Maximum curve control

HyFlex EDM Max Curve sequence and sizes breakdown (COLTENE)

The internal anatomy of human teeth often consists of a highly complicated network of multi-planar curved and anastomotic canals. Thus, reaching the biological and design objectives of root canal instrumentation in severely curved canal systems might be extremely challenging. In this paper, Dr Antonis Chaniotis discusses a safer and more predictable instrumentation technique.

The ultimate aim of endodontic therapy is the prevention of periradicular disease or the promotion of healing response. To achieve these objectives, mechanical instrumentation and chemical disinfection are considered the basic principles (Schilder,
1974), whereas the former essentially determines the efficacy of all subsequent procedures (Peters, 2004).

For gutta-percha fillings, the shaping of the canal should satisfy the following criteria:

  • i. The shape of the main root canal resembles a continuously tapering funnel from orifice to apex
  • ii. The cross-sectional diameter of the main canals should narrow apically
  • iii. Preparation follows the original shape
  • iv. The position of the apical foramen is preserved
  • v. The apical opening should retain its dimensions as much as possible (Schilder, 1974; Hulsmann et al., 2005).

The biological objectives of root canal instrumentation are:

  • i. Confinement of instrumentation to the limits of the roots themselves
  • ii. Avoidance of extruding necrotic debris into the periradicular tissues
  • iii. Removal of all organic tissue from the main and lateral canals
  • iv. Creation of sufficient space to allow irrigation and medication by simultaneously preserving enough circumferential dentin for the tooth to function (Hulsmann et al., 2005)

Achieving the aforementioned objectives in straight canals is considered a straightforward procedure. Problems arise when canals are severely curved or even bifurcated and anastomotic (Fig. 1). In such teeth, the basic endodontic techniques and instrumentation protocols might be challenging to follow. For a safer and more predictable instrumentation, a newly introduced NiTi file sequence can be applied in the so-called TCA technique.


Based on canal curvature, Nagy et al. (1995) classified root canals into four categories:

  • i. straight or I form (28% of root canals)
  • ii. apically curved or J form (23%)
  • iii. entirely curved or C form (33%)
  • iv. multicurved or S form canals (16%)

Schafer et al. (2002) found that 84% of root canals studied were curved while 17.5% of them presented a second curvature and were classified as S-shaped. From all curved canals studied, 75% had a curvature of less than 27 degrees, 10% a curvature with an angle between 27 and 35 degrees and 15% a severe curvature of more than 35 degrees.

Traditionally, root canal curvatures were described using the Schneider angle (Schneider, 1971): root canals presenting an angle of five degree or less could be classified as straight canals; an angle between 10 and 20 degrees as moderately curved; and a curve greater than 25 degrees as severely curved.

Decades later, Pruett et al. (1997) reported that two curved root canals might have the same Weine angle, but sport totally different abruptness of curvature. To define the abruptness, they introduced the radius of a curvature: the radius of a circle passing through the curved part.

In rotary instruments, the number of cycles before failure significantly decreases as the radius of curvature decreases and the angle of curvature increases. Further attempts to mathematically describe curvatures in two-dimensional radiographs introduced parameters such as the length of the curved part (Schaffer et al., 2002) and the location as defined by curvature height and distance (Gunday et al., 2005).

Recently, Estrela et al. (2008) described a method to determine the radius of root canal curvatures using CBCT images analysed by specific software. Three categories got classified: small radius (r≤4mm), intermediate (r>4 and r≤8mm) and large (r>8mm).

The smaller the radius of a curvature is, the more abrupt the curvature becomes. All those attempts to describe the root canal curvature had one goal: to preoperatively assess the risk for transportation and unexpected instrument separation.

Continue reading here. Published in Dental Asia May June 2021 issue.