Instrument failure.
During the preparation of the curved canals, none of the stainless steel K-Flexofiles, and five K3 (two 0.04 taper size 30 and three 0.04 taper size 35) nickel-titanium instruments separated (w2 = 1.733, P = 0.191). All nickel-titanium instruments separated at the tip region (Fig.1).

Canal cleanliness.
The scores for debris and smear layer are detailed in Tables 2 and 3. Completely cleaned root canals were never found. On average, more effective cleaning was observed in the coronal and middle thirds of canals (Fig. 2).
In general, the use of K-Flexofiles resulted in significantly less debris (P < 0.001) compared to canal preparation with K3 instruments (Table 2). In terms of smear layer (Fig. 3), the K-Flexofiles resulted in 24.4% and the K3 system in 17.3% of specimens having scores 1 and 2 (Table 3); no statistically significant differences were apparent (P > 0.05).

Instrumentation results.
The mean time taken to prepare the canals with the two types of instruments is shown in Table 4. There were no statistical significant differences between the two instruments (P = 0.274).
All canals remained patent following instrumentation, thus none of the canals were blocked with dentine. With both types of instruments, one canal showed overextension of preparation, whereas a loss of working distance was found in two canals prepared with K3 and four canals enlarged with K-Flexofiles. The mean changes of working length that occurred with the different instruments are listed in Table 4. The differences between the two instrument types were not statistically significant (P = 0.544).
The mean straightening of the curved canals is shown in Table 5. The use of K3 instruments resulted in significantly less straightening (1.368) during instrumentation (P < 0.0001) compared to the K-Flexofiles (6.918; Fig. 4).

Figure 1. Separated K3 file in the apical portion of a curved canal. Notice the agglomeration of debris (original magnification 40x).



Table 2. Summary of scores for debris.



Table 3. Summary of scores for smear layer.



Figure 2. Canal wall after preparation with K3 rotary nickel-titanium instruments.
(a) Nearly clean canal wall with small agglomerations of debris particles in the middle portion of the prepared canal (score2, original magnification 40x).
(b) Apical portion of the canal: complete or nearly complete covering of the canal wall by debris after preparation (score 5, original magnification 40x).



Figure 3. Canal wall after preparation with K3 rotary nickel-titanium instruments: small amount of smear layer and some open dentinal tubules (score 2, original magnification 640x).



Table 4. Mean preparation time (min) and SD and mean changes of working distance (mm) and SD with the two different instruments.



Table 5. Mean degree of straightening of curved canals (8) and SD after canal preparation with the two different instruments (n = 30 canals in each group).



Figure 4. Straightening of the curved canals after preparation with the two different instruments (n = 24 canals in each group): combined box-and-whisker and dot plot, each dot represents a reading of the difference between canal curvature prior to and after instrumentation.