The International Journal of Prosthodontics, 06/2002

Purpose: This study examined the stresses developed during loading in a first premolar metal-ceramic crown made of different metal cores, and used them to anticipate the locations and form of the most likely failure modes. The maximum principal stresses in the porcelain are indicators of fracture, and the von Mises stresses in the metal core are indicators of the location of yielding. Materials and Methods: Two-dimensional axisymmetric models with different core metals were analyzed using finite element analyses. An axial load of 600 N was applied vertically downward, over a circular area around the crown's fissure. Results: The peak maximum principal tensile stress in the porcelain existed on the surface of the crown, partially outside the cusp, with the greatest peak in the gold-porcelain system (15.8 MPa). An inverse relationship between the peak maximum principal tensile stress of each system and the elastic modulus of each core material was found. According to evaluation of the critical flaw size for each system, even a crack completely through the thickness of the porcelain was not critical. The maximum von Mises stress existed in the metal coping, on the radial edge at the axial/occlusal line angle, with the highest maximum in the nickel-chromium system (143.9 MPa). There existed a proportional relationship between the maximum von Mises stress in each metal and their respective elastic moduli. All maximums were well below the yield strength of the metal alloys used. Conclusion: A greater understanding of the influence of an axial load on the resulting stresses has been achieved, showing that the phenomena of fracture and yielding are unlikely for the crown experiencing this axial load.

Purpose: This study evaluated a newly developed visual shade-matching apparatus, Shademat Visual+, as well as the influence of tab arrangement, clinician gender, and years in practice on shade-matching quality. Materials and Methods: A group of 129 colornormal evaluators-dental students and general dentists-matched the shade of four ceramometal crowns using the Vitapan Classical shade guide. Crowns were positioned onto the upper member of an artificial head and fastened to the headrest of the dental chair or Shademat Visual+. Shade tabs were arranged according to the manufacturer's suggestion or according to ΔE* in relation to the "lightest" tab and divided into groups (alternative arrangement). The evaluators matched the shade of four ceramometal crowns both in daylight and using the Shademat Visual+ artificial light source. The color coordinates were determined using digital image analysis. Based on ΔE* values, each shade-matching result was ranked from 1 (the worst match) to 16 (the best match) points. Results: The evaluators achieved a better result in Shademat Visual+ trials than in daylight trials (13.2 points vs 12.4 points; P .001). For the same ceramometal crown, 12.7 points were recorded in daylight trials with the manufacturer-suggested arrangement, and 13.7 were recorded with the alternative arrangement (P .001). Corresponding values for Shademat Visual+ trials were 13.6 points and 14.6 points, respectively (P .001). Female-male and student-dentist evaluator differences were not significant. Conclusion: The Shademat Visual+ apparatus enabled better shade-matching results than daylight. The alternative tab arrangement enabled better results than the manufacturer-suggested one. Gender and years in practice did not influence shade-matching quality.

Purpose: The aim of this study was to evaluate the interfacial bond strength between different types of silicone facial elastomers and denture resins. Materials and Methods: The facial materials studied were Cosmesil and Ideal, whereas SR 3/60, SR 3/60 Quick, and Triad were included in the denture resins group. The "overlap-joint" model was used to evaluate the bond strength, and the samples were placed in tension until failure. The bonding surfaces were treated with a primer. Ten samples for each silicone/resin group were tested. The results were subjected to two-way analysis of variance and Tukey's test for comparison. Results: The bond strength was affected by the type of silicone elastomer and denture resin. An interaction between them was also noted. The bond strength ranged from 0.03 to 0.23 MPa. Conclusion: Cosmesil condensation-type silicone always showed higher bond strength with the three different types of denture resins compared to Ideal addition silicone, keeping other variables associated with the silicone-resin bond fixed.

Purpose: The aim of the present 5-year follow-up was to clarify the nature of occlusal support status and radiographic changes in condyles of the elderly, and the association between these two variables. Materials and Methods: The present study is part of a comprehensive medical survey of a random sample born in 1904, 1909, and 1914. A total of 364 subjects living in Helsinki participated in the dental part of the examination during 1990 to 1991, and after 5 years a total of 103 were reexamined. Comprehensive data on occlusal support status were available for 94 subjects, and radiographic data were available for 88 subjects. Occlusal support status was assessed on the basis of the Eichner index, radiographic changes were assessed from panoramic radiographs, and symptoms of temporomandibular disorders were assessed using Helkimo's anamnestic index. Results: The most frequent radiographic finding in the mandibular joint was flattening of the articular surface of the condyle associated with osteoarthrosis, found at baseline in 17% and during follow-up in 13% of the subjects. During the 5-year followup, Eichner index for natural dentition remained unaltered in 94% of the subjects and in 85% of the subjects when removable dentures were included. There were no radiographic changes in 92% of the cases. No differences based on age or gender were found. A logistic regression model revealed associations between the selected baseline factors. The odds ratio for baseline Helkimo's anamnestic index was 4.1, 5.7 for Eichner index with the support of removable dentures, and 356 for radiographic findings. Conclusion: Radiographic changes in condyles of elderly people were small during the 5-year follow-up, but baseline radiographic findings, Helkimo's anamnestic index, and Eichner index with removable dentures were risk factors for radiographic findings at the end of the follow-up.

Purpose: This study evaluated the 5-year clinical and radiographic performance of fixed implant-supported maxillary prostheses with either welded titanium or conventional cast- gold alloy frameworks. Materials and Methods: Fifty-eight consecutive patients were provided with 349 osseointegrated Brånemark system implants in the edentulous maxilla at six different implant centers. Twenty-eight of the patients received, at random, prostheses with laser-welded titanium frameworks, and the remaining 30 patients had prostheses with conventional cast-gold alloy frameworks. Clinical and radiographic data were collected for 5 years after prosthesis placement. Results: The titanium and cast-gold framework groups exhibited similar cumulative survival and success rates (CSR). The 5-year implant CSR from time of placement was 91.4% and 94.0%, respectively, and from prosthesis delivery the rate was 94.9% and 95.6%, respectively. The corresponding 5-year prosthesis CSRs were 96.4% and 93.3%. One patient from each group lost all the implants and turned to complete dentures within the first year of function. Another patient with a cast-gold framework had the prosthesis replaced after 4 years, basically because of problems with the veneering material. No fractures of implant components were observed during the follow-up period. Bone loss was on average 0.59 mm (SD 0.97 mm) during 5 years, with no statistical difference between the two groups. Conclusion: Welded titanium frameworks presented a similar favorable clinical performance as conventional cast-gold alloy frameworks in fixed implant-supported prostheses in the edentulous maxilla after 5 years in function. Implant failures were concentrated in only a few patients in each study group.

Purpose: In this study, seven denture cleansers were evaluated for their mode of action of tea stain removal from Perspex acrylic resin material. Materials and Methods: Perspex acrylic resin specimens were stained using a combination of chlorhexidine and tea solution. The specimens were also treated with saliva to form an initial pellicle layer and facilitate the uptake of the stain. The capacity for stain removal was determined by measuring the optical density of the treated specimens by using a spectrophotometer. These were then exposed, for 5 minutes, to seven denture cleansers to demonstrate the percentage stain-removal ability of each cleanser. The same procedure was applied for roughened Perspex acrylic resin specimens. Water was used as a control in the two parts of the experiment. Results: The products containing alkaline hypochlorite had the highest ability for removing stain from the acrylic resin material together with a bleaching effect. The results varied when the same denture cleansers were used on the roughened Perspex surface, and this was attributed to the presence of irregularities and porosities on the blocks. Conclusion: Denture cleansing agents containing hypochlorite and Boots Denture Cleaning Powder (constituents not stated by the manufacturer) exhibited the ability to remove stain from the denture base materials most effectively. Irregularities and porosities present on the denture surface played a major role in reducing the activity of denture cleaning agents and hence increased stain and plaque retention.

Purpose: Suction is achieved when complete dentures are first fitted, but it fades during the months following insertion, when it is increasingly complemented by muscular denture control. The aim of this study was therefore to investigate the extent to which patterns of mastication are altered by experimental failure of physical retention. Materials and Methods: Seven edentulous volunteers had maxillary replica dentures made, with fine tubes incorporated so that alternating positive and negative air pressure could be generated under the fitting surface. Mandibular movements, EMG, and pressure under the denture base were recorded during unilateral chewing. Results: The change of physical retention passed undetected by all subjects; they continued chewing apparently undisturbed. However, during air injection, the mandibular excursions were slightly slower (nonsignificant) and demonstrated a reduced variance (P = .05). The muscle activity showed significantly longer bursts (P = .05), and peak activity occurred later in the cycle (nonsignificant). Conclusion: Sudden failure of denture retention seems to initiate an immediate and effective modulation of centrally generated muscular activity patterns.

Purpose: This study was conducted to expand the knowledge on the incidence of complete cusp fractures of posterior teeth in Dutch general practices. Materials and Methods: During a 3-month period, data were obtained from 28 general practitioners, representing 46,394 patients. For each new case of complete cusp fracture, clinicians recorded information using a standard form with questions relating to location of the fracture, cause of fracture, and restorative status of the tooth prior to the cusp fracture. Results: There were 238 cases of complete cusp fracture recorded. The results of this study indicate an incidence rate of cusp fractures of 20.5 per 1,000 person-years at risk. Molars were more frequently registered with cusp fractures than premolars (79% vs 21%). Maxillary molars presented more fractures of buccal cusps (66% vs 34%), while mandibular molars presented more fractures of lingual cusps (75% vs 25%). Almost 77% of the cases had been restored on three or more surfaces. Statistical analysis revealed a positive correlation between history of endodontic treatment and subgingival fracture location. Mastication was most frequently reported as the cause for fracture (54%), although one can argue whether the occlusal force was the cause or the immediate reason. Conclusion: This study revealed that complete cusp fracture is a common phenomenon in dental practice and has shown differences in cusp fracture with respect to tooth type and restorative status of the tooth. Teeth with a history of endodontic treatment are susceptible to unfavorable subgingival fracture locations.

Purpose: This study compared the long-term effects of treatment with a stabilization appliance and treatment with a control appliance in patients with temporomandibular disorders (TMD). Materials and Methods: In a controlled trial, 60 TMD patients with temporomandibular joint (TMJ) pain were evaluated after 10 weeks of treatment with either a stabilization appliance or a control appliance. At the 10-week follow-up, the 60 patients were assigned to one of three groups according to their demand for treatment. Group T, the treatment group, comprised 30 patients treated with a stabilization appliance; group C, the control group, comprised nine patients treated with a control appliance; and group M, the mixed treatment group, comprised 21 patients treated with first a control appliance and then a stabilization appliance. Signs and symptoms were evaluated in all three groups at 6- and 12-month follow-ups. Results: At the 6- and 12-month follow-ups, a significant reduction in TMJ pain as measured on a visual analogue scale was found in all three groups, and a significant decrease in signs and symptoms was found in groups T and M. Conclusion: After 6 and 12 months of use, the stabilization appliance was found to still be effective in the alleviation of signs and symptoms in patients with TMD. Many patients in group C changed to a stabilization appliance at the 10-week follow-up, which significantly reduced the number of patients in this group. Most patients reported positive change in overall subjective symptoms in this trial. The stabilization appliance can therefore be recommended for patients with TMD.

Purpose: This study evaluated fracture resistance of five different metal framework designs for metal-ceramic restorations. Materials and Methods: Twenty-five artificial crowns were fabricated with different degrees of facial metal reduction: metal collar (group 1), and 0, 0.5, 1, and 1.5 mm (groups 2 to 5, respectively). All of the crowns were thermocycled in two different baths of 5 and 60°C for 20 seconds and had a dwell time of 10 seconds in a resting bath at 37°C for 510 cycles. The artificial crowns were then subjected to vertical loading until fracture with a Shimadzu testing machine at a crosshead speed of 1 mm/min. The load at fracture was recorded. Results: There were no statistically significant differences between mean fracture strength of groups 1 and 2 or between groups 4 and 5. There were significant differences between group 3 and groups 1, 2, 4, and 5. Conclusion: As the amount of metal reduction increased, the vertical fracture resistance decreased. Failure loads for all test groups exceeded normal biting forces.

Purpose: The aim of this study was to assess the outcome of treatment with implantsupported mandibular overdentures in terms of biting and chewing, in entirely satisfied and not fully satisfied patients. Materials and Methods: Twelve edentulous patients who had worn dentures for at least 5 years participated. They were in good health but had retention problems with their mandibular dentures. First, all patients received new dentures. After 3 months, two Astra Tech implants were placed in the anterior part of the mandible, and 6 months later the abutments were connected. Patient assessment (questionnaire) and functional recordings (chewing ability, bite force, electromyographic activity) were performed with the new dentures, and again 3 months, 1 year, and 5 years after overdenture treatment. Results: After treatment, all patients were able to comminute hard and tough food, the maximum bite force and the chewing activity increased in parallel, and the duration of the chewing cycle was reduced. Every patient felt improved function and reduction of chewing pain. However, the seven patients not fully satisfied with the function of the implant-supported mandibular overdentures were characterized by lower muscle activity, even before implant placement, than the entirely satisfied patients. Conclusion: Implant-supported mandibular overdenture treatment permits better biting and chewing function than conventional complete dentures.

Purpose: This study investigated temperature changes at the implant-bone interface during simulated implant surface decontamination with an Er:YAG laser. Materials and Methods: Stepped cylinder implants with three different surfaces (titanium plasma sprayed, sandblasted and acid etched, and hydroxyapatite coated) were placed in bone blocks cut from freshly resected pig femurs. An artificial periimplant bone defect with a size of 6 mm2 provided access for laser irradiation in the coronal third of the implant. A 540-µm periimplantitis application tip was used at a distance of 0.5 mm from the implant surface. Pulse energy was varied between 60 and 120 mJ at 10 pps. The bone block was placed into a 37°C water bath to simulate in vivo thermal conductivity and diffusitivity of heat. K-type thermocouples connected to a digital meter were used to register temperature changes at three levels of the periimplant bone. Results: The temperature at the implantbone interface did not exceed 47°C after 120 seconds of continuing laser irradiation. Temperature elevations were significantly higher at the hydroxyapatite-coated implants than in the two titanium surface groups (P .001). Conclusion: Decontamination of implant surfaces by means of the Er:YAG laser did not excessively heat the periimplant bone within the energy range investigated. This technique therefore seems clinically safe, at least when used with the surfaces studied.