There have been many studies about post and core techniques, though most of them were in vitro studies. The purpose of this study was to make clear the present situation concerning clinical studies on the post and cores.
We reviewed many articles associated with this topic having published for these 20 years, mainly in Journals of the Japan Prosthodontic Society and Journals of Prosthetic Dentistry. As a result, we could find very few articles that pursued clinical longitudinal outcomes of post and core restorations, by repeating periodic examinations under a recall system. Study designs of the selected articles were first introduced, and then various factors that might affect failure rates of the restorations were discussed, including locations of teeth, post designs, and types of abutment.
The failure rate of the post and core technique was almost 2% of all restorations, and root fracture occupied 20 to 30% of all failures. Upper anterior teeth were the most risky in the mouth, and abutments of fixed partial dentures revealed a higher failure rate than single crowns.
However, the following fundamental questions remained to be answered: Which is preferable, a cast post or a prefabricated one? A parallel-sided post or a tapered one? That means well-designed clinical studies are absolutely necessary about this topic.

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This paper reviews the factors of root fracture of endodontically treated teeth with a foundation restoration and designs of the foundation restoration for avoiding root fracture.
The strength and elastic modulus of metallic materials for a foundation restoration are stronger and greater than those of dentin, respectively; the significant strength of the metallic post is not easy to break; however, the great elastic modulus distributes occlusal load to the radicular dentin, and as a result, the stress is concentrated at the apex of the post and cervical dentin close to the crestal bone ridge. The intensity of the stresses at these stress-concentrated points varies by such as the post length, crestal bone ridge height, and residual intact tooth amount. Root fracture does not occur when the amount of radicular dentin at these stress concentrated points is sufficient. If the elastic modulus of the post material is similar to that of the radicular dentin, stress distribution to the radicular dentin will decrease, however great flexural strength of the post is required. The actual fracture pattern is analyzed by the fracture test under static loading. The fracture pattern of the fracture test is not completely identical to the clinical fracture pattern. The actual fracture crack propagates by repeated loading; therefore, evaluation of the fracture test under cyclic loading is now a research concern. Minimal tooth grinding is essential to prevent root fracture; nevertheless, the design of the foundation restoration without root fracture is associated with dropping out of the fundamental restoration. The dentist must design a suitable foundation restoration with adequate retention while minimizing subsequent root fracture.

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To prevent root fracture related to post and core build-up, its physio-mechanical characteristic has to be considered, so that the stress within the root strucure is equally distributed. This is especially of great importance with regard to “the main masticatory teeth, ” which have to bear constant masticatory stress. In this article, based on clinical fracture cases and research on “the main masticatory teeth, ” particular attention was paid to the buccal frenum, especially its relation with “the main masticatory teeth” and root fracture.
1.“The main masticatory teeth” often coincide with the dominant arm side.
2.“The main masticatory teeth” are primarily the first molars in both the upper and lower arch, and then the second molars.
3. In the upper teeth, the second premolar or first molar to which the buccal frenum is attached tend to be “the main masticatory teeth.”
4. In the lower teeth, no relation is found between the buccal frenum and “the main masticatory teeth.”
5. In the upper teeth, root fracture always occurs in teeth with buccal frenum attachment, whereas in the lower teeth, there is no relation between fractured teeth and frenum attachment.
6. Thus, particular attention has to be paid in the post and core build-up of the upper teeth with the frenum attachment.

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Purpose: The purpose of this experiment was to clarify the difference in the occlusal contact condition between the two representative patterns of movement path in mastication of healthy subjects.
Methods: Twenty healthy subjects who have a pattern that opens from centric occlusion (CO) toward the working side, and closes toward CO in a convex fashion (group I), and 20 healthy subjects who have a pattern that opens from CO toward the balancing side first, and then toward the working side, and closes toward CO in a convex fashion (group III), in their 20s, were employed. The occlusal contact conditions at the lateral occlusal positions when the mandibular incisal point was moved laterally from CO for 1mm (L1), 2mm (L2), and 3mm (L3) were compared betweenthe groups and among the lateral positions.
Results: The number of contacting teeth decreased significantly, in order, from L1 to L2 to L3, on both the working and balancing sides. Group I tended to have fewer tooth distribution for the frequency of occlusal contacts for both the working and balancing sides, and a highly significant difference was found between the two groups on the working side and the balancing side of L 2 and the working side of L3.
Conclusion: It was concluded that, though the occlusal contact condition varied according to the distance of lateral movement, in general, group I had lesser distribution of contacting teeth than group III, and this was especially evident when the mandibular incisal point was moved 2mm from centric occlusion. And the difference in this occlusal contact condition probably contributed to the difference between these two masticatory path patterns.

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Purpose: Intercuspal position (ICP) is a static tooth contact position, but in vivo, there is a relationship between the degree of bite force and the degree of intercuspation and associated tooth movement within the periodontal space. The purpose of this study was to examine how the mandible fluctuates during intercuspation.
Methods: To observe minute mandibular fluctuations, we developed a new sensor system using a silicon optoelectric position sensitive detector (PSD) and an infrared LED. A clutch-mounted PSD was attached to the buccal and labial aspects of the maxillary teeth, and an LED was attached to the mandibular teeth with the clutch. The mandibular position was measured in 11 subjects with natural dentition. We performed the experiments under two conditions: on-the-model and in-vivo conditions. Both PSD and LED were fixed with a plaster model of each subject on the articulator, to evaluate the precision of this sensor system (on-the-model condition). All subjects occluded their teeth without clenching in ICP for more than 30 seconds (in-vivo condition). The experiments were repeated three times, and we measured antero-posterior and lateral ranges and variances of mandibular fluctuations in ICP for 30 seconds. The distance of mandibular positions at the beginning of, and at the end of, occluding for 30 seconds, was also measured.
Results: There were significant differences in ranges, variances, and distances of ICP between the two conditions. The range of ICP on-the-model condition was about 6 μm, whereas the range of ICP in-vivo condition was about 40 μm. The variance and distance of the ICP in-vivo condition were significantly larger than those of the on-the-model condition.
Conclusions: It is apparent that the mandible fluctuates within antero-posterior and lateral ranges of about 40 μm during intercuspation. We speculate that the fluctuation represented the physiological motion to maintain ICP.

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Purpose: To understand the relationship between anterior disc displacement of the temporomandibular joint (TMJ) and the angle of the condylar and incisal path, mandibular movements during protrusive and lateral excursions were analyzed.
Methods: Eighty-one joints of 53 subjects (41 females and 12 males, mean age of 32.0±12.7 y) having an anteriorly displaced disc (DD), confirmed by magnetic resonance images, were recruited for this study. Twenty-five subjects (17 females and 8 males, mean age of 37.0±13.6 y) having no disc displacement were also enrolled, as a control group (ND). Protrusive and lateral excursions of subjects were recorded with a six-degree-of-freedom jaw tracking device, to obtain the difference of the angles between the condylar and incisal paths (40).
Results: Results showed that the angle of the condylar path was larger than that of the incisal path in DD, while the condylar angle was smaller than the incisal angle in ND. The condylar angle being larger than the incisal angle was possibly thought to generate reversal rotational movement under incisal teeth guided movement, to make the disc slip out into forward position.
Conclusions: The relationship between the anterior and posterior guidance is thought to be a contributing factor for anterior disc displacement in TMJ.

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Purpose: The purpose of this study was to examine displacement of the condyle in unilateral clenching, and to clarify effects of disc position on the non-clenching side on temporomandibular joint loading.
Methods: The subjects were 20 patients diagnosed with TMD who had previously agreed to be examined. They were asked to clench a plastic stick unilaterally between the upper and lower second molars. The resulting mandibular movements were then measured with a six-degree-of-freedom jaw-tracking device, and analyzed with a computer, using software designed specifically for this purpose. The subjects were divided into 2 groups according to the disc position on the non-clenching side: the NDD group (no disc displacement) and the DD group (disc displacement). The 2 groups were compared with each other. We also investigated the relationship between condylar displacement and evoked pain in the TMJ of the non-clenching side.
Results: In the NDD group, the position of the condyle on the non-clenching side distributed around a point located slightly above the starting position. In the DD group, it distributed around a point located slightly antero-superior to the starting position. In the NDD group, the condyle on the non-clenching side was displaced further upward than the condyle on the clenching side in the majority of cases; in the DD group, this occurred in a smaller number of cases. In most patients who complained of evoked pain on the non-clenching side, the condyle on the non-clenching side displaced further upward than the condyle on the clenching side. In other words, differences in disc position seemed to correlate with condylar displacement.
Conclusion: We conclude that differences in disc position affect condylar displacement in unilateral clenching, and this suggests that disc position also affects temporomandibular joint loading.

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Purpose: A color-changeable chewing gum was developed as a simple method to evaluate masticatory ability. The purpose of this study was to determine the capability of the chewing gum to measure masticatory performance of complete denture wearers.
Methods: Twenty-seven edentulous patients, who wore newly fabricated complete dentures, volunteered to participate in this study. One month after the completion of denture adjustments, the following experiments were performed. The subjects chewed the color-changeable chewing gum until the color changed completely to red. The gradation of the color was checked by a dentist at every five chewing strokes. The number of chewing strokes at the completion of the color gradation was confirmed, using the color scale for the color-changeable chewing gum, and recorded. Masticatory performance was also obtained using the sieving test reported by Manly and Braley. Spearman's correlation coefficient, between the number of chewing strokes and masticatory performance, was calculated and statistically analyzed.
Results: Mean values and standard deviations of the number of chewing strokes and masticatory performance were 107.0±23.7 and 48.2±16.3, respectively. The number of chewing strokes showed a significant negative correlation to the masticatory performance (ρ=-0.767, p<0.0001).
Conclusion: It was suggested that a color-changeable chewing gum might be useful to measure masticatory performance of complete denture wearers.

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Purpose: We previously reported that alumina sandblasting had an effect on shear bond strength for ceramic abutment CerAdapt (Nobel Biocare), which is that made from densely sintered high-purity alumina. The purpose of this study was to evaluate the shear bond strength of CerAdapt treated by tribochemical coating of ROCATEC system (ESPE) compared with alumina sandblasting.
Methods: The surface of CerAdapt was sandblasted by the ROCATEC system for 10 seconds, with 110 pm alumina particles, to clean the surface (ROCATEC-Pre), and then for 15 seconds with both special silica particles and 110μm silica coating alumina (ROCATEC-Plus, tribochemical coating). In contrast, the surface was sandblasted with 110μm alumina for 10 and 25 seconds.Prosthetic ceramic specimens (φ3.0×2.0mm) were made of IPS Empress (Ivoclar). The surface roughness and surface analysis of CerAdapt were evaluated after treatment of each group. Specimens for shear bond strength were made as follows: prosthetic ceramic specimens were bonded to CerAdapt sandblasted with resin cement Variolink II (Vivadent). Shear bond strength was measured using an Instron (cross head speed 1.0mm/min). The data were analyzed by one-way ANOVA (α=0.05).
Results: While there was a significant difference for surface roughness, there was no significant difference for shear bond strength in alumina particle size. On the other hand, there was no significant difference in both surface roughness and shear bond strength as to treatment time. Silica was detected on the surface of CerAdapt after tribochemical coating of the ROCATEC system. Shear bond strength of the ROCATEC system was significantly higher than those of alumina sandblasting.
Conclusions: The tribochemical coating of the ROCATEC system was superior to alumina sandblasting in shear bond strength. However, shear bond strength of CerAdapt was not influenced by differences in each of particle size and treatment time.

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