In continuous C-shaped canals XP4 prepares more of the wall than ProTaper Next

The same anatomy, two different risk profiles

The C-shaped root canal in mandibular second molars poses a particular problem for endodontics: thin walls, pronounced curvature and an unpredictable cross-section. A micro-CT study in the journal Clinical and Experimental Dental Research compared two nickel-titanium rotary systems, XP4 and ProTaper Next, on a sample that removes the usual confounder of such research — the difference between individual teeth.

The authors took two extracted mandibular second molars with C-shaped canals, one with a continuous cross-section (Melton category I) and one with a discontinuous cross-section (category II/III), and produced replicas by 3D printing; the fidelity of the replicas was verified by superimposition on the original. Sixteen replicas were made per specimen, randomly divided into two groups of eight, and preparation followed the manufacturers' instructions. Micro-CT scans before and after preparation were compared by three-dimensional registration, with measurement of the change in volume and surface area and of the proportion of unprepared canal wall.

In the continuous C-shaped canals ProTaper Next removed significantly less material — 2.7 mm³, or 7.4% of the initial volume, against 4.3 mm³, or 11.6%, for XP4. At the same time XP4 left a significantly smaller proportion of unprepared surface, meaning it followed the irregular cross-section better. In the discontinuous canals the proportions of unprepared surface were comparable, but the patterns of material removal and the risks differed substantially. Complications appeared exclusively in the discontinuous anatomy: two cases of critical wall thinning with ProTaper Next, and two instrument fractures with XP4.

Limitations

This is an in vitro study on resin replicas of two teeth, so the results do not transfer directly to dentine or to the range of anatomies seen in practice; the groups are small (eight replicas per system), and clinical treatment outcomes were not measured.

For your practice

• Before preparing a mandibular second molar with C-shaped anatomy it is worth establishing from the cross-section whether the canal is continuous or discontinuous — on these data that changes both the choice of instrument and the type of risk.
• In discontinuous configurations both systems carry risk, but a different one: in this sample ProTaper Next thinned the wall critically twice, and XP4 fractured twice.
• Dentine thickness and curvature, not habit, should guide the choice of system — that is the authors' conclusion, and the measurements support it for these two anatomies.