The cancer foci invasively transfer and spread along the vertebral blood vessels. The irregular bone destruction ‘crack’ is formed, which is the anatomical basis for the migration of bone cement. The immature blood vessels within the tumor and the rich blood vessels in the tumor lesions of the vertebral body are connected to form a ‘Straight Road’, which can communicate with the peripheral vascular system of the vertebral body. Following bone cement injection, this is the main reason for the formation of branched seepage. Particularly when bone cement flows along the vascular access, thrombosis vital organs will be induced (11), oppressing the surrounding organs in the vertebral body. The tumors forming in the spinal canal and intervertebral foramen should be concerned with resulting in the compression and damage of the spinal cord and nerve root.

During the process of implantation, the metastatic lesions in the space of defects are easily moved from the defected part, and thus, planted metastases and spinal cord compression would be formed.

Minimally invasive techniques are acceptable for the majority of patients with bone metastases, which has less trauma, effective results and fewer complications. Minimally invasive techniques have gradually become an important treatment for spinal metastases and are increasingly used in clinical application. The treatment methods commonly used in minimally invasive interventional therapy include percutaneous vertebroplasty (PVP), percutaneous kyphoplasty (PKP), percutaneous radiofrequency ablation (RFA), vascular embolization, a small incision in the spinal fixation surgery and radioactive seed implantation.

PVP is a percutaneous puncture injection of bone cement (polymethymethacrylate) into the vertebra guided by a digital subtraction angiography (DSA) machine, and can enhance the intensity of vertebrae and stability of the spine, prevent collapse, relieve waist and back pain and restore partial vertebral body height (21). In 1987, Galibert et al (22) first reported the successful treatment by PVP of one case of patients with chronic pain caused by C2 vertebral hemangioma. In 1989, Kaemmerlen et al (23) used the technology to treat the patients with metastatic carcinoma of the vertebral body. In recent years, the technology has been gradually extended to treat spinal metastases worldwide, and has been recognized and approved by clinicians and patients for the superior effect (24). Currently, posterior edge defect is considered as the PVP surgery contraindication by the majority of investigators, as vertebral defects are prone to induce bone cement leakage leading to the narrow spinal canal or intervertebral foramen stenosis, and even certain serious consequences, such as spinal cord or nerve root compression (25). Thus, there are a significant number of patients for whom it will be difficult to avoid the unbearable pain and paralysis.

Yang et al (26) believed that precautions in surgical procedures are as follows: i) It is recommended to complete with the beveled puncture needle, as it is conducive to accurately put the tip into the tumor lesion or the edge of tumor foci; ii) puncturing with the bevel needle, and the bevel back on the canal, making the injection pressure of the bone cement toward the tumor foci and back to the spinal vessle. iii) Target contrast should be finished prior to injection of bone cement and the comprehensive assessment of the distributions of the contrast agent in the region should be made; iv) bolus injection of bone cement solidification phase should be delayed and bone cement dispersion minimized to avoid the distribution of bone cement into the trailing edge cortex of the vertebral body; v) when necessary, a small amount of bone cement should be injected into the ‘normal’ cancellous bone in the periphery of the tumor foci, which played the anchor role, to avoid the bone cement moving into the spinal canal following surgery; and vi) limit the movement of the spine following surgery and make the regular radiographic observation. The patients with the posterior edge defects of spinal metastases can still implement PVP surgery.

PVP to kyphoplasty was another minimally invasive treatment of spinal metastases that was developed on the basis of PVP. The basic therapy methods and procedures were the same with PVP; it is expanded with a balloon to form a compartment in the target vertebral body, which can partially restore vertebral height. Subsequently, bone cement was injected into the vertebral body by needles (27). PKP requires a higher viscosity bone cement compared to PVP and the vertebral bone cement was slowly injected under fluoroscopic guidance, reaching the edge of the vertebral body, or two-thirds of the vertebral body. Due to the presence of lacuna formed by balloon dilatation, bone cement can be injected at the conditions of relatively low pressure and the incidence of bone cement leakage was reduced in theory (28). PKP indications include compressed fractures caused by osteoporosis of the vertebral body; more with vertebral compression fractures with kyphosis after 6 months; and pathological fractures and kyphotic deformities induced by metastases of the vertebral body. Although PVP can significantly alleviate severe pain induced by spinal metastases, it cannot restore kyphotic deformity and abnormal changes in spinal biomechanics, which may lead to a poorly analgesic effect at a long-term time. The pain remission rate of PKP is similar with that of PVP, but PKP has the role to increase the bone strength of the vertebral body, restore vertebral height, correct kyphosis and restore normal biological force lines of spinal body (29). As PKP can increase the vertebral defect area, it would not be recommended to the patients with vertebral defects caused by metastatic cancer of the spine (30).

RFA technology uses the radiofrequency ablation device. Under the guidance of DSA, CT, B ultrasonic imaging equipment and percutaneous ablation needle (radiofrequency electrode) punctures to the inside of the tumor, the middle or high-frequency radio waves were excited by the electrode and the surrounding tissue is subjected to plasma and shocked to produce heat. An oval area is formed with high temperature. The central temperature can reach 90–100°C and the temperature is ≤50°C, at which, coagulative necrosis of the cells arises. Thus, the tumor cells were inactivated. The blood vessels around the tumor lesions are coagulated to form a ‘reaction zone’ to interrupt the blood supply by the blood vessels within the tumor (31). Recently, RFA technology is being applied to treat bone tumor lesions. Clinical studies have shown that RFA can effectively alleviate the pain caused by bone tumors, such as osteoid osteoma, ossifying fibroma, vertebral hemangioma and vertebral metastases (32). Bone tumors are significantly different with substantive organs in organizational structure, biological and physicochemical properties, thus the range of radiofrequency ablation lesions, shape and distribution of the thermal field will be different from the parenchymal organs. The RFA diameters of the single electrode are 0.9–1.3 cm within the bone tissue, and cortical bone can effectively limit the heat conduction with a significant thermal insulation, which can protect the vital organs from thermal damage (33). The integrity of rear vertebral body bone cortex has important significance for the RFA ablation of spinal metastases. Theoretically, RFA induced the tumor tissue, paravertebral venous plexus or venous plexus within the vertebral body to coagulate to form a ‘reactive zone’, which can reduce the risk of bone cement leakage, making the bone cement more evenly distributed within the tumor tissues (34). The combination of RFA and PVP technology can largely overcome their own limitations and enhance the complementarity between them (35).

The vascular thrombosis technique is another commonly used method for minimally invasive treatment of spinal tumors, and it can be carried out by arterial cannulation and also by percutaneous puncture. The main indications for vas embolism operation are the tumors with a rich blood supply. Prior to open surgery, tumor embolization is performed to reduce blood loss during surgery. For the patients with spinal metastases who cannot tolerate surgery, vascular embolization can also be used as a local control of the tumor to relieve pain symptoms in palliative treatment, which is particularly appropriate for the tumors with a sufficient blood supply, such as renal cell carcinoma and thyroid cancer. Polyvinyl alcohol is the most commonly used embolic material and other materials, such as gelatin and sponge, are also included. The vessels can be completely embolised in 80% of patients; however, the major complication of vascular thrombosis technology is nerve damage. Cervical tumor embolization may cause cerebellar or brainstem infarction, but they usually have no symptoms. The embolization therapy on thoracic spine may damage the spinal cord, leading to motor and sensory disorders of the limbs. Koike et al (36) evaluated the effect of transcatheter arterial chemoembolization/embolization for symptomatic bone metastases, particularly in palliation. The data demonstrated that 75% of targeted lesions underwent sufficient devascularization without any serious complication and there was a positive correlation between the blocking degree of the blood supply and pain relief. Thus, vas embolism operation is an effective treatment method that is palliative for symptomatic bone metastases. Truumees et al (37) believed that >60% of spinal metastases were hypervascular and preoperative embolization was considered to decrease the hemorrhage risk and improve outcomes. Vas embolism operation can suppress tumor growth, but is not able to restore the biomechanics of the affected vertebrae. Vas embolism in combination with PVP surgery can inhibit the tumor progress, and also restore the biomechanics of the affected vertebrae to prevent the affected vertebrae to collapse.

Due to extensive destruction of the vertebral body in patients with spine metastases, spine instability severely occurred and spinal cord compression may be induced. When PVP surgery did not provide enough stability for the spine, spine internal fixation with a small incision has a unique therapeutic value. The commonly used open surgeries for vertebral metastases include removal and reconstruction surgery of the anterior tumor lesion and resection and reconstruction of the whole posterior spine. These types of surgery have large trauma, significant bleeding and a high incidence of complications. Generally, patients undergoing open surgery often require a longer postoperative rehabilitation phase. By palliative posterior spinal internal fixation, the upper and lower vertebral segments were fixed by pedicle screw, thus, the spine is rapidly stable and pain is relieved, the trauma by open surgery is avoided by the patient and they can recover in the short-term following surgery and receive further radiotherapy and chemotherapy as soon as possible to control the progression of the cancer. In the posterior approach surgery, if necessary, the vertebral plate can be decompressed to avoid or delay the damage to the spinal cord. For the thoracic spine metastasis, traditional anterior thoracic surgery has a serious impact on respiratory function. Previously, certain investigators have attempted an anterior approach for tumor removal surgery assisted by thoracoscopic surgery. Compared with undergoing thoracic surgery, internal fixation has a significantly shorter postoperative rehabilitation time and lower incidence of complications (38).

Radioactive seed implantation is a brachytherapy, which has become one of the important therapies for malignant tumors. The most commonly observed clinical radioactive particle is ¹²⁵I, which is mainly due to its characteristics. Radioactive seed implantation has unique advantages compared with external irradiation. The radiological area can get a more precise positioning. Particles are conformally distributed based on tumor size and shape. Particles are planted beyond the target range and the radiation dose decreased rapidly, which makes a high dose and long half-life of particles in the target area, and allows the spinal cord tissue to achieve a sufficient amount for the prescription (39). Yang et al (40–42) conducted a comparative study of the treatment method for spinal metastases: Simple PVP and PVP combined with ¹²⁵I seed implantation. The analgesic effect and vertebral-body changes were observed 1 day, and 1, 3 and 6 months after surgery, respectively. The results demonstrated that the analgesic effect in the simple PVP group is relatively slow, and the combination group therapy has a rapid effect to inhibit the pain and the highest complete remission rate and remission rate. PVP in combination with ¹²⁵I seed implantation has a significant effect in the prevention of vertebral collapse or the occurrence of new compression fractures with no significant complications, suggesting that the combination therapy of ¹²⁵I PVP and spinal metastases is safe and effective (43).