Current status of filter implantation The current treatment of DVT is mainly based on anticoagulation, but it has been reported in the literature that PE can occur in about 1/3 of patients with DVT even after receiving adequate amount of regular anticoagulation therapy, while some patients with DVT cannot undergo anticoagulation therapy due to surgery, trauma, pregnancy, etc. Therefore, inferior vena cava filters are undoubtedly an effective means to prevent fatal PE in patients with DVT. It has been reported that the incidence of PE in patients with DVT of the lower extremities can be reduced from 60-70% to 0.9%-5.0% after filter implantation, and the Mobin-Uddin umbrella filter ushered in a new era of inferior vena cava filters in 1967, and since then, with the continuous updating of interventional devices and the increasing simplicity of implantation techniques, the number of filter types and the number of filters used each year have gradually increased. According to statistics, only 2,000 filters were implanted in the United States in 1979, but the number increased to 49,000 in 1999; worldwide, approximately 100,000 filters were implanted in 2003, and the number is increasing at an annual rate of 16%. At the same time, the indications for filter placement have been gradually relaxed. According to statistics, 10 years ago, 67.7% of the indications for filter implantation were contraindications to anticoagulation, while only 4.7% were for prophylactic applications. In the past 10 years, the proportion of patients with contraindications to anticoagulation has decreased to 48%, while the proportion of prophylactic applications has increased to 16.4%. The types of filters currently used and their respective characteristics There are two main categories of vena cava filters: permanent filters and non-permanent filters, the latter also including retrievable filters and temporary filters. The permanent vena cava filter is the first form of filter to be introduced and widely used in clinical practice with FDA approval. The body of the filter is conical in shape and has a barb that attaches to the wall of the vena cava to prevent the filter from shifting. The main permanent filters used internationally are Greenfield (Boston), Vena Tech LP (B. Braun), Simon (Bard), and Birds neck (Cook). Some studies have confirmed that filter implantation can effectively reduce the incidence of PE, but its promotion is limited by complications such as inferior vena cava perforation, filter displacement, secondary thrombosis leading to inferior vena cava occlusion and DVT recurrence due to long-term implantation. Commonly used permanent vena cava filters Non-permanent filters are a new product aimed at replacing permanent filters and are still in the initial stage of promotion in China. It mainly includes retrievable filters and temporary filters. The retrievable filters are designed with a barb to secure the filter to the vena cava wall and a small hook at the top of the filter that can be removed by a goose neck grabber. There are three main types of FDA-approved retrievable filters: the OptEase filter (Cordis), the Recovery filter (Bard) and the Gunther Tulip filter (Cook). The advantage is that it can be placed in the body as a permanent filter for a long time, and can also be removed as a non-permanent filter. However, the disadvantage is that the time window for non-permanent filters to be placed in the body is relatively short, generally only 2 weeks, and it is not easy to remove them due to the endothelialization problem caused by the barb for too long, and less than 30% of the retrievable filters can be successfully removed, and most of them remain in the body, thus the advantages of the optional filters are not highlighted. Moreover, their removal requires more complicated operation techniques and additional equipment, which increases the cost of treatment. Commonly used retrievable vena cava filters None of the temporary filters have been approved by the FDA due to the lack of results from randomized controlled clinical studies. Unlike permanent filters, temporary filters are not designed with barbs and cannot be secured to the vena cava wall, relying instead on a connecting catheter and the force of gravity to hold them in place. Currently, the TempoFilter II is used in China, mainly produced by Beltran. The main advantage of the TempoFilter II is that it does not have a fixed claw to damage the vein wall, and the filter is allowed to remain in place for a longer period of time, from 4 to 6 weeks and up to 12 weeks. No additional device is required for recovery, as the catheter can be recovered directly, thus simplifying the operation and saving medical costs. However, if the filter intercepts a large thrombus (>2 cm), it is necessary to rethrombolyze or place another permanent filter, which is economically expensive and more difficult to perform. Evaluation of different types of filters The choice of filter type for patients with indications for filter implantation is still controversial because there are no long-term randomized clinical studies directly comparing the effectiveness and safety of permanent, temporary, and retrievable filters, and comparing the differences between different types of filters. Prior to the introduction of retrievable and temporary filters, the majority of patients in clinical practice were treated with permanent filters, which once placed could not be removed unless the inferior vena cava was surgically incised. RCTs have demonstrated a reduced incidence of PE in patients with DVT after permanent filter implantation compared to anticoagulation alone, but a significant increase in the incidence of venous thrombosis. Also permanent filter implantation requires lifelong anticoagulation. Therefore, permanent filters are more suitable for patients with VTE who are of advanced age, have a definite primary cause of VTE (e.g., thrombophilia, immune disorders, etc. that cause blood hypercoagulation) that are difficult to resolve in the short term, have long-term anticoagulation contraindications (e.g., hematologic disorders), and have oncologic disease with a short expected survival time. In patients with only temporary risk factors for VTE or contraindications to anticoagulation, permanent filter implantation may reduce the risk of PE in the short term, but the increased risk of thrombotic disease in the long term and the reduced quality of life associated with long-term anticoagulation are undoubtedly a financial and emotional burden for the patient. Non-permanent filters are becoming more popular among physicians because of their ability to reduce the risk of thrombotic complications. Several cohort studies have demonstrated the effectiveness of retrievable filters in preventing PE over a relatively short follow-up period. There are no prospective studies comparing retrievable versus permanent filters, and there is only one retrospective study comparing permanent versus retrievable filters. This study showed no significant differences in the safety and efficacy of these two filters. However, most retrievable filters, despite being called “retrievable”, have to be left in place for a longer period of time due to the short window of time they are allowed to remain in the body, often 2-3 weeks, during which time the risk of VTE or contraindication to anticoagulation is not completely eliminated in many patients, and once this window of time is exceeded, the filter is often left in place due to thrombosis and Once this window is exceeded, it is often difficult to remove the filter due to thrombosis and endothelialization (although repositioning and reimplantation can attempt to resolve such problems), thus becoming a permanent filter in the body and facing a host of problems that come with permanent filter placement. The International Society of Interventional Radiology recommends retrievable filters only for patients with a short-term risk of PE and contraindications to anticoagulation, while expecting the filter to be normally removed within 2-3 weeks. Because temporary filters do not have fixed barbs, they can be left in place for longer periods of time, such as up to 3 months with the TempoFilter II, making them sufficient to cover most patients with temporary contraindications to anticoagulation or risk of VTE. Therefore, in patients with severe trauma and major surgery, where the risk of VTE is temporary, placement of a temporary filter is most appropriate. After these risk factors are removed, removal of the temporary filter achieves prevention of PE risk while avoiding the risk of long-term anticoagulation, thrombosis and vena cava obstruction after permanent filter placement. However, it is still not FDA-approved due to its high number of post-implantation complications (e.g., infection, filter displacement, fracture, etc.). Due to the lack of prospective studies, the use of temporary filters is currently controversial, focusing on the treatment of thrombus on the filter before removal, the risk of endothelialization associated with prolonged implantation, and filter migration. Therefore, when selecting a temporary filter, the chance of thrombus dislodgement should be judged. If the chance of dislodgement is high or anticoagulation is contraindicated for a long time, such as right central unstable thrombus or combined thrombus after trauma, a retrievable filter should be preferred when anticoagulation is not effective, while if the chance of dislodgement is judged to be reasonable, a simple temporary filter implantation is feasible. In summary, inferior vena cava filter implantation only reduces the risk of PE in patients with DVT, but does not completely eliminate it. Before considering filter implantation, a comprehensive assessment of the risks and benefits of filter implantation should be performed. The choice of the type of filter should take into account the indication for filter implantation, the site of thrombus, the risk of PE from thrombus dislodgement, the possibility of recurrent thrombosis, and the patient’s health status and personal wishes. In addition, further clinical RCT results are needed to confirm the effectiveness and safety of each type of filter.