Root and canal morphology of permanent teeth showed close associations with age and gender [21,22]. The pulp-dentinal complex change over the course of a lifetime with physiological deposition of secondary dentine, which contributing to a reduction of the pulp chamber size and root canal diameter [23,24]. Consequently, canals were sharply defined and narrow, sometimes too narrow in older adults, while young patients tend to have large single canals and pulp chambers [23]. In addition, the cementum deposition with time in people and peaks in old age, resulting in a complex and changeful root morphology in old age [25]. Therefore, it is accepted that calcific changes of the pulp-dentinal complex over time pose challenges for the clinician [26].
The dangerous zone of the root canal preparation is the weakest thickness zone of the root canal wall. Root thickness tends to decrease considerably in danger zone during root canal shaping. It is particularly prone to excessive weakness and undesirable side effects [10], especially Nickel-titanium (NiTi) instruments are extensively used in endodontic treatment [27]. Stress concentration of tooth root should be concerned during the dental treatment, because it is closely related to vertical root fracture. In term of stress concentration, canal curvature seems more important than external root morphology, and that reduced dentine thickness increases the magnitude but not the direction of maximum tensile stress [28]. Versluis et al. [29] reported that external distal and mesial surfaces of roots with oval canals showed moderate stress concentrations that were minimally affected by preparations, while stress concentrations emerged on roots with round canals when preparation sizes increased. Therefore, better understanding of the danger zone anatomy may serve to decrease the risk of mishaps.
There are some reports on the radicular wall thicknesses of danger zone in MFMs, which showed that the mean thickness of dentin ranges from 0.78 to 1.27 mm, with the minimal thicknesses of 0.4 mm [3,7,9-11,13, 30]. For example, Bryant et al. [31] reported that the mean size of the danger zone for 200 canals used was 0.79 mm. Keles et al. [32] reported that the thinnest canal walls of MB canals were 1.16 ± 0.20 mm and ML canals were 1.19 ± 0.18 mm. De‐Deus et al. [33] found that the danger zone values in the MB canals varied from 0.67 to 1.93 mm with an average of 1.13 ± 0.21 mm, and in the ML canals varied from 0.77 to 1.89 mm with an average of 1.10 ± 0.21 mm, locating up to 4 mm under the furcation area. These results vary slightly because the researchers had used different methods of measuring the thickness of the root canal wall in the danger zone, and they selected different ranges of the danger zone or different human species for studies. Moreover, there is little information in the literatures correlating these measurements with other features of the teeth, such as patient’s age and gender.
In the present study, the minimal distal dentine thicknesses associated with the MB and ML canals below the furcation 1, 2, 3, 4, 5 mm of Chinese population were measured. The results showed that the minimal distal dentine thicknesses of MB and ML canals are located 3∼4 mm below the furcation for both men and women, with a mean range of 0.78∼0.80 mm, and there are no differences between MB and ML canals. The result indicated that the danger zone of MFMs is located at the same position for both men and women.
In the present study, the minimal distal dentine thicknesses of MB and ML canals were higher in men than women (P<0.05), except at 1 and 3 mm of ML canals (P>0.05). These results confirm that the minimal distal dentine thicknesses of MB and ML canals with differs between men and women. Gender is an important factor to influence the distal wall thickness of the mesial root of the MFMs. MFMs of women are more probability to strip perforation during root canal shaping and post space preparation procedures. Therefore, thinner or smaller instruments are suitable for women during endodontic treatment and post space preparation procedures.
The results of this study showed that the minimal distal dentine thicknesses of MB and ML canals increased with age in every age group in both men and women at each location (P<0.05). Age is another important factor to influence the distal wall thickness of the MRs of the MFMs. MFMs of younger people have larger canals and thinner root canal walls than these of older people.
The results of this study showed that the minimum distal dentine thickness at every location was significantly different between long teeth and short teeth both in men and women (P < 0.05), with short teeth being smallest. These results are different with previous reports by Sauáia et al. [5] and Dwivedi et al. [14] in which the distal wall thickness and distal concavity of the MR of the MFMs were found to be thinner in longer teeth compared with shorter teeth. Possible explanation is that ethnic difference is an important factor to influence the distal wall thickness of the MR of the MFMs.
The decrease of the dentine thickness is an important point during the evaluation of root canal instrumentation because excessive enlargement of the root canal space can lead to accidents such as perforations. According to Lim and Stock, 200~300 µm dentine thickness should be retained after preparation in order to withstand compaction forces during obturation and to prevent perforation or vertical root fracture [13]. Based on the results of the present study, root canal preparation in danger zone decrease dentine should not more than 0.5mm, otherwise the possibility of perforation increases. To prevent strip perforations, firstly, the selection of great taper NiTi instruments should be cautious for the "danger zone" of insufficient dentin thickness of root canal wall. Secondly, coronal flaring should be limited and instruments should be directed towards the lateral and mesial canal walls that have much thicker dentine and away from the danger zone [9]. Finally, dentists should pay more attention to shorter teeth of young women during endodontic treatment and post space preparation procedures.
This study provided a detailed description of the distal wall thickness of the MRs of the MFMs in a large sample of a Chinese population. These findings are very important for clinicians because they will help to increase the success rates for endodontic treatment and post space preparation of patients of different gender and ages. In the study, some middle mesial canals (MMCs) of MRs were found in MFMs. However, due to the small number of MMCs in every age group, no measurements were made. In addition, the danger zone was mainly towards the distal region of the roots and towards the mesial region in few MB and ML canals, so the mesial wall thickness was not measured. Further studies will be conducted to investigate these issues.