was about 52.3mm along the direction of electrodes; the long diameter of 50℃isotherm was about 34.3mm. In the coronary surface, the isotherms showed type round distribution, the maximum diameter of 42℃isotherm was about 43,4mm; the long diameter of 50℃isotherm was about 26.7mm. (3) During the heat molding process of bone cement in the vertebral kyphosis, the thermal field distribution and the distribution of bone cement are basically the same shape, furthermore the distribution range was significantly less than the thermal field of RF.2. The SAR of every measuring points keep correlation with measuring the RF power and the distance pitch with electrode. The higher of RF power, the smaller of SAR value. Closer to the electrode, the larger of SAR value, and reaching a higher saturation temperature in advance. In the vertebral body, the average distribution diameter of SAR field was 3.6cm. Coagulation along with the electrode area of bone marrow showed an oval-shaped longitudinal. The vertical direction of electrode diameter was 22.2±1.23mm; the long diameter along with the electrode direction was 28.6±1.34 m; appearing color of dark yellow; texture hard, clear boundaries; and with central mild carbonation.3. All patients were successfully treated and kept postoperative 24-hour observation of vital signs parallel anti-infective therapy.3 days Post-operation, VAS was significantly dropped. Then,1 week,1 month; 3 months; and 6 months Post-operation, the symptoms do not recur. VAS scores seem to be quite lower, which should have the statistical significance (P<0.01).Conclusions1. The thermal field distribution measured by the Finite Element Method simulation of bone single-electrode radiofrequency and the thermal field distribution in the thermal field have perfect coincidence. The finite element analysis and the simulation of thermal field basically reflect the thermal field distribution and the variation of bone radio-frequency ablation.2. The Finite Element Analysis and computer simulation provide a new and effective method in areas of thermal field research. Using computers for real-time simulation and reconstruction of three-dimensional temperature field of human beings, which is expected to make clinical oncology hyperthermia program more rational, effective and safe.3. The exothermic thermal field generated by bone cement during the period of vertebral kyphosis is relevant to patterns of bone cement distribution. 4. The thermal field distribution generated by RF electrode relates to the power, time of RF and the impedance of the bone tissue.5. The image-guided RFA combined with PKP in the treatment of vertebral metastatic is a simple, minimally invasive, safe and effective technology which requires the precise positioning and multi-departments collaboration. |
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