Hedstrom ﬁle having a double helix cross-section and a variation of the Uniﬁle is:
Modification of Hedstrom ﬁle-
Hyﬂex ﬁle. It has a cross-section which presents an “s” shaped instead of the traditional single-helix  teardrop cross-section of the Hedstrom ﬁle.
Uniﬁles. They are machined from round stainless steel wire by cutting two superﬁcial grooves to  produce ﬂutes in a double-helix design. They resemble the Hedstrom ﬁle in appearance, are less  eﬃcient. They are longer marketed
S-ﬁles. It has a double-helix cross-section and is a variation of the uniﬁle.
Textbook of Endodontics Nisha GARG 3rd ED
Hyflex file
S-file
Unifiles
All of the above