Cross-SectionProTaper NEXT and Instruments of Critical Path are a unique set of nickel-titanium endodontic instruments, which form a helical wave within the root canal and cut on a precessional axis. This feature differentiates these files from all other endodontic files. More than a dozen prototypes were produced and tested by Maillefer or Dentsply International (Ballaigues, Switzerland) over an 8 year period, and are nominally referred to as X-Files or Swaggering Files.
Numerous patents describe the “critical path” of the root canal space and a theoretical or “critical set” of instruments, whereby the diameter of the files increases as a function of the expansion of the area of a circle. This feature distributes the work requirement for each instrument more evenly, when compared to the ISO file size system, and designed to mitigate cyclic fatigue and instrument failure.

The cutting edges of the flutes are sharp without radial lands. The transverse cross-sections of the instruments are rectilinear and bi-symmetrical, which are off-set, placing the center of mass of the instrument at a specified distance away from the axis of rotation. The off-set center of mass allows the instruments to cut via a helical wave and on a precessional axis, which mimics Schilder’s Envelope of Motion. These designs create cutting envelopes, which are wider than the cross-sectional areas of the instruments themselves. The reduction in the cross-section area of the file, improves instrument flexibility, and facilitates the preparation of severely curved and complex canals. The critical path sequence, improved flexibility and precessional cutting axis enables the operator to create minimally invasive preparations with only 2 or three instruments.
These designs also display wider clearance angles (see video above), which improves hauling capacity, and the unique feature of compressibility, which contributes to the safety and versatility of the file. Other applications for the designs are orthopedic drills and industrial and directional boring.