Discussion.
The aim of this study was to evaluate the influence of clinical experience on the time required for shaping root canals with the ProFile rotary instrumentation technique. In a previous study, Mandel et al. (1999) identified the events that could explain instrument fracture during the shaping of root canals with ProFile instruments. They showed that, when other factors such as geometry of the canal, instrument sequence and rotary speed were constant, the operator’s ability and experience were important factors in instrument failure. That study also identified two different periods: the ‘learning period’ and the ‘application period’ (see Fig. 2).
The present study was based on the premise that the most important factor was not the duration of each instrument passage but its length of penetration. The advantage of this experimental method was to render the shaping ability of the instrument uniform for all operators. Indeed, the ‘penetration level’ factor tended to consider the pressure applied on the instrument during shaping as being important. This pressure can obviously differ between various operators. However, ProFile instruments are more efficient with a decreasing movement which optimizes deeper penetration in the canal. This allows the operator’s own pressure to be eliminated. In the present study, besides the preparation time for each simulated canal, we also studied the evolution of operating time during the preparation of 25 samples in relation to the level of clinical experience of each practitioner. The results showed variations of preparation time according to the clinical expertise of the operator.
For experienced operators A and B, distribution of operating times above the mean for the first 12 canals did not statistically differ from operating times for the last 13 canals (P = 0.25). Whereas, the nonexperienced group (C + D) showed that greater than mean values for the first 12 canals were more numerous than for the last13 canals (P < 0.001).
In the case of operator A, there was no statistically significant correlation between the number of simulated canals already prepared and the operating times. Unlike the three other operators, this operator neither gained nor lost time between the first and the last sample.
When comparing the values obtained for the four operators during the preparation of the 25 samples (Fig.1), the results showed two distinct periods. Indeed, for two operators (C and D), the time necessary for canal preparation decreased during the preparation of the second half of the samples. The similarity was less clear for operators A and B (experienced practitioners).

Figure 1. Differences for the mean results of each operator relating to operating times.
A and B: Experienced practitioners;
C and D: Inexperienced practitioners.



Figure 2. Outcome of study in diagrammatic format.

References.

Baumann MA, Roth A (1999) Effect of experience on quality of canal preparation with rotary nickel-titanium files. Oral Surgery, Oral Medicine and Oral Pathology 88, 714-8.
Bishop K, Dummer PMH (1997) A comparison of stainless steel Flexofiles and  nickel-titanium Ni-Ti Flex files during the shaping of simulated canals. International Endodontic Journal 30, 25-34.
BouDagher FE, Yared GM (1995) Comparison of three files to prepare curved root canals. Journal of Endodontics 31, 264-5.
Esposito PT, CunninghamCJ (1995) A comparison of canal preparation with nickel-titanium and stainless steel instruments. Journal of Endodontics 21, 173-6.
Glosson CR, Haller RH, Dove SB, DelRio CE (1995) A comparison of root canal preparation using Ni-Ti hand, Ni-Ti engine driven and K-Flex endodontic instrument. Journal of Endodontics 2, 146-51.
LoddTharuni S, Parameswarn A, SukumaranVG (1996) A comparison of canal preparation using the K-Files and Lightspeed in resin blocks. Journal of Endodontics 22, 474-6.
Mandel E, Adib-Yazdi M, Benhamou LM, Lachkar T, Mesgouez C, Sobel M (1999)  Rotary Ni-Ti profile systems for preparing curved canals in resin blocks: influence of operator on instrument breakage. International Endodontic Journal 32, 436-43.
Pruett JP, Clement DJ, Carnes DL (1997) Cyclic fatigue testing of nickel-titanium endodontic instruments. Journal of Endodontics 23, 77-85.
Thompson SA, Dummer PMH (1997a) Shaping ability of ProFile 0.4 taper Series 29 rotary nickel-titanium instruments in simulated root canal. Part I. International Endodontic Journal 30, 1-7.
Thompson SA, Dummer PMH (1997b) Shaping ability of ProFile 0.4 taper serie 29 rotary nickel-titanium instruments in simulated root canals. Part II. International Endodontic Journal 30, 8-15.
Walia H, Brantley WA, Gernstein H (1988) An initial investigation of the bending and torsional properties initial root canal files. Journal of Endodontics14, 346-51.
Zmener O, Balbachan L (1995) Effectiveness of nickel-titanium files for preparing curved root canals. Endodontics and Dental Traumatology11, 121-3.
ZuoloMI,Walton RE (1995) Instrumentation deterioration with usage: nickel-titanium versus stainless steel. Quintessence International 28, 397-40.