What is a Simon nitinol inferior vena cava (IVC) filter?

Updated: Oct 31, 2020
  • Author: Gary P Siskin, MD; Chief Editor: Kyung J Cho, MD, FACR, FSIR  more...
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Answer

Simon nitinol filter

Nitinol is an alloy consisting of 53% nickel, 45% titanium, and 2% cobalt that was developed in the 1960s at the US Naval Ordnance Laboratory. It is pliable at room temperature but has thermal memory and reverts to its preformed shape at body temperature. Nitinol resists corrosion, is nonferromagnetic, and is stronger than steel.

Simon et al performed experimental studies with this material in the 1970s, and ultimately, they developed a filter that was approved by the FDA in 1990. [38] The filter is 3.8 cm long and has 2 sections. The filter mesh, which is directed toward the heart, consists of 7 overlapping loops of 0.015-in–diameter (0.038-cm–diameter) nitinol wires fused together at 2 points. This part has an outer diameter of 28 mm. The anchoring basal part of the filter is formed by 6 diverging limbs in a radial array. Each has a small hook at the end that engages the caval wall for a depth of 1 mm (see the image below).

Simon nitinol filter. Simon nitinol filter.

The deployed filter can adapt to an IVC with a diameter as large as 28 mm. The legs also function as a coarse filter; the dome forms a fine filter and is designed to trap emboli larger than 5 mm, as well as a significant portion of smaller emboli. In vitro studies have demonstrated that the Simon nitinol filter traps more of the smaller clots than the other filters. Compared with the Greenfield filter, clot-trapping capability is not affected by filter tilt.

The Simon nitinol filter can be placed via the femoral, jugular, and antecubital [39] approaches. The delivery system for the jugular kit is longer, and the orientation of the filter is reversed in comparison with that in the femoral kit. Initially, a cold saline infusion through the storage tube via the side port was recommended to soften the filter. Later, modification of this filter eliminated the need for cold saline infusion. As a result of the flexibility of this filter at low temperature, placement through the right external jugular vein and right antecubital vein approaches have been successful using the internal jugular kit. Upon deployment, shortening of approximately 1-2 cm occurs as the dome of the filter is formed. In patients with small IVCs, incomplete formation of the dome can be seen after deployment; this does not alter filter function.

In a long-term study of 114 consecutive Simon nitinol filter insertions, Poletti et al reported a rate of PE recurrence of 4.4%. [40] Comparing different filters used during the past 26 years, Athanasoulis et al reported that the Simon nitinol filter had the lowest rate for recurrent PE (3%) and fatal recurrent PE (2%). [5] Poletti et al also reported a 3.5% rate of IVC thrombosis and a 3.5% rate of thrombosis at the femoral vein access site. [40] Aswad et al reported a higher incidence of access site and IVC thrombosis, but the rate was not significantly different from rates compared with the other types of filters studied. [41]

In a series of 103 patients, Simon et al found 5 patients with nonoccluding thrombus in the IVC, as well as 3 patients each with asymptomatic and symptomatic IVC occlusion. [42] Using MRI, Grassi et al raised the possibility of malignancy contributing to IVC thrombosis in 50% of patients with underlying malignancy undergoing Simon nitinol filter placement. [43] Filter migration or IVC perforation was not seen by Simon et al in their series of 103 patients. [42] In addition, Poletti et al found no filter migration in 38 patients in a study with a mean follow-up period of 32.2 months. [40] Strut fracture, axial deviation of the filter, and IVC perforation have been reported with the Simon nitinol filter; however, none of these complications appears to affect filter function adversely.


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