The International Journal of Prosthodontics, 05/1999

Purpose: The aim of this article is to describe the indication criteria and the treatment planning for a maxillary implant-supported removable overdenture. Prostheses are designed according to the requirements of the bar system and the factors influencing the extension of the prosthesis base. Materials and Methods: The decisive factors in determing whether a bar-retained overdenture prosthesis is indicated should be evaluated during the initial clinical examination and with the helpf of a reformatted computed tomographic (CT) scan that is performed with a radiologic template in place. Titanium markers represent the ideal location of the denture teeth in the diagnostic setup so that the implant position can be selected and the available space for the bar system can be assessed vertically and horizontally. Results: For the overdenture prosthesis that is solely implant supported 6 to 8 implants are placed ideally at a distance of about 10 to 14 mm from center to center. A prefabricated bar system that allows the clips to be inserted between the implants can then be used. When the available bone restricts implant placement to adjacent tooth positions an individually milled bar the includes additional frictional pins and/or retentive elements needs to be planned. The prosthesis design, in particular its buccal and palatal flange extension, is determined during setup try-in, taking into account the patient's smile line, their need for facial support, and their phonetic requirements. Conclusion: The removable implant-supported overdenture offers flexibility in placing implants in either adjacent tooth positions or with greater distances between them depending on the available bone, as either conventional bar and clip systems or individually milled bars can be used. Adjustment of the buccal prosthesis flange and the palatal prosthesis base is made to fulfill the patient's requirements concerning esthetics, phonetics, comfort, and function.

Purpose: The aim of this article was to present the oral rehabilitation of a patient with a large, created intraoral defect after the surgical extirpation of a pathologic process. Materials and Methods: On osseointegrated oral implants, a bar was fabricated to support a partial overdenture by means of attachments. Results: The treatment was successful and the construction has been in service for more than 5 years. Conclusion: In cases with large intraoral defects removable dentures may have clinical advantages over fixed partial dentures. In this patient the restoration of missing teeth, alveolar crest, and soft tissue was accomplished at the same time oral function was restored, and good oral hygiene access was obtained.

Purpose: This study tested the hypothesis that a scanning laser 3-dimensional digitizer is a precise and accurate instrument to measure chamfered and beveled margins of partial-coverage tooth preparations for computer-aided design/computer-aided manufacturing (CAD/CAM). Materials and Methods: The margins were measured by the digitizer on stone dies and calculated by triangulation into a 3-D representation. Instrument precision was defined as the ability to reproduce the same margin in repeated measurements and expressed as the coefficient of variation as a percentage. Instrument accuracy for chamfered and beveled margins was estimated by correlating their measurements to the measurement of the margin of a spherical calibration 'phantom' with known dimensions. Accuracy was expressed as the standard deviation. Results: The precision errors for the box- and cusp-chamfered margins and cusp-beveled margins were 3.9%, 3.4%, and 2.4%, respectively. With regard to accuracy the standard deviations of the measurements of the box- and cusp-chamfered margins and cusp-beveled margins were 19 µm, 21 µm, and 24 µm, respectively, compared to 15 µm for the phantom. Conclusion: Measurements of chamfered and beveled margins by a scanning laser 3-D digitizer for CAD/CAM are (1) precise (error 4%) and (2) accurate, with a standard deviation of less than 9 µm compared to optimal measurements of the spherical margin of the phantom.

Purpose: There is considerable interest in the European dental research literature about the problem of tooth wear and specifically about dental erosion, but this interest does not appear to be matched in North America based on the volume of the literature there. The purpose of this article is to consider the possible explanations for this difference. Materials and Methods: This article examines the reasons for this disparity and attempts to explain the difference by reviewing the North American and European literature on the etiology, pathogenesis, and prevalence of tooth wear. Results: It would appear from the literature that the reason for the difference in interest between the 2 continents is a reflection of how the appearance, etiology, and terminology are interpreted and used to define tooth wear, attrition, and erosion. Conclusion: Attrition is the wear of teeth against teeth; therefore, by definition any worn surface that does not contact the opposing tooth must have another etiology. An appropriate descriptive term is 'tooth wear' when the etiology is multifactorial or cannot be determined. A search of the literature shows more studies in the European literature of the etiology and prevalence of tooth wear than in the North American literature. The thrust of the European studies supports the view that erosion is more important than attrition in the etiology of tooth wear.

Purpose: The purpose of this article was to compare the wear behavior of precision attachments with plastic inserts to conventional metal-alloy precision attachments. Materials and Methods: In a comparative study attachments of various designs were subjected to alternating load cycles in a wear simulator. In addition to conventional adjustable attachments with metal-alloy matrix and patrix elements, attachments with female elements that are lined with plastic inserts were investigated for the first time. In each wear test 10,000 separating and joining movements were performed in an axial direction under a continuous spray of artificial saliva at 37°C. Results: The attachments with metal surfaces showed a rapid loss of approximately 60% of the required separating/joining forces during the first 1,000 cycles; after a further 9,000 cycles these forces fell to 25% and 35%, respectively, of the initial value. The attachments with plastic inserts, by contrast, showed only a 4% and 8% loss, respectively, of the required separating/joining forces even after 10,000 wear cycles. With one attachment type a reproducible 20% increase of retention occurred during the testings. Conclusion: The precision attachments with plastic female inserts showed only negligible amounts of wear and the most consistent retentive force in comparison with conventional precision attachments consisting of metal-alloy matrix and patrix components.

Purpose: In a previous article the authors examined the evolution of the bond strengths of 2 dental feldspathic ceramics. The objective of the present study was to evaluate the effect of surface modifications with hydrofluoric acid gel (concentration 10%) on the surface energy of 2 dental feldspathic ceramics (GC and PVS). Materials and Methods: For an energy characterization, 30 samples of GC and 30 samples of PVS were built. This study comprised the measurement of contact angles to determine the work of adhesion (WA) of the 2 ceramics. The evolution of the work of adhesion depended on the action of the hydrofluoric acid gel on the roughness of the surface of the 2 ceramics. Results: In a polished state PVS presented a higher work of adhesion than GC. Etching the ceramics with hydrofluoric acid gel increased the work of adhesion, especially for GC, but this treatment was not sufficient to obtain a high work of adhesion. Conclusion: Etching with hydrofluoric acid gel was not sufficient to raise the work of adhesion of the 2 ceramics. Silanization is preferable to etching.

Purpose: The purpose of this study was to test the hypothesis of a minimum electromyographic (EMG) rest position based on masseter surface EMG recordings of incremental opening and closing of the mandible with simultaneous audio EMG biofeedback. Materials and Methods: Nineteen alert subjects in an upright seated position opened and closed the mandible in 1-mm increments 20 mm interincisally. An electronic recording device allowed each subject to maintain the vertical dimension of each increment while simultaneously reducing right masseteric muscle activity to the minimum possible level using audio EMG biofeedback. Integrated EMG masseteric activity was recorded at each static opening and closing increment. Results: A mean plateau of integrated EMG output for all subjects with no minimum EMG point or circumscribed minimum EMG range for any of the nineteen subjects was shown. Analysis of variance for repeated measures showed no difference in opening and closing EMG levels (P = 0.27) and no interaction between opening, closing, and change in vertical dimension (P 0.0001). Conclusion: These results, with those of other studies, raise questions regarding the validity of the concept of a unique physiologic rest position of the mandible with the masseter or associated muscles at minimum muscle activity. The idea of overlapping postural ranges appears to be more appropriate.

Purpose: The purpose of this study was to analyze dentists' evaluations of factors related to the choice between crown therapy and filling and to possibly explain this by social and demographic attributes, job situation, and dentists' attitudes. Materials and Methods: Questionnaires were sent to a random sample of 2,059 Swedish dentists. The response rate was 76%. In the questionnaire the choice between crown therapy and filling in a clinical situation was presented. The dentists were asked to mark their assessments of the relative importance of the different items on 14-item visual analogue scales (VAS). Multiple regressions were run for all 14 items. Results: Large individual variations were seen among the dentists regarding the stated importance of the various items. The items rated as the most important were 'patient's wish' and 'treatment prognosis,' and the items rated as least important were 'treatment time required' and 'number of visits required for treatment.' The differences between groups were small, and for no item exceeded 0.7 step on the 8-grade VAS scale. No data reduction was possible using principal components analysis. Conclusion: The study showed great variations among individual dentists. The dentist-related factors explained little of the variance. The results indicated that the questionnaire instrument did not fully capture the real influences on the treatment choice between a filling and an artificial crown.

Purpose: The purpose of this investigation was to evaluate the relative 2-body abrasive wear and degree of conversion of 4 laboratory-processed composites (Targis, Concept, belleGlass, and Artglass) and 2 direct placement composites (Herculite and Heliomolar) after 7 days of storage. Materials and Methods: Human enamel was used as a positive control for 2-body abrasive wear, and 10 cylindric specimens (3.5-mm diameter, 8-mm height) of each material were prepared and stored in distilled water at 37 ± 2°C for wear testing. Relative 2-body abrasive wear rates were determined using a 30-µm diamond disk and a 2-body pin-on-disk apparatus. Subsequently, 3 polymerized specimens that had been stored in sealed polyethylene vials for 7 days were prepared for degree of conversion testing. The degree of conversion was determined on an infrared spectrometer using standard baseline techniques and various internal standards. Results: Statistical analysis using analysis of variance and the Tukey-Kramer multiple range test indicated significant differences between several of the materials tested for both 2-body abrasive wear and degree of conversion. Conclusion: Concept exhibited significantly less 2-body abrasive wear compared to the direct and indirect composites (P 0.01). Concept and belleGlass exhibited a mean degree of conversion that was significantly higher than any of the other composites tested (P 0.01).

Purpose: The purpose of this study was to determine pressure changes in the pulp space during tooth preparation with either diamond or tungsten carbide burs in a high-speed dental handpiece in the laboratory. Materials and Methods: Twenty premolar teeth were collected and randomly assigned to two groups: group 1 received preparation with diamond burs and group 2 with tungsten carbide burs. The teeth were mounted on a pressure transducer and the pulp chambers were filled with saline. A 0.1-mm thickness of tooth tissue was removed from the proximal surface of each tooth, alternating dry and wet cutting, until the pulp chamber was exposed. Pressure and temperature changes were recorded during tooth preparation. Results: At 0 to 1 mm of remaining dentin depth dry cutting with diamond and tungsten carbide burs generated a mean positive pulpal pressure of 12 kPa and 6 kPa, respectively. Wet cutting under the same conditions produced 0.6 kPa and 0.15 kPa, respectively. The difference between wet and dry cutting was highly significant (P 0.001). Diamond burs produced significantly higher pressure increases than carbide burs at all levels for both wet and dry techniques (P 0.05). When cutting farther than 2 mm from the pulp, tooth preparation created a mean 0.09-kPa pressure increase, which was not influenced by either coolant use or bur type. The temperature change was minimal during wet cutting and only minor temperature increases were recorded during dry cutting. Conclusion: From this laboratory study it is concluded that significant pressure changes occur in the pulp chamber during tooth preparation of extracted teeth when the remaining dentin thickness is less than 2 mm.

Purpose: It has been questioned whether the surface and color of the ceramic and the metal-ceramic bond strength of a titanium-ceramic system are comparable to those of a conventional noble alloy-ceramic system. It was therefore the aim of this study to carry out an intraindividual clinical comparison between crowns fabricated according to the Procera system (titanium copings veneered with a low-fusing ceramic) and noble-alloy copings veneered with a medium-fusing ceramic. Materials and Methods: Twenty-one crown pairs were fabricated for eighteen patients; three of the patients were each provided with two crown pairs. After 2 years nineteen crown pairs in sixteen patients could be compared. Clinical examinations were performed by two calibrated dentists who are long experienced in prosthetic dentistry. The crowns were rated according to the California Dental Association system. In addition, Bleeding Index and Margin Index were evaluated. Results: After 2 years the quality of surface and color of the ceramic material seemed to have deteriorated more in titanium-ceramic crowns than in conventional metal-ceramic crowns, although the difference was not statistically significant. Regarding anatomic form, margin integrity, Bleeding Index, and Margin Index the differences between the two crown systems were small. Conclusion: The low-fusing ceramics have been subject to improvements during the last few years. Their bond strength to titanium seems to be comparable to that of conventional metal-ceramic systems. However, in the long run one problem may be the surface and color stability of low-fusing ceramics. To make extended long-term comparisons between the two metal-ceramic systems possible the present patient material will be followed for a longer period than the current 2 years.