Occlusal force pattern during chewing and biting in dentitions restored with fixed bridges of cross-arch extension. I. Bilateral end abutments

Effect of preparation design and endodontic access on fracture resistance of zirconia overlays in mandibular molars: An in vitro study

PURPOSE

To evaluate the fracture resistance of zirconia overlays, considering various preparation designs and the presence of endodontic access.

MATERIALS AND METHODS

Ninety translucent zirconia (5Y-PSZ) overlay restorations were divided into six groups (n = 15/group) based on different preparation designs, with and without endodontic access: chamfer margin 4 mm above the gingival level without (group 1) and with endodontic access (group 2); margin 2 mm above the gingival level without (group 3) and with endodontic access (group 4); overlay with no chamfer margin without (group 5) and with endodontic access (group 6). Restorations were bonded to mandibular first molar resin dies, and the groups with endodontic access were sealed with flowable resin composite. All restorations underwent 100,000 cycles of thermal cycling between 5°C and 55°C, followed by loading until fracture. Maximum load and fracture resistance were recorded. ANOVA with Tukey post-hoc tests were used for statistical comparison (α < 0.05).

RESULTS

Fracture resistance significantly varied among overlay designs with and without endodontic access (p < 0.001), except for the no-margin overlays (groups 5 and 6). Overlays with a 2 mm margin above the gingival margin with endodontic access (group 4) exhibited significantly higher fracture resistance compared to both the 4-mm supragingival (group 2) and no-margin (group 6) designs, even when compared to their respective intact groups (groups 1 and 5). There were no significant differences between the no-margin and 4-mm supragingival overlays.

CONCLUSION

The more extensive zirconia overlay for mandibular molars is the first choice since the 2 mm margin above the gingival level design withstood considerable loads even after undergoing endodontic access. A no-margin overlay is preferred over the 4-mm supragingival design as it preserves more tooth structure and there was no outcome difference, irrespective of endodontic access. Caution is warranted in interpreting these findings due to the in vitro nature of the study.

Biomechanical analysis of stress distribution and failure risk in mandibular incisors restored with resin-bonded fixed partial dentures using CAD/CAM materials and restoration designs

Background

Computer-aided design/manufacturing (CAD/CAM) materials are widely used in resin-bonded fixed partial dentures (RBFPDs), but their suitability across different designs has not been fully assessed. This study compares the stress distribution and failure probability of mandibular incisors restored by RBFPDs with various CAD/CAM materials.

Materials and methods

Finite-element models of single- and double-ended RBFPDs were created using cone-beam computed tomography (CBCT) data. Five CAD/CAM materials (IPS e.max CAD, IPS e.max ZirCAD, Vita Enamic, Lava Ultimate, Vitablocs MarkII) were tested under vertical and oblique (45°) loading with a 100 N force. Stress distribution and failure risk were evaluated for each material and design.

Results

Oblique loading produced the highest stress and displacement for single-ended RBFPDs. Lava Ultimate had the largest displacement and principal stress, while IPS e.max ZirCAD showed the highest equivalent stress. IPS e.max CAD exhibited the lowest displacement and principal stress among double-ended RBFPDs under oblique loading.

Conclusion

This study demonstrated that double-ended RBFPDs experience lower stress and strain compared to single-ended designs, particularly under oblique loading. Vita Enamic had the highest failure risk, while IPS e.max ZirCAD had the lowest. These insights into stress distribution and material performance offer valuable guidance for material selection and restoration design, aiming to improve the longevity and success of RBFPDs in mandibular incisor restorations.

Quantitative evaluation of the proximal contact area gap change characterization under intercuspal occlusion by intraoral 3D scanning: Food impaction with tight proximal contact

OBJECTIVE

This study aimed to present three indicators that represent the proximal contact area gap change under intercuspal occlusion and to see if and how these indicators influence food impaction with tight proximal contact.

MATERIALS AND METHODS

Ninety volunteers were recruited for bite force measurement and intraoral scanning. Three-dimensional surface data and buccal bite data were obtained for 60 impacted and 60 non-impacted teeth. The scanning data were imported into the Geomagic Studio 2013 to measure three indicators, which included the gap change maximum (Δdm, μm), the buccolingual position of Δdm (P), and the gap expanded buccolingual range (S, mm). The difference between two groups of three indicators and their relationship with food impaction with tight proximal contact were analyzed by the t test, the Pearson chi-squared test, the nonparametric Mann-Whitney U test, and the binary logistic regression analysis (a = 0.05).

RESULTS

All indicators (Δdm, P, and S) were statistically different (p < 0.001, p = 0.002, and p < 0.001) in the impacted and non-impacted groups. Food impaction with tight proximal contact was affected by Δdm and S (p < 0.001, p = 0.039), but not by P (p = 0.409).

CONCLUSION

The excessive increase of the gap change maximum and the gap expanded buccolingual range under bite force promoted the occurrence of food impaction with tight proximal contact.

CLINICAL SIGNIFICANCE

The use of intraoral scanning to measure the characteristics of the proximal contact area gap change under bite force may help to deepen our understanding of the pathogenesis of food impaction with tight proximal contact. Importantly it can provide a reference basis for individualizing and quantifying occlusal adjustment treatment.

Marginal integrity in minimally invasive molar resin composite restorations: Impact of polymerization shrinkage

OBJECTIVES

This study utilized non-linear finite element (FE) models to explore polymerization shrinkage and its impact on marginal integrity in molars following both selective caries removal (SCR) and conventional treatment. Specifically, we performed 2D in silico simulations to study residual stresses post-resin polymerization shrinkage and their influence on the marginal integrity of various restoration types.

METHODS

Initially, FE models were developed based on a cohesive zone framework to simulate crack propagation along the bonded interfaces between restoration and tooth structure in SCR-treated molars with class I and class II restorations. The modeled resin composite restorations first underwent polymerization shrinkage and were then subjected to various occlusal loading conditions. Stress magnitudes and distributions were identified to evaluate the margin integrity and predict the mechanism and location of interfacial failure.

RESULTS AND DISCUSSION

The FE models computed polymerization shrinkage stresses of less than 1 MPa, exerting a minor influence on the composite/tooth interface. Occlusal loading, however, significantly impacted the load-bearing capacity of the composite/tooth (c/t) interface, potentially jeopardizing the restoration integrity. Especially under bi-axial occlusal loading, interfacial debonding occurred in the vertical cavity walls of the class I restorations, increasing the risk of failure. Notably, SCR-treated teeth exhibited better margin integrity than restored teeth after complete caries removal (NCR). These findings provide valuable insights into the mechanical behavior of SCR-treated teeth under different loading conditions and highlight the importance of considering the load scenarios that may lead to failure at the c/t interface. By investigating the factors influencing crack initiation and delamination, this novel research contributes to the optimization of restorative treatments and aids in the design of more resilient dental restorations.

Influence of occlusal thickness on the fracture resistance of chairside milled lithium disilicate posterior full-coverage single-unit prostheses containing virgilite: A comparative in vitro study

PURPOSE

To evaluate the fracture resistance of chairside computer-aided design and computer-aided manufacturing (CAD-CAM) lithium disilicate mandibular posterior crowns with virgilite of different occlusal thicknesses and compare them to traditional lithium disilicate crowns.

MATERIALS AND METHODS

Seventy-five chairside CAD-CAM crowns were fabricated for mandibular right first molars, 60 from novel lithium disilicate with virgilite (CEREC Tessera, Dentsply Sirona), and 15 from traditional lithium disilicate (e.max CAD, Ivoclar Vivadent). These crowns were distributed across five groups based on occlusal thickness and material: Group 1 featured CEREC Tessera crowns with 0.8 mm thickness, Group 2 had 1.0 mm thickness, Group 3 had 1.2 mm thickness, Group 4 with 1.5 mm thickness, and Group 5 included e.max CAD crowns with 1.0 mm thickness. These crowns were luted onto 3D-printed resin dies using Multilink Automix resin cement (Ivoclar Vivadent). Subsequently, they underwent cyclic loading (2,000,000 cycles at 1 Hz with a 275 N force) and loading until fracture. Scanning electron microscopy (SEM) assessed the fractured specimens. Statistical analysis involved one-way ANOVA and the Kruskal-Wallis Test (α = 0.05).

RESULTS

Fracture resistance varied significantly (<0.001) across mandibular molar crowns fabricated from chairside CAD-CAM lithium disilicate containing virgilite, particularly between crowns with 0.8 mm and those with 1.2 and 1.5 mm occlusal thickness. However, no significant differences were found when comparing crowns with 1, 1.2, and 1.5 mm thicknesses. CEREC Tessera crowns with 1.5 mm thickness exhibited the highest resistance (2119 N/mm2), followed by those with 1.2 mm (1982 N/mm2), 1.0 mm (1763 N/mm2), and 0.8 mm (1144 N/mm2) thickness, whereas e.max CAD crowns with 1.0 mm occlusal thickness displayed the lowest resistance (814 N/mm2).

CONCLUSIONS

The relationship between thickness and fracture resistance in the virgilite lithium disilicate full-coverage crowns was directly proportional, indicating that increased thickness corresponded to higher fracture resistance. No significant differences were noted among crowns with thicknesses ranging from 1 to 1.5 mm. This novel ceramic exhibited superior fracture resistance compared to traditional lithium disilicate.

Use of an optical jaw tracking system to capture the envelope of function when designing interim and definitive prostheses: A dental technique

Jaw tracking systems can record mandibular motion for incorporation into programs used for designing dental prostheses. However, the protocol for data acquisition and design using the recorded mandibular motion is unclear. The envelope of function recorded in a patient with acceptable occlusal function provides important functional information that can be integrated into the design of dental prostheses. A protocol for recording a patient's digital data, including the envelope of function using a jaw tracker, for incorporation into the design procedures and a delivery protocol are described. This technique may simplify the delivery of prostheses by reducing the adjustments needed to the definitive prostheses.

Fracture Resistance of Posterior Tooth-Supported Cantilever Fixed Dental Prostheses of Different Zirconia Generations and Framework Thicknesses: An In Vitro Study

The rehabilitation of free-end situations is a frequent indication in prosthetic dentistry. Cantilever fixed dental prostheses (cFDPs) made of 1st and 2nd generation zirconia are one treatment option. Due to a unique gradient technology, combinations of different zirconium dioxide generations are thus feasible in one restoration. However, data about these materials are rare. The purpose of this study was therefore to investigate the fracture resistance and fracture modes of tooth-supported cFDPs fabricated from different zirconia materials (gradient technology) and different framework thicknesses. A total of 40 cFDPs were fabricated using the CAD/CAM approach and belonged to five test groups. The different groups differed in the yttria content, the proportion of the tetragonal/cubic phases, or in wall thickness (0.7 mm or 1 mm). After completion, the cFDPs were subjected to thermal cycling and chewing simulation (1.2 × 106 load cycles, 108 N load). Afterwards, cFDPs were statically loaded until fracture in a universal testing machine. A non-parametric ANOVA was compiled to determine the possible effects of group membership on fracture resistance. In addition, post-hoc Tukey tests were used for bivariate comparisons. The mean fracture loads under axial load application ranged from 288 to 577 N. ANOVA detected a significant impact of the used material on the fracture resistances (p < 0.001). Therefore, the use of cFDPs fabricated by gradient technology zirconia may not be unreservedly recommended for clinical use, whereas cFPDs made from 3Y-TZP exhibit fracture resistance above possible masticatory loads in the posterior region.

Gene expression profiling of the masticatory muscle tendons and Achilles tendons under tensile strain in the Japanese macaque Macaca fuscata

Both Achilles and masticatory muscle tendons are large load-bearing structures, and excessive mechanical loading leads to hypertrophic changes in these tendons. In the maxillofacial region, hyperplasia of the masticatory muscle tendons and aponeurosis affect muscle extensibility resulting in limited mouth opening. Although gene expression profiles of Achilles and patellar tendons under mechanical strain are well investigated in rodents, the gene expression profile of the masticatory muscle tendons remains unexplored. Herein, we examined the gene expression pattern of masticatory muscle tendons and compared it with that of Achilles tendons under tensile strain conditions in the Japanese macaque Macaca fuscata. Primary tenocytes isolated from the masticatory muscle tendons (temporal tendon and masseter aponeurosis) and Achilles tendons were mechanically loaded using the tensile force and gene expression was analyzed using the next-generation sequencing. In tendons exposed to tensile strain, we identified 1076 differentially expressed genes with a false discovery rate (FDR) < 10-10. To identify genes that are differentially expressed in temporal tendon and masseter aponeurosis, an FDR of < 10-10 was used, whereas the FDR for Achilles tendons was set at > 0.05. Results showed that 147 genes are differentially expressed between temporal tendons and masseter aponeurosis, out of which, 125 human orthologs were identified using the Ensemble database. Eight of these orthologs were related to tendons and among them the expression of the glycoprotein nmb and sphingosine kinase 1 was increased in temporal tendons and masseter aponeurosis following exposure to tensile strain. Moreover, the expression of tubulin beta 3 class III, which promotes cell cycle progression, and septin 9, which promotes cytoskeletal rearrangements, were decreased in stretched Achilles tendon cells and their expression was increased in stretched masseter aponeurosis and temporal tendon cells. In conclusion, cyclic strain differentially affects gene expression in Achilles tendons and tendons of the masticatory muscles.

Path pattern and movement during mastication on habitual and non-habitual chewing sides

PURPOSE

To clarify the presence or absence of differences in path pattern and movement during mastication between the habitual and non-habitual chewing sides.

METHODS

Participants were 225 healthy adults with natural dentition. Mandibular movement while chewing gummy jelly on each side was recorded, and masticatory path pattern was classified into five types (one normal and four abnormal). The frequency of each pattern was measured and compared between chewing sides. The amount, rhythm, velocity, and stability of movement and masticatory performance were measured and compared between chewing sides.

RESULTS

A normal pattern was observed on the habitual chewing side in 84.4% of participants. There was a significant difference between chewing sides in masticatory path pattern (χ² = 35.971, P < 0.001). Values of parameters regarding the amount and velocity of movement and masticatory performance were significantly higher on the habitual chewing side. Values of parameters regarding rhythm and stability of movement were significantly lower on the habitual chewing side.

CONCLUSION

The present findings of functional differences between chewing sides in terms of path pattern and movement during mastication suggest that these factors should be analyzed on the habitual chewing side.

Is the Diagnosis of Generalized Stage IV (Severe) Periodontitis Compatible with the Survival of Extended Stabilizing Prosthetic Restorations? A Medium-Term Retrospective Study

The aim of the study was to identify the most relevant patient-related factors directly involved (alone or in combinations) in the long-term survival and functionality of the abutment teeth of extensive stabilizing bridges and removable prosthesis, in patients treated for Stage IV periodontitis, adhering to SPT over a period of at least 5 years. Seventy-six patients treated between 2000-2022, rehabilitated with FDPs and RDPs, adhering to SPT for at least 5 years were included. Patient-related factors influencing retention of RDPs and FDP, survival rates in regular (RCs) and irregular compliers (ICs), and incidence of biological and technical complications were assessed. During a follow-up of 69 months, from 57 patients with FDPs and 19 patients with RDPs, 39 (51.32%) were ICs, while 37 (48.68%) were RCs. An overall statistically significant association (p = 0.04) was identified between biological complications and the type of prostheses. The RDP patients had more complications than FDP patients. In 5.26% of the RDP patients, root caries were identified, and 10.53% were diagnosed with a periapical (endodontic) lesion, while 3.51% of the FDPS patients presented root caries. In five (6.57%) cases, abutment loss resulted in the loss of the prosthesis. Statistically significant correlations were observed between systemic diseases and tooth loss, and between type of tooth lost and the reason for tooth loss, irrespective of the type of prosthesis. A total of 66.67% of the lost incisors, 85.71% of the lost premolars, and 88.89% of the lost molars occurred due to periodontal causes. Furthermore, 93% of the FDPs and RDPs were still in place and in function.

Comparative evaluation of transpalatal arch and vertical holding appliance at different heights

Objectives

An effective method for controlling vertical dysplasia remains a concern for orthodontists. This study aims to compare the stresses on the maxillary first molars while changing the height of the loop/acrylic pad of the transpalatal arch (TPA) and vertical holding appliance (VHA) during swallowing using finite element analysis.

Methods

Head and neck computed tomography (CT) of a patient with levelled and aligned maxillary teeth was taken for a three-dimensional reconstruction. Six models comprising the maxilla, molars, and surrounding periodontal ligament (PDL) with different heights of loop/acrylic pads of the appliances were constructed. A force of 112 g/cm2 was applied to the loop/acrylic pads of both appliances to simulate the average tongue pressure. The distribution of von Mises stresses occurring at the PDL due to the tongue pressure was mapped at the cervical area, furcation, distobuccal, mesiobuccal, and palatal root apices. Separately, subjects requiring TPA and VHA at different heights during orthodontic treatment were asked to rate their comfortability with the appliance on the visual analogue scale (VAS).

Results

The VHA demonstrated higher values of stresses at all heights from the palatal floor compared to the corresponding heights of the TPA. The 8 mm model of VHA was found to be functionally effective. This height was also perceived to be acceptable for most patients on the VAS.

Conclusions

The acrylic pad of VHA when kept at a distance of 8 mm from the palatal floor enhances the functional efficiency of the appliance with an acceptable comfortability for the patient.

Gender differences in masticatory function in elderly adults with natural dentition

The purpose of this study was to clarify the presence or absence of gender differences in masticatory function in elderly adults with completely natural dentition. Fifty-six elderly adults aged 65 years and older (elderly group) and 60 young adults in their 20-40 s (young group) with natural dentition participated in this study. The maximum occlusal force on the habitual chewing side was measured using dental prescale. Next, the amount of glucose extraction when subjects chewed a gummy jelly on their habitual chewing side for 20 s was measured. The maximum occlusal force and masticatory performance in the elderly and young groups were compared between males and females. The relationship between the maximum occlusal force and masticatory performance was also investigated. The maximum occlusal force in the elderly and young groups was larger in males than in females, and the difference was significant. The amount of glucose extraction in the elderly and young groups was higher in males than in females, and the difference was significant. Regarding the relationship between the maximum occlusal force and the amount of glucose extraction, there was a significant positive correlation. These results suggest that there are gender differences in the masticatory function of the elderly adults with completely natural dentition, and these differences need to be noted in the evaluation of masticatory function.

Maximum voluntary molar bite force in subjects with malocclusion: multifactor analysis

OBJECTIVE

This study was performed to determine the maximum voluntary molar bite force (MVMBF) in relation to age, sex, lip competency, midline shifting, dental and skeletal malocclusion, overjet, overbite, and crowding.

METHODS

One hundred Saudi patients with orthodontic malocclusion aged 14 to 25 years (51 male and 49 female patients) were investigated in this cross-sectional study. The baseline MVMBF on the right and left side was evaluated in all patients before commencing any orthodontic treatment. The MVMBF was registered with a portable occlusal force gauge in the first molar region during maximal clenching.

RESULTS

The MVMBF significantly varied with respect to all nine confounding variables. The MVMBF significantly increased with an age of >18 years, male sex, right-side lip competency, no midline shift, dental and skeletal Class I malocclusion, normal overjet, normal overbite, and mild crowding.

CONCLUSION

All nine variables examined in the present study significantly influenced the MVMBF.

Construction of virtual intercuspal occlusion: Considering tooth displacement

Common impressions cannot accurately duplicate the dental occlusion under occlusal force due to tooth displacement and mandibular deformation. To establish new methods to construct virtual intercuspal occlusion and assess their construction accuracy. The intraoral occlusal contacts of posterior teeth of 15 subjects were recorded with 8 μm and 100 μm articulating paper, respectively, and the marked teeth and buccal bite data were scanned with an intraoral scanner. The virtual dental occlusions were separately determined by buccal bite registration (BBR) method, and 3 new methods, namely segmented tooth registration (STR), occlusal contact areas (marked by 8 μm articulating paper) registration (OCR) and mixing registration (MR) methods. With the intraoral contact areas marked by 100 μm articulating paper set as reference and contact areas of the 4 virtual occlusions as tests, sensitivity, positive predictive value (PPV) and the ratio of overlapping areas were calculated. Kruskal-Wallis test or 1-way ANOVA was used to analyse the difference among groups. The sensitivity ranged from 0.69 to 0.94 and the PPV from 0.67 to 0.90. Sensitivity of OCR group and PPV of STR and OCR groups were different from that of BBR group at overlapping threshold of 50% (P = .028, .028 and .006). There was statistical difference of the ratio of overlapping areas over reference areas, and the values of STR and OCR groups were higher than that of BBR group (P = .045 and .021). The ability of STR and OCR methods to construct virtual intercuspal occlusion was better than BBR method.

Application Solid Laser-Sintered or Machined Ti6Al4V Alloy in Manufacturing of Dental Implants and Dental Prosthetic Restorations According to Dentistry 4.0 Concept

This paper presents a comparison of the impact of milling technology in the computer numerically controlled (CNC) machining centre and selective laser sintering (SLS) and on the structure and properties of solid Ti6Al4V alloy. It has been shown that even small changes in technological conditions in the SLS manufacturing variant significantly affect changes from two to nearly two and a half times in tensile and bending strengths. Both the tensile and bending strength obtained in the most favourable manufacturing variant by the SLS method is over 25% higher than in the case of cast materials subsequently processed by milling. Plug-and-play SLS conditions provide about 60% of the possibilities. Structural, tribological and electrochemical tests were carried out. In vitro biological tests using osteoblasts confirm the good tendency for the proliferation of live cells on the substrate manufactured under the most favourable SLS conditions. The use of SLS additive technology for the manufacturing of dental implants and abutments made of Ti6Al4V alloy in combination with the digitisation of dental diagnostics and computer-aided design and manufacture of computer-aided design/manufacturing (CAD/CAM) following the idea of Dentistry 4.0 is the best choice of technology for manufacturing of prosthetic and implant devices used in dentistry.

Failure loads of all-ceramic cantilever fixed dental prostheses on post-restored abutment teeth: influence of the post presence and post position

The influence of a fiber post-restored abutment tooth on the load capability of a three-unit zirconia framework cantilever fixed dental prosthesis (cFDP) was evaluated after simulated clinical function. Human lower sound premolars (n = 64) were distributed, in equal numbers, to four experimental groups: two vital abutment teeth (group I; control); mesial abutment tooth post-restored (group II); distal abutment tooth post-restored (group III); and mesial and distal abutment teeth post-restored (group IV). All specimens received an adhesively luted three-unit cFDP of veneered zirconia. Simulated clinical function was performed by two subsequent sequences of thermal-cycling (2 × 3,000 cycles) and mechanical loading (1.2 × 10⁶ load cycles from 0 to 50 N) (TCML). Four specimens failed during TCML (one in each of groups I and IV and two in group II). The maximum load capability ranged from 365 to 538 N and was not significantly different between groups. Specimens with post-restored abutments failed mainly because of abutment tooth fracture of the distal abutment. The presence or position of post-restored abutment teeth has no significant impact on load capability of all-ceramic three-unit cFDPs. The risk of tooth fracture of the distal abutment teeth of a cFDP was significantly increased when one abutment tooth, irrespective of its position, was post-and-core restored.

Occlusal loading during biting from an experimental and simulation point of view

OBJECTIVES

Occlusal loading during clenching and biting is achieved by the action of the masticatory system, and forms the basis for the evaluation of the functional performance of prosthodontic and maxillofacial components. This review provides an overview of (i) current bite force measurement techniques and their limitations and (ii) the use of computational modelling to predict bite force. A brief simulation study highlighting the challenges of current computational dental models is also presented.

METHODS

Appropriate studies were used to highlight the development and current bite force measurement methodologies and state-of-the-art simulation for computing bite forces using biomechanical models.

RESULTS

While a number of strategies have been developed to measure occlusal forces in three-dimensions, the use of strain-gauges, piezo-electric sensors and pressure sheets remain the most widespread. In addition to experimental-based measurement techniques, bite force may be also estimated using computational models of the masticatory system. Simulations of different bite force models clearly show that the use of three-dimensional force measurements enriches the evaluation of masticatory functional performance.

SIGNIFICANCE

Hence, combining computational modelling with three-dimensional force measurement techniques can significantly improve the evaluation of masticatory system and the functional performance of prosthodontic components.

FEA of Peri-Implant Stresses in Fixed Partial Denture Prostheses with Cantilevers

PURPOSE

To compare stresses in the peri-implant bone produced by fixed partial prostheses with mesial and distal cantilevers, when cemented or screwed onto implants.

MATERIALS AND METHODS

The experimental design consisted of four 3D models obtained by volumetric computerized tomography and analyzed using finite element analysis (FEA). Stresses were simulated in two stages. Stage 1 consisted of application of the preload. A 288 N load was applied to pillar screws of the cemented prosthesis, resulting in 389 MPa peak; 257 N was applied to the intermediate screw (multi-unit) resulting in 390 MPa peak; and 111 N was applied to the prosthesis infrastructure screw in the screwed prostheses, resulting in 390 MPa peak. In stage 2, the axial and oblique loads were applied. The axial load consisted of 50 N for molar implants and 30 N for premolar implants. The oblique load (on the buccal slope of the buccal cusp of each crown) consisted of 30 N for premolar implants and 50 N for molar implants, with a buccolingual vector at a 45° angle with the occlusal plane. The response variables were the axial and oblique stresses on the peri-implant bone, according to the following groups: group 1-models of fixed partial prostheses cemented onto implants with mesial or distal cantilevers; group 2-models of fixed partial denture screwed onto implants with mesial or distal cantilevers. Data were recorded, evaluated, and compared.

RESULTS

Oblique loads produced higher peak values than axial loads, and were located at the bone crest of the implant closest to the cantilever, regardless of the prosthesis type. In the cemented prosthesis models, oblique loads produced the highest peaks in the primary implant region when compared with the screwed prosthesis models.

CONCLUSION

The screwed fixed prostheses caused less stress on peri-implant bone regardless of whether the cantilever was located mesially or distally.

Evaluation of stress patterns in bone around dental implant for different abutment angulations under axial and oblique loading: A finite element analysis

Introduction:

The replacement of missing anterior teeth presents peculiar challenges to the Prosthodontist. Implants are increasingly gaining favour for the same. The morphology of existing bone in the premaxilla often dictates that implants are placed at angles that are difficult to restore with conventional abutments. However, the angulated abutments might transfer unfavourable forces to the implant or bone, thereby compromising the prognosis of the treatment. Because, it is difficult to assess the generated forces clinically, a finite element analysis was chosen for the present study as it is useful tool in estimating stress distribution in the contact area of the implant with the bone.

Materials and Methods:

In this study, the frontal region of the maxilla was modelled with a cortical layer 1.5 mm thick containing an inner cancellous core. The implant was cylindrical, round ended, with length 13 mm and diameter 4.1 mm. The abutment was modelled as 7 mm in height with a 5 degree occlusal taper. The different abutment angulations used were 0°, 10°, 15° and 20°. The amount of loads used were 100, 125, 150, 175 and 200 N axially, and 50 N in oblique direction, to approximate the kind of loads seen in clinical situations.

Result:

It was seen that, as the abutment angulation changes from 0° to 20° both the compressive as well as tensile stresses increased; but, it is within the tolerance limit of the bone.

Conclusion:

It seems reasonably safe to use angled abutments in anterior implant supported prostheses, in the maxillary arch.

The influence of various core designs on stress distribution in the veneered zirconia crown: a finite element analysis study

PURPOSE

The purpose of this study was to evaluate various core designs on stress distribution within zirconia crowns.

MATERIALS AND METHODS

Three-dimensional finite element models, representing mandibular molars, comprising a prepared tooth, cement layer, zirconia core, and veneer porcelain were designed by computer software. The shoulder (1 mm in width) variations in core were incremental increases of 1 mm, 2 mm and 3 mm in proximal and lingual height, and buccal height respectively. To simulate masticatory force, loads of 280 N were applied from three directions (vertical, at a 45° angle, and horizontal). To simulate maximum bite force, a load of 700 N was applied vertically to the crowns. Maximum principal stress (MPS) was determined for each model, loading condition, and position.

RESULTS

In the maximum bite force simulation test, the MPSs on all crowns observed around the shoulder region and loading points. The compressive stresses were located in the shoulder region of the veneer-zirconia interface and at the occlusal region. In the test simulating masticatory force, the MPS was concentrated around the loading points, and the compressive stresses were located at the 3 mm height lingual shoulder region, when the load was applied horizontally. MPS increased in the shoulder region as the shoulder height increased.

CONCLUSION

This study suggested that reinforced shoulder play an essential role in the success of the zirconia restoration, and veneer fracture due to occlusal loading can be prevented by proper core design, such as shoulder.

Effect of core thickness differences on post-fatigue indentation fracture resistance of veneered zirconia crowns

PURPOSE

Despite the excellent esthetics of veneered zirconia crowns, the incidence of chipping and fracture of veneer porcelain on zirconia crowns has been recognized to be higher than in metal ceramic crowns. The objective of this investigation was to study the effect of selected variations in core thickness on the post-fatigue fracture resistance of veneer porcelain on zirconia crowns.

MATERIALS AND METHODS

Zirconia crowns for veneering were prepared with three thickness designs of (a) uniform 0.6-mm thick core (group A), (b) extra-thick 1.7 mm occlusal core support (group B), and (c) uniform 1.2-mm thick core (group C). The copings were virtually designed and milled by the CAD/CAM technique. Metal ceramic copings (group D) with the same design as in group C were used as controls. A sample size of N = 20 was used for each group. The copings were veneered with compatible porcelain and fatigue tested under a sinusoidal loading regimen. Loading was done with a 200 N maximum force amplitude under Hertzian axial loading conditions at the center of the crowns using a spherical tungsten carbide indenter. After 100,000 fatigue cycles, the crowns were axially loaded to fracture and maximum load levels before fracture was recorded. One-way ANOVA (P < 0.05) and post hoc Tukey tests (α = 0.05) were used to determine significant differences between means.

RESULTS

The mean fracture failure load of group B was not significantly different from that of control group D. In contrast, the mean failure loads of groups A and C were significantly lower than that of control group D. Failure patterns also indicated distinct differences in failure mode distributions. The results suggest that proper occlusal core support improves veneer chipping fracture resistance in zirconia crowns.

CONCLUSIONS

Extra-thick occlusal core support for porcelain veneer may significantly reduce the veneer chipping and fracture of zirconia crowns. This is suggested as an important consideration in the design of copings for zirconia crowns.

Hierarchical decisions on teeth vs. implants in the periodontitis-susceptible patient: the modern dilemma

It is estimated that advanced periodontitis typically affects about 10% of most adult populations studied. These individuals can be considered highly susceptible to periodontitis and often present difficulties for clinicians in therapeutic decision making, especially when dental implants are involved. Poor plaque control and smoking are well established risk factors for periodontitis, as well as for peri-implant disease. Long-term follow-up studies have clearly demonstrated that treatment of periodontal disease, even if advanced, can be successful in arresting disease progression and preventing (or at least significantly delaying) tooth loss. With the increasing development of implant dentistry, traditional well documented and evidence-based therapies to treat periodontal diseases may sometimes not be used to their full potential. Instead, there appears to be an increasing tendency to extract periodontally compromised teeth and replace them with implants, as if implants can solve the problem. However, peri-implant diseases are prevalent, affecting between 28% and 56% of people with implants, and (at the implant level) 12-43% of implants. A history of periodontal disease, smoking and poor oral hygiene are all risk factors for developing peri-implantitis. Unlike periodontitis, there are currently no predictable means for treating peri-implantitis, although resective surgery seems to be the most effective technique. Consequently, if implant treatment is considered in patients who are susceptible to periodontitis, it should be preceded by appropriate and adequate periodontal treatment or re-treatment to control the condition, and should be followed by a stringent supportive maintenance program to prevent the development of peri-implant disease. The decision whether implant treatment should be performed should be based on an assessment of the patient's risk profile at the subject level, as well as at the site level.

Assessment of various factors for feasibility of fixed cantilever bridge: a review study

Cantilever fixed partial dentures are defined as having one or more abutments at one end of the prosthesis while the other end is unsupported. Much controversy without documentary evidence has surrounded this prosthesis. Despite negative arguments, the cantilever prosthesis has been used extensively by the clinicians. If used nonjudiciously without following proper guidelines these might lead to some complications. Although complications may be an indication that clinical failure has occurred, this is not typically the case. It is also possible that complications may reflect substandard care. Apart from the substandard care, the unique arrangement of the abutments and pontic also accounts for the prime disadvantage: the creation of a class I lever system. When the cantilevered pontic is placed under occlusal function, forces are placed on the abutments. There are various criteria and factors necessary for a successful cantilever fixed partial denture (FPD). The purpose of this paper is to discuss briefly various factors involved in the planning of a cantilever fixed partial denture.

Three-dimensional finite element analysis of posterior fiber-reinforced composite fixed partial denture Part 2: influence of fiber reinforcement on mesial and distal connectors

The aim of this study was to evaluate the influence of connectors under two different loading conditions on displacement and stress distribution generated in isotropic hybrid composite fixed partial denture (C-FPD) and partially anisotropic fiber-reinforced hybrid composite fixed partial denture (FRC-FPD). To this end, two three-dimensional finite element (FE) models of three-unit FPD from mandibular second premolar to mandibular second molar - intended to replace the mandibular first molar - were developed. The two loading conditions employed were a vertical load of 629 N (applied to eight points on the occlusal surface) and a lateral load of 250 N (applied to three points of the pontic). The results suggested that the reinforcing fibers in FRC framework significantly improved the rigidity of the connectors against any twisting and bending moments induced by loading. Consequently, maximum principal stress and displacement generated in the connectors of FRC-FPD were significantly reduced because stresses generated by vertical and lateral loading were transferred to the reinforcing fibers.

Ante's (1926) law revisited: a systematic review on survival rates and complications of fixed dental prostheses (FDPs) on severely reduced periodontal tissue support

BACKGROUND

In subjects suffering from generalized severe periodontitis, only a few teeth may be treated and used as abutments for fixed dental prostheses (FDPs).

OBJECTIVE

To systematically review the impact of severely reduced, but healthy periodontal tissue support on the survival rate and complications of FDPs after a mean follow-up time of at least 5 years.

SEARCH STRATEGY

Publications considered for inclusion were searched in MEDLINE (PubMed) and relevant journals were hand searched. The search was performed in duplicate and was limited to human studies published in the dental literature from 1966 up to and including September 2006. Only publications in English, in peer-reviewed journals, were considered. Abstracts were excluded.

SELECTION CRITERIA

Prospective and retrospective cohort studies were included. The primary outcome measure included survival rates of FDPs and abutment teeth, whereas biological and technical complications of FDPs and abutment teeth represented secondary outcome measures.

DATA ANALYSIS

Summary estimates of survival rates and of biological and technical complications were calculated after 5 and 10 years.

RESULTS

The search provided 860 titles of which six publications were included. A total of 579 FDPs were incorporated and followed up to 25 years. Meta-analysis yielded an estimated FDP survival rate of 96.4% [95% confidence interval (95% CI): 94.6-97.6%] after five and of 92.9% (95% CI: 89.5-95.3%) after 10 years, respectively. After 10 years, the estimated rate of abutment teeth without endodontic complications amounted to 93% (95% CI: 62.6-98.9%). The 10-year estimated rate of caries-free abutment teeth was 98.1% (95% CI: 88.2-99.7%). FDPs without loss of retention were estimated to occur in 95.4% (95% CI: 92.6-97.2%) of cases after 10 years.

CONCLUSIONS

These results showed that (i) masticatory function could be established and maintained in subjects receiving FDPs on abutment teeth with severely reduced but healthy periodontal tissue support and (ii) FDPs survival rates compared favourably with those of FDPs incorporated in subjects without severely periodontally compromised dentitions.

Modulation of mandibular loading and bite force in mammals during mastication

Modulation of force during mammalian mastication provides insight into force modulation in rhythmic, cyclic behaviors. This study uses in vivo bone strain data from the mandibular corpus to test two hypotheses regarding bite force modulation during rhythmic mastication in mammals: (1) that bite force is modulated by varying the duration of force production, or (2) that bite force is modulated by varying the rate at which force is produced. The data sample consists of rosette strain data from 40 experiments on 11 species of mammals, including six primate genera and four nonprimate species: goats, pigs, horses and alpacas. Bivariate correlation and multiple regression methods are used to assess relationships between maximum (epsilon(1)) and minimum (epsilon(2)) principal strain magnitudes and the following variables: loading time and mean loading rate from 5% of peak to peak strain, unloading time and mean unloading rate from peak to 5% of peak strain, chew cycle duration, and chew duty factor. Bivariate correlations reveal that in the majority of experiments strain magnitudes are significantly (P<0.001) correlated with strain loading and unloading rates and not with strain loading and unloading times. In those cases when strain magnitudes are also correlated with loading times, strain magnitudes are more highly correlated with loading rate than loading time. Multiple regression analyses reveal that variation in strain magnitude is best explained by variation in loading rate. Loading time and related temporal variables (such as overall chew cycle time and chew duty factor) do not explain significant amounts of additional variance. Few and only weak correlations were found between strain magnitude and chew cycle time and chew duty factor. These data suggest that bite force modulation during rhythmic mastication in mammals is mainly achieved by modulating the rate at which force is generated within a chew cycle, and less so by varying temporal parameters. Rate modulation rather than time modulation may allow rhythmic mastication to proceed at a relatively constant frequency, simplifying motor control computation.

ZrO2three-unit fixed partial dentures: comparison of failure load before and after exposure to a mastication simulator

This study evaluated the failure load of different zirconia-based all-ceramic fixed partial dentures (FPD) before and after artificial aging. Forty-eight zirconia frameworks for three-unit FPDs were fabricated using different manufacturing systems [(DCS), Procera (Nobel Biocare) and Cerec inLab (Sirona)], veneered using Vita VM9 (Vita Zahnfabrik) opaque ceramic and conventionally cemented on human teeth. The restorations were divided according to the system used for manufacturing the frameworks into three groups of 16 specimens each (DCS, Procera and Vita YZ-Cerec). Half of each group was artificially aged through dynamic loading and thermal cycling, whereas, the other half was not subjected to artificial aging. Subsequently, all specimens were loaded occlusally until fracture occurred using a universal testing machine. Pair-wise Wilcoxon rank tests were performed to test for differences in failure loads at a statistical significance level of 0.05. All specimens subjected to artificial aging survived with no failures. The median (IQR=x0.25-x0.75) failure loads (N) before and after artificial aging were, respectively, as follows: group DCS, 2131 (1948-2239) and 1797 (1590-2074); group Procera, 1684 (1615-1873) and 1394 (1275-1495); and group Vita YZ-Cerec, 1845 (1621-1923) and 1625 (1521-1747). No significant differences were found for comparisons between different groups before artificial aging. After artificial aging, group Procera showed significantly smaller values than group DCS (P=0.042). All tested restorations have the potential to withstand occlusal forces applied in the posterior region and can therefore represent interesting alternatives to replace metal-ceramic restorations. Further assessments are needed before recommending these restorations for daily practice.

Fracture resistance of different zirconium dioxide three-unit all-ceramic fixed partial dentures

OBJECTIVE

This study evaluated the fracture resistance of different zirconia three-unit posterior all-ceramic fixed partial dentures before and after fatigue loading.

MATERIAL AND METHODS

Forty-eight zirconia three-unit posterior all-ceramic fixed partial dentures were fabricated using different manufacturing systems and conventionally cemented on human teeth. The restorations were divided according to the system used for manufacturing the frameworks into 3 groups of 16 specimens each (DCS, Procera and Vita CerecInlab). Half of each group was artificially aged through dynamic loading and thermal cycling, whereas the other half was left with no artificial aging. Afterwards, all specimens were tested for fracture resistance using compressive load on the occlusal surface. Non-parametric ANOVA using the Kruskal-Wallis and Wilcoxon rank tests was performed to test for differences in fracture resistance values with a global significance level of 0.05.

RESULTS

All artificially aged specimens survived with no failures. The median fracture resistance values (before; after artificial aging) were: DCS (1683 N; 1618 N), Procera (1522 N; 1256 N), and Vita CerecInlab (1702 N; 1556 N). No significant differences were found for comparisons between different groups before artificial aging. Artificial aging did not significantly influence the fracture resistance of different groups. After artificial aging, group Procera showed significantly lower fracture resistance than the Vita CerecInlab (p=0.015) and DCS (p=0.038) groups.

CONCLUSIONS

All tested restorations have the potential to withstand occlusal forces applied in the posterior region and can therefore represent interesting alternatives for replacing porcelain-fused-to-metal restorations. Further assessment is needed before recommending such restorations in daily practice.

Zirconia-implant-supported all-ceramic crowns withstand long-term load: a pilot investigation

OBJECTIVES

The purpose of this pilot investigation was to test whether zirconia implants restored with different all-ceramic crowns would fulfill the biomechanical requirements for clinical use. Therefore, all-ceramic Empress-1 and Procera crowns were cemented on zirconia implants and exposed to the artificial mouth. Afterwards, the fracture strength of the all-ceramic implant-crown systems was evaluated. Conventional titanium implants restored with porcelain-fused-to-metal (PFM) crowns served as controls.

MATERIAL AND METHODS

Sixteen titanium implants with 16 PFM crowns and 32 zirconia implants with 16 Empress-1 crowns and 16 Procera crowns each--i.e., three implant-crown groups--were used in this investigation. The titanium implants were fabricated using the ReImplant system and the zirconia implants using the Celay system. The upper left central incisor served as a model for the fabrication of the implants and the crowns. Eight samples of each group were submitted to a long-term load test in the artificial mouth (1.2 million chewing cycles). Subsequently, a fracture strength test was performed with seven of the eight crowns. The remaining eight samples of each group were not submitted to the long-term load in the artificial mouth but were fracture-tested immediately. One loaded and one unloaded sample of each group were evaluated regarding the marginal fit of the crowns.

RESULTS

All test samples survived the exposure to the artificial mouth. Three Empress-1 crowns showed cracks in the area of the loading steatite ball. The values for the fracture load in the titanium implant-PFM crown group without artificial loading ranged between 420 and 610 N (mean: 531.4 N), between 460 and 570 N (mean: 512.9 N) in the Empress-1 crown group, and in the Procera crown group the values were between 475 and 700 N (mean: 575.7 N) when not loaded artificially. The results when the specimens were loaded artificially with 1.2 million cycles were as follows: the titanium implant-PFM crowns fractured between 440 and 950 N (mean: 668.6 N), the Empress-1 crowns between 290 and 550 N (mean: 410.7 N), and the Procera crowns between 450 and 725 N (mean: 555.5 N). No statistically significant differences could be found among the groups without artificial load. The fracture values for the PFM and the Procera crowns after artificial loading were statistically significantly higher than that for the loaded Empress-1 crowns. There was no significant difference between the PFM crown group and the Procera group.

CONCLUSIONS

Within the limits of this pilot investigation, it seems that zirconia implants restored with the Procera crowns possibly fulfill the biomechanical requirements for anterior teeth. However, further investigations with larger sample sizes have to confirm these preliminary results. As three Empress-1 crowns showed crack development in the loading area of the steatite balls in the artificial mouth, their clinical use on zirconia implants has to be questioned.

Effect of incomplete crown ferrules on load capacity of endodontically treated maxillary incisors restored with fiber posts, composite build-ups, and all-ceramic crowns: an in vitro evaluation after chewing simulation

OBJECTIVE

The aim of this study was to compare the fracture resistance of endodontically treated maxillary central incisors with incomplete crown ferrules after chewing simulation.

MATERIAL AND METHODS

Forty caries-free maxillary central incisors were divided into 4 groups (n = 10). Endodontic treatment was performed. Teeth were decoronated 2 mm above the cemento-enamel junction (CEJ). Group I (control) provided a 360 degrees circumferential 2-mm ferrule. In group II, a 2-mm ferrule was present on the palatal aspect (status after occlusal overload), and in group III on the facial aspect (status after traumatic injury). In group IV, the ferrule was interrupted by bi-proximal cavitations (simulating caries treatment). The teeth received glass fiber reinforced posts and composite core restorations. All-ceramic crowns were adhesively cemented. Specimens were simultaneously exposed to thermal cycling and mechanical loading (1.2 million cycles; 6,000 cycles 5 degrees/55 degrees C) and finally statically loaded until failure in a universal testing machine (crosshead speed = 1 mm/min). For statistical analysis, the Kruskal-Wallis test was applied followed by the Mann-Whitney U-test as post hoc testing.

RESULTS

The median fracture load values (min./max.) were: group I = 502 (326/561), group II = 658 (280/827), group III = 899 (396/1176), and group IV = 360 (279/646). Analysis revealed statistically significant differences between test groups, except between groups I and IV and groups II and IV.

CONCLUSIONS

The fracture resistance of endodontically and post/core restored teeth is dependent on the degree of tooth conservation. An incomplete crown ferrule is associated with greater variation in load capacity and, despite high fracture values, inclines to fracture.

Finite element analysis of fixed partial denture replacement

The purpose of this study was to investigate, by means of the finite element method the mechanical behaviour of three designs of fixed partial denture (FPD) for the replacement of the maxillary first premolar in shortened dental arch therapy. Two-dimensional, linear, static finite element analyses were carried out to investigate the biomechanics of the FPDs and their supporting structures under different scenarios of occlusal loading. Displacement and stress distribution for each design of FPD were examined, with particular attention being paid to the stress variations along the retainer-abutment--and the periodontal ligament-bone interfaces. The results indicated that displacement and maximum principal stresses in the fixed-fixed, three-unit FPD were substantially less than those in the two-unit cantilever FPDs. Of the two cantilever FPDs investigated, the distal cantilever design was found to suffer less displacement and stresses than the mesial cantilever design under similar conditions of loading. The highest values for maximum principal stress in the cantilever FPDs were found within the connector between the pontic and the retainer, and within the periodontal ligament and adjacent bone on the aspect of the retainer away from the pontic.

A 6-Year Prospective Study of 620 Stress-Diversion Surface (SDS) Dental Implants

A 6-year prospective study was conducted to assess the clinical success rates and crestal bone response of a dental implant system with a stress-diversion design. Mathematical modeling, digital radiography with applied isodensity, and finite element analysis were used to highlight the effect of the stress distribution design. A total of 386 hydroxyapatite-coated prototypes and 234 commercial grit-blasted external hex implants were placed in virgin bone as well as various grafted maxillary regions, with 36% of the posterior implants being immediately loaded. Prototypes achieved 96.6% survival over a 3-year period. The grit-blasted implant, placed from 2000 to 2003, showed a 95% survival rate. There were no significant changes in crestal bone levels after the first 12 months of prosthetic loading. Engineering evaluations suggested that undesirable stresses were distributed from the crest of the ridge down through the center of the implant body.

Mechanical strain on the human skull in a humanoid robotic model

Patterns of strain were analyzed in a dry human skull at 15 different regions on the lateral and medial surfaces of the mandible. The strains were induced with a human robotic system that represented each of 8 bilateral muscles by a DC servomotor connected to a wire and pulley. The tractions of the simulated muscles (masseter, medial pterygoid, anterior temporalis, and posterior temporalis) were increased from 1x to 4x with each representing different levels of traction or force (5, 3, 4, and 4 N, respectively). The study was done with the teeth in maximum intercuspal occlusion. Bite forces were also measured with a transducer and reached a maximum of 40 N on the posterior teeth with less force on the anterior dentition. The smallest traction level (1x) developed some small strains. At 2x, compressive strains developed more on the medial (lingual) side beneath the molars through the corpus and radiated into the anterior ramus. Strains at 3x to 4x significantly increased both the tensile and compressive strains throughout the mandible with more strains developing in the ramus. The increased bilateral traction and loading developed significant compressive forces on both sides of the mandible. Evaluation of disparities between compressive and tensile strains at one site, and comparison between the medial and lateral sides of strain, suggested some visible distortion of portions of the mandible under the higher loads.

3-D Finite element analysis of all-ceramic posterior crowns

The purpose of this study was to evaluate the stress distribution under various loading conditions within posterior all-ceramic crowns. A three-dimensional finite element model representing a lower first molar was constructed. Variations of the model had two types of single layer all-ceramic crowns (Dicor and Empress) and two types of double layer all-ceramic crowns (In-Ceram and Empress2) cemented. A load of 600 N, simulating the maximum bite force, was applied vertically to the crowns. Loads of 225 N, simulating masticatory force, were applied from three directions (vertically, at a 45 degrees angle, and horizontally). In the test simulating maximum bite force, the maximum tensile stresses on all crowns (17.4-19.4 MPa) concentrated around the loading points. In the masticatory force simulation test, the specimens experienced maximum tensile stresses of 19.7-27.0 MPa under a horizontal load and 10.8-10.9 MPa under a vertical load. When the load was applied horizontally, the maximum tensile stress was observed around the loading points on the surface in the case of the single layer crowns, and of the cervical area of the inner core of the double layer crowns. Within the limitation of this study, it was found that the strength of occlusal contact points is important to the integrity of posterior all-ceramic crowns and that bite forces applied from the horizontal direction are a critical factor.

Influence of filler loading on the two-body wear of a dental composite

The purpose of the study was to explore the fundamental wear behaviour of a dental composite with different filler loadings under two-body wear conditions. The parent resin and filler components were mixed according to different weight ratios to produce experimental composites with filler loadings ranging from 20 to 87.5% by weight. A two-body wear test was conducted on the experimental composites using a wear-testing machine. The machine was designed to simulate the impact of the direct cyclic masticatory loading that occurs in the occlusal contact area in vivo. The results showed that there was little increase in the rate of wear with filler loadings below 60 wt%, but a sharp increase between 80 and 87.5 wt% in filler loading. Wide striations and bulk loss of material were apparent on the wear surfaces at higher filler loadings. Coefficients of friction increased with filler loading and followed the increase in rate of wear loss closely. It was concluded that, under two-body wear conditions, addition of high levels of filler particles into the resin matrix could reduce the wear resistance of dental composites. This finding may help when designing future dental composites for use in particular clinical settings.

Correlation between periodontal status and biting ability in Chinese adult population

This study investigated the relation between periodontal condition and biting ability in a Chinese population using the pressure-detecting sheet. A total of 142 subjects residing in Nanchang, Jiangxi province, China, participated in the study. The examination included probing pocket depth (PD), clinical attachment level (CAL), bleeding on probing and coronal caries teeth. Biting abilities per person, biting force, biting pressure and occlusal contact area were measured using the sheet. Number of teeth present showed positive correlation with biting force and occlusal contact area, but were negatively correlated with biting pressure. No significant correlation was observed between the mean PD, percentage of pocket > or = 4 mm, bleeding index and any of biting abilities. The mean CAL showed a significantly negative correlation with biting force and occlusal contact area. Multiple stepwise regression analysis selected the number of teeth present, sex and age as the significant factors affecting the biting ability. However, no periodontal indices were picked up as the significantly contributing factor. There appears to be little effect of periodontal condition on biting ability. Ageing was selected as a contributing factor to reduction in the Chinese adults, but not in the Japanese population.

Maximal bite force in patients with reduced periodontal tissue support with and without splinting

BACKGROUND

Chewing and biting forces are supposed to be limited by sensory input from periodontal mechanoreceptors. This is why the threshold level of those receptors should be lower in teeth with reduced periodontal tissue support. The purposes of the present study were to evaluate the influence of reduced periodontal tissue support on maximal bite force in natural dentitions and to study the effect of splinting on maximal bite force.

METHODS

In 10 patients with reduced periodontal tissue support (test), as well as in 10 periodontally healthy subjects (control), maximal bite force was measured. The remaining periodontal ligament area in the test group was calculated from x-rays. Bite force was assessed at 4 mm mouth opening in the premolar region without and following splinting of the posterior teeth and transduced using a strain-gauge (full-bridge circuit). Maximal bite force measured with and without splinting was compared between test and control subjects, and within each group.

RESULTS

In test subjects, the mean periodontal ligament area was 48.5% (SD = 10.1) across first premolars and 50.0% (SD = 12.2) across posterior teeth (control: 100%). In test subjects, the mean maximal bite force without splinting was 357 N (SD = 70), and in control subjects, 378 N (SD = 66; P > 0.05). After splinting, the bite force in test subjects was 509 N (SD = 75), and in control subjects, 534 N (SD = 49; P > 0.05). Bite force before and after splinting was P < 0.05 within each group.

CONCLUSIONS

Reduced periodontal tissue support does not seem to limit bite force with maximal strength in natural dentitions as measured by a device that opens the bite by 4 mm. Furthermore, maximal biting forces at 4 mm mouth opening are increased when molar teeth are included in a posterior splint.

Task-dependence of activity/ bite-force relations and its impact on estimation of chewing force from EMG

Estimation of chewing force from electromyograms (EMGs) calibrated in isometric biting yielded strikingly high force values. We tested the hypothesis that EMG-based force predictions are excessive because of differing activity/bite-force relations in mastication and isometric biting. In nine patients, unilateral bite forces and EMGs of 4 elevator muscles were recorded during chewing and isometric clenching on a bite-fork. We estimated chewing force by substituting chewing EMGs of each muscle into isometric activity/bite-force regressions. The estimates were compared with actual chewing forces recorded by intra-oral transducers. In all muscles except the balancing-side masseter, the activity/bite-force ratio was significantly higher in chewing than in isometric biting. The actual mean chewing force amounted to 220 N, while EMG-based estimates ranged from 273 to 475 N, depending on the muscle used for estimation. The results indicate that different activity/force characteristics in dynamic and isometric biting can cause overestimation when chewing force is predicted from masticatory EMGs.

Clinical experiences with a new maintenance-free shock absorbing element in titanium implants

Until now, the biokinetic elements of one implant system were to be substituted once a year in order to prevent complications of fractures of fixation screws. In this article a new implant with a maintenance-free shock absorbing element was examined. During the last 6 years, 384 dental implants with a biokinetic element (mobile-implant, SIS Inc., Klagenfurt, Austria) were placed in 138 patients. The implants were loaded with prosthetic superstructures 4 months after implantation. For comparison, 160 patients were treated with 494 conventional titanium implants of the same design without biokinetic elements. All patients were examined radiologically and clinically. Periimplant probing depth, periimplant bleeding, Periotest-values (Siemens, Bensheim, Germany) at the time of prosthetic treatment and 3, 6, 9, 12, and 24 months after implant loading were registered. Implantation was successful in 97.2% of mobile-implants and 98% of conventional implants. There was a low degree of sulcus bleeding and high degree of physiological periimplant probing depths in both patients groups. In mobile-implants, the Periotest-values were positive and similar to that of the control. There was no difference between the values in the maxilla and mandible. In the group with conventional implants, the Periotest-values were negative and showed a low degree of negativity during the first 12 months after implant loading. Periotest-values in the upper jaw were higher than in the lower jaw. There was a lower degree of periimplant bone loss after implant loading in patients with mobile-implants. In conclusion, mobile-implants demonstrate the positive effects of implants with shock absorbing elements. They are maintenance free.

The effect of normal occlusal forces on fluid movement through human dentine in vitro

Receptors inside human incisors appear to respond to stress (comparable to pressure as opposed to force) on the crown. This ability may be used to limit the stress applied to teeth or to discriminate between the hardness of objects clenched between upper and lower teeth. Here the hypothesis that these receptors are activated by fluid squeezed out of dentinal tubules when the loaded tooth is stressed was tested. Vertically compressing the crowns of extracted human teeth with loads of from 20 to 120 N, similar to those used in other studies and during natural chewing, did indeed displace fluid into the pulp. The fluid was displaced away from the crown immediately after the tooth had been loaded and moved back into the crown when the load was removed. The volume ranged from 3.5 to 22.2x10(3) pl, similar to that known to stimulate single pulpal nerve fibres. Thus, normal chewing forces could displace sufficient fluid out of dentine to excite putative mechanoreceptors somewhere inside the dentine/pulp complex.

Finite element stress analysis on the effect of splinting in fixed partial dentures

STATEMENT OF PROBLEM

Long-span fixed partial dentures usually require splinting of multiple abutments to overcome mechanical problems associated with the long edentulous span. Most information and indications for the use of multiple splinted abutments have been empirically derived.

PURPOSE

This study analyzed the stress levels in the teeth and supporting structures of a fixed prosthesis and ascertained how the addition of multiple abutments in a fixed prosthesis modifies the stresses and their deflection.

MATERIAL AND METHODS

The finite element method was used to analyze mechanical behaviors of a prosthesis and its supporting structures when a fixed prosthesis with several designs replaced a mandibular second premolar and a first molar. Variations of the standard finite element model were made by changing the number of splinted teeth and the level of bone support.

RESULTS

A reduction of stress and deflection was observed in the supporting structures when a fixed partial denture was fabricated and teeth were splinted together. Increasing the number of splinted abutments did not reveal a proportional reduction of stress in the periodontium. Stress concentrations were seen in the connectors of prosthesis and in the cervical dentin area near the edentulous ridge.

CONCLUSION

Increasing the number of the splinted abutment did not compensate for the mechanical problems of a long-span fixed partial denture sufficiently.

Clinically relevant approach to failure testing of all-ceramic restorations

STATEMENT OF PROBLEM

One common test of single-unit restorations involves applying loads to clinically realistic specimens through spherical indenters, or equivalently, loading curved incisal edges against flat compression platens. As knowledge has become available regarding clinical failure mechanisms and the behavior of in vitro tests, it is possible to constructively question the clinical validity of such failure testing and to move toward developing more relevant test methods.

PURPOSE

This article reviewed characteristics of the traditional load-to-failure test, contrasted these with characteristics of clinical failure for all-ceramic restorations, and sought to explain the discrepancies. Literature regarding intraoral conditions was reviewed to develop an understanding of how laboratory testing could be revised. Variables considered to be important in simulating clinical conditions were described, along with their recent laboratory evaluation.

CONCLUSIONS

Traditional fracture tests of single unit all-ceramic prostheses are inappropriate, because they do not create failure mechanisms seen in retrieved clinical specimens. Validated tests are needed to elucidate the role(s) that cement systems, bonding, occlusion, and even metal copings play in the success of fixed prostheses and to make meaningful comparisons possible among novel ceramic and metal substructures. Research over the past 6 years has shown that crack systems mimicking clinical failure can be produced in all-ceramic restorations under appropriate conditions.

Splinting osseointegrated implants and natural teeth in rehabilitation of partially edentulous patients. Part II: principles and applications

Lone standing splinted implant segments are accepted as optimal. However, in the treatment of partial edentulism, clinical reality often predicates the consideration of splinting teeth and implants due to variables of tooth/implant location and available bone support. This article presents a review on biomechanical aspects of splinting teeth and some considerations of splinting teeth and implants. A proposed classification of splinting applicable to both teeth and implants is presented as well as a discussion of the clinical aspects of splinting illustrated with clinical cases.

The association among occlusal contacts, clenching effort, and bite force distribution in man

The contact area during habitual biting can vary according to the activity of the jaw musculature. Forceful masticatory muscle activity may also induce deformations of the dento-alveolar tissues and the supporting skeleton, yielding various tooth loads despite an apparently even distribution of tooth contacts. To investigate this variability, we measured bite forces simultaneously at multiple dental sites during maximum-effort clenching tasks. In each of four healthy adults with complete natural dentitions, four strain-gauge transducers in the right side of an acrylic maxillary appliance occluded with the lower canine, second premolar, and first and second molars. These, and matching contralateral contacts, were balanced by means of articulating paper and a force monitor (type F appliance). Bite forces were recorded when the subjects, without visual feedback, clenched maximally on the appliance. Similar recordings were made when contralateral molar and all contralateral contacts were removed (type R and type U appliances, respectively). Although the relation between individual forces often changed during the initial increase in force, it was generally constant around the maximum. The maximum forces at the four dental locations varied in distribution between subjects, but were characterized by posteriorly increasing forces. Forces in the anterior region (especially at the canine) significantly increased (up to 10 times) when clenching took place on unilateral contacts only (type U) as compared with fully balanced ones (type F). Bite force distribution thus changed with biting strength and the location of occlusal contacts. Increased force in the canine region during unilateral clenching seems related to the pattern of jaw muscle co-activation and the physical properties of the craniomandibular and dental supporting tissues which induce complex deformations of the lower jaw.