Physical and mechanical characterization and the influence of cyclic loading on the behaviour of nickel-titanium wires employed in the manufacture of rotary endodontic instruments

AIM: To analyse the influence of cyclic loading on the mechanical behaviour of nickel-titanium (NiTi) wires employed in the manufacture of ProFile rotary endodontic instruments. METHODOLOGY: Nickel-titanium wires, 1.2 mm in diameter, taken from the production line of ProFile rotary endodontic instruments before the final machining step, were tensile-tested to rupture in the as-received condition and after 100 load-unload cycles in the superelastic plateau (4% elongation). The wires were characterized by X-ray energy-dispersive spectroscopy, X-ray diffraction and by differential scanning calorimetry and compared with new size 30, .06 taper ProFile instruments. The fracture surfaces of the wires were observed by scanning electron microscopy. RESULTS: The mechanical properties of the as-received wires, their chemical composition, the phases present and their transformation temperatures were consistent with their final application. Only small changes, which decreased after the first few cycles, took place in the mechanical properties of the cycled wires. The stress at maximum load and the plastic strain at breakage remained the same, while the critical stress for inducing the superelastic behaviour, which is related to the restoring force of the endodontic instruments, decreased by approximately 27%. CONCLUSIONS: The mechanical behaviour of the NiTi wires was modified slightly by cyclic tensile loading in the superelastic plateau. As the changes tended towards stabilization, the clinical use of rotary NiTi ProFile instruments does not compromise their superelastic properties until they fracture by fatigue or torsional overload, or are otherwise discarded.     

Assessing sealant retention using a Poisson frailty model

OBJECTIVE: This paper utilises a Poisson frailty regression model with clustering to report on the failure rates of sealants placed and repaired during a three-year follow-up school-based comprehensive preventive programme. BASIC RESEARCH DESIGN: The programme included annual application/reapplication of light-cured fissure sealants in 210 12-year-old subjects over three years. Sealants placed at the commencement of each year of the programme were assessed for retention 12 months later and repair/reapplication was undertaken at that time for those surfaces judged to require it. An application of a Poisson regression model with random effects ("frailties") and clustering was used in this paper to allow for heterogeneity between subjects. Whilst it appears that the direct utilization of frailty models in determining fissure sealant failure rates has not been reported, it is clearly an appropriate use of this statistical method. A total of 1,544 sealants were placed during the study period. Of these, 1,038 (67.2%) were first placements at baseline and 506 (32.8%) were repairs/reapplications. Of these repairs/reapplications, 206 (40.7%) were performed at first year follow-up and 300 (59.3%) at second year follow-up. RESULTS: The Failure Rate Ratios were statistically significantly greater for second molars cf. first molars (FRR=1.33, p=0.008) and for lower molars cf upper molars (FRR=1.32, p<0.001). Failure rate ratios for sealants placed in the distal occlusal pit and fissures cf. mesial occlusal pit and fissures were significantly higher (FRR=1.33, p<0.001). A significant interaction was found for molar type versus pit and fissure site (p=0.009). There was no significant interaction between molar type and arch. CONCLUSIONS: Sealants placed on second molars, those placed on lower molars and those placed on the distal fissure sites have a higher failure rate. As individual tooth sites in the mouth are not independent, any statistical analysis should allow for these associations when assessing sealant retention.     

The effect of sectioning the trigeminal sensory root on the periodontally-induced jaw-opening reflex

The experiment was designed to determine the pathway taken to the brain stem by periodontal afferents responsible for the digastric jaw-opening reflex induced by tooth-tapping. Cutting the trigeminal sensory roots of anesthetized decerebrate cats eliminated the ipsilateral periodontally-induced reflex, although the stretch reflexes of the jaw-closing muscles were undiminished. These results suggest that periodontal afferents causing the jaw-opening reflex reach the brainstem through the Vth sensory root, and confirm that muscle spindle afferents travel through the Vth motor root.     

Relationship between periodontal furcation involvement and molar cervical enamel projections

A total of 78 individuals ages 21 to 61 years with periodontal furcation involvement was examined for the presence of cervical enamel projections on the buccal surfaces of molar teeth. The furcal defects and cervical enamel projections (CEPs) of molars were diagnosed by probing, periapical roentgenographs, flap operation and inspection. Plaque index (PlI) and gingival index (GI) were recorded for the buccal and lingual surfaces of molars examined. The percentage of CEPs in the 78 individuals examined was 67.9%. The prevalence of CEPs in all molars examined was 45.2%. The prevalence of CEPs in molars with and without furcal involvements were 82.5% and 17.5%, respectively. The frequency of CEP in molars occurred in the following order: mandibular first molars, maxillary first molars, mandibular second molars and maxillary second molars. Statistical analyses (Chi-square test) revealed a significant difference between periodontal furcation involvements and the presence of CEPs. Results of this study also indicated that the furcal involvements with CEPs were associated with poor oral hygiene as measured by GI and PlI.     

Squamous cell carcinoma arising in the lining of odontogenic cysts. Report of 5 cases

The lining of odontogenic cysts seldom gives rise to the development of a carcinoma. In a review of the literature by Gardner in 1975, 25 cases were accepted as such. In the present paper, 5 new cases are reported. These cases were encountered during a 14-year-period in which 292 oral carcinomas were registered, emphasizing the rarity of carcinomas arising from odontogenic cysts.     

Inhibition of plaque growth by zinc salts

Certain metal salts inhibit the growth of dental plaque. The effect of zinc salts was investigated because of contradictory reports in the literature. Using cultures of human plaque, artificial dental plaques were grown in vitro on aluminium strips. The effect of various zinc salts was examined by treating the strips intermittently with solutions of the salts during the period of growth. The action of mouthwashes containing zinc salts in vivo on plaque growth was determined in human volunteers.
All the water-soluble zinc salts that were tested gave a similar concentration-dependent inhibition of plaque growth in vitro . The effectiveness of the treatment was reduced by increasing the time allowed for growth after the final application of zinc.
Solutions of zinc (17–19 mM) as the citrate or phenolsulphonate salt, gave about 30% reduction in the extension of plaque along the gingival margin in vivo over 16 hours. Subjects with high rates of plaque growth with the placebo mouthwash were most affected by zinc salts. Increasing the zinc concentration to 30 mM did not further increase the activity. Previous reports of low activity of zinc against plaque growth in vivo are explained by too low a concentration of zinc or too low a frequency of application. The results confirm that zinc salts, including zinc citrate, inhibit the growth of plaque in vivo and in vitro .     

Layered silicone rubber technique for flasking removable partial dentures.

A method for flasking removable partial dentures using a layered silicone rubber technique has been described. The silicone investment protects the stone teeth from fracture during removal of the removable partial denture from the investment. Then the occlusal changes can be accurately corrected following processing.     

A survey of human genetics in the dental school curriculum.

A survey of American dental schools was conducted to determine the extent to which genetics is currently taught. Results indicate that there is a stronger emphasis at the postdoctoral level than at the predoctoral level and that the approach to teaching genetics tends to multidisciplinary. Some dental schools have already given organizational status to genetics and have instituted clinical activities in orofacial or craniofacial genetics. Considerable interest was shown in audiovisual materials and autotutorial programs in genetics. A greater emphasis on the teaching of genetics at the predoctoral, postdoctoral, and continuing education level is recommended. More trained orofacial geneticists are needed to support didactic and clinical activities within the dental schools.