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Aim: To use micro-CT technology and metrology software to validate the use of contralateral premolars as samples in endodontic comparison studies by comparing them before and after canal instrumentation with one instrumentation system. Furthermore, to determine whether contralateral premolar roots (CPRs) will yield non-significantly different outcomes regarding shaping ability (volume), degree of twisting and three-dimensional shape changes. The null-hypothesis (H 0 ) is that there are no differences between the CPRs pre- or post-instrumentation., Methodology: Twenty-eight extracted human contralateral premolars (n = 44 contralateral roots) from 12 donor patients were scanned with microcomputed tomography before and after instrumentation. Root canal lengths (RCLs) were measured visually using a dental-operating microscope, electronic apex locator and micro-CT scans. Data were analysed statistically for differences between pre- and post-instrumentation., Results: Instrumentation increased the volume of the canals significantly (P < 0.05). Degree of twisting for a majority (83%) of the contralateral roots pairs did not change significantly (P > 0.05). There was no significant difference (P > 0.05) in the shape deviation analysis between contralateral pairs. There was no significant difference (P > 0.05) for RCL between the contralateral pairs for any of the three endometric methods., Conclusion: Contralateral premolar root canals were associated with similar changes in terms of volume, three-dimensional shape and degree of twisting from pre- to post-instrumentation. There was no difference between the CPR pairs pre- and post-instrumentation, and the study validates contralateral premolars as samples for root canal comparison studies. The null-hypothesis (H 0 ) could not be rejected., (© 2018 International Endodontic Journal. Published by John Wiley & Sons Ltd.)

In a previous study, we used muscle functional magnetic resonance imaging to show that the anterior movement of the occlusal point increased the activity of the superior head of the ipsilateral lateral pterygoid muscle (ipsilateral SHLP) during unilateral single-tooth clenching. The purpose of this study was to verify the hypothesis that the increased activity of the ipsilateral SHLP described above serves to antagonise the occlusal force acting on the condyle. In total, 9 healthy volunteers were requested to perform left unilateral clenching at the first molar or first premolar region for 1 minute at 20% or 40% maximum voluntary clenching force. Changes in the mean proton transverse relaxation time (∆T2) were examined from the magnetic resonance images obtained before and after each clenching act as an index of the activity in all masticatory muscles. Correlation analyses of the mean ΔT2 for each volume of interest were performed with the first molar or premolar clenches to analyse the correlation between the activities in each muscle. A statistically significant correlation was exhibited between the ipsilateral temporal and ipsilateral SHLP (r = .651, P = .003) during first premolar clenching. However, no significant correlations were observed in the ipsilateral SHLP during first molar clenching. The results of this study suggest that the ipsilateral SHLP may contribute to the pulling of the mandibular condyle forward against the occlusal force generated by the ipsilateral temporal muscle., (© 2017 John Wiley & Sons Ltd.)

Aim: To obtain temperature distribution data through human teeth focusing on the pulp-dentine junction (PDJ)., Methodology: A three-dimensional tooth model was reconstructed using computer-aided design software from computed tomographic images. Subsequently, temperature distribution was numerically determined through the tooth for three different heat loads. Loading type I was equivalent to a 60° C mouth temperature for 1 s. Loading type II started with a 60° C mouth temperature, decreasing linearly to 37° C over 10 s. Loading type III repeated the pattern of type II in three consecutive cycles, with a 5 s resting time between cycles., Results: The maximum temperatures of the pulp were 37.9° C, 39.0° C and 41.2° C for loading types I, II, and III, respectively. The largest temperature rise occurred with the cyclic loading, that is, type III., Conclusion: For the heat loads considered, the predicted peak temperatures at the PDJ were less than the reported temperature thresholds of irreversible pulpal damage., (© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd.)

The aim of this study was to test the hypothesis that experimental and reversible changes of occlusion affect the levels of surface electromyographic (SEMG) activity in the anterior temporalis and masseter areas during unilateral maximal voluntary biting (MVB) in centric and eccentric position. Changes were achieved by letting 21 healthy subjects bite with and without a cotton roll between the teeth. The placement alternated between sides and between premolar and molar areas. The SEMG activity level was lower when biting in eccentric position without than with a cotton roll between teeth (P < 0.043). It was always lower with premolar than with molar support when biting with a cotton roll (P < 0.013). In the anterior temporalis areas, the SEMG activity was always lower on the balancing than on the working side (P < 0.001). Such a difference was also found in the masseter areas but only during molar-supported centric biting (P = 0.024). No differences were found when comparing the SEMG levels in masseter areas between centric and eccentric biting (P > 0.05). In the anterior temporalis area, the balancing side SEMG activity was lower in eccentric than in centric but only in molar-supported biting (P = 0.026). These results support that the masseter and anterior temporalis muscles have different roles in keeping the mandible in balance during unilateral supported MVB. Changes in occlusal stability achieved by biting with versus without a cotton roll were found to affect the SEMG activity levels.

The aim of the present study was to underline the difference in premolar-molar functional areas according to both age and gender in a human adult population. Premolar and molar series were marked manually on opposed casts with articulating paper and functional areas were evaluated by quantification of these markings. It was noticed that on average larger functional areas were displayed by the older group (19 individuals, mean age 71.2 years, minimum 68 years, maximum 73 years) in comparison with the younger one (24 individuals, mean age 27.4 years, minimum 22 years; maximum 33 years). It was also observed that the average total functional area (4 hemiarches) was larger in the male subgroup (n: 23; mean age 45; young adults, 14; old adults, 9) compared with the female one (n: 20; mean age 49; young adults, 10; old adults, 10). Nevertheless, this gender difference in premolar-molar functional areas was related to the individual occlusal areas. As a matter of fact, when the individual occlusal area was taken into account for each series in the form of a ratio (functional area/occlusal area), the values obtained were not different between both gender subgroups studied. When comparing premolar and molar functional ratios (functional area/occlusal area) between age groups, it appears that the values obtained were slightly higher for the premolar series (ratios for young and older adults, respectively: Premolar series: 23.6+/-7.5%, 30+/-6.5%; Molar series: 23.1+/-6.8%, 27.3+/-6.7%). This difference was interpreted in relation to the reference (occlusal area) chosen.

We conducted a series of studies with the purpose to investigate the locations of tooth contacts in the retruded contact position (RCP) and to discuss their significance in the stomatognathic system. In the present study, the relationship between the locations of RCP contacts and mandibular positioning during retrusion was examined. Thirty dentists and clinical residents were selected as subjects. One specialist in prosthetic dentistry examined each subject for the location of the RCP contacts. The mandibular positioning during retrusion was measured using a mandibular movement analysis system with six degrees of freedom. Originally programmed software was developed. Five reference points were selected: the central lower incisor (point I), the first molars on both sides (points RM and LM) and the condyles on both sides (points RC and LC). Tooth contact was observed most frequently at the second molar, followed by the first premolar. Points I, RM and LM all moved in an inferior-posterior direction, whereas points RC and LC moved in various directions ranging from superior-posterior to inferior-posterior. When the subjects were divided into two groups according to the most anterior tooth of occlusion in the RCP, the condylar positioning tended to be more superior in the group with molar contact than that with premolar contact. These results suggest that the locations of RCP contacts could be an important factor in jaw guidance during retrusion.

The aim of this study was to investigate the relationship between occlusal tooth contact patterns and the tightness of proximal tooth contact (TPTC) during clenching. Twenty young adult volunteers with healthy dentition participated in the study. TPTC between the left second premolar (P2) and the first molar (M1) was measured during clenching at the 50% maximum voluntary contraction level in the intercuspal position (ICP). A silicone impression material was used to make an interocclusal record at the ICP in each subject, and interocclusal records were analysed using an image-processing system. Subjects were classified according to the presence or absence of each type (A, B or C) of occlusal contact. Statistical analysis was performed using the Mann-Whitney U-test. The results of this study exhibited a relationship between B-type contact and the TPTC of maxillary teeth. The experimental group with a lack of B-type contact on maxillary P2 and/or M1 showed a statistically greater TPTC than the group with B-type contact on both of these teeth (P < 0.01). These results suggest that occlusal tooth contact patterns have an influence on TPTC during clenching.

Aim: To measure excessive electrical stimulus time during pulp testing via electromyography (EMG) in the anterior belly of the digastric muscle, voice and finger movement, and to determine whether excessive stimulus time could be attenuated by a specially designed automatic circuit breaker on the basis of the EMG signal., Methodology: The signals from three human responses (EMG, finger and voice), induced by the Digitest (Parkell Inc., Farmingdale, NY, USA) electric pulp tester, were captured using a MP100 (Biopac System Inc., Goleta, CA, USA) and recorded into a personal computer. The excessive stimulus time from activation to the end of electrical stimulation was calculated for each of these three responses. The automatic circuit breaker was designed to disconnect the electrical output of the electric pulp testing (EPT) unit immediately after detecting the preset EMG level (100 mV). Each of the right central incisors and first premolars of 23 healthy individuals (16 males and seven females) was tested to see whether there was a difference in tooth type or gender. This was analysed by Wilcoxon signed rank test (nonparametric paired t-test) and Mann-Whitney test (nonparametric independent t-test), respectively., Results: Amongst three human responses, the electrical onset occurred in the order of EMG, finger and voice. Excessive stimulus time was 347.8 +/- 78.3 ms when observed by the EMG, 264.9 +/- 63.9 ms when observed by finger span and 229.4 +/- 41.8 ms when observed by the voice, which were all found to be significantly different (P < 0.05). When the automatic circuit breaker was used, the excessive stimulus time was 61.0 ms, which was 286.8 ms shorter than that measured from EMG onset when using the conventional EPT., Conclusions: When the automatic circuit breaker was used, excessive stimulus time on the basis of EMG was attenuated on average by 286.8 ms.

Aim: The aims of this study were to define the average time of eruption of permanent teeth, the range of variation of the eruption time of each tooth, and their sequence of eruption amongst boys and girls in Tehran, Iran., Methods: The population under study comprised 3744 pupils and students in the 4-15-year-old age group (1786 girls and 1958 boys) who were randomly selected from schools and training centres in the 20 districts covered by the General Department of Education and Training in Tehran. The research was carried out on a cross-sectional basis. The average age at eruption of permanent teeth, excluding third molars, was given as the mean (+/- SD) and median in months for each gender. A table of percentiles of the eruption time was also determined., Results: The results show that, with the exception of the maxillary second premolars, the average age at eruption of permanent teeth in girls is less than in boys. The results also show that mandibular teeth have an earlier eruption time than maxillary teeth in both boys and girls. By the age of 96 months, 97% of the girls had all their first permanent molars. In boys, the corresponding age was 99 months. Amongst girls, maxillary canines erupt earlier than maxillary second premolars., Conclusion: On average, girls have their permanent teeth erupt earlier than boys. The sequence of eruption differs between girls and boys for maxillary canines and maxillary second premolars.

The aims of this study were to develop a device for measuring the tightness of proximal tooth contact and to evaluate the proximal contact tightness using this device at rest and during clenching. Twenty young adult volunteers with healthy dentition participated in this experiment. The tightness of proximal tooth contact between the second premolar and the first molar of both the maxilla and the mandible was measured by pulling a stainless steel strip between them at rest, and at 20 and 50% clenching levels of maximum voluntary contraction of masseter muscles at intercuspal position. Proximal contact tightness increased as the clenching levels of both the maxilla and the mandible increased. At rest, proximal contact tightness was less in the maxilla than in the mandible, whereas during clenching it was less in the mandible. These results indicate that during clenching, the teeth are displaced and they contact appropriately with adjacent teeth, making it possible to exert sufficient occlusal force while maintaining the integrity of dental arches.

The assessment of bite forces on healthy single tooth appears essential for a correct quantification of the actual impact of single implant oral rehabilitations. In the present study, a new single tooth strain-gauge bite transducer was used in 52 healthy young adults (36 men, 16 women) with a complete permanent dentition. The influences of tooth position along the dental arch, of side, and of sex, on maximum bite force were assessed by an ANOVA. No significant left-right differences were found. On average, in both sexes the lowest bite force was recorded on the incisors (40-48% of maximum single tooth bite force), the largest force was recorded on the first molar. Bite forces were larger in men than in women (P < 0.002), and increased monotonically along the arch until the first or second permanent molar (P < 0.0001). The present data can be used as reference values for the comparison of dental forces in patients.

This study utilized the finite element method (FEM) to predict distribution of stresses in dentin of an endodontically treated tooth, restored with cast post and cores. For this investigation an axisymmetric model of a maxillary second pre-molar that included an alveolar bone was analysed. The three tooth models evaluated were Ti-Ti alloy, NiCr-AuPd alloy and Ti-NiCr alloy as post-material and crown material with porcelain. A load of 200 N at an angle of 45 degrees to the longitudinal axis was applied on the occlusal margin of each model. The tooth was assumed isotropic, homogenous and elastic. The author prepared a calculation program using fortran 77. Investigation of the stress distributions was made in five regions; namely bottom of post, top of post, cole, metal-cement interface and metal-porcelain interface. The distributions of radial and axial stresses were plotted with length of radial.

Aims: This study investigated the effect of stainless steel bands on cuspal flexure and fracture resistance of extracted maxillary premolars., Methodology: Twenty extracted maxillary premolars (10 matched pairs) with mesio-occluso-distal (MOD) cavities and endodontic access were subjected to occlusal loading tests (100 N) using a servo-hydraulic testing machine. Cuspal deflections were measured by an extensometer, with and without the band present. Ten teeth (one of each pair) then had the band removed, and all teeth were subjected to loading until fracture., Results: Mean cuspal flexure of teeth with bands was one-half of flexure without bands (P < 0.001). Teeth with bands fractured at higher load than their matched pairs with the band removed (P < 0.001), with mean loads at fracture of 1282 N and 729 N, respectively., Conclusions: The study showed that stainless steel bands used in endodontics reduce the cuspal flexure of maxillary premolars and increase their fracture resistance.

The objective of this study was to calculate stress distribution in a maxillary second premolar tooth which occurred by the mastication force. The tooth model was crowned with Au-Pd alloy, Ni-Cr alloy and porcelain. A load of 450 N, at an angle of 45 degrees to the longitudinal axis was applied on the occlusal margin of the crown tooth. The tooth was assumed isotropic, homogenous, elastic and unsymmetrical. This numerical study was carried out using three-dimensional finite element models and calculation programs were prepared by the authors using FORTRAN 77. The distribution of compressive, tensile and shear stress were plotted for the dentine, dentine-metal and metal-porcelain interfaces. The highest stress values were observed when Ni-Cr alloy and porcelain was used.

Objectives: The need for an adequate quantity of saliva to maintain oral health has been widely reported. It is not only the quantity that is important, but also how the saliva is distributed once it enters the mouth. Several studies have looked at how saliva is distributed around adult mouths. The aim of this experiment was to describe the distribution of stimulated saliva around the mouth in children., Sample and Methods: In order to demonstrate the distribution of saliva around the mouth under stimulated conditions, 25 child subjects were asked to chew a piece of chewing gum containing 1.5 mg erythrosin for 3 min on one side of the mouth. The distribution of dye in the mouth was recorded on standardized charts., Results: In all cases, dye was present ipsilateral to the chewing side. In 13 cases (52%) the dye did not cross the midline and of the remaining 12 children the dye only reached the contralateral canines in four of them (16%)., Conclusions: Evidence is presented which suggests that during unilateral chewing in children, saliva is not well distributed around the mouth and has a tendency to stay on the side of the mouth from where it was secreted (P < 0.01).

The use of an mesial-occlusal-distal (MOD) restoration in repairing a large carious lesion depends on many factors. Biomechanical performance is one of the most important. It has been recognized that resistance to restoration failure is not solely a biological concern (e.g. toxicity), but that the cavity shape, dimensions, and the state of stress must all be taken into account. In the present study, a newly developed auto-mesh program was used to generate 30 three-dimensional (3D) finite element (FE) models simulating the biomechanics for multiple factorial design of the MOD gold restoration in a maxillary second premolar. Stress levels were related to individual design factors (e.g. pulpal wall depth [P], isthmus width [W] and interaxial thickness [T]) and to their interactions under the worst physiological scenario: a concentrated bite force acting on lingual cusp with debonded interfaces between cavity walls and restorations. The results showed that enlarging the volume of the MOD cavity significantly increased stresses in enamel but did not intentionally affect stresses in dentin. The alternation of individual design parameters significantly changed the peak stresses (P < 0.05). For all three parameters, except for the width, the peak stress increased as the cavity dimension increased. Stress elevation rate (termed as 'volumetric stress rate'--stress elevation by increasing one unit volume of the restored materials) was different among three design factors. Depth was the most critical factor governing the stress elevation in enamel (1.76 MPa mm(-3)) while length (interaxial thickness) was the most important parameter in dentin (0.49 MPa mm(-3)). Width was the least compromising factor to the remaining tooth, 0.32 MPa mm(-3) for enamel and -0.23 MPa mm(-3) for dentin. The findings, at its core, did not fully agree with the traditional concept that the preservation of tooth substances will reduce risk of tooth fracture. This study leaves open possibility for the structural optimization of the MOD restoration.

Many finite element analyses studies have been published in the dental literature. Some of these have analysed just the crown of the tooth, others have included part or all of the root, while others have included the supporting periodontal ligament and alveolar bone. The aim of this study was to examine which of the supporting structures was important to the model when analysing the stress distribution within a tooth. A two-dimensional plane strain finite element model of a lower second pre-molar was developed, which included the supporting periodontal ligament and alveolar bone. Two 50 N loads were applied to simulate the effect of a load in centric occlusion. The nodal x, y and shear stresses were recorded along two horizontal planes, one in the crown and one in the cervical region. Each of the supporting structures was systematically removed and the remaining structures reanalysed. It was found that it was particularly important to include both the periodontal ligament and alveolar bone when undertaking the finite element analyses of teeth.

This paper presents the stress analysis of the maxillary second premolar tooth under thermal loading as a result of hot/cold liquid in the mouth using the three-dimensional (3D) finite element method (FEM). The tooth was considered to be in a restored state with composite resin and amalgam on glass-ionomer as the base material. In the first step of the study, the temperature changes as a result of hot/cold liquid in the mouth were calculated. The thermal stress distributions owing to the temperature changes were then obtained. All calculation programs were prepared by the authors using FORTRAN 77. The tooth was assumed to be isotropic, homogeneous, elastic and unsymmetric. The distribution of temperature and stress were plotted for some critical points.

The purpose of this study was to evaluate the relationship between the clenching level and the intercuspal contact area in different regions of the dental arch. Twenty-five healthy subjects with natural normal dentitions and good occlusal support performed clenching tasks in the intercuspal position at four different levels (10, 30, 70 and 100% levels of maximum voluntary contraction) through EMG visual feedback from bilateral masseter and anterior temporal muscles. Simultaneously, the occlusal contacts were recorded with a silicone occlusal contact checking material (Black Silicone, GC Dental Industrial Corp., Tokyo, Japan). The occlusal records were analysed by an image analyser. Every area of the thickness less than 50 microm was determined to be an occlusal contact area. The occlusal contact areas on the anterior teeth (incisors and canines), the premolars (first and second premolars) and the molars (first and second molars) were calculated separately. The posterior occlusal contact area increased with an increase in the clenching levels but that of the anterior did not. The results of this study indicate that the increase in clenching forces affects the anterior and posterior occlusal contact areas differently.

The purpose of this study was to explore the occlusal force distribution on the dental arch in the intercuspal position and to evaluate the relationship between the clenching strength and the occlusal force distribution. These variables were recorded using the Dental Prescale System in 16 healthy young adults. The number of tooth contacts, occlusal force and occlusal contact area increased linearly as clenching strength increased. The distribution of the occlusal force was greatest at the molar region followed by the premolar and anterior teeth region. The proportion of occlusal force (occlusal force at each region/total occlusal force) on molar regions increased as clenching strength increased. On the contrary, the proportion of occlusal force on the premolar and anterior teeth regions decreased as clenching strength increased. These findings suggest that control of occlusal force is important in diagnosis of the nature of occlusal contacts.

The relationship between anterior guidance and the mesiodistal inclination of the lower incisors relative to the existence of temporomandibular joint (TMJ) noise was investigated. One hundred and thirty-four individuals, 21 to 40 years of age (mean age 23.8; 104 men and 30 women), were examined. A highly significant correlation was observed between the mean lower-incisor inclination and the difference in steepness between the right and left anterior guidances in the subject group showing TMJ noise (P<0.001), whereas no significant correlation was observed in the group without TMJ noise.

The study was carried out in two stages: (1) MOD amalgam cavities were prepared on maxillary second premolars and three strain gauges were attached to the palatal surface of each tooth. The teeth were filled using amalgam with and without base material (glass-ionomer). Stresses occurred during the hardening phase and also through mastication and were measured by strain-gauge rosettes. (2) The stresses which occurred at the same points were calculated by the finite element method and compared with the values obtained experimentally. The highest stress values were observed when no base material was used.

In the first part of this study some physical properties of restorative materials, amalgam, glass-ionomer and composite resin were measured experimentally. In the second part a numerical study was carried out, for which the maxillary second premolar tooth was used. The tooth model was restored with composite resin and amalgam on glass-ionomer, which was used as the base material. The stress distribution investigated was the resultant of the stresses which come from the mastication force and those resulting from the contraction and expansion of restorative materials. All calculations were carried out using the finite element method and programs were written using FORTRAN 77. A load of 450 N, at an angle of 45 degrees to the longitudinal axis was applied on the occlusal margin of the tooth. The tooth was assumed isotropic, homogenous, elastic and unsymmetrical. The distribution of compressive, tensile and shear stresses were plotted for the whole tooth structure.

The effect of measuring bite force with different patterns of transducer on different occasions was studied. Maximum voluntary bite force was measured in eight volunteers. Three transducer positions, each with a different pattern of transducer, were used; between the anterior teeth, between the second premolar and the first molar on one side and between the second premolars and first molars on both sides. Visual feedback of force was provided. Two sets of five maximum clenches were recorded with a rest period in between. This sequence was repeated for each transducer and the experiment was repeated on three different days. The highest forces were measured with the bilateral posterior transducer (mean 580 N, s.d. 235) and the lowest on the anterior transducer (mean 286 N, s.d. 164). The standard deviations of the bite force mean values were used as an indication of the variability and were subjected to a non-parametric anova (Kruskal-Wallis). The forces recorded with each transducer position were significantly different between the transducers (P < 0.01) and the maximum bite force showed least variability when measured between the posterior teeth on one side only. There was little difference in bite force between the three different sessions (P > or = 0.05) when measured in the same position within the dental arch, whichever of the three positions that may be.

The purpose of this study was to assess the influence of an experimentally induced light premature occlusal contact on tooth sensation. This assessment involved monitoring the electric tooth pain threshold (TPT) at multiple times before and after altering the occlusal contact. This alteration was produced by placing high inlays, which were measured with a custom made measuring device in maximum intercuspation. Data were collected on 10 teeth in 5 subjects, all whom were male and between the ages of 24 and 30 years. The contact area of the high inlays varied from 75 microm to 193 microm. The TPT changes in these teeth ranged from -43 to +21% of their baseline level. Seven of these teeth showed a significant decrease (P < 0.05) in the TPT, one showed an increase and two did not show a significant change. Two of the teeth with a decreased TPT had cold water triggered occlusal pain, and 2 teeth had only occlusal pain. After several inlay adjustments to eliminate interference in maximum intercuspation, all tested teeth returned to their TPT baseline level and all symptoms disappeared. These results suggested that a light premature occlusal contact may change tooth sensation.

The aim of the study was to obtain data on the dynamic strength of both intact and prepared premolars for partial veneer crowns. Cyclic loading was performed in a modified Amslers high-frequency pulsator with varying force (between 400 and 1400 N). The greatest dynamic strength was exhibited by the intact samples. The greater the load the fewer the cycles necessary to fracture the tooth. A statically significant difference was found (P < 0.05) between the magnitude of cyclic loads and the different types of specimens tested.

The variable properties of photoelastic models and the difficulty of fabrication of the model limit experimental designs. The 'material fringe value' is introduced into the definition of the 'stress concentration index' by multiplying the fringe orders. Three levels of periodontal support in three photoelastic models (nine cases) were compared quantitatively by determining the stress concentration induced in periodontium by point loading vertically on a span of the fixed partial denture of four units length. The results indicate that comparison among photoelastic models is possible and useful. The stress concentration effects produced by the second molar were more prominent and determinant than that by the first premolar in the periodontium of the abutments.

The purpose of this study was to investigate occlusal schemes (canine guidance and group function) in relation to masticatory muscle activity. It was performed on 30 subjects, 15 with canine guidance and 15 with group function. Bilateral electromyographic recordings of masseter and anterior temporal muscles were obtained by surface electrodes during gum chewing and sliding laterally from centric relation while the teeth were in contact. The obtained data for both groups were compared statistically. Electromyograms were also evaluated visually. There was no significant difference in results between the occlusal schemes. However, canine guidance showed a narrower chewing model than group function and reduced anterior temporal muscle activity during sliding. This suggests that in the presence of healthy and good supportive canine teeth, canine guidance occlusion will be preferable.

A two-dimensional finite element model of a mandibular quadrant was used to examine differences in magnitude of the principal stresses from the placement of three- and four-unit bridges. The area of interest spanned the first premolar to the second molar. Loading conditions were (i) vertical and distributed and (ii) 30 degrees to the vertical and concentrated. The principal stresses were calculated and compared for: (i) the first molar removed with the remaining bone either cancellous or cancellous surrounded by a cortical shell; (ii) as in (i) but with the second premolar and first molar removed; (iii) a three-unit bridge spanning the second premolar to the second molar; and (iv) a four-unit bridge spanning the first premolar to the second molar. Each tooth was supported by periodontal ligaments, cortical and cancellous bone with each assigned the appropriate physical constants. Removal of the first molar resulted in considerable variation of the stresses especially when the cortical shell was replaced by cancellous bone. Because of the lower modulus of cancellous bone and its lower load-bearing capabilities the stresses were three to ten times lower and more uniform within the cancellous bone. Generally, the addition of a bridge resulted in lower and better distributed sigma min stresses. The bridge also resulted in higher tensile stresses distal to the abutment teeth which theoretically could result in bone deposition. No significant differences in magnitude were observed between the three- and four-unit bridge. From a stress standpoint the bridges resulted in more uniform stress distribution around the abutments and an increase in the tensile stress distal to the abutments. Such findings support the placement of a fixed bridge to maintain bone in an edentulous area.

In the opinion of some authors cusps are functionally efficient when worn down: others view cusps as efficient structures in themselves which should be retained. Such opinions are reviewed, and some conclusions drawn as to whether cusps should be retained or imitated when restoring an occlusion.

The greater forces generated between posterior teeth compared with between anteriors, may be explained by interpreting the behaviour of the mandible as a lever system, with the condyle as a fulcrum. This study used a semi-conductor strain gauge transducer to measure load, and EMG activity to index muscular effort. A posterior and an anterior load position were compared by using parallel metal platforms which maintained the occlusal vertical dimension for different biting positions. Reductions in muscular effort were observed for the posterior position of a magnitude that was consistent with the greater mechanical advantage predicted by the level model.

A two-dimensional finite element model of a mandibular quadrant was used to examine the stresses and displacements resulting from a 100 N load placed as follows: (i) distributed on the second molar, (ii) concentrated at 30 degrees to the vertical on the second molar, and (iii) distributed on the second premolar and second molar. Each tooth was supported by periodontal ligaments, cortical and cancellous bone. The modulus and Poisson's ratios for each material were selected from accepted values. The principal stresses were determined throughout the model, with special emphasis being placed for elements in the immediate vicinity of the teeth mentioned above. It was found that for the case where the load was uniformly distributed on the second molar the stresses sigma min were mostly compressive (-) in nature while sigma max were mostly tensile (+). In the case where the load was concentrated and at 30 degrees to the vertical, the magnitude of the stresses were three to five times greater and the pattern less uniform than those resulting from the distributed load. The concentrated load at 30 degrees to the vertical resulted in higher bending stresses than the distributed load and hence higher sigma max stresses.