Mesial Marginal Ridge Research Articles

Effect of Incorporating Chitosan to Resin Modified Glass Ionomer Cement on Shear Bond Strength to Dentin (An In vitro Comparative Study).

Resin-modified glass ionomer cement tends to shrink due to polymerization of the resin component. Additionally, they are more prone to syneresis and imbibition during the setting process. This in vitro study evaluates the impact of chitosan, a biopolymer that is, both biomaterial and biocompatible, on the strength of dentin bonding and compares it with ACTIVA Bio-ACTIVE Restorative. The present study was aimed to assess the impact of including chitosan into Fuji II on the shear bond strength between. the restoration material and tooth dentin, in contrast to Bioactiva in permanent teeth. A total of 30 premolar teeth were recently extracted. The study involved three distinct sample groups. Group 1 (10 teeth) is the negative control (Fuji II), Group 2 (10 teeth) is the positive control (ACTIVA Bio-ACTIVE Restorative), and Group 3 (10 teeth) is treated with a mixture of Chitosan and Fuji II (CH-Fuji II). Each tooth's buccal and palatal cusps were eliminated to achieve a horizontal surface. Using a periodontal probe, 1.5 mm from the mesial pit to the mesial marginal ridge were removed. Restoration was implemented in all groups following manufacturer directions. Thermocycling the teeth by immersing them in a water bath with temperatures ranging from 5°C to 55°C (± 1-2°C) for 30 s (500 cycles). Each sample was attached to the universal testing machine's jig at a cross-head speed of 1 mm/min. Shear force was used until breakage, and the bond's adhesive strength was then calculated. Statistical analysis using ANOVA with Dunnett's T3 post hoc test. Results were significant at P < 0.05. Statistically significant difference was present between Chitosan and Fuji II and between Chitosan and Activa by reducing the shear bond strength. Addition of chitosan to Fuji II had a negative effect on the shear bond with a significant difference while Activa and Fuji II exhibited favorable shear bond strength.

Associations between 44 nonmetric permanent dental traits or anomalies with skeletal sagittal malocclusions and sex, besides correlations across the variations or abnormalities

IntroductionNonmetric dental traits and the shape, size, or number of dental anomalies are essential to various dental fields such as orthodontics, dental anatomy, anthropology, pathology, and forensic dentistry. Nonetheless, many are not well assessed worldwide. Moreover, most studies are limited to a few nonmetric traits. Therefore, we aimed to examine several nonmetric dental traits/anomalies.MethodsIn this cross-sectional epidemiological study, ~ 9000 permanent teeth of 331 non-syndromic orthodontic patients (radiographs and dental casts) with fully erupted permanent dentitions (except the third molars and some cases of a few teeth missing or excluded) were evaluated by two observers, each twice, in search for 62 nonmetric traits/shape-number-size anomalies. The traits/anomalies of interest were supernumerary, microdontia, peg-shaped lateral, shovelings, talon cusps, Carabelli cusps, fifth/sixth/seventh cusps on the molars, hypocone/hypoconulid absence, protostylid, deflecting wrinkle‏s, canine mesial ridge, distal trigonid crest, canine distal accessory ridge, accessory cusps in the mesial/distal marginal ridges, mesial/distal accessory ridges, and accessory cusps in the lingual of the mandibular premolars and second molars). Data, at both patient/quarter levels, were analyzed regarding the associated factors (skeletal Angle classes, crowding, sex, and sides) as well as the correlations among traits, using the chi-square test and Spearman correlation coefficient (α = 0.05).ResultsPrevalence rates of 44 traits/anomalies were reported (18 out of the 62 searched traits/anomalies were not found [prevalence = 0%]). Microdontia and accessory cusps on the marginal ridge of the second mandibular molars were significantly more common in women (P < 0.05). Canine talon cusp and distal trigonid crest of the second mandibular molars were more prevalent in men (P < 0.05). Shoveling, canine talon cusp, canine distal accessory ridge, and accessory cusp in the first premolar might be more prevalent in skeletal Angle class II; whereas, accessory cusp in the mesial marginal ridge of the second premolar might be rather more prevalent in skeletal Angle class I (P < 0.05). Few dental traits were positively and moderately or strongly correlated with each other (Spearman Rho ≥ 0.4, P < 0.0005).ConclusionsSex dimorphism was uncommon in nonmetric dental traits/anomalies. Skeletal malocclusions may be associated with a few dental abnormalities or variations.

The Effect of Varying Occlusal Loading Conditions on Stress Distribution in Roots of Sound and Instrumented Molar Teeth: A Finite Element Analysis

IntroductionThis study aimed to investigate whether the direction of force applied to the occlusal surface influenced the pattern of tensile stresses in roots of sound and root canal–prepared mandibular molar teeth. The effect of obturation forces on the development of apical stress was also investigated. To this end, models were constructed using micro–computed tomographic imaging and investigated using finite element analysis. MethodsMicro–computed tomographic data established boundaries of internal and external model surfaces to allow finite element analysis. Individually segmented components were modeled based on mechanical properties in precedent literature. The following conditions were considered: axial force directed over the mesial marginal ridge, a mesial or a distal tipping force, a combination of both a torquing force and axial loading, and hydrostatic pressure. The maximum principal stresses were determined. ResultsThe highest root stress occurred in the cervical third of root surfaces (ie, not apically) under all loading conditions. Importantly, mesial tipping forces resulted in tension on distal roots, whereas distal tipping resulted in tension in the mesial roots. Intracanal pressures produced tensile stress on the internal root canal walls in the cervical third of the root. Stresses were calculated to be less than the fatigue tensile strength of dentin. ConclusionsStatic loading, under the conditions modeled, does not result in stress concentration at the root apices that would cause root fracture under normal masticatory loads. Stress patterns developing from mesial and distal tipping forces help to explain the appearance of vertical root fractures reported in sound nonrestored molar teeth.

Residual stress estimated by nanoindentation in pontics and abutments of veneered zirconia fixed dental prostheses.

Glass ceramics’ fractures in zirconia fixed dental prosthesis (FDP) remains a clinical challenge since it has higher fracture rates than the gold standard, metal ceramic FDP. Nanoindentation has been shown a reliable tool to determine residual stress of ceramic systems, which can ultimately correlate to failure-proneness.Objectives:To assess residual tensile stress using nanoindentation in veneered three-unit zirconia FDPs at different surfaces of pontics and abutments.Methodology:Three composite resin replicas of the maxillary first premolar and crown-prepared abutment first molar were made to obtain three-unit FDPs. The FDPs were veneered with glass ceramic containing fluorapatite crystals and resin cemented on the replicas, embedded in epoxy resin, sectioned, and polished. Each specimen was subjected to nanoindentation in the following regions of interest: 1) Mesial premolar abutment (MPMa); 2) Distal premolar abutment (DPMa); 3) Buccal premolar abutment (BPMa); 4) Lingual premolar abutment (LPMa); 5) Mesial premolar pontic (MPMp); 6) Distal premolar pontic (DPMp); 7) Buccal premolar pontic (BPMp); 8) Lingual premolar pontic (LPMp); 9) Mesial molar abutment (MMa); 10) Distal molar abutment (DMa); 11) Buccal molar abutment (BMa); and 12) Lingual molar abutment (LMa). Data were assessed using Linear Mixed Model and Least Significant Difference (95%) tests.Results:Pontics had significantly higher hardness values than premolar (p=0.001) and molar (p=0.007) abutments, suggesting lower residual stress levels. Marginal ridges yielded higher hardness values for connectors (DPMa, MMa, MPMp and DPMp) than for outer proximal surfaces of abutments (MPMa and DMa). The mesial marginal ridge of the premolar abutment (MPMa) had the lowest hardness values, suggesting higher residual stress concentration.Conclusions:Residual stress in three-unit FDPs was lower in pontics than in abutments. The outer proximal surfaces of the abutments had the highest residual stress concentration.

Distinct mandibular premolar crown morphology in Homo naledi and its implications for the evolution of Homo species in southern Africa

Homo naledi displays a combination of features across the skeleton not found in any other hominin taxon, which has hindered attempts to determine its placement within the hominin clade. Using geometric morphometrics, we assess the morphology of the mandibular premolars of the species at the enamel-dentine junction (EDJ). Comparing with specimens of Paranthropus, Australopithecus and Homo (n = 97), we find that the H. naledi premolars from the Dinaledi chamber consistently display a suite of traits (e.g., tall crown, well-developed P3 and P4 metaconid, strongly developed P3 mesial marginal ridge, and a P3 > P4 size relationship) that distinguish them from known hominin groups. Premolars from a second locality, the Lesedi Chamber, are consistent with this morphology. We also find that two specimens from South Africa, SK 96 (usually attributed to Paranthropus) and Stw 80 (Homo sp.), show similarities to the species, and we discuss a potential evolutionary link between H. naledi and hominins from Sterkfontein and Swartkrans.

Influence of cavity depth and restoration of non-carious cervical root lesions on strain distribution from various loading sites

BackgroundWe aimed to investigate the load-induced strain variation in teeth with unrestored and resin-based composite restored non-carious cervical lesions (NCCLs).MethodsTwelve extracted premolars were provided for measuring buccal-side root NCCLs. Strain gauges were fixed at four measuring sites of each tooth, two at the buccal surface and two at the lingual surface. NCCLs were prepared with occlusal margins at the cemento-enamel junction. A static 9-kg load was applied at seven occlusal loading points: buccal cusp tip (BC), inner inclination of the BC, lingual cusp tip (LC), inner inclination of the LC, center of the mesial marginal ridge or distal marginal ridge, and center of the central groove. The strain was detected at each site in teeth with NCCL depths of 0 (control), 0.5, 1.0, and 1.5 mm. Each NCCL was restored using an adhesive composite resin, and the strains were re-measured.ResultsThe strains at the NCCL occlusal and gingival margins decreased with increasing defect depths, and the effect was significant when the depth of the defect was 1.5 mm. Loading on the buccal and lingual cusps induced prominent strain variation. The strains at all depth distribution recovered to nearly intact conditions when the NCCLs were restored.ConclusionsNCCLs at 1.5 mm depth are detrimental, but they can be restored using resin composites.Clinical significanceThe existence of NCCLs should not be ignored. The depth of the NCCL may affect the progression of the lesion. Resin composite restoration is an appropriate method for preventing persistent NCCL deterioration.

Endostructural morphology in hominoid mandibular third premolars: Discrete traits at the enamel-dentine junction

The mandibular third premolar (P3) exhibits substantial differences in size and shape among hominoid taxa, and displays a number of discrete traits that have proven to be useful in studies of hominin taxonomy and phylogeny. Discrete traits at the enamel-dentine junction (EDJ) can be accurately assessed on moderately worn specimens, and often appear sharper than at the outer-enamel surface (OES). Here we use microtomography to image the P3 EDJ of a broad sample of extant apes, extinct hominins and modern humans (n = 100). We present typologies for three important premolar discrete traits at the EDJ (transverse crest, marginal ridge and buccal grooves), and score trait frequencies within our sample. We find that the transverse crest is variable in extant apes, while the majority of hominins display a transverse crest which runs directly between the two major premolar cusps. Some Neanderthals display a unique form in which the transverse crest fails to reach the protoconid. We find that mesial marginal ridge discontinuity is common in Australopithecus anamensis and Australopithecus afarensis while continuous marginal ridges largely characterize Australopithecus africanus and Paranthropus. Interrupted mesial and distal marginal ridges are again seen in Homo sapiens and Neanderthals. Premolar buccal grooves, previously identified at the OES as important for hominin systematics, are again found to show a number of taxon-specific patterns at the EDJ, including a clear difference between Australopithecus and Paranthropus specimens. However, their appearance may be dependent on the morphology of other parts of the crown such as the protoconid crest, and the presence of accessory dentine horns. Finally, we discuss rare variations in the form of dentine horns that underlie premolar cusps, and their potential homology to similar morphologies in other tooth positions.

Treatment Decision-making of Cracked Teeth: Survey of American Endodontists

The aim of this study is to survey the opinion of endodontists in the United States regarding their approach to treatment of cracked teeth. A survey assessing the opinions of 3,500 endodontists in the United States was administered over a 4-week period. It consisted of a hypothetical clinical case and eight different treatment scenarios. Participants were asked to select one of the two treatment options: (A) informing the patient of the presence of a crack, reinforcing the tooth, and continuing with endodontic treatment; and (B) extraction of the tooth followed by replacement with an implant supported crown or fixed partial denture. When the crack extended across the distal marginal ridge with no associated probing depths, 92.65% endodontists preferred to continue with endodontic treatment and reenforcing the tooth. When the crack extended across the distal marginal ridge and was associated with a probing depth of 6 mm, 80% preferred extraction of the tooth. When the crack extended across the mesial marginal ridge up to the orifice of the mesiolingual canal with no associated probing depth, 82.78% preferred to continue with the treatment. When the crack was associated with a 6-mm probing depth, 83.79% preferred tooth extraction. When the crack extended across the mesial marginal ridge and down into the mesiolingual canal with/without associated probing depth, 91.13% and 63.54% preferred tooth extraction, respectively. When the crack involved both mesial and distal marginal ridges and extended across the pulp chamber, 79.74% preferred tooth extraction. When a split tooth was presented, 98.48% preferred tooth extraction. It appears that the presence of a 6-mm periodontal pocket is considered an important factor by most American endodontists when deciding whether to preserve the cracked tooth or extract it. There is no consensus among dentists regarding the best approach to treat cracked teeth. Conclusive studies evaluating clinical approaches of endodontists regarding treatment of cracked teeth are lacking. Therefore, surveyed opinion of endodontists in the United States regarding their approach to treatment of cracked teeth was done to try to reach the best clinical decision regarding this dilemma.

A cone-beam computed tomography study of the mesial cervical concavity of maxillary first premolars

ObjectiveTo investigate the mesial cervical concavity of maxillary first premolars and its relationship with root and canal configuration using cone-beam computed tomography (CBCT). DesignImages of maxillary first premolars (n = 1056) were collected from patients (n = 601) who had undergonein vivo CBCT scanning. The root and canal number and morphology were evaluated. The following measurements of the mesial cervical concavity of the maxillary first premolars were evaluated in section images: dentine thickness (in concavity at the cemento-enamel junction), concavity angle, depression depth (distance from mesial dentinal surface at concavity to mesial proximity), concavity position (distance from mesial dentinal wall at invagination to the top of the mesial marginal ridge). The reliability of the data was analyzed with an unpaired Student’s t test and Fisher’s exact test. ResultsThe percentages of maxillary first premolars with one root, two, and three roots were 55.5%, 43.7%, and 0.8% respectively. Mesial cervical concavity was recorded in 64.5% of single-root maxillary premolars. The prevalence of two-root maxillary first premolars with mesial cervical concavity was 73.8%. The means of the aforementioned four measurements were 1.705, 147.9, 1.640, and 5.247 mm. The values of dentine thickness (mm), depression depth (mm), and concavity position (mm) of the mesial cervical concavity were largest in two-root maxillary first premolars. The smallest concavity angle of the mesial cervical concavity was found in three-root maxillary first premolars. ConclusionsThere is a high prevalence of mesial cervical concavity among maxillary first premolars. The mesial root concavity is more prevalent in single-rooted maxillary first premolars when there are two canals present, and its prevalence and degree of concavity increase with the number of roots.

A Neanderthal deciduous human molar with incipient carious infection from the Middle Palaeolithic De Nadale cave, Italy.

The aim of the study is the assessment of Nadale 1, a Neanderthal deciduous tooth recently discovered in Northeastern Italy in the De Nadale cave (Middle Palaeolithic). Together with the clear archaeological context of the site, this study brings new insight on Neanderthal behavior and dental morphological variability. We used microCT data to provide a morphological description and morphometric analysis (diameter measurements and dental tissue volumes) of the Nadale 1 human tooth. Microwear analysis, taphonomical investigation and caries identification were performed using a stereomicroscope and Scanning Electron Microscope. In terms of morphology (i.e., incipient tuberculum molare, marked mesial marginal ridge and well-developed mid-trigonid crest connecting the protoconid and the metaconid, deep anterior fovea) and size, Nadale 1 presents features frequently observed in Neanderthal lower first deciduous molars. Microscope investigations reveal the presence of a small pit which could be correlated to an incipient caries. Nadale 1 expands the Italian Middle Palaeolithic fossil record and provides further information on Neanderthal dm1s in terms of dimensional and morphological variability. Furthermore, the presence of an incipient caries brings further data on Neanderthal diet.

Cracked Tooth Syndrome in an Unrestored Maxillary Premolar: A Case Report

Cracked tooth syndrome is known to occur most frequently in heavily restored teeth. Nevertheless, when the symptoms occur in intact teeth, there is difficulty in obtaining a correct diagnosis because it is difficult for the dentist to find where the crack is located. This clinical report describes the diagnostic procedures and the direct bonded composite restoration used to restore an incompletely fractured unrestored maxillary premolar in a 22-year-old female patient. To achieve a correct diagnosis, the following were performed: periapical and bitewing radiographs, percussion and thermal vitality tests, a bite test, and the placement of a stainless steel band. Once the symptoms ceased with band placement, cone beam computed tomography, transillumination, macro photographs, and isolation with a rubber dam helped to visualize the crack line along the occlusal surface involving distal and mesial marginal ridges. The crack was traced using a high-speed tungsten carbide bur until the fracture line was not visible. The tooth was restored with a direct composite resin, associated with a total-etch adhesive system, and the symptoms were immediately eliminated.

Assessment of first molars sagittal and rotational position in Class II, division 1 malocclusion

This study assessed the anterior-posterior positioning of the upper and lower first molars, and the degree of rotation of the upper first molars in individuals with Class II, division 1, malocclusion. Asymmetry I, an accurate device, was used to assess sixty sets of dental casts from 27 females and 33 males, aged between 12 and 21 years old, with bilateral Class II, division 1. The sagittal position of the molars was determined by positioning the casts onto the device, considering the midpalatal suture as a symmetry reference, and then measuring the distance between the mesial marginal ridge of the most distal molar and the mesial marginal ridge of its counterpart. With regard to the degree of rotation of the upper molar, the distance between landmarks on the mesial marginal ridge was measured. Chi-square test with a 5% significance level was used to verify the variation in molars position. Student's t test at 5% significance was used for statistical analysis. A great number of lower molars mesially positioned was registered, and the comparison between the right and left sides also demonstrated a higher number of mesially positioned molars on the right side of both arches. Theaverage rotation of the molars was found to be 0.76 mm and 0.93 mm for the right and left sides, respectively. No statistically significant difference was detected between the mean values of molars mesialization regardless of the side and arch. Molars rotation, measured in millimeters, represented ¼ of Class II.

A comparative evaluation of fracture resistance of endodontically treated teeth, with variable marginal ridge thicknesses, restored with composite resin and composite resin reinforced with Ribbond: An in vitro study

The anatomic shape of maxillary premolars show a tendency towards separation of their cusps during mastication after endodontic treatment. Preservation of the marginal ridge of endodontically treated and restored premolars can act as a strengthening factor and improve the fracture resistance. To evaluate the effect of varying thickness of marginal ridge on the fracture resistance of endodontically treated maxillary premolars restored with composite and Ribbond reinforced composites. One hundred and twenty, freshly extracted, non carious human mature maxillary premolars were selected for this experimental in vitro study. The teeth were randomly assigned in to twelve groups ( n = 10). Group 1 received no preparation. All the premolars in other groups were root canal treated. In subgroups of 3 and 4, DO cavities were prepared while MOD cavities were prepared for all subgroups of group 2, the dimensions of the proximal boxes were kept uniform. In group 3 and 4 the dimensions of the mesial marginal ridge were measured using a digital Vernier caliper as 2 mm, 1.5 mm, 1 mm and 0.5 mm in the respective subgroups. All samples in groups 2.2 and all the subgroups of 3 were restored with a dentin bonding agent and resin composite. The teeth in group 2.3 and all subgroups of 4 were restored with composite reinforced with Ribbond fibers. The premolars were submitted to axial compression up to failure at 45 degree angle to a palatal cusp in universal testing machine. The mean load necessary to fracture was recorded in Newtons and the data was analysed. There was a highly significant difference between mean values of force required to fracture teeth in group 1 and all subgroups of group 2, 3 and 4 (i.e., P < 0.01) CONCLUSION: On the basis of static loading, preserving the mesial marginal ridge with thicknesses of mm, 1.5 mm, 1 mm and 0.5 mm, composite restored and Ribbond reinforced composite restored maxillary premolars can help preserve the fracture resistance of teeth.

일부 대학생의 상악 대구치 임상치관의 형태와 크기

이 연구는 상악 대구치 임상치관의 형태 및 크기를 계측하기 위해 건강한 영구치열을 갖는 C보건대학생 100명을 대상으로 다음과 같은 결론을 얻었다. 1. 임상치관의 교두높이, 치관폭, 치관두께는 좌우 대칭관계를 보였다. 2. 협면구의 강한 발육은 우측 제1대구치에서 우세함을 보였고, 협면소와는 우측 제1대구치에서 높은 출현률을 보였다. 3. 4교두형은 좌 우측 제1대구치에서 100%, 좌 우측 제2대구치에서 각각 78%, 75%로 나타났다. 4. 4교두형일때 원심설측교두 크기는 좌, 우 대칭관계를 보였다. 5. 교두정간 거리는 좌 우측 제1,2대구치 모두 근심 교두정간 거리가 컸다. 6. 카라벨리 교두의 발달은, 좌 우측 모두 제1대구치에서 높은 발육을 보였다. 7. 사주융선의 출현률은 우측 제1대구치에서 87.0%, 우측 제 2대구치에서는 73.0%의 출현율을 보였고, 좌측 제1대구치에서는 88.0%, 좌측 제2대구치에서는 73.0%로 이는 제1대구치에서 약한 치관우식증을 가지고 있는 사람 때문인것으로 사료되었다. 8. 근심변연융선결절 출현율은 좌 우측 모두 제1대구치에서 높은 출현율을 보였다. 원심부 결절은 우측 제1대구치에서 16.0%, 우측 제 2대구치에서 26.0%였고, 좌측 제1대구치에서 14.0%, 좌측 제2대구치에서 21.0%의 출현율을 보였다. This study was implemented among 100 students of C College of Public Health who have healthy permanent dentition in order to measure the morphology and sizes of clinical crowns of permanent maxillary molars. The following are conclusions of this study. 1. The cusp height, crown width, crown thickness of clinical crowns appeared to be bilaterally symmetrical. 2. The strong development of the buccal groove showed to be superior in the right first molar. The appearance rate of the buccal pit was high in the right first molar also. 3. The type 4th cusp appeared as 100% in the left and right first molars, and 78%, 75% in the left and right second molars respectively. 4. The distal lingual cusp(DLC) size were bilaterally symmetrical in the type 4th cusp. 5. As for the distance between two cusp tips, it was large between mesial cusp tips in all of the first and second molars at both sides. 6. Development of the Carabelli's cusp was high in both the left and right first molars. 7. The appearance rate of the oblique ridge was 87.0% in the right first molar, 73.0% in the right second molar, 88.0% in the left first molar, and 73.0% in the left second molar. This is considered to be caused by people who have mild dental crown caries in their first molars. 8. The appearance rate of the mesial marginal ridge tubercle(DMRT) was high in both of the left and right first molars. That of the distal tubercle was 16.0% in the right first molar, 26.0% in the right second molar, 14.0% in the left first molar, and 21% in the left second molar.

Increased Occurrence of Dental Anomalies Associated with Infraocclusion of Deciduous Molars

To test the null hypothesis that there is no relationship between infraocclusion and the occurrence of other dental anomalies in subjects selected for clear-cut infraocclusion of one or more deciduous molars. The experimental sample consisted of 99 orthodontic patients (43 from Boston, Mass, United States; 56 from Jerusalem, Israel) with at least one deciduous molar in infraocclusion greater than 1 mm vertical discrepancy, measured from the mesial marginal ridge of the first permanent molar. Panoramic radiographs and dental casts were used to determine the presence of other dental anomalies, including agenesis of permanent teeth, microdontia of maxillary lateral incisors, palatally displaced canines (PDC), and distal angulation of the mandibular second premolars (MnP2-DA). Comparative prevalence reference values were utilized and statistical testing was performed using the chi-square test (P < .05) and odds ratio. The studied dental anomalies showed two to seven times greater prevalence in the infraocclusion samples, compared with reported prevalence in reference samples. In most cases, the infraoccluded deciduous molar exfoliated eventually and the underlying premolar erupted spontaneously. In some severe phenotypes (10%), the infraoccluded deciduous molar was extracted and space was regained to allow uncomplicated eruption of the associated premolar. Statistically significant associations were observed between the presence of infraocclusion and the occurrence of tooth agenesis, microdontia of maxillary lateral incisors, PDC, and MnP2-DA. These associations support a hypothesis favoring shared causal genetic factors. Clinically, infraocclusion may be considered an early marker for the development of later appearing dental anomalies, such as tooth agenesis and PDC.

Effect of restoration type on the stress distribution of endodontically treated maxillary premolars; Three-dimensional finite element study

The purpose of this study was to investigate the effects of four restorative materials under various occlusal loading conditions on the stress distribution at the CEJ of buccal. palatal surface and central groove of occlusal surface of endodontically treated maxillary second premolar, using a 3D finte element analysis. A 3D finite element model of human maxillary second premolar was endodontically treated. After endodontic treatment, access cavity was filled with Amalgam, resin, ceramic or gold of different mechanical properties. A static 500N forces were applied at the buccal (Load-1) and palatal cusp (Load-2) and a static 170N forces were applied at the mesial marginal ridge and palatal cusp simultaneously as centric occlusion (Load-3). Under 3-type Loading condition, the value of tensile stress was analyzed after 4-type restoration at the CEJ of buccal and palatal surface and central groove of occlusal surface Excessive high tensile stresses were observed along the palatal CEJ in Load-1 case and buccal CEJ in Load-2 in all of the restorations. There was no difference in magnitude of stress in relation to the type of restorations. Heavy tensile stress concentrations were observed around the loading point and along the central groove of occlusal surface in all of the restorations. There was slight difference in magnitude of stress between different types of restorations. High tensile stress concentrations around the loading points were observed and there was no difference in magnitude of stress between different types of restorations in Load-3.

Effects of Splinting on Displacement of Maxillary Canines as Abutments of Removable Partial Dentures: a Finite Element Analysis

Purpose: The purpose of this study was to investigate the relationship between the number of splinted teeth and the effect of splinting on the movement of canines as abutments for maxillary distal-extension removable partial dentures.Methods: A three-dimensional finite element method was used for the analysis. Abutments with normal bone height and with bone height lowered by 3 mm were prepared. The numbers of splinted teeth were two, three and six units. A load of 50 N or 200 N was applied in the vertical, buccal-oblique and lingual-oblique directions at the node corresponding to the buccolingual center of the mesial marginal ridge of the left first molar. Different material properties of the mucosa and periodontal ligament were used depending on the amount of loading. The ligament was defined as an orthotropic material.Results: The results showed that horizontal displacement of the canine in the “two-unit splint” model was approximately one-third to one-half of that in the single abutment model. There was little difference in the horizontal displacement between the two-unit splint and “three-unit splint” models. In the “six-unit splint” model, the displacement was reduced to approximately half of that of the two-unit splint model. In all of the splinted models, the vertical displacement was reduced to approximately half of that of the single abutment model.Conclusion: In the cases of reduced periodontal support or large and/or laterally directed occlusal force, abutments should be splinted. In cases of a coincidence of these unfavorable conditions, splinting six anterior teeth was recommended.

Morphometrical Study of the Dentino-enamel Junction of the Labial Surface of the Maxillary Central Incisor

The labial surface of the upper central incisor has mesial and distal marginal ridges, and these vary in form and development. We measured the height of the labial marginal ridges of the upper central incisors, which had double-shovel (DS) and no double-shovel (NDS) on the outer enamel surface (OES) and dentino-enamel junction (DEJ) using a 3D scanner. Enamel thickness of the labial surface was also measured in these specimens. Mesiodistal and buccolingual diameters on both OES and DEJ were measured for the comparison of OES and DEJ structures. We obtained the following results ; 1. On the DEJ, DS incisors had more sharp and clear ridges than on the OES. 2. The mesial marginal ridge (MMR) was significantly higher than the distal marginal ridge (DMR) on both OES and DEJ of DS incisors. 3. There were no significant differences of the enamel thicknesses at the three points of the labial surface, such as mesial and distal marginal ridges and the central ridge. 4. The enamel thickness of the DS was significantly thicker than that of the normal incisor at the center of the labial surface, the average of which was 0.91 mm in DS and 0.73 in NDS incisors. 5. No significant differences of both mesiodistal and buccolingual diameters of OES and DEJ were found between averages of NDS group and DS group. 6. Both mesiodistal diameter and buccolingual diameter on OES had a strong correlation to the corresponding diameters on DEJ. The enamel substance at the central labial surface of the DS was significantly thicker than that of the NDS, and marginal ridges in DS observed in OES were clearer and more developed in the DEJ, which was an anatomical base for crown development.

The size of occlusal rest seats prepared for removable partial dentures

Objective The aim of this study was to test whether rest seats cut by a group of general dental practitioners for a removable partial denture differed in size and shape from those prepared by either a group of postgraduate students or their academic teachers. Method The occlusal surfaces of a number of plastic teeth were scanned by a laser profilometer. Each tooth was then placed in a set of articulated phantom head dental arches. 30 dental practitioners, 16 postgraduates and 11 dental academics were asked to cut a rest seat preparation in the mesial marginal ridge suitable for the construction of a removable chromium cobalt partial denture. The tooth was removed from the models, rescanned, and this data converted to grey level images for measurement of the width, length and area of each rest seat. Depth was calculated as the difference between the pre and post preparation scanned profiles. Results There was a wide variation in the size of the individual rest seat preparations. There was no significant difference between the measured parameters from the images of the preparations made by academic staff and postgraduate students. The two sets of data were therefore combined. The length, width and area of the rest seats prepared by the staff and postgraduate group were significantly greater than those cut by the dental practitioner group. However, there was no significant difference in the depths measured. The outline form of the rests prepared by the dental practitioners was often round with sharply defined margins contrasting with the smooth triangular preparation the staff and postgraduates prepared. Conclusion A 'refresher' in tooth modification for GDPs designing partial dentures would improve the long term success of the prosthesis.

The size of occlusal rest seats prepared for removable partial dentures

The aim of this study was to test whether rest seats cut by a group of general dental practitioners for a removable partial denture differed in size and shape from those prepared by either a group of postgraduate students or their academic teachers. The occlusal surfaces of a number of plastic teeth were scanned by a laser profilometer. Each tooth was then placed in a set of articulated phantom head dental arches. 30 dental practitioners, 16 postgraduates and 11 dental academics were asked to cut a rest seat preparation in the mesial marginal ridge suitable for the construction of a removable chromium cobalt partial denture. The tooth was removed from the models, rescanned, and this data converted to grey level images for measurement of the width, length and area of each rest seat. Depth was calculated as the difference between the pre and post preparation scanned profiles. There was a wide variation in the size of the individual rest seat preparations. There was no significant difference between the measured parameters from the images of the preparations made by academic staff and postgraduate students. The two sets of data were therefore combined. The length, width and area of the rest seats prepared by the staff and postgraduate group were significantly greater than those cut by the dental practitioner group. However, there was no significant difference in the depths measured. The outline form of the rests prepared by the dental practitioners was often round with sharply defined margins contrasting with the smooth triangular preparation the staff and postgraduates prepared. A 'refresher' in tooth modification for GDPs designing partial dentures would improve the long term success of the prosthesis.