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What is Hedstrom ﬁle having a double helix cross-section and a variation of the Uniﬁle is: and explain in detail?
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