Distolingual Cusp Research Articles

Monson’s sphere in Chinese young adult females with individual normal occlusion: a preliminary study using digital models

BackgroundThis study investigated the characteristics of Monson’s sphere in Chinese young adult females with individual normal occlusion to provide a reference for oral rehabilitation in prosthodontic and orthodontic treatments.MethodsPoints at the dental cusps and incisal edges were selected from 51 digital mandibular dental models of Chinese young adult females (aged 18–22 years) with individual normal occlusion. Monson’s spheres were fitted to the selected points based on the least-squares principle and the radii were calculated. The deviation of each selected point from its relative spherical surface was also calculated. The radii and deviations of these points were examined using conventional descriptive statistics and distributions of the most deviated points inside and outside the spheres were analyzed.ResultsThe mean radius of Monson’s sphere in Chinese young adult females was 79.60 ± 14.13 mm. The deviation of each selected point from its relative sphere surface was 0.38 ± 0.30 mm. The maximum deviations inside and outside the sphere were 0.93 ± 0.25 mm and 0.95 ± 0.30 mm, respectively. The most deviated points outside the spheres were mainly distributed at the distolingual cusps of the mandibular second permanent molars (31.37%), while those inside the spheres were mainly distributed at the mesiolingual cusps of the mandibular first permanent molars (45.10%).ConclusionsThe radius of Monson’s sphere in Chinese young adult females was smaller than the classic four-inch value suggested by Monson. Deviation was observed from all selected points to their Monson’s sphere surface, with the most deviated points distributed primarily in the molar region.

New material (Tritylodontidae, Mammaliamorpha) from the lower Jurassic of Lufeng and its implication on the taxonomy of Lufengia

ABSTRACT The Lufeng locality of China was a major biodiversity hotspot for sauropods and tritylodontids during the Early Jurassic. We describe a partly crushed cranium with complete dentition and dentaries from the Early Jurassic Lufeng Formation of China that we refer to Lufengia. The upper postcanine row length is 10.27 mm, representing one of the smallest tritylodontids to date. Lufengia and Dianzhongia are comparable in the upper postcanine dental pattern in having upper postcanine teeth with the cusp formula 2-3-3 and a small distolingual cusp. Based on the new discovery, Dianzhongia is here considered to be an older individual of Lufengia. Through the micro-computed tomography (micro-CT) of the fossil, differing paths of upper and lower dental replacements alongside new details about tooth replacement in tritylodontids have been revealed.

Oral Functional Behaviors and Tooth Factors Associated with Cracked Teeth in Asymptomatic Patients

IntroductionA cracked tooth may occur due to excessive applied force or tooth weakness. However, there is scant information concerning the cracked tooth risk factors. This study aimed to explore the oral functional behaviors and tooth factors associated with posterior cracked teeth. MethodsFifty-six patients underwent their oral functional behavior assessment via a questionnaire. The intraoral parameters at the patient level (remaining teeth, occluding tooth pairs, overbite, overjet, and occlusal guidance type) and tooth level (remaining marginal ridge number, restored surface number, restorative materials, and cuspal inclination) were examined. The posterior teeth were stained with methylene blue dye and inspected for cracks using a microscope. The correlations between each patient-level parameter and the cracked tooth number/subject were determined using linear regression analysis. The cracked teeth were matched with their contralateral noncracked teeth, and binary regression analysis was used to analyze the association between tooth-level parameters and a cracked tooth. Multivariate regression analysis was performed if more than 1 parameter had a P value ≤.1. ResultsOne hundred thirty-five cracked teeth were found. Eating hard food was significantly related to the cracked tooth number (P < .05). In molars, the occlusal surface restoration and cuspal inclination were significantly related to a cracked tooth, except the mesiobuccal cusp. In the multivariate analysis, the distolingual cusp inclination significantly predicted a cracked tooth (P < .05). In premolars, the lingual cusp inclination was associated with a cracked tooth (P < .05). ConclusionsEating hard food, occlusal surface restoration, and steep cuspal inclination were associated with posterior cracked teeth.

Senescence-accelerated mouse prone 8 (SAMP8) mice exhibit reduced entoconid in the lower second molar

ObjectiveThe position and size of the major cusps in mammalian molars are arranged in a characteristic pattern that depends on taxonomy. In humans, the cusp which locates distally within each molar is smaller than the mesially located cusp, which is referred to as “distal reduction”. Although this concept has been well-recognized, it is still unclear how this reduction occurs. Current study examined whether senescence-accelerating mouse prone 8 (SAMP8) mice could be a possible animal model for studying how the mammalian molar cusp size is determined. DesignSAMP8 mice were compared with parental control (SAMR1) mice. Microcomputed tomography images of young and aged mice were captured to observe molar cusp morphologies. Cusp height from cement-enamel junction and mesio-distal length of molars were measured. The statistical comparison of the measurements was performed by Mann-Whitney U test. ResultsSAMP8 mice showed reduced development of the disto-lingual cusp (entoconid) of lower second molar when compared with SAMR1 mice. The enamel thickness and structure was disturbed at entoconid, and aged SAMP8 mice displayed severe wear of the entoconid in lower second molar. These phenotypes were observed on both sides of the lower second molar. ConclusionsIn addition to the general senescence phenotype observed in SAMP8 mice, this strain may genetically possess molar cusp phenotypes which is determined prenatally. Further, SAMP8 mice would be a potential model strain to study the genetic causes of the distal reduction of molar cusp size.

A 3D finite element analysis of maxillary molar distalization using unilateral zygoma gear and asymmetric headgear.

The aims of this study were (a) to evaluate the displacement and stress distribution in the maxillary dentition associated with the use of two different unilateral distalization appliances-unilateral zygoma gear (UZG) appliance and asymmetric headgear (AHG)-in non-growing patients; and (b) to assess the effects of fully erupted maxillary third molars on the distalization of the first molar with both appliances by using three-dimensional finite element analysis. Two 3D models of the maxilla were created: one with third molars and one without. Next, two distalizing appliances (UZG and AHG) were added to each model to create four models. Distalization forces were applied, and the resultant displacements were recorded at the mesiobuccal and distolingual cusps and palatal root apex of each molar and the incisal edge root apex of the central incisors. The resulting von Mises stress distributions were evaluated. With the UZG, the first molar showed greater root distalization than the crown in the model with the third molar, whereas the model without the third molar showed distalization and distal tipping of the first molar. With the AHG, the first molar showed a large amount of distal tipping in the model without a third molar. However, this tipping was less than that in the model with third molars. The presence of completely erupted third molars decreased the amount of uncontrolled distal tipping in both appliances. UZG can be considered as an effective option for maxillary molar distalization.

Anatomical insights of the mylohyoid for clinical procedures in dentistry.

The purpose of this study is to provide useful data by conducting a comprehensive study of the mylohyoid muscle and its related structures. Fifty-eight mandibles and 30 mylohyoid muscles from Korean adult cadavers were used. The shape and location of the mylohyoid line were analyzed by using digital calipers. The mylohyoid muscle and its herniation were observed using ultrasonography. After dissection, morphometric measurements of the muscle and herniation were conducted. The distribution pattern of the nerve to mylohyoid muscle was confirmed. The proportion of the distance between the cementoenamel junction and the mylohyoid line decreased from the mesiolingual cusp of the mandibular first molar (1:0.57) to the distolingual cusp of the mandibular second molar (1:0.41). The mylohyoid muscle was large, thick, and deep in men. Herniation was observed in 16 (53.3%) cases, and it was concentrated in the anterior one-third (52.2%) of the muscle. The richest arborization of the nerve to mylohyoid muscle was in the middle one-third (52.9%) of the muscle. The results of the mylohyoid line can be applied in the reconstruction of the occlusal plane in edentulous patients. Differences between the sexes should be considered in the morphological characteristics of the mylohyoid muscle. Differential diagnosis of herniation is particularly important in the anterior one-third of the muscle. In the case of treatment with botulinum toxin in the mylohyoid muscle, it is recommended to inject into the middle one-third area considering the depth and thickness of the muscle in that area.

Influence of long and short arms of X chromosome on maxillary molar crown morphology.

Although genes on the human X chromosome reportedly influence tooth crown morphology, little is known about X chromosome activation or inactivation systems relevant to morphological variations. We assessed the relationships between tooth crown size and crown morphological traits in females with Turner syndrome, the variants of which include complete absence of one X chromosome, lack of the short arm (Xp), or duplication of the long arms (Xq), and then estimated the functions of Xp and Xq in the process of unilateral X chromosome inactivation during tooth crown development. The mesiodistal and buccolingual diameters in the maxillary first (M1) and second (M2) permanent molars were compared among X chromosome karyotypes by multiple regression analyses, and their relationships with the development of Carabelli’s cusp and the distolingual cusp were analyzed using logistic regression analysis.The crown sizes increased in the order of the 46,X,i(Xq) karyotype, 45,X and 45,X/46,XX karyotypes, and control group. A lower frequency of Carabelli’s cusp and higher frequency of a reduced distolingual cusp in M1 were characteristics of Turner syndrome. The overall M1 and M2 crown sizes differed among the X chromosome karyotypes, whereas a smaller crown size was associated with a reduced distolingual cusp but not with Carabelli’s cusp. Considering the differences in chromosome arrangement among females with Turner syndrome and the process of unilateral X chromosome inactivation, the observed results can be considered in terms of quantity or number of promoters/inhibitors during tooth crown development.The X chromosome karyotypes have a strong influence on the overall crown sizes of the M1 and M2 molars because those karyotypes with variable numbers of active gene regions directly influence tooth germ development in an early stage of human odontogenesis. The later forming cusps, such as the distolingual cusp and Carabelli’s cusp, may be affected by this developmental prerequisite.

Crown morphology of the mandibular first molars with distolingual roots

Background/purposeMost mandibular first molars have two roots. A major common variation of this tooth is the presence of a distolingual root, which is a common Mongoloid trait in certain populations. The aim of this article was to examine crown morphology in relation to the presence of the distolingual root. Materials and methodsUsing dental casts, the crown morphology of 141 mandibular first molars from 71 Taiwanese individuals was analyzed. Periapical radiographs were used to detect distolingual roots. The length and width of the crowns and the crown units (i.e., trigonid and talonid) were measured. Ten intercuspal distances and five cusp angles were examined. ResultsThe buccolingual dimension of the crown and its ratio to the mesiodistal dimension were significantly increased in molars with a distolingual root, compared to molars without a distolingual root. Mesiodistal crown dimensions were similar; however, the crown unit dimensions were different: molars with a distolingual root had a shorter mesiodistal trigonid dimension but a longer talonid dimension, compared to molars without a distolingual root. The intercuspal distances from the three buccal cusps to the distolingual cusp were significantly longer, however, the distance between the mesiobuccal cusp and mesiolingual cusp was significantly shorter in teeth with a distolingual root than in teeth without a distolingual root. A significantly wider mesiolingual angle and narrower distolingual angle were observed in molars with a distolingual root, compared to molars without a distolingual root. ConclusionThe presence of a distolingual root significantly increased the buccolingual dimension of the crown and the location of distolingual cusp is significantly closer to the lingual side.

Crown shape of maxillary molars with delayed eruption.

The purpose of this study was to explore the crown shapes of maxillary molars with delayed eruption (DEMo1) at the position distal to the maxillary second premolar. Included teeth erupted later than the average for the maxillary first molar eruption in Japanese females (6.58 ± 0.67 years) by more than two standard deviations. Crown shapes of 12 four-cusped left DEMo1 teeth were compared with those of 25 four-cusped left maxillary first molars (U6n) and 25 four-cusped left maxillary second molars (U7n) from different patients with normal eruption. Seven landmarks were established on the reference plane containing the mesiobuccal, distobuccal and mesiolingual cusp tips of the molars; the origin was defined as the center of gravity of these three points. According to the obtained discriminant function (percentage of correct classifications, 84%), five DEMo1 teeth were classified as U6n and the other seven as U7n. The DEMo1 teeth were also classified into two subgroups, the U6n-close and U7n-close groups, according to the location of the distolingual cusp tip. These results suggest that DEMo1 teeth could include U6 and U7 with delayed eruption or could be an intermediate between U6 and U7, according to their crown shapes.

Lifetime comparison of Y-TZP/porcelain crowns under different loading conditions

ObjectivesTo compare the lifetime of Y-TZP/porcelain crowns under three different load conditions using step-stress accelerated lifetime testing. MethodsThe Y-TZP frameworks were milled using CAD/CAM, veneered with a porcelain and cemented onto dentine analogue dies. Specimens were divided according to the occlusal load condition (n=20): central fossa load (CFL), cusp tip load (CTL) and sliding contact (SC). For CFL and CTL, the cyclic load was applied parallel to the long axis of the preparation using a ceramic piston. For SC, the axial load was associated to 1mm lateral displacement at the disto-lingual cusp. Different stress profiles were used. Failures were detected with an acoustic system. A Weibull distribution (95% confidence boundary) was used to analyse the data, and fractographic principles were used to evaluate fractured specimens. ResultsThe acoustic monitor was able to detect the initial crack. The probability of failure (at 300N load and 200,000 cycles) was statistically greater for CTL (0.63; 0.44–0.81) compared to CFL (0.23; 0.12–0.43). The Weibull modulus of CFL (2.1; 1.5–3.6) was greater than for SC (0.7; 0.5–1.2), with no difference in the lifetime. All specimens failed by chipping, which originated predominantly at the contact (66.7%) on CTL, and in the bulk of the porcelain on CFL (100%) and SC (80%). ConclusionsContact at the cusp tip is more harmful than at the central fossa. Data from sliding contact are less consistent than from axial contacts, but more clinically relevant. Clinical significanceThe loading condition of Y-TZP/porcelain crowns can influence on the probability of failure and failure mode. The contact at the cusp tip is more harmful than at the central fossa, where the stress is better distributed. Sliding contact is a better simulator of the chewing cycle compared to axial contacts.

Evaluation of the effectiveness of the new tooth wear measurement parameters

Nowadays, there has been an increasing interest in the preservation of natural dentition and the proper occlusion related to tooth wear for quality of life. To overcome the problems of the existing qualitative tooth wear analysis method, virtual three-dimensional models have been used. This study was designed to develop and validate a new quantitative method using tooth wear measurement parameters with angles obtained from virtual vectors and planes of the three-dimensional models. Sixteen parameters were evaluated in the virtual models of 20 students (7.57±1.55 years old) and 20 adults (56.85±6.34 years old). There were 12 angle and 4 height parameters, and the number of parameters measured from the virtual planes and vectors were 10 and 6, respectively. For each parameter, means and standard deviations were calculated, and an unpaired sample t test was performed to compare the young and the adult groups. Also, differences between the means were determined and expressed as percentages. The results were statistically significant between the two groups (P<0.001). In general, parameters using virtual vectors showed greater change than virtual plane. Although there were statistically significant differences among all parameters using virtual planes (P<0.001), the changes of the three angles were similar, except distolingual cusp angle. It was found that the parameters using virtual vectors were effective and tooth wear took place in both buccal and lingual cusps. Likewise, the validation of the new measurement parameters suggests that they can also be applied in the assessment of tooth wear related to dental biomaterials.

Morphological changes in the crown of mandibular molars with an additional distolingual root

ObjectiveThe mandibular molars typically have two roots placed mesiodistally, but they occasionally have an additional distolingual (DL) root. This study was to determine the morphological characteristics of the crown of such mandibular molars. DesignRadiographic records and study models were collected from a Korean population (n=86 patients). Each molar was assigned to either the experimental group (i.e. with a DL root) or the control group (i.e. without a DL root; n=41 patients) based on the radiographic findings. The intercuspal distances of the first permanent molars (total, n=100; control/experimental, n=50/50) and primary second molars (46, 23/23), and the largest buccolingual/mesiodistal widths of those molars and primary first molars (42, 21/21) were measured for molars with and without a DL root. In addition, the correlation between the existence of a sixth cusp and a DL root was examined. ResultsThe crowns of first permanent and primary second molars with DL roots had significantly larger intercuspal distances between the distobuccal–distolingual cusp tips and a larger distal-area buccolingual width than those without the DL root (t-test; p<0.05). There was no significant correlation between the existence of a sixth cusp and the presence of a DL root. ConclusionsThe existence of a DL root was associated with larger buccolingual dimensions, especially in the distal area.

Case Report: Pre-eruptive intra-coronal radiolucencies revisited

Pre-eruptive intra-coronal radiolucency (PEIR) describes a radiolucent lesion located in the coronal dentine, just beneath the enamel-dentine junction of unerupted teeth. The prevalence of this lesion varies depending on the type and quality of radiographic exposure and age of patients used for assessment. The aetiology of pre-eruptive intra-coronal radiolucent lesions is not fully understood, but published clinical and histological evidence suggest that these lesions are resorptive in nature. Issues around the diagnosis, treatment planning and clinical management of this lesion are explored using previously unreported cases. Case 1: A ten-year-old girl attended for a routine check-up. An intra-coronal radiolucency in the unerupted lower right second premolar was an incidental finding on orthopantomograph (OPT). The tooth erupted and removal of enamel revealed a space filled with soft red tissue, unlike carious dentine in appearance. The tooth was restored with an indirect pulp cap, resin modified glass ionomer base and composite resin. Tissue from the lesion was removed for histopathological investigation. Root development continued to completion and the tooth remained asymptomatic and vital. Case 2: A six-year-old girl attended for her first dental visit. An intra-coronal radiolucency in a lower right first permanent molar was noted on baseline bitewing radiographs. The lesion was monitored and fissured sealed upon eruption. The lesion was monitored annually radiographically. The tooth remained symptom free for 5 years. The patient presented on an emergency basis having fractured the distolingual cusp overlying the lesion. There was no pain and the tooth was vital. The softened dentine was removed and the tooth was restored using a preformed metal crown. Case 3: A 12-year-old girl was referred for restoration of mandibular left second permanent molar. Clinically there was extensive occlusal destruction. Review of a previous OPT showed that an intra-coronal radiolucency was present in tooth 37 at least one year prior to its eruption. The large mass of coronal soft tissue was removed, the remaining enamel shell was deemed to be unrestoreable and the tooth was extracted. The patient was referred back to an orthodontist for completion of orthodontic treatment. Early detection and classification of the PEIR lesion allows an array of individualised treatments to be provided for successful outcome.

Are there two distinct types of hypocone in Eocene primates? The ‘pseudohypocone’ of notharctines revisite

Upper molars of modern humans and most extant primates have four cusps that have evolved from the original tribosphenic tooth of therian mammals. These include the three cusps of the original trigon (e.g., paracone, metacone, and protocone), and the addition of the distolingual cusp or hypocone. Among Eocene primates of the family Adapidae, a distinction has long been made between a “true” hypocone associated with the lingual cingulum (adapine form) and a “pseudohypocone” associated with the distal margin of the protocone (notharctine form). The developmental processes underlying these two types of distolingual cusp are unknown, and the validity of the distinction is based on phylogenetic utility and homology rather than cusp position, as in other mammalian groups. To address this issue we use micro-computed tomography to reveal the morphology of the hypocone and associated cusps and crests on the enamel-dentine junction (EDJ). The EDJ preserves the initial steps of tooth crown development and can be used to clarify detailed aspects of crown morphology in variably worn or damaged fossil teeth. Our study sample includes both adapine species from Europe and notharctines from North America. We confirm that the pseudohypocone found among notharctines is a true cusp since it forms as a dentine horn during crown development. Our results also confirm that these two forms of hypocone are developmentally distinct and have evolved convergently in these two primate clades. A review of the paleontological literature suggests that, in spite of the fact that homoplasy is rampant among mammalian clades with respect to the development of the hypocone, only among the notharctines do we find an alternative name for this cusp.

상악 제2유구치와 상악 제1대구치의 치관 형태: 치아계측학적 연구

유치 치관의 기본적인 구조는 대개 계승 영구치를 닮는다. 그러나 제2유구치는 계승 영구치인 제2소구치보다 오히려 제1대 구치와 더 유사한 경향이 있다. 이에 본 연구는 한국인에서 상악 제2유구치와 제1대구치의 치아계측학적 자료(odontometric data)를 얻어 두 치아의 형태와 크기 사이의 연관성을 살펴보고, 나아가 남녀간의 차이점 유무를 고찰해 보고자 하였다. 남아 150명, 여아 150명 총 300명의 모형을 이용하여 상악 제2유구치와 상악 제1대구치의 근원심 치관 폭경, 협설측 치관 폭경, 각 교두의 직경, 교두정간 거리를 계측하였고, 치관의 교합면 사진을 촬영하여 각 교두정 사이의 각도를 측정하여 다음과 같은 결과를 얻었다. 1. 남아에서 Protocone index, Paraconeindex, Distobuccal cusp(DBC)각, Mesiobuccal cusp(MBC)각은 상악 제1대 구치와 제2유구치 사이에 통계적 유의차가 관찰되지 않았고(p>0.05), 중등도의 상관관계가 확인되었다. 2. 여아에서 crown index, Distobuccal cusp(DBC)각, Mesiobuccal cusp(MBC)각은 상악 제1대구치와 제2유구치 사이에 통계적 유의차가 관찰되지 않았고(p>0.05), 중등도의 상관관계가 확인되었다. 3. 남, 여 모두에서 Distobuccal cusp(DBC)각, Mesiobuccal cusp(MBC)각은 두 치아 사이에 유의할만한 차이를 보이지 않았고(p>0.05), Distolingual cusp(DLC)각에서는 통계적 유의차가 관찰되었다(p<0.01). 4. 대부분의 계측치에서 남녀 차이를 나타냈지만, Distobuccal cusp (DBC)각은 상악 제1대구치와 제2유구치 모두에서 남녀 차이를 보이지 않았다(p>0.05). The basic structure of the primary crowns usually resemble their succeeding permanent teeth. However, maxillary second primary molars resemble ipsilateral first permanent molars. Accordingly in this study, odontometric data of the two teeth was obtained, then analyzed to verify the morphological relationship and sex difference between the two teeth. Dental study casts were examined for their mesiodistal and buccolingual width of the crowns, diameter of each cusps, and intercuspal distances. Photographs of the crowns were taken to measure the angles between each cusp tip. The results are as follows : 1. In boys, PrI, PaI, DBC angle, and MBC angle did not show any statistically significant difference between the two teeth(p>0.05), and moderate level of correlations were observed. 2. In girls, crown index, DBC angle, and MBC angle of the two teeth did not show any statistically significant difference(p>0.05), and moderate level of correlations were recognized. 3. Measurements that did not show statistically significant difference between the two teeth in both boys and girls were DBC angle and MBC angle(p>0.05). In DLC angle, however, statistically significant difference was observed(p<0.01). 4. Most of the measurements showed sex differences, except DBC angle, which did not show any sex difference in both teeth(p>0.05).

Anesthetic technique for inferior alveolar nerve block: a new approach

BackgroundEffective pain control in Dentistry may be achieved by local anesthetic techniques. The success of the anesthetic technique in mandibular structures depends on the proximity of the needle tip to the mandibular foramen at the moment of anesthetic injection into the pterygomandibular region. Two techniques are available to reach the inferior alveolar nerve where it enters the mandibular canal, namely indirect and direct; these techniques differ in the number of movements required. Data demonstrate that the indirect technique is considered ineffective in 15% of cases and the direct technique in 1329% of cases.ObjectiveObjective: The aim of this study was to describe an alternative technique for inferior alveolar nerve block using several anatomical points for reference, simplifying the procedure and enabling greater success and a more rapid learning curve.Materials and MethodsA total of 193 mandibles (146 with permanent dentition and 47 with primary dentition) from dry skulls were used to establish a relationship between the teeth and the mandibular foramen. By using two wires, the first passing through the mesiobuccal groove and middle point of the mesial slope of the distolingual cusp of the primary second molar or permanent first molar (right side), and the second following the oclusal plane (left side), a line can be achieved whose projection coincides with the left mandibular foramen.ResultsThe obtained data showed correlation in 82.88% of cases using the permanent first molar, and in 93.62% of cases using the primary second molar.ConclusionThis method is potentially effective for inferior alveolar nerve block, especially in Pediatric Dentistry.

Molar Cusps in Southern Chinese

Aim: To investigate the number of molar cusps of the Southern Chinese and compare these with studies in dif- ferent populations. Materials and Methods: The number of molar cusps of study casts of an unselected sample from a 12 year old Hong Kong Oral Health Survey of 12 year old children (n=459; 295 boys and 164 girls) were studied. Results: For upper first molars, 5-cusp molars were the most prevalent in males (39%) while 4-cusp molars were the most prevalent in females (39%). For lower first molars, 5-cusp molars were the most prevalent in both sexes (63%-72%). For lower second molars, 5-cusp molars were the most prevalent in both sexes (43%-53%). Conclusion: Different from traditionally described, the Southern Chinese has more 5-cusp lower second molars than the 4- cusp ones. In modern man, the number of cusps of the upper molars is frequently reduced from four to three. The formula for the number of cusps of the upper molar series is changing from 4-4-4 to 4-4-3 and 4-3-3. The trend is more marked in fe- males than in males of all populations that have been stud- ied. In most studies, males generally showed a lesser ten- dency than females to reduction in the number of cusps of the upper molars. The process of reduction occurs through the elimination of the hypocone or distolingual cusp, with a subsequent decrease in the mesiodistal crown diameter and in the occlusal area.

Developmental Pattern of Human Milk Teeth

The development of mammalian tooth cusps or ridges has been explained by phylogenetic homology, such as the ‘Tritubercular theory’ (Gregory 1934). However, descriptions of the incisor and canine are lacking, and many questions remain unanswered. In this paper, the authors examine crown development in human teeth in embryos and skulls of young children by micro computed tomography (μ-CT), and discuss these problems. The results were as follows :1) In milk incisors and canines, the first swelling developed into the central marginal ridge, and new swellings became mesial and distal sites and also buccal and mesial sites. Mesial and distal prominences developed into mesial and distal mamelons, but labial and lingual swellings formed cingulums.2) In upper first milk molars, the first swelling developed into a buccal cusp and new swellings around it became buccal and lingual, and also mesial and distal sites symmetrically. The lingual prominence formed a lingual cusp, and the mesial and distal prominences formed mesial and distal cones or marginal ridges, but only the labial prominences developed into a mesio-buccal ridge.3) In lower first milk molars, the first swelling developed into a buccal cusp, and new swellings became mesial and distal, and also buccal and lingual sites symmetrically. The lingual prominence developed into a lingual cusp. The mesial ridges and conules developed from the mesial prominence, and the distal prominence developed into three cones known as hypoconids. The buccal prominence developed into a mesio-buccal ridge similar to the first upper milk molars.4) In upper second milk molars, the first swelling developed into a mesio-buccal cusp, and new swellings formed in the mesial and distal, and also buccal and lingual sites symmetrically. The lingual prominence developed into a mesio-lingual cusp, and the distal swelling into a disto-buccal cusp. The mesial prominence developed into mesial cones or mesial ridges, and the buccal prominence developed into a cingulum or a mesio-buccal ridge. Next, the disto-lingual cusp formed from a distal extension of the mesio-lingual and disto-buccal cusps.5) In lower second milk molars, the first swelling developed into a mesio-buccal cusp, and new swellings spread mesially and distally, and also to buccal and lingual sites. The lingual prominence developed into a mesio-lingual cusp, the mesial prominence into mesial ridges, and the buccal prominence formed a cingulum or a mesio-buccal ridge. Subsequently, new swellings added two more mesial cusps, which developed into three hypoconids.6) Many swellings appeared temporarily and then disappeared during the development process between and around those main cusps.7) Stable main cusps or mamelons developed in wide spaces formed during jaw development. However, small swellings and invaginations of the crown, such as cingulums, marginal ridges and other structures, formed in narrow spaces of the jaw. The tooth and jaw morphogenesis have a close relationship.8) Those developments suggested that the crown developed with symmetrical divergence patterns such as widening of concentric circular ripples. However, some of the new structures were inhibited by other factors. The authors dubbed this the ‘Divergence’ and ‘Convergence’ processes of tooth morphogenesis. These patterns originated in mammalian evolution.9) This fundamental developing pattern of the crown is the same in both the upper and lower, and also the right and left sides of jaws.10) All structures could be understood from the symmetry in ‘Divergence’ and ‘Convergence’ factors.

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

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

Incidence rates for complete cusp fracture.

Although complete cusp fracture is acknowledged to occur frequently, incidence rates have been reported rarely. This study determined incidence rates for complete coronal cusp fracture per person and per tooth type. All fractures presenting among enrollees in a dental health maintenance organization using two geographically isolated clinics were noted for 105 days. For a sample of these enrollees, likelihood of attending the clinic in the event of a fracture was assessed through a telephone survey, and the at-risk status of all teeth was determined through a record survey. Incidence rates were calculated for persons, and for individual tooth types for all complete fractures and for non-carious complete fractures. In addition, for posterior teeth the distribution of fractured cusps, and the severity of fractures were examined. Per-person incidence rates for complete coronal fractures for all teeth were 89.0 and 72.7 per 1000 person years, respectively, for all fractures and for non-carious fractures. The rates for all anterior and all posterior teeth were 10.2 and 69.9, respectively, for all non-carious fractures. In mandibular posterior teeth, lingual cusps fractured twice as frequently as facial cusps, while the opposite was true for maxillary premolars. Among maxillary molars, the mesiofacial and distolingual cusps fractured most frequently. The large majority of fractures exposed dentin (95%), while pulpal exposure occurred infrequently (3%). A minority of fractures extended below the gingival crest (24%) or the DEJ (25%). This is the first report of fracture incidence rates for enumerated persons and teeth at risk and as such helps define the magnitude of the problem for dentists and their patients.