Complete denture impression techniques: evidence-based or philosophical.
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Code of practice is dangerous and ever-changing in today's world. Relating this to complete denture impression technique, we have been provided with a set of philosophies--"no pressure, minimal pressure, definite pressure and selective pressure". The objectives and principles of impression-making have been clearly defined. Do you think any philosophy can satisfy any operator to work on these principles and achieve these objectives? These philosophies take into consideration only the tissue part and not the complete basal seat, which comprises the periphery, the tissues and the bone structure. Under such circumstances, should we consider a code of practice dangerous or should we develop an evidence-based approach having a scientific background following certain principles, providing the flexibility to adapt to clinical procedures and to normal biological variations in patients rather than the rigidity imposed by strict laws? (+info)
Maximal bite force in young adults with temporomandibular disorders and bruxism.
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Parafunctional habits, such as bruxism, are contributory factors for temporomandibular disorders (TMD). The aim of this study was to evaluate the maximal bite force (MBF) in the presence of TMD and bruxism (TMDB) in young adults. Twelve women (mean age 21.5 years) and 7 men (mean age 22.4 years), composed the TMDB group. Ten healthy women and 9 men (mean age 21.4 and 22.4 years, respectively) formed the control group. TMD symptoms were evaluated by a structured questionnaire and clinical signs/symptoms were evaluated during clinical examination. A visual analogical scale (VAS) was applied for stress assessment. MBF was measured with a gnatodynamometer. The subjects were asked to bite 2 times with maximal effort, during 5 seconds, with a rest interval of about one minute. The highest values were considered. The data were analyzed with Shapiro-Wilks W-test, descriptive statistics, paired or unpaired t tests or Mann-Whitney tests when indicated, and Fisher's exact test (p < 0.05). TMDB women presented lower values of MBF as compared to those presented by TMDB men and by the control group. MBF for TMDB men was similar to that of the control group. The proportion of TMDB women with muscle pain and facial/teeth/head pain upon waking up was significantly higher than that of men. Control women presented significantly lower stress scores than the others. It was concluded that MBF was reduced in TMDB women, as they presented more signs and symptoms. Men presented higher MBF values than women, but TMD and bruxism did not significantly decrease MBF. Stress was not an influencing factor for TMD and bruxism in men. (+info)
Influence of unilateral disc displacement on the stress response of the temporomandibular joint discs during opening and mastication.
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The temporomandibular joint plays a crucial role in human mastication acting as a guide of jaw movements. During these movements, the joint is subjected to loads which cause stresses and deformations in its cartilaginous structures. A perfect balance between the two sides of the joint is essential to maintain the physiological stress level within the tissues. Therefore, it has been suggested that a derangement of the joint is a contributing factor in the development of mandibular asymmetry, especially if problems of the temporomandibular joint start in childhood or adolescence. To analyze the movement of the mandible and the stresses undergone by the discs, two finite element models of the human temporomandibular joint including the masticatory system were developed, one corresponding to a healthy joint and the other with a unilateral anterior disc displacement with their movement controlled by muscle activation. A fibre-reinforced porohyperelastic model was used to simulate the behaviour of the articular discs. The stress distribution was analyzed in both models during free opening and closing, and during the introduction of a resistant force between incisors or molars. It was found that a slight unilateral anterior disc displacement does not lead to mandibular asymmetry but to a slight decrease of the maximum gape. With the introduction of a restriction between incisors, the maximum stresses moved to the anterior band in contrast to what happened if the restriction was imposed between molars where maximum stresses were located more posteriorly. Finally, the presence of a unilateral displacement of the disc involved a strong change in the overall behaviour of the joint including also the healthy side, where the maximum stresses moved to the posterior part. (+info)
Clinical performance and wear characteristics of veneered lithia-disilicate-based ceramic crowns.
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OBJECTIVES: The objectives of this study were to characterize the clinical performance and wear characteristics of lithia-disilicate-based ceramic crowns. METHODS: Thirty posterior crowns were made using the heat-pressing technique and lithia-disilicate-based core ceramic. Subjects were recalled annually. The quality of crowns and adjacent gingival tissues were examined using nine criteria for acceptability. All crowns were examined and ranked from 4 (Excellent) to 1 (Unacceptable) for each criterion. Impressions were made for replica models at each appointment. Wear characteristics of dental ceramic and enamel were obtained by comparing the surface of the original model with the follow-up model using a laser scanner. RESULTS: Twenty-nine subjects returned for the 1-year recall examination. The maximum clenching force for the 30 subjects ranged from 125 to 815 N. All clinical criteria were ranked good to excellent at the 1-year recall exam and no fractures were observed. The mean occlusal wear volumes for the ceramic crowns after 1 year were 0.19 (0.065)mm3 for premolar sites and 0.34 (0.08)mm3 for molar sites. The mean occlusal wear volumes of opposing enamel after 1 year were 0.21 (0.06)mm3 for premolar teeth and 0.50 (0.22)mm3 for molar teeth. The mean occlusal wear volume of ceramic molar crowns was significantly lower than the volume of enamel wear of the opposing teeth (p+info)
Computer simulation of feeding behaviour in the thylacine and dingo as a novel test for convergence and niche overlap.
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The extinct marsupial thylacine (Thylacinus cynocephalus) and placental grey wolf (Canis lupus) are commonly presented as an iconic example of convergence. However, various analyses suggest distinctly different behaviours and specialization towards either relatively small or large prey in the thylacine, bringing the degree of apparent convergence into question. Here we apply a powerful engineering tool, three-dimensional finite element analysis incorporating multiple material properties for bone, to examine mechanical similarity and niche overlap in the thylacine and the wolf subspecies implicated in its extinction from mainland Australia, Canis lupus dingo. Comparisons of stress distributions not only reveal considerable similarity, but also informative differences. The thylacine's mandible performs relatively poorly where only the actions of the jaw muscles are considered, although this must be considered in the light of relatively high bite forces. Stresses are high in the posterior of the thylacine's cranium under loads that simulate struggling prey. We conclude that relative prey size may have been comparable where both species acted as solitary predators, but that the dingo is better adapted to withstand the high extrinsic loads likely to accompany social hunting of relatively large prey. It is probable that there was considerable ecological overlap. As a large mammalian hypercarnivore adapted to taking small-medium sized prey, the thylacine may have been particularly vulnerable to disturbance. (+info)
Applied mechanics of the Puricelli osteotomy: a linear elastic analysis with the finite element method.
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BACKGROUND: Surgical orthopedic treatment of the mandible depends on the development of techniques resulting in adequate healing processes. In a new technical and conceptual alternative recently introduced by Puricelli, osteotomy is performed in a more distal region, next to the mental foramen. The method results in an increased area of bone contact, resulting in larger sliding rates among bone segments. This work aimed to investigate the mechanical stability of the Puricelli osteotomy design. METHODS: Laboratory tests complied with an Applied Mechanics protocol, in which results from the Control group (without osteotomy) were compared with those from Test I (Obwegeser-Dal Pont osteotomy) and Test II (Puricelli osteotomy) groups. Mandible edentulous prototypes were scanned using computerized tomography, and digitalized images were used to build voxel-based finite element models. A new code was developed for solving the voxel-based finite elements equations, using a reconditioned conjugate gradients iterative solver. The Magnitude of Displacement and von Mises equivalent stress fields were compared among the three groups. RESULTS: In Test Group I, maximum stress was seen in the region of the rigid internal fixation plate, with value greater than those of Test II and Control groups. In Test Group II, maximum stress was in the same region as in Control group, but was lower. The results of this comparative study using the Finite Element Analysis suggest that Puricelli osteotomy presents better mechanical stability than the original Obwegeser-Dal Pont technique. The increased area of the proximal segment and consequent decrease of the size of lever arm applied to the mandible in the modified technique yielded lower stress values, and consequently greater stability of the bone segments. CONCLUSION: This work showed that Puricelli osteotomy of the mandible results in greater mechanical stability when compared to the original technique introduced by Obwegeser-Dal Pont. The increased area of the proximal segment and consequent decrease of the size of lever arm applied to the mandible in the modified technique yield lower stress values and displacements, and consequently greater stability of the bone segments. (+info)
Morphology and kinematics of feeding in hagfish: possible functional advantages of jaws.
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As in gnathostomes, the hagfish feeding apparatus includes skeletal, dental and muscular components. In the present study, we examined feeding morphology and kinematics in two hagfish species, Eptatretus stoutii and Myxine glutinosa, representing the two major hagfish lineages. E. stoutii have larger dental plates, larger basal plates, and stronger clavatus muscles (the major dental plate retractor) than M. glutinosa. Despite morphological differences, kinematic profiles are similar in E. stoutii and M. glutinosa. When protracted, the dental plate unfolds and exposes keratinous teeth, which are then embedded in the prey. Once food is grasped, the dental plate is retracted into the mouth. During retraction, the clavatus muscle can generate up to 16 N of force, which exceeds the bite force of some gnathostomes of similar size. In addition to producing high forces with the feeding muscles, hagfish can evert their dental plates to 180 degrees , exceeding the gape angles attained by virtually all gnathostomes, suggesting vertebrate jaws are not the prerequisites for muscle force generation and wide gapes. We propose that dental plate protraction and retraction can be modeled as a fixed pulley that lacks the speed amplification occurring in gnathostome jaws. Hagfish gape cycle times are approximately 1 s, and are longer than those of gnathostomes, suggesting that a functional advantage of jaws is the speed that allows gnathostomes to exploit elusive prey. (+info)
Tooth structure and fracture strength of cavities.
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This study evaluated, in vitro, the loss of tooth substance after cavity preparation for direct and indirect restorations and its relationship with fracture strength of the prepared teeth. Sixty sound human maxillary first premolars were assigned to 6 groups (n=10). MOD direct composite cavities (Groups I, II and III) and indirect inlay cavities (Groups IV, V and VI) were prepared maintaining standardized dimensions: 2-mm deep pulpal floors, 1.5-mm wide gingival walls and 2-mm high axial walls. Buccolingual width of the occlusal box was established at 1/4 (Groups I and IV), 1/3 (Groups II and V) or 1/2 (Groups III and VI) of the intercuspal distance. Teeth were weighed (digital balance accurate to 0.001 g) before and after preparation to record tooth substance mass lost during cavity preparation. The prepared teeth were submitted to occlusal loading to determine their fracture strength using a universal testing machine at a crosshead speed of 0.5 mm/min. Data were analyzed by two-way ANOVA and Tukey test (alpha= 0.05). 1/4-inlay cavities had higher percent mean mass loss (9.71%) than composite resin cavities with the same width (7.07%). 1/3-inlay preparations also produced higher percent mean mass loss (13.91%) than composite resin preparations with the same width (10.02%). 1/2-inlay cavities had 21.34% of mass loss versus 16.19% for the 1/2-composite resin cavities. Fracture strength means (in kgf) were: GI = 187.65; GII = 143.62; GIII = 74.10; GIV = 164.22; GV = 101.92; GVI = 50.35. Statistically significant difference (p<0.05) were observed between Groups I and IV, II and V, III and VI. Higher tooth structure loss and lower fracture strength were recorded after preparation of inlay cavities, regardless of the width of the occlusal box, compared to the direct composite resin cavities. (+info)