The International Journal of Prosthodontics, 02/1998

Purpose: Previous studies deisclosed the ability of a light-activated denture reline material to strengthen a particular denture base material. The present study investigated the ability of the reline material to strengthen denture base materials processed by other means. Materials and Methods: Four denture base materials were tested for flexural strength at proportional limit (FSpl) with or without being relined. Results: The reline material maintained the FSpl of the microwave-processed and the autopolymerizing denture base materials. It increased the FSpl of the heat-processed and the light-activated denture base materials. Conclusion: The light-cured reline material increased the flexural strengths of the heat-processed and the light-activated denture base materials and maintained those of the microwave-processed and the autopolymerizing denture base materials.

Purpose: This study investigated the masticatory efficiency and ability in girls with normal occlusion and Class II malocclusion. Furthermore, it investigated the association between masticatory efficiency and ability on one hand and signs and symptoms of temporomandibular disorders (TMD) on the other hand. Materials and Methods: A total of 183 girls, aged 11 to 15 years, were included in this study. Sixty subjects had normal occlusion and 123 subjects had Class II malocclusion. The examination included registration of signs and symptoms of TMD. Masticatory efficiency was evaluated with a masticatory efficiency test, while masticatory ability was self-assessed on a visual analog scale. Results: Subjects with normal occlusion presented statistically significantly better masticatory effiency and ability than subjects with Class II malocclusion. Thirty percent of the variation in masticatory efficiency was explained in a multiple regression analysis. Few occlusal contacts and a large overjet predicted a reduced masticatory efficiency. Subjects who reported frequent temporomandibular joint clicking and subjects who estimated their overall symptoms of TMD as moderate or severe also had reduced masticatory efficiency. Conclusion: The authors concluded that masticatory efficiency and ability were partly dependent on the occlusion and that symptoms of TMD influenced the masticatory efficiency and ability.

Purpose: The objective of this study was to identify failures and to assess the success rate for fixed partial dentures constructed 20 years earlier by general practitioners in Sweden. Materials and Methods: The original group consisted of 164 patients who had received prosthodontic treatment in 1974 comprising fixed partial dentures with at least five units. They had all been selected at random from the files of the Swedish Federal Dental Insurance Register. Twenty years later, 98 of them could be located and 72 of them agreed to participate in a clinical examination. The majority of drop-outs of the original group had died, could not be traced, or could not participate because of medical reasons. The examination focused on removed prostheses and/or units and a number of other clinically valid variables that will be reported in a forthcoming article. The 26 patients not participating in the clinical examination were interviewed by telephone. Results: The examined participants had a total of 140 fixed partial dentures, 557 retainers, and 383 pontics still in service. When the number of failed and removed prostheses was calculated, a cumulative success rate of 65% was found, ie, one third had been lost since 1974. There was no difference in failures between fixed partial dentures with or without cantilevers during the previous 8 years compared to the first 12 years in service. Loss of reten tion together will periodontal and esthetic problems, wear, and discoloration of acrylic veneers were the most frequently reported reasons for removal. Conclusions: The cumulative success rate for fixed partial den tures placed in 1974 ws 65%. Failed prostheses had in most cases been replaced by a new fixed partial denture.

Purpose: The purpose of this study was to investigate the effect of an altered inclination of the canine guidance on the pattern of the condylar movements during laterotrusion. Materials and Methods: The inclination of the canine guidance was steepened approximately 10 degrees by attaching a metal overlay to the lingual surface of the maxillary working-side canine. The laterotrusive movements of 20 young adults were measured using a three-dimensional mandibular movement analyzing system before and after alteration of the canine guidance. The difference between anterior, lateral, and inferior components of the working- and nonworking-side condylar movements, the Bennett angle, and the sagittal inclination of the nonworking-side condyalr path before and after steepening of the canine guidance was evaluated by a paired t test. Results: After the canine guidance was steepened, the working-side condyle moved even more anteriorly and inferiorly. In contrast, little change was found in the direction and the amount of the nonworking-side condylar movement. Conclusion: The results of this study suggest that there is a relationship between the steepness of canine guidance and the pattern of the working-side condylar movement, but not between the steepness of the canine guidance and the pattern of the nonworking-side condylar movement.

Purpose: The aim of this prospective study was to determine whether a machine-fabricated titanium fixed partial denture veneered with a low-temperature fused porcelain is an acceptable treatment modality. Materials and Methods: The study originally included 25 patients with 37 fixed partial dentures of three to seven units, with a total of 151 units including 98 abutments. A clinical examination using the California Dental Association quality evaluation system and radiologic examinati on and registration of complications was conducted at the time of insertion after 1 and 2 years. Results: After 2 years, 123 units (including 80 abutments) rem ained for examination. Twenty-eight units 9including 18 abutments) were lost during the follow-up period. The losses were because of drop-out (eight cases), root fracture (two cases), caries (six cases), and periodontitis (two cases). One fixed partial denture fractured after 18 months and was therefore remade. In all, porcelain fractuers occurred in two patients, both with severe parafunctional habits. At baseline, the California Dental Association rating system for marginal integrity was excellent for 72 abutments and acceptable for 26 abutments. At the 2-year follow-up the cor ersponding rating was excellent for 46 abutments, acceptable for 30 abutments, and not acceptable for four abutments. The latter marginal defects were corrected with restorations. For surface and color the corresponding rat ings were excellent for 137 units and acceptable for 14 nits at baseline, and excellent for 80 units and acc eptable for 43 units at the 2-year follow-up. Bleeding index increased from 24% at baseline to 33% after 2 years. Conclusion: The study confirms that for an observation period of 2 years, fixed partial dentures made by the procera method seem to be an acceptable treatment modality. They are a suitable alternative to fixed partial dentures with porcelain fused to high-noble alloys.

Purpose: The aim of this study was to determine the noise levels made by different clinical handpieces, laboratory engines, and other significant equipment such as ultrasonic scalers, amalgamators, high-speed evacuation, and other items. Materials and Methods: Sound levels were measured at four dental practices and three dental laboratories selected as representative of a variety of workplaces to reveal a range of noise. The noise levels were determined using a precision sound level meter, which was positioned at ear level and also at 2 meters distance from the operator. Results: Virtually all noise levels at the dental clinics were below 85 dB(A). The noise levels in the dental laboratories had much higher maxima, with some cutting activities, steam cleaning, and sandblasting up to 90 dB(A), and compressed air blasts with a maximum of 96 dB(A). Conclusions: The noise levels in the dental clinics are considered to be below the limit of risk of hearling loss. However, technicians and other personnel who spend many hours in noisy dental laboratories may be at risk if they choose not to wear ear protection.

Purpose: This study assessed the effect of using an occlusal stabilization splint in the maxilla for 6 weeks on certain parameters of chewing movements in subjects with and without temporomandibular disorder symptoms Materials and Methods: Twelve male and 30 female temporomandibular patients with and without a prior whiplash incidence, and individuals without signs and symptoms of temporomandibular disorders participated. The participants formed three groups matched according to gender and age (n = 3 x 14). A maxillary stabilzation splint was used during sleep for 6 weeks. An optoelectronic system (MacReflex, Qualisys) was used to record chewing movements at baseline, before using the splint, and after 6-weeks' use of the splint. Calculated parameters were the duration of the chewing cycles, the spatial displacement, and the mean velocity of the mandible while chewing paraffin wax for 20 seconds. Results: On a group basis, the use of an occlusal stabilization splint for 6 weeks did not change the jaw movement parameters in a predictable pattern as recorded under the conditions of this study. On an intraindividual basis, large variations in changes of chewing parameters over time were observed. Conclusion: The use of an occlusal stabilization splint for 6 weeks did not alter the jaw movements when chewing a substance with a soft consistency.

Purpose: This study investigated the metal-ceramic bonding of treated metal surfaces. Materials and Methods: The study was divided into two parts. In Part I, the depth of tin diffusion from a tin-plated bone metal alloy surface was measured using an energy-dispersive sprctrometer. In Part II the metal-ceramic bond strength was determined using a shear test. Results: The weakest bonding was observed in the directly tin-plated group, and the strongest metal-ceramic bonding was maintained in the tin-diffused group. A controlled oxidation produced the greatest bond strengths. Conclusions: With the base metal alloys tested, diffusion under the argon environment was conducive to a stronger metal-ceramic bond. The metal oxidation rate should approximate the ceramic vitrification rate, and the diffusion rate of the metal elements should be slower than the vitrification rate to obtain the strongest metal-ceramic bond.

Purpose: This study investigated the usefulness of a newly developed color-changeable chewing gum for evaluating masticatory performance. Materials and Methods: The color changes of the gum after chewing were measured using the L*a*b* color space defined by the Commission Internationale de l'Eclairage with a spectrophotometer. Numerical scores and a color scale were developed to easily evaluate color change. Numerical scores were assigned corresponding to the degree of color change, and a color scale was made by using the numerical scores. Four examiners graded the gum chewed by 18 fully denta te subjects by using the color scale. Results: As the number of chews increased, the degree of color change increased significantly. The percentages of correct answers by examiners using the color scale method ranged from 67% to 83%. Conclusion: The results indicate that this simple method may be used to quantitatively evaluate masticatory performance and may be reliable enough to be used clinically

Purpose: THe object of this study was to evaluate retention of complete cast crowns cemented with different luting cements and to dtermine whether recommended castings are as retentive as when initially placed. Materials and Methods: Thirty standardized Rexillium III complete crowns were made on extracted human molar teeth prepared with a modified milling machine using conventional laboratory techniques. The crowns were randomly assigned into three groups according to the luting cement used: zinc phosphate (Fleck's, Mizzy) adhesive r esin (Panavia-EX, J. Morita), and glass-ionomer (Ketac-Cem, Aplicap, ESPE-Premier Sales). Retention was evaluated by measuring the tensile force required to dislodge the crowns from tooth preparations with an Instron testing machine after thermocycling (1,500 cycles between 5 degrees C and 55 degrees C with 1-minute dwell times). Subsequently, the tooth preparations were scraped clean and polished with flour of pumice (prophylaxis paste), and the fitting surfaces of the artificial crowns were ultrasonically cleaned and air abraded using 50-um alumin um oxide powder prior to recementation. Results were subjected to two-way analysis of variance and Tukey's Studentized Range test. Results: The analysis showed the highest mean retention for cast crowns cemented with Panavia-EX cement (314 N), but teh difference was not statistically significantly different from crowns cemented with glass-ionomer cement (307 N). However, crowns cemented with zinc phosphate cement (233 N) had significantly lower values (P 0.05). Restorations recemented with zinc phosphate had significantly lower retention (190 N) than their initial cementation (P 0.05). Conclusions: Panavia-EX and glass-ionomer cements yeilded the highest initial and recementation retentive strength, with values almost 32% higher than those obtained with zinc phosphate cement during the initial cementation and 59% higher than zinc phosphate for recementation. Zinc phosphate cement exhibited significantly lower retention for the initial and second trial of cementation.

Purpose: This paper describes the mechanical testing of dental ceramic core materials in combination with porcelains to simulate the real service conditions for dental applications. Materials and Methods: The study included Procera AllCeram (densely sintered high-purity alumina); Vita In-Ceram (glass-infiltrated presintered alumina); three dental porcelains (Procera Porcelain AllCeram, Vitadur-N, and Vitadur Alpha); and densely sintered alumina-Procera Porcelain AllCeram two-layer composites, densely sintered alumina-Vitadur-N two-layer composites, and glass-infiltrated presintered alumina-Vitadur Alpha two-layer composites, with different thicknesses of densely sintered alumina or glass-infiltrated presintered alumina, respectively. The flexural tests were performed in ring-on-ring biaxial bending. The flexural failure stresses were determined using both Timoshenko's biaxial loading equation and Shetty's ring-on-ring stress equation. The bending theory of bilayer plates was used to evaluate the failure stresses in the two-layer composites. Weibull statistics were applied to all stress data, and the information was used to estimate the failure probability at selected applied stresses for a comparison of different two-layer densely sintered alumina-porcelain composites. Results: The failure stress of densely sintered alumina is significnatly higher than that of glass-infiltrated presintered alumina as a core dental material under the same testing conditions. The failure stresses of the three commercial dental porcelains are statistically the same. Conclusion: The results show that the densely sintered alumina-Procera Porcelain AllCeram two-layer composite has int ersting dental applications.