An extensive literature survey of the subject of gear dynamics is undertaken and the increasing recognition of the role of the lubricating oil film in this field, especially as a damping source, is highlighted. The oil film separating the mating surfaces of involute spur gears is analysed assuming hydrodynamic conditions, rigid teeth (as far as the film shape is concerned) and pressure dependent viscosity. Gear tooth mesh stiffness is expressed as a function of the dynamic load and the position of contact. simple model of a pair of spur gears is subjected to a transient response analysis and the behaviour of the lubricating oil film observed. According to the motions of equivalent masses of the apars under these transient conditions damping due to the oil film is determined. The numerical solutions obtained at various operating conditions are combined to form an approximate formula to predict the damping ratio in terms of the dynamic tooth load, rolling speed of the tooth surfaces and the viscosity of the lubricating oil. A digital computer simulation of the dynamic motion of the pair of gears is carried out incorporating the above damping ratio formula. The actual load sharing between the pairs of teeth (when more than one pair of teeth are in mesh), considering the tooth deflections, pitch errors, oil film thicknesses and the differences in mesh stiffnesses, is taken into account. The variations of the total maximum dynamic load and the maximum tooth load are studied under different nominal loads, contact ratios, oil viscosities and pitch errors over a wide range of speeds covering the resonance area. The variations of the dynamic load, individual tooth load, mesh stiffness and the oil film thickness during complete mesh cycles are also analysed under different operating conditions to identify particular areas where high loads and minimum film thicknesses occur. Theoretical results are compared with the experimental results obtained on a back-to-back gear test rig.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Background: Removal of teeth results in both horizontal and vertical changes of hard and soft tissue dimensions. The magnitude of these changes is important for decision-making and comprehensive treatment planning, with provisions for possible solutions to expected complications during prosthetic rehabilitation. Objectives: to review all English dental literature to assess the magnitude of dimensional changes of both the hard and soft tissues of the alveolar ridge up to 12 months following tooth extraction in humans. Materials and methods: An electronic MEDLINE and CENTRAL search complemented by manual searching was conducted to identify randomized controlled clinical trials and prospective cohort studies on hard and soft tissue dimensional changes after tooth extraction. Only studies reporting on undisturbed post-extraction dimensional changes relative to a fixed reference point over a clearly stated time period were included. Assessment of the identified studies and data extraction was performed independently by two reviewers. Data collected were reported by descriptive methods. Weighted means and percentages of the dimensional changes over time were calculated where appropriate. Results: The search provided 3954 titles and 238 abstracts. Full text analysis was performed for 104 articles resulting in 20 studies that met the inclusion criteria. In human hard tissue, horizontal dimensional reduction (3.79 ± 0.23 mm) was more than vertical reduction (1.24 ± 0.11 mm on buccal, 0.84 ± 0.62 mm on mesial and 0.80 ± 0.71 mm on distal sites) at 6 months. Percentage vertical dimensional change was 11-22 % at 6 months. Percentage horizontal dimensional change was 32% at 3 months, and 29-63% at 6-7 months. Soft tissue changes demonstrated 0.4-0.5 mm gain of thickness at 6 months on the buccal and lingual aspects. Horizontal dimensional changes of hard and soft tissue (loss of 0.1 mm to 6.1 mm) was more substantial than vertical change (loss 0.9 mm to gain 0.4mm) during observation periods of up to 12 months, when study casts were utilised as a means of documenting the changes. Conclusions: Human re-entry studies showed horizontal bone loss of 29-63% and vertical bone loss of 11-22% after 6 months following tooth extraction. These studies demonstrated rapid reductions in the first 3-6 months that was followed by gradual reductions in dimensions thereafter. / published_or_final_version / Dental Surgery / Master / Master of Dental Surgery

Background: Removal of teeth results in both horizontal and vertical changes of hard and soft tissue dimensions. The magnitude of these changes is important for decision-making and comprehensive treatment planning, with provisions for possible solutions to expected complications during prosthetic rehabilitation. Objectives: to review all English dental literature to assess the magnitude of dimensional changes of both the hard and soft tissues of the alveolar ridge up to 12 months following tooth extraction in humans. Materials and methods: An electronic MEDLINE and CENTRAL search complemented by manual searching was conducted to identify randomised controlled clinical trials and prospective cohort studies on hard and soft tissue dimensional changes after tooth extraction. Only studies reporting on undisturbed post-extraction dimensional changes relative to a fixed reference point over a clearly stated time period were included. Assessment of the identified studies and data extraction was performed independently by two reviewers. Data collected were reported by descriptive methods. Weighted means and percentages of the dimensional changes over time were calculated where appropriate. Results: The search provided 3954 titles and 238 abstracts. Full text analysis was performed for 104 articles resulting in 20 studies that met the inclusion criteria. In human hard tissue, horizontal dimensional reduction (3.79 ± 0.23 mm) was more than vertical reduction (1.24 ± 0.11 mm on buccal, 0.84 ± 0.62 mm on mesial and 0.80 ± 0.71 mm on distal sites) at 6 months. Percentage vertical dimensional change was 11-22 % at 6 months. Percentage horizontal dimensional change was 32% at 3 months, and 29-63% at 6-7 months. Soft tissue changes demonstrated 0.4-0.5 mm gain of thickness at 6 months on the buccal and lingual aspects. Horizontal dimensional changes of hard and soft tissue (loss of 0.1 mm to 6.1 mm) was more substantial than vertical change (loss 0.9 mm to gain 0.4mm) during observation periods of up to 12 months, when study casts were utilised as a means of documenting the changes. Conclusions: Human re-entry studies showed horizontal bone loss of 29-63% and vertical bone loss of 11-22% after 6 months following tooth extraction. These studies demonstrated rapid reductions in the first 3-6 months that was followed by gradual reductions in dimensions thereafter. / published_or_final_version / Dental Surgery / Master / Master of Dental Surgery

Delayed diagnosis of dental anomalies in the permanent dentition can lead to severe occlusal, functional and aesthetic problems. Early diagnosis is not feasible in the primary dentition stage without screening radiography, which is controversial due to possible mutagenic effects. However, some dental anomalies in the primary dentition of Caucasians have been found to be followed by anomalies of the permanent successors. Therefore, identifying individuals at high risk of having anomalies of their permanent teeth by screening children for dental anomalies in the primary dentition, will facilitate early diagnosis and appropriate treatment. The objectives of this study were to determine the prevalence of dental anomalies in the primary dentition of a southern Chinese child population, and to investigate the relationship between the presence of a talon cusp, missing or supernumerary tooth in the primary dentition and the number of the permanent successor teeth. A total of 1513 children from 12 kindergartens were invited to participate in the prevalence study. School children visiting a regional school dental clinic during a four-year period and having talon cusp, supernumerary tooth or congenitally missing tooth in the primary dentition were included in the study to determine the relationship to the permanent successors. A total of 1333 children, aged from 2 years 11 months to 5 years 5 months (mean age 4 years 4 months) were included in the prevalence study. The prevalence of the commonest dental anomaly, double tooth, was 4.28%. Almost 95% of which were in the mandibular anterior region; one third of the double teeth involved the central and lateral incisors while the other two-thirds involved a lateral incisor and canine. The prevalence of congenitally missing teeth was 3.53%; all but one were mandibular incisors, the majority of which were lateral incisors. Girls were affected twice as often as boys. Both talon cusp and a supernumerary tooth occurred rarely (0.15%) and they were only found in the maxillary incisor region. A study of 57 cases of talon cusp on primary maxillary incisors showed that, when there was a talon cusp on the lateral incisor, 78.3% of the permanent successors exhibited odontogenic abnormalities; mostly in the form of supernumerary teeth. However, a talon cusp on a maxillary central incisor had no effect on the permanent successor. A study of 30 children with supernumerary primary teeth revealed that half of the children with supernumerary primary maxillary lateral incisors also had supernumerary permanent successors. A further study of 182 children with missing primary mandibular incisors showed that normal permanent successors were present in 50% of the cases with missing central incisors but only 8.5% of cases with missing lateral incisors. The prevalence of dental anomalies in the primary dentition of a local southern Chinese population differed from that of Caucasians. Children with talon cusp on primary maxillary lateral incisors, missing mandibular incisors and supernumerary maxillary lateral incisors were found to be at high risk of having an odontogenic abnormality in the permanent dentition. Clinicians should be aware of these anomalies and prepared to implement appropriate treatment. / published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

This study aimed to compare the effectiveness of an 8% arginine and calcium carbonate in-office desensitizing prophylaxis paste with that of a home-use toothpaste having the same active ingredients in reducing dentine hypersensitivity. The study was a randomized controlled clinical trial involving 55 subjects who had at least one tooth with sound exposed dentine on the cervical region of the buccal surface displaying hypersensitivity. The subjects were recruited from students and staff, other than those from the Faculty of Dentistry, of The University of Hong Kong. After receiving scaling performed by a dental hygienist at baseline, subjects were randomly assigned to either the in-office desensitizing prophylaxis paste treatment group or the home-use desensitizing toothpaste group. Clinical assessments of dentine hypersensitivity to tactile and to thermal stimuli were performed prior to the treatment at baseline, week 4 and week 12 by a blinded examiner and subjective global assessments of dentine hypersensitivity were recorded. The responses of the subjects to both stimuli were recorded by using a pain score scale from 0 to 10 (from no pain to unbearable pain). Reponses to both tactile and thermal hypersensitivity assessments in the home-use desensitizing toothpaste group showed statistically significant differences between baseline and week 12 (tactile: 2.4 vs. 0.9; cold: 5.8 vs. 3.5, both p<0.05). There was a statistically significant difference in the tactile assessment scores (1.8 vs. 1.0, p<0.05) but not in the thermal hypersensitivity assessment scores (5.5 vs. 5.3, p>0.05) for the in-office desensitizing prophylaxis paste group. There was also a statistically significant difference in the change in thermal assessment scores between the two groups (in-office: -0.2 vs. home-use: -2.3, p<0.05) but not in the tactile hypersensitivity assessment scores (in-office: -0.9 vs. home-use: -1.2, p>0.05). There were no statistically significant differences in global subjective responses to external stimuli in the telephone interviews. Daily use of an 8% arginine and calcium carbonate toothpaste was able to reduce dentine hypersensitivity over 12 weeks but a single in-office application of prophylaxis paste of similar content was not able to provide instant and long-lasting benefits for subjects with a mild-to-moderate degree of dentine hypersensitivity. / published_or_final_version / Dentistry / Master / Master of Philosophy

No description available.

No description available.

No description available.

No description available.