The aim of this study was to evaluate root canals instrumented by dental students using the modified double-flared technique, nickel-titanium (NiTi) rotary System GT files and NiTi rotary ProTaper files by micro-computed tomography (MCT). A total of 36 root canals from 18 mesial roots of mandibular molar teeth were prepared; 12 canals were prepared with the modified double-flared technique, using K-flexofiles and Gates-Glidden burs; 12 canals were prepared using System GT and 12 using ProTaper rotary files. Each root was scanned using MCT preoperatively and postoperatively. At the coronal and mid-root sections, System GT and ProTaper files produced significantly less enlarged canal cross-sectional area, volume and perimeter than the modified double-flared technique (P < 0.05). In the mid-root sections there was significantly less thinning of the root structure towards the furcation with System GT and ProTaper (P < 0.05). The rotary techniques were both three times faster than the modified double-flared technique (P < 0.05). Qualitative evaluation of the preparations showed that both ProTaper and System GT were able to prepare root canals with little or no procedural error compared with the modified double-flared technique. Under the conditions of this study, inexperienced dental students were able to prepare curved root canals with rotary files with greater preservation of tooth structure, low risk of procedural errors and much quicker than with hand instruments.

AIM To investigate the prevalence of root canal isthmuses in the apical 5 mm of the mesial root of mandibular molars by means of micro-computed tomography (MCT) and to describe the morphology of the isthmuses. METHODOLOGY Twenty extracted mandibular first molars that had fully formed roots were selected. The mesial roots were sectioned from the distal roots and subjected to MCT. The number of sections showing isthmuses in each of the apical 5 mm of the root canals was recorded. In each one of the apical 5 mm of the 20 roots examined, 40 sections were observed, giving a total of 800 sections observed in each group. Data were analysed as a contingency table using the chi-square statistic to test the null hypothesis that location of the sections in each of the apical 5 mm and presence of the isthmus were independent. RESULTS Isthmuses were found to be present at all levels with prevalence figures between 17.25 and 50.25%. The chi-square test indicated a significant difference in the distribution of isthmuses with section (P = 0.001). It was found that sections in the first millimetre from the apex had fewer isthmuses than expected and that sections in the third millimetre from the apex had more isthmuses than expected under the null hypothesis. Calcifications were found to be present in most isthmuses, occasionally lateral canals originated from the central part of the isthmuses. CONCLUSIONS Isthmuses were present in the vast majority of roots observed. The third millimetre from the apex showed more isthmuses than expected. The results of clinical and surgical endodontic procedures performed in the mesial root of mandibular molars may be affected by this aspect of the root canal anatomy.

Reduced tillage management is being adopted at an accelerated rate on the Canadian prairies. This may influence soil quality and productivity. A study conducted on a clay soil (Udic Haplustert) in southwestern Saskatchewan, Canada, to determine the effects of fallow frequency [fallow-wheat (F-W) vs. continuous wheat (Cont W)] and tillage [no-tillage (NT) vs. conventional (CT) or minimum tillage (MT)] on yields of spring wheat (Triticum aestivum L.), was sampled after 3, 7 and 11 years to assess changes in selected soil quality attributes. Tillage had no effect on amount of crop residues returned to the land, but the tilled systems had significantly (P<0.05) lower total organic C and N in the 0-7.5 cm soil depth, though not in the 7.5-15 cm depth. Further, these differences were observed after only 3 years and persisted for the entire II years of the study. For example, in the 0-7.5 cm depth, organic C in F-W (MT) after 3 years was 10 480 kg ha(-1) and in F-W (NT) 13 380 kg ha(-1), while in Cont W (CT) and Cont W (NT) corresponding values were 11 310 and 13 400 kg ha(-1), respectively. After 11 years, values for F-W (MT) and F-W (NT) were 11 440 and 14 960 kg ha(-1), respectively, and for Cont W (CT) and Cont W (NT), 12 970 and 16 140 kg ha(-1), respectively. In contrast to total organic matter, two of the more labile soil quality attributes [i.e., C mineralization (C-min) and N mineralization (N-min)] did not respond to fallow frequency until after 7 years and only in the 0-7.5 cm depth. Microbial biomass (MB) and the ratio of C-min to MB [specific respiratory activity (SRA)], two attributes also regarded as labile, were not influenced by the treatments even after 11 years. After 11 years, only C-min and N-min among the labile soil quality attributes responded to the treatments. Surprisingly, the labile attributes were no more sensitive to the treatments than was total organic C or N. More research is required to determine why responses in this soil differed from those reported elsewhere. (C) 1998 Elsevier Science B.V. All rights reserved.

The economic performance of continuous wheat (Triticum aestivum L.) and fallow-wheat rotations grown under conventional, minimum- and zero-tillage management practices on silt loam, sandy loam and heavy clay in southwestern Saskatchewan was determined during the relatively dry period of 1982–1988. The costs and returns for each rotation-tillage system were evaluated annually based on 1989–1990 price and cost conditions, and for various other plausible scenarios. Gross returns on silt loam were higher for continuous wheat (average 228 $ ha−1) than for fallow-wheat systems (average 155 $ ha−1). On the sandy loam, gross returns were similar for all cropping systems (average 112 $ ha−1); on the heavy clay, they were higher for fallow-wheat than for continuous wheat (139 versus 119 $ ha−1). Conservation tillage management increased gross returns over that obtained with conventional tillage only in years when growing season temperatures were high and precipitation was poorly distributed, or when the 21-month summerfallow period was droughty. On silt loam, gross returns were significantly lower with conservation tillage in as many as 3 of 7 years. On silt loam, net returns were highest for conventionally tilled continuous wheat when wheat prices were> 175 $ t−1; at lower wheat prices, conventionally tilled fallow-wheat was the most profitable. On the other soils, minimum- and zero-tillage fallow-wheat provided the highest net returns at all wheat prices tested, with minimum tillage being slightly better at low wheat prices, but at these sites conventionally tilled fallow-wheat was not studied. The cost of production was highest for continuous wheat and for zero-tillage management. For fallow-wheat systems, conservation tillage required lower expenditures than conventional tillage for fuel, labor, machine repair and machine overheads; costs for minimum tillage averaged 9 $ ha−1 and for zero tillage 15 $ ha−1 lower on the silt loam. These savings were more than offset by increased herbicide costs which averaged 26 and 64 $ ha−1 higher for minimum-tillage and zero-tillage systems, respectively. We concluded that producers in southwestern Saskatchean who are motivated primarily by short-term profit will find little incentive to adopt conservation tillage systems for spring wheat production, unless they are situated on soils that have already incurred severe soil loss or the soils are highly prone to further erosion losses.

In semi-arid Saskatchewan, the major incentives for adoption of conservation tillage are water and soil conservation. Zero and minimum tillage systems for spring wheat (Triticum aestivum L. or Triticum turgidum L.) grown continuously and on summer fallow were studied on sandy loam, silt loam, and clay soils between 1982 and 1988. Effects on water recharge, soil fertility, crop yields, and soil erosion risks were measured. Zero-tillage fallow reduced evaporative losses during dry summers and conserved 8% more spring soil water after 21 months compared with conventional fallow (232 mm vs. 214 mm) on the silt loam soil; minimum tillage fallow conservea an extra 3% spring soil water (225 vs. 214 mm). Soil nitrogen fertility under zero-tillage fallow was generally lower than under conventional tillage in the first years of the study, but these trends disappeared with time. Yields of spring wheat grown on fallow on the silt loam soil were greater with zero-tillage (1477 kg ha−1) compared with conventional tillage (1198 kg ha−1) in two dry years, lower in one year (1248 vs. 1558 kg ha−1, respectively), and otherwise equal (2725 kg ha−1 on average). Grain yields on minimum tillage fallow were generally equal to or lower than with zero tillage on all soil textures. When spring wheat was grown continuously, yields were similar whether sown directly in stubble or after preseeding tillage. Elimination of some or all tillage operations for fallow and prior to seeding sufficed to maintain the fraction of wind-erodible aggregates (diameter < 0.84 mm) below the 60% threshold value in most years on the silt loam and clay soils. Based on crop residue conservation and soil aggregation, only chemical fallow can ensure adequate wind erosion protection for coarse- or fine-textured soils in low wheat yield areas, while minimum tillage can provide sufficient wind erosion protection on a silt loam.