Effect of traction direction and pressure load on the palatal plate on retentive force.

BACKGROUND: Recently, a denture adhesive that is easy to clean and contain moisturizing ingredients have been developed for patients with dry mouth. Although the retentive force produced by conventional denture adhesives and oral moisturizers have been compared on models, no study has reported their comparison in the oral cavity. In this study, we aimed to clarify the effects of different directions of traction and loads at the time of pressure contact on the retentive force on a palatal plate made from a dentulous jaw model. METHODS: A palatal plate was fabricated with thermoplastic resin on a dentulous jaw model, and a loop-shaped traction device was attached to the centre of the palate. The test samples were a cream-type denture adhesive, a denture adhesive for dry mouth, an oral moisturizer, and a denture moisturizer. The specimens were applied to the inner surface of the plate, which was then mounted under vertical pressure. Then, the retentive force was measured, using a digital force gauge, while the plate was pulled in different directions and with different loads. RESULTS: No significant difference in retentive force was observed in any of the test samples when the direction of traction was between 45° and 60°. The retentive force of the denture adhesive for dry mouth was significantly higher with a direction of traction of 90° than that of 45° or 60°. The retentive force when oral moisturizer was used was significantly higher in the 90° traction direction than in the 45° direction. There was no significant difference between a force of 4.0 kgf or 5.5 kgf when using a denture adhesive for dry mouth. Comparing the four load size conditions, the larger the load, the higher was the retentive force. Similar results were obtained for the cream-type denture adhesive and denture moisturizer. Significantly higher retentive force was observed for larger loads when oral moisturizer was used. CONCLUSIONS: The results suggest that the direction of traction and the pressure load affect the retentive force on a palatal plate.

Comparison of oral hypofunction tests and determination of reference values for a subjective masticatory function test.

BACKGROUND: In Japan, oral hypofunction has been recognized as a disease since 2018. An alternative to occlusal force testing for assessing oral hypofunction is the evaluation of the number of natural teeth. Subjective masticatory function testing, which evaluates the ease or difficulty in chewing foods, is an effective alternative to occlusal force testing. However, no reference values have been established for this test. We determined the reference values of the subjective masticatory function test and evaluated its potential as a substitute for the number of natural teeth for assessing oral hypofunction. METHODS: The sample consisted of 184 older adults who visited the Department of Geriatric Dentistry, Showa University Dental Hospital, from July 2018 to January 2020. The subjective masticatory function test (table for evaluation of chewing function in complete denture wearers [Chewing Score 20]) was performed using 20 foods. The occlusal force test and a receiver operating characteristic curve were used to determine the reference values for Chewing Score 20. The sensitivity, specificity, and positive and negative predictive values were calculated and compared with the occlusal force test and the number of natural teeth. RESULTS: A significant correlation (r) was found between the occlusal force test and the Chewing Score 20 (r = 0.526, p < 0.001). The reference value for Chewing Score 20 was < 85. Although the Chewing Score 20 was less sensitive than the number of natural teeth, it demonstrated a higher specificity and a positive predictive value. CONCLUSION: Herein, a score of < 85 on the subjective masticatory function test was determined to be the optimal quantitative reference. The subjective masticatory function test may be used as an alternative for assessing oral hypofunction.

Changes in denture retention with denture adhesives and oral moisturizers for the oral cavity: an in vitro study.

BACKGROUND: It is difficult to maintain complete dentures during meals in place. This in vitro study aimed to assess changes in denture retention between rest and function using denture adhesives and oral moisturizers in an oral cavity model. METHODS: The following test samples were applied between the palatal plate and the edentulous jaw ridge model: denture adhesive, denture adhesive for dry mouth, oral moisturizer, and denture moisturizer. The retentive force was measured under two conditions: at rest while immersed in water and during function with a 2.5-kg load applied. The plate was pulled perpendicular to the occlusal plane and the retentive force was measured using a digital force gauge. RESULTS: Under dry conditions, denture adhesive for dry mouth and oral moisturizer had a significantly higher retentive force than denture adhesive and denture moisturizer. After 30 min of immersion in water, the retentive force of the denture adhesive increased while that of the oral moisturizer decreased. After 30 min of function, the retentive force of the denture adhesive and denture adhesive for dry mouth remained high, while that of the oral moisturizer and denture moisturizer significantly decreased. Between rest and function, the retentive force of the denture adhesive and denture adhesive for dry mouth was high, and that of the oral moisturizer was low. CONCLUSIONS: Immediately after use, denture adhesive for dry mouth exhibited high retentive force, but retention gradually decreased due to its water content.

Experimental study on the factors affecting torque of beam-type implant torque wrenches.

BACKGROUND: Screw breakage and loosening are the most common mechanical complications associated with implant treatment, and they may occur due to excess or inadequate screw tightening torque. When fastening and fixing the implant superstructure, screws are tightened using a torque wrench, which is essential for an accurate tightening force. However, the characteristics of the torque wrench have not been fully verified. Therefore, we aimed to clarify the factors affecting the torque with a focus on beam-type torque wrenches, which are the main types of wrenches. METHODS: The torque values generated by beam-type torque wrenches from eight manufacturers were measured using a torque gauge. To investigate the influence of the location of the beam relative to the scale, measurements were performed with a scale aligned with the trailing edge, center, and leading edge of the beam respectively. Additionally, measurements were taken at 90°, 60°, and 30° to examine the effect of the angle at which the examiner read the torque value. Under each condition, a single examiner applied the recommended torque to each manufacturer's screws five times in a clockwise direction. The average measured torque, standard deviation, bias, and coefficient of variation were calculated and compared accordingly. RESULTS: Wrenches from six manufacturers demonstrated excellent accuracy for measurements at the center of the beam (bias within ± 4%). For measurements at 90°, equipments from five manufacturers displayed excellent accuracy (bias within ± 7%), and seven showed excellent repeatability (coefficient of variation ≤ 2%). CONCLUSION: The scale should be aligned with the center of the beam and read from 90° while using a torque wrench. The accuracy and repeatability torques generated by the wrenches differed according to the manufacturer, scale width, scale line width, beam width, and distance between the scale and beam center. Based on these results, we suggest that a torque wrench must be selected after determining the difference in the structure of the torque wrench.

Influence of denture surface roughness and host factors on dental calculi formation on dentures: a cross-sectional study.

BACKGROUND: Dental calculi formation on dentures can worsen the oral cavity environment by complicating oral hygiene. However, few studies have investigated the effect of how patients use and manage their dentures, denture surface roughness, and host factors such as oral cavity dryness and saliva properties on denture cleanliness and denture dental calculi formation. Accordingly, we conducted the present survey to evaluate these factors to clarify the strength of the influence of each factor. METHODS: We enrolled 53 patients who had used dentures for at least 3 months and used a dental prosthesis that covered at least the six front teeth including the left and right mandibular canines. After staining the dentures, we divided the participants into a group that was positive for dental calculi (DCP group) and a group that was negative for dental calculi (DCN group). After removing all the stains, we evaluated the surface roughness of the dentures. A questionnaire was used to survey how the participants used and managed their dentures. Oral cavity dryness was evaluated, and resting saliva samples were collected to assess saliva properties. Correlations between the presence or absence of dental calculi and denture use and management were evaluated using a chi-square test. Correlations with denture surface roughness, oral cavity dryness, and saliva properties were evaluated using the Mann-Whitney U test. Correlations between the presence or absence of dental calculi and all factors were analyzed using multivariate analysis (quantification II). RESULTS: Surface roughness was significantly greater in the DCP group (p < 0.01), and the DCP group members wore their dentures during sleep significantly more often and used a denture cleaner when storing their dentures significantly less often (both p < 0.01). No significant differences were observed for oral cavity dryness or saliva properties. The multivariate analysis showed significant correlations of dental calculi formation with denture surface roughness and items related to denture use and management, but not for oral cavity dryness or saliva properties. CONCLUSIONS: Our findings indicate that dental calculi formation is influenced by how dentures are used and managed and by denture surface roughness, but not by oral cavity dryness and saliva properties.

Relationship between retention forces and stress at the distal border in maxillary complete dentures: Measurement of retention forces and finite-element analysis in individual participants.

STATEMENT OF PROBLEM: Although the retention force of maxillary complete dentures has been measured in numerous studies with different devices, the biomechanical mechanism associated with the generation of this retention force cannot be determined. PURPOSE: The purpose of this clinical study was to investigate whether 3-dimensional finite-element analysis can be used to estimate the retention force of maxillary complete dentures. MATERIAL AND METHODS: The study included 12 participants (6 men and 6 women, mean 77.5 years of age). Replicas of the maxillary complete dentures of all the participants were made using scanning resin. The denture replicas were scanned using cone-beam computed tomography (CBCT), and 3-dimensional finite-element models were constructed (dentures, mucosa, and jig). The tensile site was located 5 mm anterior from the central point of the denture's posterior border, and the loading site was located at the central point of the central incisor edge and the right first premolar buccal cusp. The load was 10 N and perpendicular to the occlusal plane. One-way ANOVA was calculated to determine any differences in the maximum principal stress value among the 3 sites. The Games-Howell test for multiple comparisons was applied to determine which sites were different. A Spearman rank correlation coefficient was used to determine any correlation between the retention force and maximum principal stress at measurement posterior site, and a Pearson correlation coefficient was used at the central incisor edge and premolar buccal cusp (all α=.05). Comparative investigations of the association between the maximum principal stress generated and the denture retention force were carried out. RESULTS: The stress distribution of the maximum principal stress at each measurement point was similar for each participant. The maximum principal stress at the posterior site measurement was significantly higher than measurements at the incisor edge and posterior buccal cusp (P<.01). The maximum principal stress the posterior and incisor edge sites were found to be correlated (P<.05). CONCLUSIONS: This study results suggest that although more factors need to be considered, a 3-dimensional finite-element analysis may be used to estimate the retention force of maxillary complete dentures.

Fabrication of a customized tray for preventing fracture of isolated abutment teeth in definitive casts.

Isolated abutment teeth with advanced bone resorption on definitive casts for a partial removable dental prosthesis can fracture when the definitive cast is removed from the impression or during subsequent laboratory procedures. This report describes a technique that avoids fracture of the definitive cast during its fabrication. A separating line is formed on the custom tray, which enables removal of the definitive cast without fracturing the isolated tooth. In addition, if the cervical line is sharp and appears compromised, then the impression can be trimmed at the clinical cervical line to enlarge the abutment diameter and increase its resistance to fracture without altering the shape of critical areas.

Associations between Oral Hypofunction Tests, Age, and Sex.

Oral function declines in older individuals due to disease and age-related changes, making them vulnerable to oral and physical frailty. Therefore, it is important to manage the decline in oral function in older outpatients. Oral hypofunction is diagnosed by seven tests related to oral function, oral hygiene, oral moisture, occlusal force, oral diadochokinesis, tongue pressure, masticatory function, and swallowing function. However, sex or age were not factored into the current reference values of these tests. We included subjects attending the dental hospital clinic for maintenance, and recorded and analyzed oral hypofunction and the factors associated with its diagnosis. Of the 134 outpatients (53 males and 81 females, mean age 75.2 ± 11.2 years), 63% were diagnosed with oral hypofunction. Oral hypofunction prevalence increased significantly with age, and significant variations were observed in all tests. Furthermore, oral hygiene and swallowing function were not associated with oral hypofunction diagnosis. All examined factors decreased with increasing age, even after adjusting sex, except for oral hygiene and moisture. Occlusal force and masticatory function were higher in men after adjusting age. This study suggested that older outpatients were likely to be diagnosed with oral hypofunction, and that the test reference value and their selection for oral hypofunction should be reconsidered.

Effectiveness of a clinical pathway for complete denture adjustment.

This study was performed to evaluate the usefulness of a developed clinical pathway (CP) to determine the sequence of dental treatment in patients who are edentulous and use maxillary and mandibular complete dentures. Sixty-two edentulous patients with maxillary and mandibular complete dentures were randomly allocated either to with (31 subjects) or without (31 subjects) a CP. The main categories of the CP were the examination, instructions, practice, idle time, evaluation sheet, preparation, and other steps. The subcategories were made with respect to the main categories. The participants were categorized as dentists, assistants, or patients. There was no significant difference in mean chair time between with a CP (27.5 min) and without a CP (28.0 min). The CP led to a significant reduction in idle time (waiting in the chair or no treatment) of dentists, assistants, and patients. In contrast, the instances of medical examination, patients' instructions, and the number of patients reporting improvement of symptoms significantly increased. These results indicated that there is an opportunity to improve the complete denture treatment with a CP.

Systematic clinical evaluation and correction procedures for support of removable partial dentures.

Support, retention, and bracing are the three main functions of a direct retainer in removable partial dentures (RPDs). RPDs must have sufficient supporting ability for proper occlusal rehabilitation. Support ability depends on the fit, size, shape, and location of the occlusal rest. Support cannot be adjusted chairside in most cases. The purpose of this article is to present systematic evaluation criteria for support in an RPD and to describe methods for adjusting cast clasps with improper support. Appropriate design of RPDs and preparation of abutments are also described.

[Effect of clinician's experience and degree of difficulty on chair time for complete denture treatment].

PURPOSE: The Japan Prosthodontic Society has presented a classification system for teeth defects, and partial and complete edentulism. The classification system consists of two major sections and four divisions. Within each division there are four levels, which relate to degree of difficulty of treatment: Level I: ideal or minimally compromised; Level II: moderately compromised; Level III: substantially compromised; and Level IV: severely compromised. The purpose of this study was to evaluate the effects of the clinician's experience and the classification of degree of difficulty on chair time required for complete denture treatment by conducting a self-assessment survey of dentists. METHODS: A questionnaire relating to clinician's experience (prosthodontic specialist certified or not) and the chair time required for each step of complete denture treatment was sent by post to clinicians. RESULTS: The questionnaire was answered by 311 clinicians from 14 universities. Higher levels of difficulty required greatly increased chair time, whereas the treatment fee/hour decreased greatly. In addition, chair time required by prosthodontic specialist was short; however, chair time increased with higher levels of difficulty. CONCLUSION: Chair time and treatment number required by prosthodontic specialists (certified by Japan Prosthodontic Society) were shorter than those appertaining to non-prosthodontic specialist. However, chair time increased with Level III and Level IV by prosthodontic specialist.

[New estimation method for displacement characteristic of teeth with occlusal force records].

OBJECTIVES: To establish adequate occlusion for prostheses, evaluation of degree of displacement of teeth and implant is essential. However, it cannot be attained without complex equipment, which limits clinical application. To develop a new estimation method for displacement characteristic of teeth and implant by both occlusal force of individual teeth and existing data on degree of displacement, the relationship between increase of total occlusal force and occlusal force of individual teeth was evaluated. METHODS: Ten male subjects (mean age: 28.5, [range: 26-31] years) with clinically normal healthy dentition were selected. Electromyograms at maximum clenching at intercuspal position were recorded as 100 MVC. Then, clenching at 80, 60, 50, 30, 20, and 10 MVC was instructed with visual feedback. Occlusal forces were recorded with pressure-sensitive sheet (Dental-Prescale). The occlusal contacts were recorded by a silicone occlusal contact checking material (Black Silicone). The change of occlusal forces at first premolar was converted to displacement with existing data of degree of displacement (Goto et al). Then displacement characteristic of second premolar and first molar was calculated. RESULTS: The displacement characteristic of second premolar was similar to existing data on first premolar. Although the displacement characteristic of first molar was similar, the degree of displacement was small, which means occlusal force at the first molar increased more than at the first premolar as increase of the displacement. CONCLUSION: The results of this study, indicating similarity to past studies with complex equipment, suggest that the displacement characteristic of teeth could be estimated with both occlusal force of individual teeth and existing data on degree of displacement.

[Evaluation of new complete denture treatment by dentists and patients].

PURPOSE: With an increase in the aged population in Japan, it is important that clinicians evaluate the outcome of complete denture treatment based on evaluation by dentists and patients to provide high-quality treatment. The purpose of this study was to evaluate the evaluation and to clarify the relationship between them by dentists and patients. METHODS: Forty patients wearing complete dentures were selected, after obtaining informed consent. Scores of complete denture, supporting tissue, chewing function, satisfaction and QOL were the factors used for evaluation. These were evaluated at four stages in complete denture treatment: at the initial visit, before insertion of new dentures (just before), first visit after insertion of new dentures (1st visit) and 1 month after insertion. We compared scores obtained just before and at the 1st visit using t-test, and analyzed changes in scores with correlation coefficient. In addition, the relationship between dentists and patients was analyzed with kappa statistic. RESULTS: Evaluation of the new denture was influenced by the evaluation of the old denture. In case of high evaluation of old dentures, evaluation of new dentures tended to decrease. Conversely, in case of low evaluation of the old dentures, evaluation of new dentures tended to increase (p < 0.01) . There was no significant correlation between the evaluation by the dentists and the patients. CONCLUSIONS: Changes after wearing new dentures and the relationships were clarified. The disagreement of the evaluation by dentists and patients suggests that both evaluations should be done comprehensively.

[A study on the utility of a scale for examination in residual ridge height based on an objective assessment].

PURPOSE: To establish a simple and objective assessment of the residual ridge height of edentulous patients, we investigated the utility of a newly developed "examination scale" based on our clinical research. METHODS: The sites corresponding to the right and left first molars in study models (100 sets) of edentulous maxillae and mandibles were examined. Subjective assessment and assessment with the three-grade examination scale (high, middle, low) were performed by 16 dentists (less than 3 years of clinical experience: 8, more than 7 years of clinical experience: 8). Objective assessment of residual ridge vertical height was performed with digital vernier calipers. Agreement among the subjective assessment, objective assessment, and assessment with the examination scale was analyzed with kappa values. RESULTS: Marked variations were noted in the relation between the subjective assessment and objective assessment regardless of the individual's clinical experience. Agreement between the subjective assessment and objective assessment was low (maxillae: less than 3 years of clinical experience: 0.38, more than 7 years of clinical experience: 0.32, mandibles: less than 3 years of clinical experience: 0.35, more than 7 years of clinical experience: 0.43). However, agreement between the assessment with the examination scale and objective assessment was high (maxillae: less than 3 years of clinical experience: 0.68, more than 7 years of clinical experience: 0.69, mandibles: less than 3 years of clinical experience: 0.57, more than 7 years of clinical experience: 0.60). CONCLUSION: The assessment by the newly developed examination scale for residual ridge height of edentulous patients agreed well with the objective assessment, indicating the usefulness of this scale.

Device for easily measuring of the torque of an implant hand driver.

BACKGROUND: As one of the loads applied in implant dentistry, managing the torque is important for the success of an implant treatment. For this purpose, it is crucial to ascertain the level of torque being exerted with a hand driver. We have developed an adapter that makes it easy to measure torque by using a standard torque wrench in one's possession, rather than with a torquemeter. PROCEDURES: The head of an abutment screw is cut into a hexagon and pushed into and fixed to the hexagonal hole (for multi-unit abutments) of a machine driver. With this, a torque wrench adapter and torque wrench are assembled. A hand driver is rotated clockwise to the limit, and the torque value is read. RESULTS: It was possible to read the torque value during screw fastening. CONCLUSIONS: This technique makes it easy for each dentist to measure the maximum torque that can be exerted by a hand driver he or she is using. It is even possible to handle different implant systems.

Biomechanical effects of offset placement of dental implants in the edentulous posterior mandible.

BACKGROUND: Proper implant placement is very important for long-term implant stability. Recently, numerous biomechanical studies have been conducted to clarify the relationship between implant placement and peri-implant stress. The placement of multiple implants in the edentulous posterior mandible has been studied by geometric analysis, three-dimensional finite element analysis (FEA), model experimentation, etc. Offset placement is a technique that reduces peri-implant load. However, few studies have used multiple analyses to clarify the value of the offset placement under identical conditions. The present study aimed to clarify the biomechanical effects of offset placement on the peri-implant bone in edentulous posterior mandibles by comparative investigation using FEA and model experimentation with strain gauges. METHODS: Three implants were embedded in an artificial mandible in the parts corresponding to the first premolar, the second premolar, and the first molar. A titanium superstructure was mounted to prepare models (experimental models). Three load points (buccal, central, and lingual) were established on the part of the superstructure corresponding to the first molar. Three types of experimental models, each with a different implant placement, were prepared. In one model, the implants were placed in a straight line; in the other two, the implants in the parts corresponding to the second premolar and the first molar were offset each by a 1-mm increment to the buccal or lingual side. Four strain gauges were applied to the peri-implant bone corresponding to the first molar. The experimental models were imaged by micro-computed tomography (CT), and FEA models were constructed from the CT data. A vertical load of 100 N was applied on the three load points in the experimental models and in the FEA models. The extent of compressed displacement and the strain in the peri-implant bone were compared between the experimental models and the FEA models. RESULTS: Both experimental and FEA models suffered the least compressed displacement during central loading in all placements. The greatest stress and compressive strain was on the load side in all types of placements. CONCLUSIONS: Offset placement may not necessarily be more biomechanically effective than straight placement in edentulous posterior mandibles.

[The relationship between subjective evaluation and objective evaluation on the examination for the residual ridge height of edentulous patients].

PURPOSE: Appropriate examination of the height of the residual ridge is critical in complete denture treatment. To clarify the possibility of establishing standards for improving the reproducibility of subjective evaluation, the relationship between conventional subjective evaluation by several dentists and objective evaluation by model measurement was evaluated. METHODS: Locations for evaluation were the sites corresponding to the central fossa of the right and left first molars in study models (100 sets) of edentulous maxillas and mandibles. Subjective evaluation with a three-grade scale was performed by some dentists and objective evaluation was performed with digital vernier calipers. The relationship between objective evaluation and subjective evaluation was studied separately in dentists with less than 3 years of clinical experience and those with more than 7 years. Discrimination values for objective evaluation were determined at changeover points of the rate of subjective evaluation. In addition, we studied the correction between subjective evaluation and objective evaluation with the discrimination values. RESULTS: Marked variations were noted in subjective evaluation depending on the individual and number of years of clinical experience. Discrimination values were obtained from subjective evaluation for dentists with more than 7 years of clinical experience: for the maxilla, "low" and "middle" were 6 mm and "middle" and "high" were 10 mm; for the mandible, "low" and "middle" were 0 mm and "middle" and "high" were 5 mm. The correlation between subjective evaluation and objective evaluation based on these discrimination values was extremely low regardless of the individual's clinical experience. CONCLUSIONS: Reproducibility of subjective evaluation of the residual ridge height was low. To improve the universality and reproducibility of the examination, subjective evaluation must be performed by referring to the discrimination values.

A biomechanical investigation of mandibular molar implants: reproducibility and validity of a finite element analysis model.

BACKGROUND: Three-dimensional finite element analysis (FEA) is effective in analyzing stress distributions around dental implants. However, FEA of living tissue involves many conditions, and the structures and behaviors are complex; thus, it is difficult to ensure the validity of the results. To verify reproducibility and validity, we embedded implants in experimental models and constructed FEA models; implant displacements were compared under various loading conditions. METHODS: Implants were embedded in the molar regions of artificial mandibles to fabricate three experimental models. A titanium superstructure was fabricated and three loading points (buccal, central, and lingual) were placed on a first molar. A vertical load of 100 N was applied to each loading point and implant displacements were measured. Next, the experimental models were scanned on micro-computed tomography (CT) and three-dimensional FEA software was used to construct two model types. A model where a contact condition was assumed for the implant and artificial mandible (a contact model) was constructed, as was a model where a fixation condition was assumed (a fixation model). The FEA models were analyzed under similar conditions as the experimental models; implant displacements under loading conditions were compared between the experimental and FEA models. Reproducibility of the models was assessed using the coefficient of variation (CV), and validity was assessed using a correlation coefficient. RESULTS: The CV of implant displacement was 5% to 10% in the experimental and FEA models under loading conditions. Absolute values of implant displacement under loading were smaller in FEA models than the experimental model, but the displacement tendency at each loading site was similar. The correlation coefficient between the experimental and contact models for implant displacement under loading was 0.925 (p < 0.01). The CVs of equivalent stress values in the FEA models were 0.52% to 45.99%. CONCLUSIONS: Three-dimensional FEA models were reflective of experimental model displacements and produced highly valid results. Three-dimensional FEA is effective for investigating the behavioral tendencies of implants under loading conditions. However, the validity of the absolute values was low and the reproducibility of the equivalent stresses was inferior; thus, the results should be interpreted with caution.

Occlusal status of implant superstructures at mandibular first molar immediately after setting.

BACKGROUND: Occlusal contact on the implant superstructures is important for successful treatment. The purpose of this study was to investigate the occlusal contact of single implant superstructures at the mandibular first molar immediately after seating from weak to strong clenching. METHODS: Subjects were nine patients who had just been fitted with an implant prosthesis in the mandibular first molar region, with no missing teeth other than in the implant region. First, while masseter muscle activity was monitored, maximum clenching strength (100 % maximum voluntary contraction (MVC)) was determined with an electromyogram. Next, occlusal load and occlusal contact area were measured three times at clenching intensities of 40, 60, 80, and 100 % MVC by the use of pressure-sensitive film for occlusal force diagnostic and Occluzer for occlusal force measurement. Finally, the occlusal contact area was measured once each at 20, 40, and 60 % MVC using a silicone testing material and BiteEye for occlusal contact measurement. A two-way analysis of variance (ANOVA) was used to determine occlusal loading and occlusal area as dependent variables, and clenching strength and presence or absence of implant as between-subject factors. A multiple comparison test was performed using the Bonferroni method. RESULTS: The occlusal contact area and occlusal load of the implant prosthesis increased with clenching strength, and the increases in occlusal contact area and occlusal load of the implant prosthesis were less than those of the contralateral tooth at high clenching strength. However, significant difference was not observed when compared with both sides of the molar region regardless of clenching strength. CONCLUSIONS: The occlusal contact area of the implant had a tendency to be adjusted smaller than the natural tooth by a dental technician and a dentist. On the other hand, despite the small tissue displaceability of the implant, occlusal load on the implant prosthesis was smaller than on the natural tooth at high clenching strength.

Influence of occlusal loading force on occlusal contacts in natural dentition.

PURPOSE: Proper occlusal contact is important for the long-term success of prosthodontic therapy. We clarified the effects of occlusal loading force on occlusal contact in natural dentition by comparing measured values for occlusal loading and occlusal contact area. METHODS: Masseter muscle activity was measured in 10 subjects (2 male, 8 female; mean age, 27 years) with natural dentition using electromyography, with clenching at full strength with nothing interposed between the upper and lower teeth defined as 100% maximum voluntary contraction (MVC). Pressure-sensitive film (Occluzer) was used to examine occlusal contact points at 20, 40, 60, 80, 100 and 120% MVC. A material for checking accuracy of fit (BiteEye) was used to examine occlusal contact points at 20, 40, 60 and 80% MVC. ANOVA and the Bonferroni method were used to assess the results, with the level of significance set at 5%. Coefficients of variation (CV) were also calculated by dividing the standard deviation by the mean. RESULTS: Occlusal loading and occlusal contact area increased with clenching strength; however, CV showed differences between the methods at low and high MVC. CONCLUSIONS: With Occluzer, testing should be carried out at clenching strength ≥ 60% MVC. With BiteEye, testing should be carried out from light clenching strength at 20% MVC to moderate clenching strengths at 40-60% MVC. Occluzer and BiteEye (10 µm) gave similar occlusal contact areas at 60-80% MVC. These results suggest that combined use of Occluzer and BiteEye gives an accurate picture of occlusion from weak to strong clenching strength.