Biomechanical impact of different isthmus positions in mandibular first molar root canals: a finite element analysis.

OBJECTIVE: This study used image-based finite element analysis (FEA) to assess the biomechanical changes in mandibular first molars resulting from alterations in the position of the root canal isthmus. METHODS: A healthy mandibular first molar, characterized by two intact root canals and a cavity-free surface, was selected as the subject. A three-dimensional model for the molar was established using scanned images of the patient's mandibular teeth. Subsequently, four distinct finite element models were created, each representing varied root canal morphologies: non-isthmus (Group A), isthmus located at the upper 1/3 of the root (Group B), middle 1/3 of the root (Group C), and lower 1/3 of the root (Group D). A static load of 200 N was applied along the tooth's longitudinal axis on the occlusal surface to simulate regular chewing forces. The biomechanical assessment was conducted regarding the mechanical stress profile within the root dentin. The equivalent stress (Von Mises stress) was used to assess the biomechanical features of mandibular teeth under mechanical loading. RESULTS: In Group A (without an isthmus), the maximum stress was 22.2 MPa, while experimental groups with an isthmus exhibited higher stresses, reaching up to 29.4 MPa. All maximum stresses were concentrated near the apical foramen. The presence of the isthmus modified the stress distribution in the dentin wall of the tooth canal. Notably, dentin stresses at specific locations demonstrated differences: at 8 mm from the root tip, Group B: 13.6 MPa vs. Group A: 11.4 MPa; at 3 mm from the root tip, Group C: 14.2 MPa vs. Group A: 4.5 MPa; at 1 mm from the root tip, Group D: 25.1 MPa vs. Group A: 10.3 MPa. The maximum stress in the root canal dentin within the isthmus region was located either at the top or bottom of the isthmus. CONCLUSION: A root canal isthmus modifies the stress profile within the dentin. The maximum stress occurs near the apical foramen and significantly increases when the isthmus is located closer to the apical foramina.

Pilocarpine improves submandibular gland dysfunction in irradiated rats by downregulating the tight junction protein claudin-4.

OBJECTIVES: To investigate the effects of radiation on paracellular pathway of rat submandibular glands (SMGs) and the mechanism of increasing secretion following treatment with pilocarpine. MATERIALS AND METHODS: In situ irradiation models of SMGs in Wistar rats were conducted, and the glands were exposed to X-radiation at a single dose of 20 Gy. Pilocarpine was intraperitoneally injected 60 min prior to radiation and continuous 6 days postirradiation for a total of 7 days. Salivary secretion, histological changes, pro-inflammatory cytokines, alterations in tight junctions (TJs), and functional membrane proteins aquaporin-5 (AQP5) and claudin-4 mediated by the muscarinic acetylcholine M3 subtype receptor were determined at 1 and 12 weeks after irradiation. RESULTS: Salivary secretion of the irradiated glands was reduced at 1 and 12 weeks. As well, acinar cell numbers, TJ width, and the levels of M3 receptor and AQP5 were decreased. In contrast, tumor necrosis factor-α, interleukin 6, interleukin 1α, and the expression of the TJ protein claudin-4 were significantly increased in irradiated SMGs. Notably, all the alterations were attenuated by pilocarpine treatment. CONCLUSIONS: Pilocarpine could improve the secretory function of irradiated rat SMGs via reducing inflammation, ameliorating the structural injury of TJs, and attenuating the up-regulation of claudin-4 expression.

[Damage on paracellular pathway and effect on tight junction protein claudin-4 in rat parotid glands caused by ionizing radiation].

PURPOSE: To investigate the effect and the potential mechanism of ionizing radiation on secretory function and tight junction (TJ) protein claudin-4 in paracellular pathway of rat parotid glands. METHODS: Twenty four 8-week-old male Wistar rats were randomly divided into one control group(n=6) and three irradiation groups (i.e., 1-week group post-irradiation, 4-week group post-irradiation, and 12-week group post-irradiation, 6 rats in each group). The experimental glands of irradiation groups were exposed to X-radiation in one-time single doses of 20 Gy. The residual salivary secretion of parotid glands was measured by Schirmer's test. The pathological changes of gland tissues were observed under light microscope after hematoxylin-eosin(H-E) staining. The changes of TJs ultrastructure were observed under transmission electron microscopy (TEM). Immunofluorescence staining and Western blot were used to detect the expression levels of muscarinic acetylcholine M3 subtype receptor, aquaporin 5 (AQP5), and claudin-4 proteins. The experimental results were analyzed with SPSS 23.0 software package for one-way analysis of variance. RESULTS: The residual salivarysecretion of irradiation group glands at 1, 4, and 12 weeks after irradiation was reduced compared with that of the control group(P<0.05), and the residual salivary secretion of irradiation group at 12 weeks was significantly lower than that at 4 weeks(P<0.05). Histologically, the dilation and congestion of interstitial vessels were observed at early stage after irradiation, and significant reduced number of acinar cells was found at late stage(P<0.05). In the irradiation groups, the ultrastructures of TJ were fuzzy, the electron density was decreased, and the TJ width at 1, 4, and 12 weeks was reduced compared with that in the control group. Immunofluorescence staining and Western blot indicated that the protein expression levels of M3 and AQP5 were down-regulated; however, the protein expression levels of claudin-4 were significantly increased at 1, 4, and 12 weeks after irradiation. CONCLUSIONS: After ionizing radiation, decreased secretory function of paracellular pathway, alterations in TJ structures, and up-regulation of claudin-4 expression may be involved in the mechanism of hyposecretion in rat parotid glands after irradiation.

Effect of ionizing radiation on the secretion of the paracellular pathway in rat submandibular glands.

OBJECTIVES: This study aims to investigate the effects of ionizing radiation on the secretion of the paracellular pathway in rat submandibular glands (SMGs) and reveal the changes in the tight junction (TJ) protein claudin-4. METHODS: A total of 24 Wistar rats were randomly divided into control and irradiation groups. The irradiation groups were further divided into 1, 4, and 12 weeks groups after irradiation. One-time 20 Gy irradiation was given to the SMG area on the experimental side of the irradiation group. At 1, 4, and 12 weeks after irradiation, the secretion of SMGs was measured using the Schirmer's test. The pathological changes in the gland tissues were observed under light microscopy after hematoxylin⁃eosin (HE) staining. The changes in the TJ ultrastructure were observed under transmission electron microscopy. The immunofluorescence staining and Western blot were used to detect the expression levels of muscarinic acetylcholine M3 receptor, aquaporin 5 (AQP5), and claudin-4 protein. RESULTS: At 1, 4, and 12 weeks after irradiation, the secretion of SMGs in the irradiation group was significantly decreased and lower than that in the control group (P<0.01). At 1 week, the interstitial edema was observed in SMG tissues. Nuclear pyknosis, decreased number of acinar cells, and small focal necrosis with inflammatory infiltration were also observed over time. However, these changes were most evident at 12 weeks after irradiation. In the irradiation group, the TJ ultrastructure of glands at different times appeared to be fuzzy, collapsed, and had decreased electron density. Moreover, the width of TJs was remarkably decreased (P<0.01). The expression levels of M3 and AQP5 were decreased in a time-dependent manner, and the fluorescence intensity was significantly reduced after irradiation. However, the expression levels and fluorescence intensity of claudin-4 were enhanced in different degrees. CONCLUSIONS: The changes in the TJ structure, the upregulation of the claudin-4 expression, and the damage in the paracellular pathway were involved in the hyposecretion of SMGs after irradiation.

Effect of ischemic preconditioning on radiation damage to the submandibular gland in rats.

To investigate the effects of radiation on rat submandibular glands and the possible protective effects of ischemic preconditioning, the submandibular glands of Wistar rats were subjected to in situ radiation after ischemic preconditioning. The glands were exposed to X-radiation at a single dose of 20 Gy. Ischemic preconditioning was achieved by three min of ischemia and three min of reperfusion, repeated three times before irradiation. Salivary secretion, histological changes, alterations in tight junctions, and the levels of oxidative stress, pro-inflammatory cytokines, and water secretion proteins mediated by the muscarinic acetylcholine M3 subtype receptor were determined at 1 and 12 weeks post-irradiation. In glands subjected to irradiation only, the secretion, superoxide dismutase activity, tight junction width, acinar cell number, and M3 receptor and aquaporin-5 levels were lower at 1 and 12 weeks than seen in the ischemically preconditioned irradiated glands. In contrast, tumor necrosis factor-α, malondialdehyde, myeloperoxidase activity, and the expression of the tight junction protein claudin-4 were significantly higher in the irradiated only glands. Our study revealed that radiation caused a series of injury-stress responses, especially damage to the water secretion pathway mediated by the M3 receptor that ultimately led to hyposecretion, which might play an important role in the dysfunction of the irradiated only glands. Ischemic preconditioning reduced the radiation-induced injury to submandibular glands and ameliorated salivary hyposecretion.

Study of root canal isthmus dentin structure and mineral content and microhardness / 口腔疾病防治

Objective@#To investigate the differences in dentin structure, mineral content and microhardness of single premolar roots with or without an isthmus.@*Methods @#Thirty single-root premolars with a canal isthmus (experimental group) and thirty single-root premolars without a canal isthmus (control group) were collected and analyzed by scanning electron microscopy, X-ray energy spectrometry and a Vickers microhardness tester. Structure, mineral content and microhardness were compared. Results In the experimental group, the dentinal tubules were not obvious, and the structure was disordered. In the control group, the dentin tubules of the root canal wall were arranged regularly, and the lumen opening was clear. The Ca and P contents and Ca/P ratio of the experimental group were lower than those of the control group (P < 0.05). The microhardness of the experimental group was less than that of the control group (P < 0.05).@* Results@# In the experimental group, the dentinal tubules were not obvious, and the structure was disordered. In the control group, the dentin tubules of the root canal wall were arranged regularly, and the lumen opening was clear. The Ca and P contents and Ca/P ratio of the experimental group were lower than those of the control group (P < 0.05). The microhardness of the experimental group was less than that of the control group (P < 0.05).@*Conclusion@#The dentin structure of the root canal isthmus was changed, and the Ca, P contents and microhardness were low. The root canal isthmus was a weak link in the root, which could weaken the resistance of the hard tissue of the root tooth, which may be one of the important factors that could cause vertical root fracture.