Background: The effect of HydroSoft coils on the prevention of recanalization and thrombosis after embolization is unclear. We herein report the results of the single-armed prospective Japanese HydroSoft Registry. Methods: Aneurysms with a diameter of <10 mm that were treated with a ≥50% length of HydroSoft coils were registered. We evaluated the safety and recanalization rate and analyzed the factors related to their recanalization and thrombosis 1 year later. Results: In total, 122 aneurysms were registered. Their mean maximum diameter and neck length were 6.4 and 3.9 mm, respectively. The mean length of the HydroSoft coils was 84.3%. No intracranial hemorrhage occurred, but 2 patients developed minor ischemic strokes. Angiographic examination immediately after the procedure showed complete obliteration, neck remnant (NR), and body filling (BF) in 20 (16.4%), 32 (26.2%), and 67 (54.9%) cases, respectively. One-year follow-up angiography showed complete obliteration, NR, and BF in 68 (55.7%), 15 (12.3%), and 15 (12.3%) cases, respectively, and 5 aneurysms (4.1%) were recanalized (4 and 1 with BF and NR as their initial angiographic result, respectively). Another 11 aneurysms still showed BF, although their thrombosis was promoted. No significant factors related to recanalization were identified. A high volume embolization ratio and small neck were significantly associated with thrombosis 1 year after embolization with HydroSoft coils. Conclusions: The safety and prevention of recanalization 1 year after the treatment appeared acceptable. The high volume embolization ratio associated with HydroSoft coils could induce progression of thrombosis for aneurysms characterized by NR and BF during the follow-up period.
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