An analysis of multiple failures of resin-bonded bridges

The effect of resin cement type and cleaning method on the shear bond strength of resin cements for recementing restorations

PURPOSE

This laboratory study assessed the effect of different dentin cleaning procedures on shear bond strength of resin cements for recementing prosthesis.

MATERIALS AND METHODS

4 × 4 flat surface was prepared on the labial surface of 52 maxillary central incisors. Metal frames (4 × 4 × 1.5 mm) were cast with nickel-chromium alloy. All specimens were randomly divided into 2 groups to be cemented with either Panavia F2.0 (P) or RelyX Ultimate (U) cement. The initial shear bond strength was recorded by Universal Testing Machine at a crosshead speed of 0.5 mm/min. Debonded specimens were randomly allocated into 2 subgroups (n = 13) according to the dentin cleaning procedures for recementation. The residual cement on bonded dentin surfaces was eliminated with either pumice slurry (p) or tungsten carbide bur (c). The restorations were rebonded with the same cement and were subjected to shear test. Data failed the normality test (P < .05), thus were analyzed with Mann Whitney U-test, Wilcoxon signed rank test, and two-way ANOVA after logarithmic transformation (α = .05).

RESULTS

The initial shear bond strength of group P was significantly higher than group U (P = .001). Pc and Uc groups presented higher bond strength after recementation compared to the initial bond strength. However, it was significant only in Pc group (P = .034).

CONCLUSION

The specimens recemented with Panavia F2.0 provided higher bond strength than RelyX Ultimate cement. Moreover, a tungsten carbide bur was a more efficient method in removing the residual resin cement and increased the bond strength of Panavia F2.0 cement after recementation.

Longevity of resin-bonded fixed partial dental prostheses made with metal alloys

Objectives

The purpose of this study was to evaluate the clinical performance of resin-bonded fixed partial dental prostheses (RBFPDPs) made with metal alloys.

Materials and methods

The retention of 311 RBFPDPs from 226 patients fabricated from 1983 to 2013 using an adhesive resin was clinically evaluated. Partial or complete debonding of the RBFPDP or framework fracture was considered a treatment failure. All data were obtained from clinical examinations, and missing data were censored at the date of the last available information. The effect of the following factors on survival rate were investigated: patient gender, location (maxilla/mandible and anterior/posterior), number of missing teeth, number of abutment teeth, framework structure, type of metal alloy, patient age at the point of cementation, cement type, and distinction of the treating dentist. Data were analyzed with the Kaplan–Meier survival tests, log-rank tests, and Cox regression analyses (α = 0.05).

Results

The Kaplan–Meier survival rate was 41.2 % ± 6.5 % (standard error) at 28.8 years (last outcome event). Significant differences were found for patient age and treating dentist (p < 0.05). The risk of failure in younger patients was 1.7 times greater than that in older patients and that of inexperienced dentists was 2.0 times greater than that of dentist experienced and specialized in adhesive dentistry.

Conclusions

When fabricating RBFPDPs for younger patients, mechanical preparation for bonding may be necessary in consideration of the risk for debonding. Experienced dentists may achieve better results.

Clinical relevance

Mastery of skills is necessary to ensure excellent prognoses for RBFPDPs.

Effect of framework design on the surface strain of zirconia fixed partial dentures

The purpose of this study was to evaluate the design of resin-bonded fixed partial dentures (RBFPDs) with zirconia frameworks. The abutment teeth were the upper central incisor and the canine. Three types of frameworks were fabricated as follows: 0.5-mm- and 0.8-mm-thick zirconia frameworks with grooves and holes (0.5ZrG, 0.8ZrG) and 0.5-mm-thick zirconia frameworks without grooves and holes (0.5Zr). The control group was designed as a 0.8-mm-thick metal framework with grooves and holes (0.8MG). Static loading was applied and the surface strain of the retainers was measured with strain gages. The magnitude of the principal strain of the 0.5ZrG framework was significantly lower than that of the 0.8MG and the 0.5Zr frameworks. This result suggests that the zirconia and retention form had a significant effect on decreasing the framework deformation, indicating that the RBFPDs that use a 0.5-mm thick zirconia framework are effective for replacing a single anterior missing tooth.

Contemporary issues in the provision of restorative dentistry

This review highlights current issues in the provision of restorative dentistry both in primary and secondary care settings. Current challenges in the provision of restorative procedures are reviewed and methods in improving care and efficiency are proposed. Possible changes to the infrastructure and delivery of care are suggested in order to improve effectiveness and the patient pathway.

NUMERICAL INVESTIGATION OF SPAN LENGTHS AFFECTING MECHANICAL RESPONSES IN ANTERIOR RESIN-BONDED FIXED PARTIAL DENTURE

The geometric shape and mechanical structure of RBFPD compared to conventional FPD are relatively complex and unstable. The low retention rate between the retainer and abutment affects the prosthesis/abutment interface de-bonding, and closely relates to the design of the prosthesis and varied occlusion status. This study used reverse engineering (RE) and computer-assisted design (CAD) to construct two solid models of anterior RBFPD with different span lengths. After mesh generation, biomechanical interactions of span length in RB prosthesis with two loading conditions (axial and lateral) were performed by FE analysis. The simulated results indicated that lateral occlusal force increased significantly 2-3 times maximum stress than that of axial occlusal force. For different span lengths simulation, the analysis on static movement finds that longer pontic would lead to high stress to the prosthesis. Thus, the length of the pontic has significant effect on the overall intensity of the prosthesis under static clenching loading, and the effect of lateral loading exceeds that of axial loading.

Resin bonded bridges: techniques for success

Resin bonded bridges are a minimally invasive option for replacing missing teeth. Although they were first described over 30 years ago, evidence regarding their longevity remains limited and these restorations have developed an undeserved reputation for failure. This article provides a brief review of the literature regarding bridge success and continues to highlight aspects of case selection, bridge design and clinical procedure which may improve outcome.

Estimation of the retainer height biomechanical contribution in posterior resin-bonded fixed partial dentures: a structural-thermal coupled finite element analysis

This study determines the RBFPD (resin-bonded fixed partial dentures) biomechanical aspects to retainer height using structural-thermal coupled finite element (FE) analysis under normal (37°C) and high (51°C) oral temperatures. Three RBFPD FE models with different retainer heights (100, 75, and 50% of the distance from 2 mm above the CE (cementum-enamel) junction to the occlusal surface) were created using image processing, contour stacking, and mapping mesh procedures. After FE model validation, the maximum first principal and von Mises stresses in the remaining tooth (σ(T)) and prosthesis (σ(P)), were recorded for all models under structural-thermal coupled analyses. The simulation results showed that the σ(T) and σ(p) values decreased with greater retainer height as a result of the increasing prosthesis stiffness and maximizing bonding area between the enamel and retainer at normal oral temperature (37°C). However, no significant stress differences were found according to the retainer height varying dimensions at high (51°C) temperatures. The RBFPD retainer height biomechanical response is dominated by the structural analysis result (at 37°C) and it is recommended that the prosthesis retainer have as great a height as possible to decrease the stress values.

In vitro exploration and finite element analysis of failure mechanisms of resin-bonded fixed partial dentures

PURPOSE

The purpose of this study was to explore the debonding mechanisms of two-unit cantilevered and straight and bent three-unit fixed-fixed resin-bonded fixed partial dentures (RBFPDs) and to measure the failure loads needed for debonding.

MATERIALS AND METHODS

Failure load tests were performed using Bondiloy beams simulating both cantilevered and fixed-fixed RBFPDs, luted onto flat-ground buccal surfaces of bovine teeth with RelyX ARC, Panavia F2.0, and UniFix resin cements. The failure loads were recorded, and the debonded surfaces of both the enamel and the restorations were examined for details of interest. Finite element analysis (FEA) was used to calculate the stress concentrations within the cement layers at failure.

RESULTS

Simulated two-unit cantilevered and straight three-unit fixed-fixed RBFPDs showed a significantly higher failure load than the simulated three-unit fixed-fixed RBFPDs with a curved appearance. The FEA models revealed the magnitude and stress locations within the cement layer, resulting in an explanation of the different failure modes.

CONCLUSIONS

The low failure loads for the three-unit bent fixed-fixed RPFPDs, compared with their straight counterparts and the two-unit cantilevered RBFPDs, indicate that clinically a reserved attitude needs to be maintained with regard to three-unit fixed-fixed RBFPDs spanning a clearly curved part of the dental arch. The FEA results make it clear which part of the tooth restoration interface is subject to the highest stress levels, making it possible to design abutment preparations that avoid high interfacial stresses to help prevent debonding.

Multi-factorial retainer design analysis of posterior resin-bonded fixed partial dentures: a finite element study

OBJECTIVES

Biomechanical aspects related to prosthesis design of RBFPD have been proposed as the predominant contributor to unpredictable clinical retention. The aim of this study was to investigate the biomechanics of multiple posterior retainer designs and their interactions using three-dimensional finite element analysis.

METHODS

To understand the interactions among the retainer design factors, three values of retainer thicknesses (1.2, 0.8 and 0.4mm), heights (100, 75 and 50% of the distance from 2mm above the CEJ to the occlusal surface) and angle of the axial surface extensions (150, 180 and 210 degrees ) were selected as the design parameters. Twenty-seven RBFPD FE models with three retainer-design parameters were created by image processing, contour stacking, and mapping mesh procedures. The maximum principal and von Mises stresses in remaining tooth and prosthesis, respectively, were recorded in 54 FE analyses (27 FE models with axial and lateral occlusal loadings).

RESULTS

The simulated results showed that the averaged stress values of the remaining tooth and prosthesis decreased with greater retainer thickness and height as a result of increasing prosthesis stiffness and maximizing of the bonding area between the enamel and the retainer, respectively. However, no significant stress differences were found according to the angle of the retainer extension because stress transmission was concentrated at the connectors.

CONCLUSIONS

The stress elevation rate (termed as VSR-stress elevation by decreasing one unit volume of the remaining tooth) addresses that the height was the most influential factor for the remaining tooth structure, followed by retainer thickness and extension angle.

Towards automated 3D finite element modeling of direct fiber reinforced composite dental bridge

An automated 3D finite element (FE) modeling procedure for direct fiber reinforced dental bridge is established on the basis of computer tomography (CT) scan data. The model presented herein represents a two-unit anterior cantilever bridge that includes a maxillary right incisor as an abutment and a maxillary left incisor as a cantilever pontic bonded by adhesive and reinforced fibers. The study aims at gathering fundamental knowledge for design optimization of this type of innovative composite dental bridges. To promote the automatic level of numerical analysis and computational design of new dental biomaterials, this report pays particular attention to the mathematical modeling, mesh generation, and validation of numerical models. To assess the numerical accuracy and to validate the model established, a convergence test and experimental verification are also presented.

An investigation of aspects of design of resin-bonded bridges in general dental practice and hospital services

PURPOSE OF STUDY

To assess the design of resin-bonded bridges (RBBs) by dentists, the quality of information provided to a dental laboratory, and aid identification of best practice.

DESIGN

A retrospective case series study of slips for RBB construction, sent by hospital and general dental practitioners to a dental laboratory based in a postgraduate dental institute in Scotland, UK.

MAIN FINDINGS

384 forms were reviewed. For single-tooth replacements, 40% of designs for upper anterior bridges and 46% for lower posterior bridges were fixed-fixed (F-F) when a cantilever design was a better option. Thirty-six (9.3%) of RBB designs involved double abutting. On the laboratory forms, fewer than five (1%) cases included instructions about the thickness of the metal framework and in 48% there was no reference to the extension of the metal framework.

PRINCIPAL CONCLUSIONS

For single-tooth replacements, a relatively high percentage of dentists prescribed a fixed-fixed design for RBBs, despite the evidence advocating the use of a cantilever design. A significant number of dentists used double abutments. The information provided to the laboratory for the construction of resin-bonded bridges was often insufficient.

Clinical longevity of resin-bonded bridges bonded using a vinyl-thiol primer

The purpose of this study is to clinically confirm the adhesive effect of a vinyl-thiol primer (V-Primer) for precious alloys. Further, we examined the failure of resin-bonded bridges (RBBs) in order to improve the clinical longevity of RBBs, and to analyse the causes of failure. Twenty-six RBBs were bonded with V-primer and Super Bond C & B between May 1987 and December 1997 in the Dental Clinic of the Health Sciences University of Hokkaido. The RBBs were made of silver-palladium-gold alloy or metal-ceramic gold alloys. V-primer was effective in bonding Super Bond C & B to the RBBs made of silver-palladium-gold alloys. Ten prostheses had functioned satisfactorily for 8-11 years. However, six of the 26 RBBs had become detached. In four of the six failed RBBs, the type of bond failure was a mixture fracture in the resin-enamel interface. Our results suggest that: (1) V-primer is effective on RBBs made of silver-palladium-gold alloys. (2) Mechanical retention is necessary for retainers to improve the longevity of RBBs when the V-primer is applied to RBBs made of precious alloys without copper. (3) The RBB should not be applied to abutment teeth that have differences in tooth mobility.

Integration of CT, CAD system and finite element method to investigate interfacial stresses of resin-bonded prosthesis

The question of whether prosthesis/abutment tooth interface debonding is associated with inappropriate occlusal force is investigated in this study. A new modeling approach was employed to perform the interfacial stress analyses. Solid models of resin-bonded (RB) prosthesis and abutment teeth were constructed by stacking serious section contours that were obtained from CT images. A 3-D finite element (FE) model of RB prosthesis/abutment teeth was generated in a CAD system after assembling and meshing procedures. An in-house program was developed to combine the FE package (ANSYS) to calculate the interfacial (normal and shear) stresses at the prosthesis/molar interface with the bonding and debonding conditions. After 10 different occlusal force(s) evaluations, three initial opening gaps at the distal margin of the retainer were assumed to examine the possible interface debonding mechanism under the worst loading case. The results indicated that a more accurate FE model of the posterior RB prosthesis/abutments could be generated through combining several computer-aided techniques. The maximum interfacial stresses were obtained when the occlusal force was applied on the buccal slope of the distolingual cusp of the molar. For interface debonding simulations, peak normal and shear stresses were found near the debonding areas and stress values increased gradually with small to large initial opening gaps. From these results, prosthesis/abutment tooth interfacial fatigue damage might arise or accelerate the interface to debond under adequate bonding or initial gap opening conditions after long-term repeated inappropriate occlusal force actions.

Clinical complications in fixed prosthodontics

The purpose of this article is to identify the incidence of complications and the most common complications associated with single crowns, fixed partial dentures, all-ceramic crowns, resin-bonded prostheses, and posts and cores. A Medline and an extensive hand search were performed on English-language publications covering the last 50 years. The searches focused on publications that contained clinical data regarding success/failure/complications. Within each type of prosthesis, raw data were combined from multiple studies and mean values calculated to determine what trends were noted in the studies. The lowest incidence of clinical complications was associated with all-ceramic crowns (8%). Posts and cores (10%) and conventional single crowns (11%) had comparable clinical complications incidences. Resin-bonded prostheses (26%) and conventional fixed partial dentures (27%) were found to have comparable clinical complications incidences. The 3 most common complications encountered with all-ceramic crowns were crown fracture (7%), loss of retention (2%), and need for endodontic treatment (1%). The 3 most common complications associated with posts and cores were post loosening (5%), root fracture (3%), and caries (2%). With single crowns, the 3 most common complications were need for endodontic treatment (3%), porcelain veneer fracture (3%), and loss of retention (2%). When fixed partial denture studies were reviewed, the 3 most commonly reported complications were caries (18% of abutments), need for endodontic treatment (11% of abutments), and loss of retention (7% of prostheses). The 3 most common complications associated with resin-bonded prostheses were prosthesis debonding (21%), tooth discoloration (18%), and caries (7%).

Resin-retained bridges re-visited. Part 2. Clinical considerations

Resin-retained bridges have been used clinically since the 1970s, and offer a more conservative approach to the restoration of edentulous spaces than conventional bridgework. They are easy to place, cheap to fabricate and have been shown to be cost-effective. Despite this, they are not frequently used in general dental practice and they have an undeserved reputation for failure. Since their initial introduction, they have undergone a number of changes to their method of retention, and the materials used in their construction. This has resulted in a predictable, aesthetic restoration which, barring the use of implants, is often the treatment of choice where teeth adjacent to an edentulous space are minimally or not restored. This article hopes to show the clinical techniques required to produce predictable resin-retained bridgework in general practice.

Resin-retained bridges re-visited. Part 1. History and indications

Resin-retained bridges have been used clinically since the 1970s, and offer a more conservative approach to the restoration of edentulous spaces than conventional bridgework. They are easy to place, cheap to fabricate and have been shown to be cost effective. Despite this, they are not frequently used in general dental practice and they have an undeserved reputation for failure. Since their initial introduction, they have undergone a number of changes to their method of retention, and the materials used in their construction. This has resulted in a predictable, aesthetic restoration which, barring the use of implants, is often the treatment of choice where teeth adjacent to an edentulous space are minimally or not restored. This first article details the history, advantages, indications, and designs of resin-retained bridges.

Risk factors and multiple failures in posterior resin-bonded bridges in a 5-year multi-practice clinical trial

OBJECTIVES

A randomized controlled clinical trial was undertaken, to study the influence of some patient- and operator-dependent variables on the survival of posterior resin-bonded bridges (PRBBs) and to assess the survival of replacement' PRBBs. This report contains some of the results of the 5-year analysis.

METHODS

Survival was defined at three levels: (1) complete survival (without any debonding), (2) functional survival (i.e. survival after one loss of retention) and (3) replacement survival (survival of 'replacement' PRBBs, inserted after rebonded bridges suffered a second dislodgement). Potential risk factors were analysed with Cox's proportional hazards model and differences were tested for significance with the Breslow test. Observed effects are expressed as conditional-relative-risk (CRR). Survival of 'replacement' PRBBs was assessed with the Kaplan-Meier method.

RESULTS

Factors showing significant influences on complete survival were: 'location' (highest risk for mandibular PRBBs: CRR = 2.2), 'aetiology' (higher risk in treatment of aplasia: CRR = 2.9), and 'time of existence' (open spaces existing less than 2 years before insertion of PRBB: CRR: 2.0). The factor 'large open spaces in the mandible' was a risk for both complete and functional survival (CCR values 3.1 and 3.5, respectively). The survival of mandibular and maxillary 'replacement' PRBBs after 5 years was 19 +/- 7% and 31 +/- 18%, respectively.

CONCLUSIONS

Risk factors for PRBBs were: 'location', 'aetiology', 'time of existence', 'isolation method' and 'large open spaces in the mandible'. Mandibular 'replacement' PRBBs showed such an unacceptably low survival rate that fabrication is not recommended.

A five-year multi-practice clinical study on posterior resin-bonded bridges

Previous clinical observations have revealed that resin-bonded bridges for posterior tooth replacements are less retentive than anterior resin-bonded bridges. Improved bonding procedures and preparation designs, however, may have a positive effect on the functional durability of these restorations. The present study reports the final analysis of a randomized controlled clinical trial in which different designs of posterior resin-bonded bridges were evaluated for a period of at least 5 years. The operational hypothesis was that the bonding system and the preparation design used in posterior resin-bonded bridges have an influence on the survival and clinical functioning of these restorations. Survival in this study was defined at two levels: (1) 'complete' survival (survival without any debonding), and (2) 'functional' survival (survival including loss of retention on one occasion and successful rebonding of the original RBB without further debonding). With regard to 'complete' survival, no significant differences were found between the bonding systems used for adherence of the restorations to abutment teeth (etching/Clearfil F2, sandblasting/Panavia EX, and silica-coating/Microfill Pontic C). The variable 'preparation form' (conventional preparation form vs. modified preparation form) for complete survival was statistically in favor of the modified preparation form (62% vs. 46%), but did not influence the functional survival. With regard to 'functional' survival, the combination of silica coating and Microfill Pontic C was more retentive than the other bonding systems (90% survival vs. 72% and 75%, p < 0.01). Factor location was found to be highly significant for both survival levels [Cox's PH model, p = 0.0002 (Cox, 1972)]: The five-year 'complete' survival rates were 65% for maxillary restorations and 40% for mandibular restorations, while the five-year 'functional' survival rates were 89% and 68%, respectively. It is concluded that preparation of grooves in abutment teeth for posterior resin-bonded bridges is beneficial to their chance of survival. Resin-bonded bridges placed in the maxilla have a better prognosis than those made in the mandible. The bonding systems used in this study appear to have no influence on the chance of failure. In rebonded posterior resin-bonded bridges, the bonding system silica-coating/Microfill Pontic C was more retentive than the other systems tested.

Clinical performance of resin-bonded fixed partial dentures and extracoronal attachments for removable prostheses

STATEMENT OF PROBLEM

It is important to evaluate the long-term clinical performance of resin-bonded fixed partial dentures and extracoronal attachments for removable prostheses.

PURPOSE

A prospective, long-term clinical study was conducted to evaluate the success of resin-bonded fixed partial dentures since 1985 and of resin-bonded extracoronal attachments from 1987.

METHODS

Until 1993, a total of 130 resin-bonded fixed partial dentures had been seated in 101 patients, as well as 12 removable partial dentures (RPDs) with 24 extracoronal retainers in 10 patients. The clinical treatment protocol and the laboratory procedures were standardized. By the end of 1993, it was possible to reexamine 98 patients with a total of 127 resin-bonded fixed partial dentures and all 10 patients with removable partial dentures. The average time in function for the resin-bonded fixed partial dentures at the time of examination was 3.4 years and 2.3 years for the removable restorations.

RESULTS

During the period of observation, one retainer failed on six of the resin-bonded fixed partial dentures, which represents a failure rate of 4.7%. Debonding of extracoronal attachments was recorded for 8.3% of the total number of retainers.

CONCLUSION

The resin-bonded fixed partial denture technique can be considered to be a clinically reliable method of treatment, and permits the expansion of indications beyond a classical three-unit resin-bonded fixed partial denture. Long-term clinical success of removable partial dentures with resin-bonded extracoronal retainers warrants additional clinical studies.

Long-term survival data from a clinical trial on resin-bonded bridges

OBJECTIVES

A clinical trial, involving 203 resin-bonded bridges (RBBs) was undertaken to investigate the influence of retainer-type and luting material on the survival of these restorations.

METHODS

For this evaluation, 157 patients were available (14% of the original sample was lost to follow-up or excluded from the study following the stopping criteria). Fifty per cent of the patients were questioned concerning the fate of the RBBs and 59% of questioned patients were examined clinically. The patients that were seen for examination were representatives of the experimental groups. The findings from the clinical examination were compared with the data obtained from the questionnaire. Missing data were censored at the date of the last available information. Kaplan-Meier estimates were calculated to assess the survivals at the endpoints and compared using Cox's proportional hazards procedure.

RESULTS

A significant difference was found between perforated (P-type) and etched (E-type) RBBs (P = 0.05) for original bonded restorations but not when rebonded RBBs were taken into account. The results of the survival analysis were: anterior P-type, 49 +/- 7% after 10.5 years: anterior E-type, 57 +/- 7% after 10.5 years; posterior P-type, 18 +/- 11% after 6.8 years; posterior E-type, 37 +/- 13% after 10.2 years. Survivals of RBBs that were rebonded once during the evaluation period were 62 +/- 9% (11.0 years) for anterior RBBs and 51 +/- 11% (10.2 years) for posterior RBBs.

CONCLUSIONS

The factor location (anterior versus posterior) was as in previous analyses, highly significant. Differences in survival between cementation materials were not significant.

Resin-bonded fixed partial dentures: a review of three decades of progress

The resin-bonded fixed partial denture has undergone significant changes in design, materials and tooth preparation since its development in 1973. The selection of resin cement and micromechanical retention have closely paralleled alloy-resin-bonding research. Despite impressive in vitro research results, only 74 percent of the prostheses are still functional at the end of four years. Understanding the limitations, indications and design of resin-bonded fixed partial dentures will result in improved longevity and patient satisfaction.

Tensile peel failure of resin-bonded Ni/Cr beams: an experimental and finite element study

Tensile bond strength data for resin-bonded bridge cements would predict failure loads in excess of 1000 N when based on available bonded surface area. Such loads are unlikely to be encountered clinically, yet these bridges frequently fail. Loading conditions resulting in a peeling action at the adhesive interface may provide a more probable explanation for failure rather than being attributable to poor clinical or laboratory technique. This hypothesis was investigated by bonding grit-blasted Ni/Cr alloy beams of different thicknesses to a block of the same alloy using a resin-bonded bridge cement, leaving half the length of the beam free. Beams were pulled off the block with a peeling action by applying a tensile load to the free end of the beam and load at failure recorded. Fractured surfaces were examined under SEM. A two-dimensional finite element model of a section through the peel test arrangement was used to produce data of the stress distribution in the adhesive layer. Load at failure was closely correlated to beam thickness. Using this data and known tensile bond strength values it was possible to predict the loads necessary to cause adhesive failure of the beam, which were found to correlate closely with the experimental measurements. It was concluded that design and the stiffness of the retainers can have a potentially profound effect on the adhesive quality of resin-bonded bridges.

Clinical relevance of laboratory studies on dental materials: strength determination--a personal view

The measurement of strength of materials is undertaken in a laboratory primarily to determine its relationship to the microstructural features of the material, as the understanding it brings will allow the production of stronger materials. Problems arise when attempts are made to derive some sort of clinical significance from these data without due regard to the functional aspects of the device from which the material is made. Three examples are presented which seek to highlight that: (1) strength, however desirable, is not necessarily the most important consideration; (2) great care has to be exercised in interpreting strength data; and (3) the most appropriate properties are considered for each individual dental application of the material.