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Purpose: The purpose of this study was to examine the mechanical influence of edentulous mandibular morphology on peri-implant bone in implant prosthetics by finite element analysis., Materials and Methods: Computed tomographic data from 25 patients with edentulous mandibles were selected and the radius of mandibular curvature and the height of the mandible at the midline were measured in order to clarify the morphological characteristics of the mandible. From the measurement, two patients with the smallest and largest radii of the mandible were selected. Two types of three-dimensional finite element models consisting of the edentulous mandible (small and large radius), superstructure (a cantilever bridge), implants, and abutments were created. Four implants were inserted between the right and left mental foramina. The upper surface of the mandibular condyle was constrained, and a vertical load of 100 N was applied on the occlusal surface of the right first molar. Three-dimensional finite element analysis of each model was performed to examine the mechanical influence of the edentulous mandibular morphology on the peri-implant bone., Results: Measurement of mandibular morphology in CT images indicated that the lower the mandibular height was, the larger was the radius of the anterior mandibular curvature. Finite element analysis revealed that a higher equivalent stress was generated in the peri-implant bone of the model with a larger radius of curvature than that of the model with a smaller radius of curvature. The highest equivalent stress in the mandible was generated in the distal margin of the peri-implant bone posterior to the loaded side of the large radius of curvature model., Conclusions: The mandibular morphology had a mechanical influence on the peri-implant bone.

Purpose: To determine the influence of partial edentulous distal extension and the use of removable partial dental prostheses in partially edentulous areas on the force exerted on the anterior teeth., Materials and Methods: A total of 83 volunteer patients participated in this study. The occlusal force was measured using an occlusal force measuring sheet in the patient's mouth. The occlusal forces and ratios were compared using the Wilcoxon signed-rank test (P < .05)., Results: The force exerted on the anterior teeth increased significantly as the number of remaining teeth decreased in an edentulous distal extension. The force exerted on the anterior teeth decreased significantly with the use of a removable partial dental prosthesis., Conclusions: In a clinical setting, when the number of remaining teeth in a partial edentulous distal extension decreases, the burden on the anterior teeth increases. Our findings suggest that, for patients with partial edentulous distal extension, using a removable partial dental prosthesis is effective in preserving the residual anterior teeth by reducing excessive force.

Purpose: To examine the influence of posterior residual ridge resorption (PRRR) and relining on bending strain around implants under an overdenture., Materials and Methods: Maxillary and mandibular edentulous models were made and covered with white silicone. Four strain gauges were placed on four sides of each implant. Two tissue-level implants were placed bilaterally in the part between the lateral incisors and canines of the mandible. A maxillary conventional denture and a mandibular overdenture were fabricated to fit the models. Three levels of bone resorption around the implant were created: 0, 0.8, and 1.5 mm. The surface of the residual ridge was uniformly removed from the area near the distal side of the implant to the retromolar section, and three levels of PRRR were created (0, 1, and 1.5 mm). After the measurement, the experimental denture of the PRRR of 1 mm and 1.5 mm were relined. Locator attachments were used. A 98-N occlusal force was applied, and bending strains around the implant were measured. The Kruskal-Wallis test and Bonferroni correction were employed to analyze the data., Results: Without relining, at each bone resorption level around the implant, the bending strain was smallest with PRRR of 0 mm and largest with PRRR of 1.5 mm. The bending strain after relining was smaller than the bending strain with ridge resorption. Bending strains after relining were smaller than bending strains with no ridge resorption, but there was no significant difference in bending strain between these three groups., Conclusion: As PRRR increased, the bending strain increased without relining. As bone resorption around the implant progressed, PRRR enhanced the increase in the peri-implant bending strain. Bending strain was reduced when relining was performed.

Purpose: Implant overdentures with splinted attachments have been used in clinical practice, and the effect of splinting on implants has been reported frequently. However, the effect of implant configuration of more than four implants and covering the palate with an overdenture has not been sufficiently examined. The purpose of this study was to reveal the effects of implant configuration and palatal coverage on both implant and denture strain in maxillary implant overdentures using splinted implants., Materials and Methods: Six implants were placed in the anterior, premolar, and molar areas in a maxillary edentulous model. Four strain gauges were attached to the implant surface, and Dolder bar attachments were used to splint implants distributed in various configurations. Two types of maxillary experimental dentures (with/without palatal coverage) were fabricated, and two strain gauges were attached at the midline. A vertical load of 98 N was applied, and the strains on the dentures and implants were measured. The strain measurements were compared using one-way analysis of variance and t test (P = .05)., Results: When comparing the implant strains, the strain was significantly smaller on the anterior and premolar implants when six implants were used, but there was no significant difference among the different implant configurations in the strain on the molar implants. Strains on anterior implants of the palateless overdenture were significantly greater than those of the overdenture with palatal coverage. When four implants were used, the strain on the palatal side of dentures without palate was significantly greater than that on those with palatal coverage. When six implants were used, there was no significant difference in the strain on either side between two types of dentures., Conclusion: When implants were splinted to each other using a denture without palatal coverage, the strain of dentures when six or two anterior and two posterior implants were used was lower. The difference of denture strains between two types of dentures was not significant when six implants were used, but was significant for all other configurations. Also, the difference of implant strains between two dentures was significant in anterior implants regardless of implant configuration.

Purpose: To examine the effect of the attachment features (abutment height, retention, and freedom of rotation) of implant overdentures on the bending strain around implants under an overdenture., Materials and Methods: Mandibular and maxillary edentulous models were fabricated and covered with silicone rubber. Strain gauges were attached to the four sides of each implant. Two implants (tissue-level implant, 4.1-mm diameter, 10-mm length) were placed bilaterally in the area between the mandibular canines and lateral incisors. A maxillary conventional denture and a mandibular overdenture were made to fit the edentulous models. Three levels of peri-implant bone resorption were created sequentially: no bone resorption, 0.8 mm, and 1.5 mm. Three kinds of attachments (magnetic, stud, and ball attachments) were used. Bending strains generated from a 98-N occlusal load through the maxillary denture were measured using a sensor interface, and the data were analyzed using the Kruskal-Wallis and Bonferroni tests. Multiple regression analysis was used to find the relationship between independent variables (peri-implant bone resorption level, abutment height, retention, and freedom of rotation) and the dependent variable (bending strain)., Results: For all attachment types, the bending strains were smallest with no bone resorption and largest with 1.5-mm bone resorption (P < .05). Multiple regression analysis showed that abutment height had the greatest impact on the reduction of bending strain (beta = .413), followed by freedom of rotation (beta = -.349), and retention (beta = .107)., Conclusion: Low abutment height, large freedom of rotation, and low retention can minimize bending strain around implants.

Purpose: The purpose of this study was to evaluate the biomechanical effects of bone loss on peri-implant bending strain underneath overdentures., Materials and Methods: One mandibular and one maxillary edentulous model were created and covered with 2 to 4 mm of silicone rubber to simulate the oral mucosa. Four strain gauges were attached to four sides of each implant. In the mandibular model, two implants were inserted, and the implant was placed bilaterally in the area between the mandibular canines and lateral incisors. A maxillary conventional denture and mandibular overdenture were fabricated with acrylic resin to fit the edentulous models. Five bone loss levels were created: no bone loss, 0.8 mm bone loss on only the left implant, 0.8 mm bone loss bilaterally, 1.5 mm bone loss on only the left implant, and 1.5 mm bone loss bilaterally. Magnetic attachments were used to retain the denture. The bending strains generated from three types of unilateral loading were measured by a sensor interface, and the data were analyzed using the Kruskal-Wallis test and Bonferroni's post hoc test., Results: Bending strain increased with decreasing bone levels on the loading side. On the nonloading side, bending strains were greater at even bone loss (0.8 mm or 1.5 mm bone loss bilaterally) than at uneven bone loss (0.8 mm or 1.5 mm bone loss on only the left implant). Strains under loading at the left first premolar were approximately equal to that under loading with a cotton roll; these strains were higher than those generated with loading at the left first molar., Conclusion: Peri-implant bending strains rapidly increased on the loading side as bone loss levels increased. Peri-implant bending strains on the nonloading side were higher at bilaterally even bone loss than at unilateral bone loss.

Purpose: To clarify the relationship between the pressure generated during impressions (impression pressure) and the presssure beneath the corresponding denture base (functional pressure) and to examine the influence of tray modifications., Materials and Methods: Impression and functional pressures were measured on models with the same appearance but different bone morphologies for comparison. Pressures with modified trays with additional relief and escape holes were also measured for comparison., Results: The correlation between impression pressure and functional pressure was high, and both could be changed by tray modifications., Conclusion: The distribution of impression pressure will be recreated beneath the denture base, and modifications of trays can control the pressure distribution.

Purpose: The purpose of this study was to determine the optimal positioning of implants for maxillary implant-supported overdentures (MIODs) using finite element analysis (FEA)., Materials and Methods: Three-dimensional (3D) finite element models were built incorporating the maxilla, mucous membrane, removable denture, and artificial teeth using FEA software. Four models were prepared: one without an implant; one with two implants in the region of the lateral incisor; one with two implants in the first premolar region; and one with four implants in the lateral incisor and first premolar regions. Occlusal load was applied to the model, and denture displacement and the distribution of stress in the surrounding bone and mucous membrane were observed and measured., Results: The maximum equivalent stresses to the bone and mucous membrane were highest in the premolar and anterior areas, respectively. Stress to the bone was concentrated around the implants, whereas stress in the mucous membrane was concentrated along the posterior margin. Denture displacement in the anterior direction was highest in the model without implants, while denture displacement in the posterior direction was less affected by the presence of implants. Overall, denture displacement in the four-implant model was smaller than that in the other models. Implant angle affected the stress distribution, with highest stress to the bone when the implant was positioned in a palatal direction and highest stress to the mucous membrane when placed mesially. However, implant angle had little impact on denture displacement, with almost the same stress for implants at all tested angles., Conclusion: Optimal implant position can stabilize overdentures and reduce stress to the bone and mucous membrane. Stress is reduced when implants are inserted in the premolar area and when the direction of the implant is perpendicular to the occlusal plane.

Purpose: To clarify the relationship between residual ridge morphology and pressure distribution of the alveolar bone surface during dental impression procedures., Materials and Methods: Seven experimental models of the mandibular posterior residual ridge with the same ridge morphology but different mucosa (ie, silicone material) thicknesses and bone (ie, plaster) shapes and sizes were fabricated. The pressure on the bone surface was recorded using a pressure sensor sheet. The data from each model were compared using the Kruskal-Wallis test, and P < .05 was considered statistically significant., Results: Even with the same ridge morphology, the distribution of the impression pressure on the bone surface differed according to the mucosal thickness and bone shape and size. Pressure tended to concentrate on sharp edges and prominences of a slope and became more widely distributed as the mucosal thickness increased., Conclusion: Within the limitations of this experimental study, the morphology of the residual alveolar ridge (bone and mucosa) appears to be a significant influencing factor for the pressure distribution during impression procedures.

Purpose: To evaluate the long-term effects of preventive measures against denture fracture using clinical surveys of denture fracture cases from 1984 and 2009., Materials and Methods: This study included 128 patients who presented with a chief complaint of denture fracture and received denture repair treatment in 2009. The following data were collected: denture repair procedure; location of denture base fracture (with or without reinforcement); and period of denture use from insertion to repair. Significant differences in characteristics between patients treated in 1984 and 2009 were determined using the chi-square test (P < .05)., Results: In 2009, denture fractures comprised 55.5% of all repair cases. The most frequent location of denture fracture was around the clasp and metal in the denture base. Approximately 45% of all dentures were reinforced. The mean period from denture insertion to repair was 37 months. The number of denture fractures significantly decreased between 1984 and 2009 (P < .05), and the number of dentures with reinforcement significantly increased (P < .05). The mean period from denture insertion to repair also increased., Conclusion: These findings suggest that denture reinforcement as a preventive measure is effective against denture fracture, allowing patients to use their dentures more effectively.

Purpose: Palateless maxillary implant overdentures are often used for patients experiencing problems with a full-palate denture. However, palateless overdentures are reported to be a risk factor for implant complications. The purpose of this study was to examine the strain on implants beneath palateless overdentures with unsplinted attachments under various implant distributions., Materials and Methods: A maxillary edentulous model with implants and an experimental palateless overdenture were fabricated. Four strain gauges were attached to each implant, positioned in the anterior, premolar, and molar areas. Three types of unsplinted attachments (ball, locator, and magnet) were set on the implants under various implant distributions. A vertical occlusal load of 98 N was applied through the mandibular complete denture, and the bending strain on the implants was compared using the Kruskal-Wallis test (P = .05)., Results: When comparing the strain among different attachments, those using a magnet attachment were the smallest and those using a ball attachment were the greatest, and this difference was significant in most situations (P < .05). When comparing the strain among different implant distributions, the strain on a four-implant distribution was significantly smaller than that on a two-implant distribution in most situations (P < .05), and those using premolar and molar implants recorded the smallest strain. The strain on implants using a locator attachment tended to be midway between those using ball and magnet attachments, regardless of the implant distribution., Conclusion: In most implant distributions, magnet attachments decrease the strain on implants more than ball and locator attachments. The most favorable unsplinted attachments for use beneath palateless overdentures to decrease the implant strain are magnet attachments and four implants placed in the premolar and molar areas.

Purpose: The aim of this study was to examine the deformation modality of palateless maxillary implant overdentures using isolated attachments under various implant configurations., Materials and Methods: A maxillary edentulous model with implants inserted in the anterior, premolar, and molar areas was fabricated, and three types of unsplinted attachments-ball, locator, and magnet-were set on the implants distributed in various configurations. Experimental palateless dentures were fabricated, and two strain gauges were attached at the anterior midline of the labial and palatal sides. A vertical occlusal load of 98 N was applied, and the shear strains of dentures were measured. The measurements of strains were compared with the Kruskal-Wallis test (P = .05)., Results: The strains of the labial side were much larger than those of the palatal side except for those using the ball attachment. The strains using the magnet attachment on anterior implants were significantly larger than those using other attachments (P < .05). Those using anterior implants were significantly smaller than those using premolar or molar implants (P < .05)., Conclusion: The strains of palateless overdentures were different according to the attachments and implant distribution. However, when using molar implants, there was no significant difference among the three attachments.

Purpose: Implant overdentures with attachments have been used in clinical practice and the effect of attachments on implant strain has been frequently reported. However, most studies have focused on mandibular overdentures; there are few reports on maxillary overdentures. The purpose of this study was to examine the influence of attachment type on implant strain in maxillary overdentures under various implant configurations., Materials and Methods: A maxillary edentulous model with implants and experimental overdentures were fabricated. Four strain gauges were attached to each implant, positioned in anterior, premolar, and molar areas. Three types of unsplinted attachments-ball, locator, and magnet-were set on the implants under various implant configurations. A vertical occlusal load of 98 N was applied through the mandibular complete denture, and implant strain was compared using the Kruskal-Wallis test., Results: Ball attachments caused the greatest amount of strain, while magnet attachments caused the least amount under all conditions. For all attachments, two anterior implants caused significantly more strain than four implants (P < .05). No significant difference was observed between subtypes in four-implant configurations except when using locator attachments., Conclusion: When using unsplinted attachments for maxillary implant overdentures, magnet attachments are recommended to reduce implant stress. Using only two implants, especially two anterior implants, is not recommended regardless of attachment type.

Purpose: The purpose of this preliminary study was to examine the influence of overdenture attachment height and shape on lateral force on implants and denture displacement., Materials and Methods: A straight model was fabricated to simulate the edentulous ridge. Two strain gauges were attached to the mesial and distal surfaces of the implant in the canine area. Five types of ball attachments and three types of Locator attachments with three types of replacement male attachments were used. A vertical load of 49 N was applied. The lateral force on the implant and the denture displacement were measured. Statistical comparisons were performed using one-way analysis of variance, Tukey post hoc comparison test, and multivariate analysis. P < .05 was considered significant., Results: Ball attachments of 4-mm height exerted the highest lateral force on the implant, and the greatest displacement was also observed with attachments of 4-mm height. Different lateral forces were observed for different shapes of ball attachments, but there were no significant differences in denture displacement among the shapes. For Locator attachments, the highest lateral force was observed with the white male type, and the greatest denture displacement occurred with attachments of 4-mm height. There were no significant differences among male white, pink, and blue types., Conclusion: The height of the stud attachments in implant overdentures had a marked effect on the lateral force on implants and on denture displacement. To protect implants supporting an overdenture and to prevent bone resorption, the height of the attachment should be carefully considered.

Purpose: To describe a method for customizing scuba diving mouthpieces, to examine their usability, and to assess divers' satisfaction with their use., Materials and Methods: A rational method for fabricating custom-made mouthpieces for scuba diving was developed. Twenty scuba divers agreed to use a custom-made mouthpiece for at least 4 weeks. After mouthpiece use, participants completed an 11-item questionnaire., Results: Custom-made mouthpieces had significantly lower scores for all items on the questionnaire except uncomfortable sensation, indicating that they were more comfortable than ready-made mouthpieces., Conclusion: It can be concluded that these novel custom-made mouthpieces for scuba diving offer greater comfort than ready-made mouthpieces.

Purpose: This study examined the effects of attachments on strain in maxillary implant overdentures supported by two or four implants., Materials and Methods: A maxillary edentulous model with implants inserted into anterior, premolar, and molar areas was fabricated, and three types of unsplinted attachments-ball, locator, and magnet-were set on the implants distributed under various conditions. Maxillary experimental dentures were fabricated, and two strain gauges were attached at the anterior midline on the labial and palatal sides. A vertical occlusal load of 98 N was applied and shear strain of the dentures was measured., Results: On both sides, magnet attachments resulted in the lowest shear strain, while ball attachments resulted in the highest shear strain under most conditions. However, differences in shear strain among the three attachment types were not significant when supported by four implants, especially molar implants., Conclusion: Shear strain of the maxillary implant overdenture was lowest when using magnet attachments. Magnet attachments mounted on four implants are recommended to prevent denture complications when using maxillary implant overdentures.

Purpose: The aim of this study was to determine the optimal number and location of mini-implants retaining an overdenture to resist lateral forces., Materials and Methods: A strain gauge was attached to a mini-implant and placed in an edentulous mandible model. The following different implant locations were evaluated: one mini-implant at the midline, two mini-implants in the lateral incisor regions, two mini-implants in the canine regions, and four mini-implants., Results: Lateral forces on mandibular overdentures retained by one mini-implant at the midline or two mini-implants in the lateral incisor regions appeared to be advantageous when compared with placing two mini-implants in the canine regions or four mini-implants., Conclusion: Fewer mini-implants resulted in less lateral stress, while four were associated with an unequal stress distribution.

Purpose: Success rates for maxillary implant treatment are lower than for mandibular treatment because of the presence of poorer bone quality or quantity in the maxilla. The purpose of this study was to determine favorable implant positions in the maxilla using implant simulation software and clinical anatomical morphology together with bone quality data obtained by computed tomography (CT)., Materials and Methods: A convenience research sample of 10 edentulous subjects was recruited, and research information from right and left edentulous sites was obtained from each subject. The height, width, angulation, and Hounsfield unit value of the maxillary alveolar bone were measured using CT data obtained from the subjects., Results: Bone height in the incisor area was significantly greater than in the molar area, and bone width in the incisor area was significantly narrower than in the molar area. The average bone quality in the maxillary molar area was significantly higher when compared with the premolar and incisor areas. The angle between the occlusal plane and the bisector of the alveolar bone in the incisor area was reduced when compared with the molar area., Conclusion: The premolar region appears to be the most favorable area in the maxillary arch for implant placement with regard to bone height, width, angulation, and quality.

Purpose: Reinforcement of maxillary implant overdentures is necessary not only for preventing prosthetic complications but also for protecting underlying structures, as it makes the denture base more rigid. However, few studies have investigated the effects of reinforcement on the underlying structure. The purpose of this study was to examine the effects of reinforcement on implant stress under various implant configurations and denture designs., Materials and Methods: A maxillary edentulous model with implants and experimental overdentures with and without palatal coverage and three types of cast reinforcement was fabricated. Four strain gauges were attached to the implants in the anterior, premolar, and molar areas. A vertical occlusal load of 98 N was applied through the mandibular complete denture, and the strain on the implants was compared using one-way analysis of variance (P = .05)., Results: In dentures with palatal coverage, reinforcement significantly decreased the strain on the anterior and molar implants. In palateless dentures, reinforcement with a palatal bar decreased the strain most on the anterior and molar implants., Conclusion: Reinforcement of maxillary implant overdentures decreased the strain on underlying implants, regardless of the denture design and implant configurations.

Purpose: This study evaluated the impact of angled abutments on strain in implants supporting a distal extension removable partial denture., Materials and Methods: An in vitro model of an implant supporting a distal extension removable partial denture was developed. The implant was positioned with a 17- or 30-degree mesial inclination, with either a healing abutment or a corrective multiunit abutment. Levels of strain under load were compared, and the results were compared using t test (P = .05)., Results: Correcting angulation with a multiunit angled abutment significantly decreased strain (P < .05) when compared with a healing abutment., Conclusion: An angled abutment decreased the strain on an inclined implant significantly more than a healing abutment when loaded under a distal extension removable partial denture.

Purpose: Maxillary implant overdentures are often designed without palatal coverage to maximize wearer comfort. Although palateless dentures have been reported to be less rigid than conventional dentures, and require reinforcement to prevent complications, there is little documentation about the effects of such reinforcement. The purpose of this study was to examine the effects of reinforcement on the strain on maxillary implant overdentures supported by implants in a variety of configurations., Materials and Methods: A maxillary edentulous model with implants inserted in the anterior, premolar, and molar area was fabricated. Five types of experimental overdentures, with and without reinforcement, were fabricated, and two strain gauges were attached at the anterior midline of the labial and palatal sides. A vertical occlusal load of 98 N was applied through a mandibular complete denture, and the shear strain on the denture was measured. The measurements were compared using the Kruskal-Wallis test (P = .05)., Results: On both the labial and palatal sides, the strain on the palateless dentures with reinforcement was significantly lower than the strain on palateless dentures without reinforcement in all implant configurations (P < .05). The labial strain on the palateless dentures with reinforcement was almost as low as the labial strain on dentures with palatal coverage in most implant configurations., Conclusion: Reinforcement of a palateless implant overdenture with residual ridge reinforcement and a palatal bar could reduce the strain in the anterior midline to almost the same level as a denture with palatal coverage. This type of reinforcement may prevent prosthetic and implant complications.

Purpose: Maxillary implant overdentures are often used in clinical practice. However, there is no agreement or established guidelines regarding prosthetic design or optimal implant placement configuration. The purpose of this study was to examine the influence of palatal coverage and implant number and distribution in relation to impact strain under maxillary implant overdentures., Materials and Methods: A maxillary edentulous model with implants and experimental overdentures with and without palatal coverage was fabricated. Four strain gauges were attached to each implant, and they were positioned in the anterior, premolar, and molar areas. A vertical occlusal load of 98 N was applied through a mandibular complete denture, and the implant strains were compared using one-way analysis of variance (P = .05)., Results: The palatolabial strain was much higher on anterior implants than on other implants in both denture types. Although there was no significant difference between the strain under dentures with and without palatal coverage, palateless dentures tended to result in higher implant strain than dentures with palatal coverage. Dentures supported by only two implants registered higher strain than those supported by four or six implants., Conclusion: Implants under palateless dentures registered higher strain than those under dentures with palatal coverage. Anterior implants exhibited higher palatolabial strain than other implants regardless of palatal coverage and implant configuration; it is therefore recommended that maxillary implant overdentures should be supported by six implants with support extending to the distal end of the arch.

Purpose: Root fracture is a common and serious cause of abutment tooth loss. No studies to date have comprehensively assessed the individual contributions of the factors that increase stress on abutment teeth. The purpose of this study was to analyze the stress distribution in abutment teeth based on a three-dimensional finite element model and to analyze the factors that affect stress distribution., Materials and Methods: Models were generated from the computed tomography data of a single patient, consisting of a mandibular second premolar abutment tooth, bone, residual mucous membrane, and a partial removable dental prosthesis (PRDP). Four models were prepared using different types of endodontic posts and cores. Akers clasps were used for the simulated PRDPs, and a vertical load was applied to the occlusal surface of the PRDPs. Debonding between the post and root was simulated. The Young modulus of the residual ridge was reduced to simulate a poor fit between the denture base and the residual ridge. Stress distribution in the abutment tooth root was observed, and the maximum principal stress was evaluated., Results: The nonmetal post model and the mesial rest model reduced stress concentration in the root. The stress increased in models simulating debonding and poor fit. The results of the multiple linear regression analysis confirmed that debonding and poor fit were significantly associated with root stress., Conclusion: Within the limitations of this study, it is suggested that the bonding integrity of posts and adequate fit of the denture base are important factors affecting the longevity of abutment teeth for PRDPs.

Purpose: The partial removable dental prosthesis (PRDP) is used widely to replace missing teeth, but it can disturb the ecology of the mouth. In this retrospective cohort study, the null hypothesis was that there was no difference between tooth loss and the type of edentulous spaces in participants wearing PRDPs., Materials and Methods: Participants were selected from patients who were treated consecutively with PRDPs between January 2003 and December 2006 by undergraduate dental students at the University of British Columbia and attended the clinic for at least 5 years following placement of the PRDP. Clinical records were analyzed for 50 men and 52 women aged 19 to 85 years at baseline, before placement of the PRDP (mean age: 63.2 years). Baseline data were obtained from the records on missing teeth and edentulous spaces, and thereafter data were collected on teeth lost over the following 5-year period. Baseline data on age, sex, occlusal contacts, number of dental rests for the dentures, carious teeth, gingival attachment loss, and teeth with endodontic fillings were also obtained., Results: The presence at baseline of teeth with endodontic fillings significantly (P < .05) increased the prediction of tooth loss over the 5 years, but the type of edentulous space did not., Conclusion: The risk of tooth loss over a 5-year period was higher in the presence of endodontically treated teeth, but no higher in participants wearing PRDPs with distally extended edentulous spaces rather than tooth-bounded edentulous spaces.

Purpose: As maxillary implant overdentures are being increasingly used in clinical practice, prosthodontic complications related to these dentures are also reported more often. The purpose of this study was to examine the influence of palatal coverage and implant distribution on the shear strain of maxillary implant overdentures., Materials and Methods: A maxillary edentulous model with implants inserted in the anterior, premolar, and molar areas was fabricated. Two kinds of experimental overdentures, with and without palatal coverage, were also fabricated, and two strain gauges were attached at the midline of the labial and palatal sides. A vertical occlusal load of 98 N was applied through a mandibular complete denture, and the shear strain in each denture was compared by analysis of variance (P = .05)., Results: In all situations, the shear strain in palateless dentures was significantly higher than in dentures with palate on both sides (P < .05). In dentures with palate, the shear strain was lower when anterior implants were present., Conclusion: Palateless maxillary implant overdentures exhibited much higher strain than overdentures with palate regardless of the implant distribution; this may cause more prosthodontic and implant complications. The most favorable configuration to prevent complications in maxillary implant overdentures was palatal coverage that was supported by more than four widely distributed implants.

Purpose: The aim of this study was to investigate strain on implants used for adjunctive support of distal extension removable partial dentures., Materials and Methods: An implant with strain gauges was used for testing purposes in two positions, parallel and inclined. Three loading scenarios--loading apparatus (LA), artificial teeth via cotton roll (CR), and artificial teeth (UT)--were studied and strains compared via the Kruskal-Wallis test (P=.05)., Results: Strain under CR was significantly larger than UT in parallel (P<.05). However, the opposite was observed in inclined., Conclusions: Strain in parallel was smallest for UT, whereas in inclined it was largest for CR.

Purpose: The purpose of this study was to examine the influence of reinforcement of an embedded cast on the strains within maxillary implant overdentures., Materials and Methods: A maxillary edentulous model with implants placed bilaterally in the canine positions, dome-shaped copings, and experimental overdentures was fabricated. Rosette-type strain gauges were attached in the canine positions and at three points along the midline of the polished surface of the denture and connected to the sensor interface controlled by a personal computer. Experimental dentures with five different reinforcements were tested: without reinforcement; with a cast cobalt-chrome reinforcement over the residual ridge and the tops of the copings; with the same reinforcement from first molar to first molar, over the residual ridge and the tops of the copings; with the same reinforcement over the residual ridge and the sides of the copings; and with the same reinforcement from first molar to first molar, over the residual ridge and the sides of the copings. A vertical occlusal load of 49 N was applied to the first premolar and then to the first molar, and the strains were measured and compared by analysis of variance., Results: In both loading situations, significantly less strain was recorded in dentures with reinforcement than in those without reinforcement. When the first premolar was loaded on dentures with and without palatal reinforcement at the first premolars, the strains on the denture with reinforcement over the tops of the copings were significantly lower than on the denture with reinforcement over the sides of the copings at the canine position., Conclusion: Cast reinforcement over the residual ridge and the top of copings embedded in an acrylic base reduced the strain from occlusal stress on maxillary implant overdentures.

Purpose: The purpose of this study was to determine the implant stress under distalextension removable prostheses (DERPs)., Materials and Methods: A mandibular distal-extension cast and denture were fabricated. Strain gauges were attached to the implant, which was placed either parallel to the distal abutment or perpendicular to the ridge crest (inclined). Occlusal load was applied in five directions, and strains were compared using the Kruskal-Wallis test (P = .05)., Results: When the implant was parallel, the strain was maximum at 20 degrees applied load direction. When the implant was inclined, the strain was minimum at 20 degrees mesial and maximum at 20 degrees applied distal load direction., Conclusions: Implant bending strain is reduced when the implant is loaded on its long axis and oriented parallel with the long axis of the most distal tooth.

This preliminary study examined laboratory-simulated differences between maxillary complete dentures with and without a palate (palateless) as well as the effect of reinforcement of the latter design. Five types of experimental dentures and three types of reinforcements were made. Strain gauges were attached, and a vertical load was applied. The strain was statistically compared using analysis of variance (P = .05). Strain recordings on the palatal side of palateless dentures without reinforcement were significantly higher than in complete dentures and palateless dentures with reinforcement (P < .05). These preliminary observations suggest that such reinforcement with a palatal bar or metal-based palate may reduce the risk of fracture and deformation.

Purpose: Although the risks of using a cantilever to treat missing teeth have been described, the mechanisms remain unclear. This study aimed to reveal these mechanisms from a biomechanical perspective. The effects of various implant sites, number of implants, and superstructural connections on stress distribution in the marginal bone were analyzed with three-dimensional finite element models based on mandibular computed tomography data. Forces from the masseter, temporalis, and internal pterygoid were applied as vectors., Materials and Methods: Two three-dimensional finite element models were created with the edentulous mandible showing severe and relatively modest residual ridge resorption. Cantilevers of the premolar and molar were simulated in the superstructures in the models. The following conditions were also included as factors in the models to investigate changes: poor bone quality, shortened dental arch, posterior occlusion, lateral occlusion, double force of the masseter, and short implant. Multiple linear regression analysis with a forced-entry method was performed with stress values as the objective variable and the factors as the explanatory variable., Results: When bone mass was high, stress around the implant caused by differences in implantation sites was reduced. When bone mass was low, the presence of a cantilever was a possible risk factor. The stress around the implant increased significantly if bone quality was poor or if increased force (eg, bruxism) was applied., Conclusion: The addition of a cantilever to the superstructure increased stress around implants. When large muscle forces were applied to a superstructure with cantilevers or if bone quality was poor, stress around the implants increased.

The aim of this study was to examine the contribution of the palate to denture base support. Four subjects with tooth- or implant-supported maxillary overdentures were enrolled. Recordings (strain values converted to load values) were performed using miniature strain gauges and force transducers for the following conditions: metal framework only (A), denture base with full palatal coverage (B), and denture base without palatal coverage (C). The palatal-supporting ratio (PSR) was calculated using the equation PSR = (B - C) / A. The PSR values were less than 10% in all subjects, suggesting that the palate plays a minimal role in denture base support.

This study evaluated how the contact height between the magnetic attachment and denture base influences stability and bending strain. An implant modified with strain gauges and a magnetic attachment mounted in an acrylic resin block were used to characterize systems with varying degrees or heights of contact with the abutment. Bending strain under lateral loading increased significantly as the contact height decreased. In the no contact and resilient contact groups, magnetic assemblies separated at reduced bending strain in all loading conditions. The contact height of the magnetic attachment influenced the stability and the amount of bending strain on the implant.

The purpose of this study was to compare the strain of complete dentures with and without full palatal coverage and to determine the effect of reinforcement. Three types of maxillary complete dentures (with palate, without palate, and without palate but with reinforcement) were fabricated, and reinforcements were made from a cobaltchromium alloy. Strain gauges were attached to the polished surface, and a vertical load was applied in the region of the first premolar and first molar. The strains were statistically compared. The greatest strain occurred on the anterior palatal surface and was tensile in the denture with palatal coverage and compressive in the denture without palatal coverage. Significantly less strain occurred in dentures with reinforcement than without reinforcement.

The purpose of this study was to examine the influence of reinforcing the structure of an overdenture on stress distribution in the residual ridge using the three dimensional finite element method. Four models of mandibular overdentures with various reinforcement methods were analyzed, including (1) without reinforcement, (2) with chrome-cobalt reinforcing wire, (3) with a reinforcing structure (a cast metal framework) and no reinforcement on the coping top, and (4) with a reinforcing structure and reinforcement on the coping top. The reinforcement adjacent to the top of the coping and the medial part reduces the stress beneath the loading side of dentures and widely and evenly distributes the stress of the residual alveolar ridge.

Purpose: This study identifies clinical factors that predict multiple tooth loss in a socioculturally diverse population of older adults., Materials and Methods: A total of 193 participants from English-, Chinese-, or Punjabi-speaking communities in Vancouver, British Columbia, with low incomes and irregular use of dental services were followed for 5 years as part of a clinical trial of a 0.12% chlorhexidine mouthrinse. The participants were interviewed and examined clinically, including panoramic radiographs, at baseline and annually for 5 years. Binary logistic regression was used to test the hypothesis that there was no difference between incidence of multiple (≥ 3) tooth loss in older people with various biologic, behavioral, prosthodontic, and cultural variables over 5 years., Results: Multiple tooth loss, which was distributed similarly among the groups in the trial, occurred in 39 (20%) participants over 5 years. The use of removable prostheses was the best predictor of loss, followed by the number of carious surfaces and number of sites with gingival attachment loss > 6 mm. The pattern of prediction was consistent across the three linguocultural groups., Conclusion: The use of removable dentures was the dominant predictor of multiple tooth loss in the three communities, but that tooth loss was not significantly associated with the cultural heritage of the participants.

The purpose of this preliminary investigation was to assess the influence of palatal morphology on deformation of maxillary complete dentures in vivo. The palatal morphology of the maxillae of eight edentulous subjects was measured. Experimental dentures were fabricated, and a strain gauge was attached at the first molar position at the midline of the polished surface of each denture. Subjects were instructed to bite a metal bar placed bilaterally at the denture's first molar region with a force of 49 N. The resultant strains were recorded, and the correlation between strain and palatal morphology was evaluated using the Pearson correlation coefficient. A strong correlation between strain and both palatal depth and radius of curvature was noted, suggesting that edentulous patients with wide, shallow palates have a higher risk of denture deformation, which may lead to material fracture.

The purpose of this study was to test intraorally the effect of a cobalt-chromium cast reinforcement embedded in the denture base of a maxillary complete denture on the distribution of stress. Eight subjects were selected randomly, and three designs of a cobalt-chromium alloy bar were embedded in the dentures. A strain gauge was attached at three positions around the midline of the polished surface of each denture. Strain in the reinforced dentures was significantly less than strain in dentures without reinforcement. These observations suggest that a cast cobalt-chromium reinforcement reduces strain and could contribute to fracture avoidance deformation in maxillary complete dentures.

Seven implants provided with strain gauges were placed in an acrylic block to evaluate the biomechanical performance of short implants in splinted restorations. Two 7-mm-long implants with the same diameter (3.8, 4.4, or 5.0 mm) were splinted together (short-short implant splinted restoration [SS]) or individually with a 4.4 ? 12.0-mm implant (short-long implant splinted restoration [SL]), and a 50-N oblique load was applied to both restorations. The strain decreased significantly with an increase in implant diameter in both the SS and SL restorations, and the observed strain was identical between splinted implants of the same diameter and those splinted with the long implant, suggesting that splinting of two short implants has the same biomechanical effectiveness as splinting with a single long implant.

The aim of this study was to establish a method for in vivo examination of the load-bearing ratio between mucosa and abutments beneath an overdenture. Two patients wearing a four tooth-supported or a four implant-supported overdenture were enrolled in this study. Recordings were performed with the metal framework only or with a metal framework and a denture base. The force value with the framework only was designated as 100%, and the tissue-supporting ratio (TSR) with the denture base was calculated. The TSR was approximately 30% to 40% in both subjects, regardless of the load. These data suggest that measurement of a TSR beneath an overdenture is feasible.