The effects of hydrochloric acid on all-ceramic restorative materials: an in-vitro study.

OBJECTIVES: The aim of this in-vitro study was to assess the long-term effects of hydrochloric acid on the surface roughness of three all-ceramic restorative materials CEREC VITABLOC Mark II CAD, IPS Empress CAD and IPS e.max CAD. MATERIALS AND METHODS: Six cylindrical specimens (10mm diameter, 3mm height) of each material type were prepared, using the CEREC CAD/CAM machine. The unpolished samples were immersed in 15ml hydrochloric acid (pH 2) at 37 degrees C. Before immersion (baseline) and at periods of 7.5 hours, 45 hours and 91 hours, the specimens were removed from the acid and two randomised areas (10 microm X 10 microm) were selected and tested on each. The atomic force microscope (Bruker Dimension icon) was used to assess surface roughness and surface area at baseline and after each exposure time. The materials were compared over time with respect to surface roughness and surface area (baseline, 1 month, 6 months, 1 year) in a repeated measures analysis of variance (ANOVA). RESULTS: Sample groups differed significantly for roughness (p < 0.0001) and surface area (p < 0.0001). For both parameters a significant interaction also existed between material and time (surface roughness: p = 0.0085; surface area: p = 0.0014). CEREC VITABLOC Mark II CAD and IPS Empress CAD had substantially higher levels of roughness and surface area than IPS e.max CAD, which was also affected to a lesser extent over time. CONCLUSION: The results showed that IPS e.max CAD was least affected by long-term exposure to hydrochloric acid.

A comparison of preload values in gold and titanium dental implant retaining screws.

This in vitro investigation compared the effect of using either gold or titanium retaining screws on preload in the dental implant- abutment complex. Inadequate preload can result in screw loosening, whilst fracture may occur if preload is excessive. These are the most commonly reported complications in implant-retained prostheses, and result in unscheduled, costly and time-consuming visits for the patient and the clinician. This study investigated changes in preload generation after repeated torque applications to gold and titanium screws. The test set-up consisted of an implant body, a cylindrical transmucosa abutment, and the test samples of gold and of titanium retaining screws. The implant bodies were anchored using a load cell, and the transmucosal abutments were attached using either gold or titanium retaining screws. A torque gauge was used to apply torque of 20Ncm, 32Ncm, and 40Ncm to the retaining screws. The preloads generated in each screw type were compared at each torque setting, and after repeated tightening episodes. In addition, the effect of applying torque beyond the manufacturers' recommendations was also examined. Gold retaining screws were found to achieve consistently higher preload values than titanium retaining screws. Preload values were not significantly different from the first to the tenth torque cycle. Titanium screws showed more consistent preload values, albeit lower than those of the gold screws. However due to possible galling of the internal thread of the implant body by titanium screws, gold screws remain the retaining screw of choice. Based on the findings of this study, gold retaining screws generate better preload than titanium. Torque beyond the manufacturers' recommendations resulted in a more stable implant complex. However, further investigations, with torque applications repeated until screw breakage, are needed to advise on ideal maintenance protocols.

Evaluation of surface characteristics of titanium and cobalt chromium implant abutment materials.

INTRODUCTION: Micro-organism adhesion and plaque formation is affected by surface free energy (SFE), surface roughness, hydrophilicity, surface chemistry, surface charge and the presence of proteins. AIMS: The aim of this study was to assess and compare surface characteristics of surgical grade cobalt chromium alloy (CCM) and of commercially pure titanium (cpTi). METHOD: Nine metallic cylinders were machined to precise standards from each material. Surface roughness was measured at four different points on each sample and the average Ra value was calculated for each material. Contact angles were obtained using the sessile-drop method and applied in calculating the SFE. Surface hardness was evaluated by means of a Vickers hardness micro-indentation. RESULTS: Surface roughness was similar for both metals, but total SFE values and Vickers surface hardness scores showed significant differences (p < 0.0001). CONCLUSION: SFE analysis showed CCM to be more hydrophobic and that oral bacteria might therefore be less adherent than to cpTi. The mean Vickers Hardness scores of the cpTi were significantly lower (p < 0.0001), suggesting that CCM may be more resistant to surface modifications and surface roughening, thus remaining smoother with less plaque accumulation than cpTi. This study demonstrated that CCM might be a suitable alternative implant abutment material.

Students' vulnerability in educational research.

Dental teaching institutions in South Africa recently implemented "learner-centred" curricula and expected educators to alter their teaching styles accordingly, but perhaps without providing adequate training in this paedagogical philosophy. At the same time, the lecturers were required to conduct evidence-based research to evaluate the outcomes. Thus, clinicians/lecturers also became researchers, using their own students or student material for assessment purposes. Previously, this form of educational research, which was carried out in normal academic settings, was not subject to review by Institutional Review Boards (IRB). However, concerns have risen that learners may be a vulnerable population due to their position in the academic institution, and the power and knowledge differentials that exist between them and the lecturer/researcher. This raises ethical concerns regarding their autonomy and ability to provide free, voluntary, informed consent to be research participants. This paper questions whether educational research may lead to student vulnerability, and proposes some recommendations for educators and institutions involved in educational research.

The accuracy of three impression transfer techniques for implant supported prostheses.

Cast framework for osseointegrated implant retained prostheses need to be passively fitting so as not to place undue forces and stress concentrations around the fixtures. Impression materials and techniques play a key role in the accuracy and fit of the metal framework. In this study, three impression procedures were investigated to establish which was the most accurate, reliable and predictable. This included using smooth sided tapered transfer copings and unsplinted and splinted square undercut transfer copings. A metal plated master model was fabricated and used as the standard against which models poured using the different impression techniques were compared. Readings were taken with a Reflex Microscope and used to calculate the mean, standard deviation and deviation index for each test model. The results of this study suggest that direct coping transfer techniques provide comparable or better results than indirect coping transfers in master cast fabrication. There was no significant difference in impression transfer accuracy between splinted and non-splinted square impression coping transfer techniques.

Flexibility of technopolymer clasps compared with cobalt-chromium and titanium clasps.

Patients often cite lack of retention and poor aesthetics as reasons for not wearing their partial dentures. Traditional metal alloy clasps have been shown to exert forces on abutment teeth that exceed those capable of producing tooth movement. In addition, metal display on anterior teeth is often unacceptable. The technopolymer materials are purported to have superior flexibility and exert less force than the metals. This study compared the flexibility and forces produced by technopolymer clasps with those exerted by cobalt-chromium and titanium clasps. The results showed that the technopolymer clasps were up to ten times as flexible as the metal clasps, and they returned to their pretest dimensions after being stretched. In addition, they exerted forces on the abutment teeth that fall within the range of those considered safe for use. This coupled with their pleasing aesthetics makes them suitable for use on periodontally compromised teeth, those with deep undercuts and on anterior teeth.

Use of technopolymer clasps in prostheses for patients due to have radiation therapy.

Patients with large oral defects as a result of a maxillectomy procedure, who are due to have postoperative radiotherapy, need to have the density of the defect restored to ensure standardised radiation distribution. This is achieved with various types of boluses that often require tissue surface positioning stents to help support them. Traditional metal-clasp retained stents were discarded as the clasps caused backscatter of the radiation beams. A radiolucent material was needed to retain these prostheses. Dental D, an acetal resin was used in the fabrication of a positioning stent. It was assessed in terms of ease of manufacture, cost, fit, retention and radiolucency. The material was found to be more costly and time-consuming to manufacture than conventional metal-retained acrylic resin prostheses, but its radiolucency made it ideal for use in patients during radiotherapy treatment. Future studies will determine if the long-term strength of the material will allow the clasps to be used in a definitive prosthesis and thus help offset the initial time and costs.

Partial denture prosthodontic procedures employed by practising graduates of the University of the Witwatersrand, Johannesburg.

This study sought answers to the following two questions related to the teaching of partial denture prosthodontics in the Department of Prosthetic Dentistry at the University of the Witwatersrand, Johannesburg (Wits). Firstly, do practising graduates from Wits assiduously apply, or do they perhaps ignore, the concepts, principles, and practices taught to them at undergraduate level? Secondly, is there any merit in providing additional tutorials on partial denture prosthodontics to undergraduate students at Wits? A questionnaire was mailed to dentists who had graduated at Wits between 1979 and 1986. This sample of 302 subjects was divided into two groups: Group I--those who had graduated between 1979 and 1982; Group II--those who had graduated between 1983 and 1986 and had received additional tutorials on partial denture design. The results suggest that a wide gap exists between faculty beliefs about partial denture design and fabrication and the methods used in general dental practice. Moreover, additional tutorials had some effect on the attitudes and practices of the respondents.