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Background: The most severe complications following soft tissue filler injections result from the intraarterial administration of the filler product. Although hyaluronic acid-based filler can be trans-arterially dissolved with hyaluronidase, no information is available on calcium hydroxylapatite (CaHA)-based fillers., Objective: The authors sought to test whether CaHA-based fillers can be trans-arterially dissolved by sodium thiosulfate (STS) when evaluated in cadaveric and in vitro models., Methods: Human cadaveric facial arterial segments were each filled with 0.2 cc of commercially available CaHA product and submerged for 24 hours in 4 different STS-containing solutions: 10 cc STS (300 mg/cc) (pure, 1:1 dilution, 1:2 dilution), 0.9% saline and 10 cc STS (300 mg/cc), and 300 IU (bovine) hyaluronidase in a 1:1 ratio., Results: Intraarterial CaHA was detected in human facial artery segments after 24 hours independent of the STS concentration employed. Submerging the arterial segments in STS (300 mg/cc) and 300 IU (bovine) hyaluronidase (1:1 ratio) also did not dissolve the intraarterial CaHA product. Gray scale analyses did show, however, that increasing concentrations of STS resulted in increased disintegration of CaHA in an in vitro experimental setting., Conclusions: The results of this study indicate that STS is limited in its potential to dissolve intraarterial CaHA of cadaveric human facial arteries, despite the fact that it appears effective when in direct contact with the CaHA. Adverse events caused by intraarterial administration of CaHA-based fillers still lack a suitable antidote., (© 2021 The Aesthetic Society. Reprints and permission: journals.permissions@oup.com.)