Shoji Kohno

Fatigue resistance of cast occlusal rests using Co-Cr and Ag-Pd-Cu-Au alloys

STATEMENT OF PROBLEM Fatigue failure in a removable partial denture framework includes fracture of the occlusal rest at the rest-minor connector angle. PURPOSE This in vitro study evaluated the fatigue resistance of 0.8-mm-thick occlusal rests cast with Co-Cr and Ag-Pd-Cu-Au alloys. MATERIAL AND METHODS The specimen consisted of occlusal rest (0.8 x 2.0 x 10 mm), vertical minor connector (0.9 x 2.0 x 5.0 mm), and denture base connector (1.5 x 5.0 x 25 mm). Twenty-five specimens each were cast with Co-Cr and Ag-Pd-Cu-Au alloys. From each group, 5 specimens were subjected to a load-deflection test conducted to determine the amount of deflection to be used in fatigue test. The fatigue test was performed such that the occlusal rest component was deflected by displacing the denture base component in a tissueward direction. Predetermined denture base displacement values of 0.25 and 0.49 mm for Co-Cr and Ag-Pd-Cu-Au specimens, respectively, were repeated at a rate of 500 cycles/min by use of a displacement-controlled fatigue testing machine, until occlusal rest failure occurred or a preset limit of 2 million cycles was achieved. The survival rates of the occlusal rests were calculated assuming a chewing cycle of 2 x 10(5) per year. Scanning electron microscopy (SEM) photographs were made to examine the fracture surfaces and to identify casting defects. The relationship between fatigue cycles and number of casting defects was determined by Spearman rank correlation analysis (P<.01). RESULTS All 20 Co-Cr specimens outran the preset limit, whereas 18 Ag-Pd-Cu-Au specimens fractured. After 3 years of simulated clinical use, only 50% of the Ag-Pd-Cu-Au occlusal rests survived. Statistical analysis showed that the fatigue cycles and number of casting defects were inversely related (P<.01). Fatigue crack initiation occurred at the inner-curvature surface of the rest-minor connector angle. The typical Ag-Pd-Cu Au fatigue fracture surface consisted of smooth propagation and dimpled and smooth final fracture areas. CONCLUSIONS This study suggests that cast Co-Cr rests are more rigid and fatigue resistant than Ag-Pd-Cu-Au rests. The fatigue resistance of 0.8-mm-thick occlusal rests may be adequate if cast with Co-Cr alloy. An increased number of casting defects may hasten the fatigue failure of occlusal rests.

New Animal Model for Studying Mastication in Oral Motor Disorders

To identify the basic parameters of oral behavior in mice, we recorded the three-dimensional jaw movement trajectories and masseter and digastric muscle activities in freely behaving mice eating foods of various textures. Results showed that: (1) there are characteristic jaw movement patterns for food intake and mastication; (2) the pattern in a chewing cycle may be divided into opening, closing, and protruding (power) strokes; and (3) food texture affects basic patterns of jaw movement, muscle activities, and chewing rhythms. The oral motor behavior of mice appears identical to those of other experimental animals, so mice are appropriate animal models for the study of mastication.

Head movements and neck muscle activities associated with the jaw movement during mastication in the rabbit authors

Rhythmical head movements and neck muscle activities associated with the masticatory jaw movement were investigated in rabbits. In natural mastication, head movements and neck muscle activities showed a rhythmical feature synchronized with jaw movement. During cortically induced rhythmical jaw movements, some neck muscle showed rhythmical activity induced by biting a wooden stick. Neck muscles may contribute to the rhythmical head movement after loading the tooth with food.

Relationship between Porotic Changes in Alveolar Bone and Spinal Osteoporosis

Epidemiological studies have shown that post-menopausal women who do not use an estrogen supplement have fewer teeth than those who do. We hypothesized that changes in the dentition of post-menopausal women might be due to alveolar bone alterations by estrogen deficiency. To clarify this, we analyzed the microstructural alveolar bone changes in ovariectomized monkeys and compared these with their lumbar bone mineral density. The % of baseline bone mineral density showed a significant decrease in the ovariectomized group as compared with the controls. The second-molar interradicular septa in ovariectomized monkeys showed a significantly decreased nodes number, cortices number, and an increased structural model index value. More pores were seen in the ovariectomized group at the top of the septa. This study demonstrated that, in such monkeys, estrogen deficiency led to fragility of the trabecular structure of the molar alveolar bone, and such fragility was inversely correlated with lumbar bone mineral density.

The effect of aging and ovariectomy on mandibular condyle in rats

STATEMENT OF PROBLEM It is important for dentists to understand the effect of systemic hormonal change on the osseous oral structures. PURPOSE This study examined the effect of aging and ovariectomy on rat mandibular condyle. MATERIAL AND METHODS Seventy-two 120-day-old female Fischer rats were killed at 7, 14, 30, and 60 days after bilateral ovariectomy or sham surgery. As the baseline control group, eight animals were killed on day 0 without surgeries. Changes in the bone mineral density and bone marrow area were detected through dual-energy x-ray absorptiometry and soft x-ray photography, respectively. RESULTS No significant difference of bone mineral density was found between the bilateral ovariectomy and sham surgery groups with dual-energy x-ray absorptiometry, probably because the thickness of cortical bone obscured any possible changes in trabecular bone. Age-related osteosclerotic changes were found in the sham group with soft x-ray photography. In contrast, the bilateral ovariectomy group showed little change in bone marrow area in relation to time course; on the other hand, the value of their bone marrow area became significantly larger than that of the sham surgery group from 14 days after ovariectomy onward. CONCLUSIONS It was inferred that estrogen deficiency caused the significantly large marrow area found in the rat mandibular condyle. Although much more research is necessary, this study allowed us to speculate that osteoporotic changes may occur in the mandibular condyle of postmenopausal women.

Region-specific Bone Mass Changes in Rat Mandibular Condyle following Ovariectomy

The effects of estrogen deficiency on the mandible have been indicated; however, there have been only a few reports on the mandibular condyle. To clarify the region-specific changes of condyles following estrogen deficiency, we histomorphometrically studied the anterior and posterior regions of condyles in sham-operated (Sham) and ovariectomized (Ovx) female Fischer rats, since it has been reported that rat occlusal force loads mainly onto the anterior part of the condyle. The anterior and posterior regions of condyles showed significantly different bone dynamics. The bone volume in the anterior region of the Sham group increased to 79.9% with aging, but that of the posterior region remained at ca. 60%. In the Ovx group, the bone volume of the posterior region significantly declined to 42.7% until 60 days post-ovariectomy, although no changes occurred in the anterior region. As compared with the Sham counterparts, both regions of the Ovx group showed high bone turnover and revealed significantly low bone volume at 60 days (p < 0.01). Regional differences between anterior and posterior regions were more emphasized in the Ovx than in the Sham group. The osteoclast number and amount of eroded surface in the Ovx were higher in the posterior than in the anterior region, with significance at 30 and/or 60 days (p < 0.05). The mineralized surface at 14 days and the bone formation rate at 60 days in the Ovx posterior region were significantly lower than those in the Ovx anterior region (p < 0.05). Consequently, a net bone loss occurred in the posterior region of the mandibular condyle in ovariectomized rats, owing to region specificity and estrogen deficiency.

Effect of intermittent administration of human parathyroid hormone (1-34) on the mandibular condyle of ovariectomized rats

Abstract: Intermittent administration of human parathyroid hormone (1-34) (PTH) increases bone mass in lumbar vertebrae and long bones of osteoporotic experimental animals. However, whether PTH has the same effect on jaw bones remains unclear. This study determined the effect of intermittent administration of PTH on rat mandibular condyle affected by estrogen deficiency. Fifty 6-month-old rats were either sham operated or ovariectomized, then divided into five groups depending on surgical procedure and hormone administration: sham + vehicle (SV), OVX + vehicle (OV), OVX + PTH 6 μg/kg once per week (OP6-1), OVX + PTH 60 μg/kg once per week (OP60-1), and OVX + PTH 20 μg/kg three times per week (OP20-3). PTH or vehicle was injected intermittently for 6 months in 5 rats of each group either immediately after surgery in a preventive administration experiment, or injected starting 6 months after surgery in a therapeutic administration experiment. The mandibles were excised, and bone morphometry was performed using confocal laser scanning microscopy and soft X-ray images. In both experiments, the bone volume of the OV groups was significantly lower than that of the SV group (P < 0.01); also, depending on dose and frequency, the bone volume of the OP group was higher than that of the OV group, particularly in the OP20-3 group. The value of mineralized surface of the OP groups was significantly higher than that of the OV group (P < 0.01), whereas the value of eroded surface of the OP groups was not significantly higher than that of the OV group. This study indicates that preventive and therapeutic intermittent administration of PTH in ovariectomized rats increase the bone formation in rat mandibular condyle without accelerating bone resorptive activity. This anabolic effect was best induced by the injection mode of 20 μg/kg three times per week.

Expression of 25 kDa heat shock protein by synovial type B cells of the mouse temporomandibular joint

Earlier studies have demonstrated immunoreactivity for heat shock protein 25 (Hsp25) in type B synovial lining cells of the rat temporomandibular joint, and also the presence of characteristic cytoplasmic processes in these cells, but it is unclear whether or not the type B cells in other animals possess such elaborate cytoplasmic projections and as there is as yet no evidence for the synthesis of this protein by these cells. For these reasons, the expression of Hsp25 was investigated in the synovial membrane of the mouse temporomandibular joint by immunocytochemistry and by in situ hybridization using a specific cRNA probe. Intense immunoreaction for Hsp25 was found in the cytoplasm of certain synovial lining cells that were identified as type B by immunoelectron-microscopy. These Hsp25-positive cells had slender cytoplasmic processes, either projecting towards or covering the synovial surface. Morphological differences between cytoplasmic processes seemed to depend on the location of the type B cell bodies. In situ hybridization showed intense signals for Hsp25 mRNA in the synovial lining cells, suggesting that the type B cells produce, rather than resorb, Hsp25. These findings indicate that Hsp25 is a useful marker for the identification of the synovial type B cells in the temporomandibular joint. It is further hypothesized that Hsp25 in type B cells is involved in maintaining their specific profile and epithelial-like arrangement, and in protecting against mechanical stress.

Bactericidal eradication from carious lesions of prepared abutments by an antibacterial temporary cement

PURPOSE The aim of this study was to observe the antibacterial potential of polycarboxylate temporary cement containing a mixture of metronidazole, ciprofloxacin, and cefaclor on carious lesions of prepared abutments that were designed to leave caries on the abutments. MATERIAL AND METHODS Antibacterial efficacy was estimated in vitro and in vivo by measuring bacterial recovery from the lesions. Bacteria counts ranged from 10(4) to 10(7) both in vitro (nine samples) and in vivo (five samples) in time-zero samples, just before the application of the antibacterial cement. RESULTS No bacteria were recovered from carious lesions in vitro (six samples) or in vivo (four samples) after the lesions were covered by the antibacterial temporary cement. For the remaining samples, some bacteria (5 to 80 counts per sample) were recovered, with one notable exception in which marginal leakage provided a bacteria count of 10(3). Bacteria counts ranging from 10(3) to 10(5) occurred in carious lesions covered by temporary cement without antibacterial agents, which indicated that temporary cement alone was not a potent disinfectant. CONCLUSIONS The results of this study indicated that the antibacterial temporary cement can be useful for eradicating bacteria from carious lesions of prepared abutments.

The relationship between retruded contact position and intercuspal position in patients with TMJ osteoarthritis.

ABSTRACT This study was conducted to investigate the relationship between posterior mandibular excursion movement and temporomandibular joint osteoarthritis (TMJ OA) in 25 orthodontic patients with Angle Class I and Class II, using a six degrees-of-freedom measuring device and helical computed tomography. There were significant differences found in three-dimensional length, antero-posterior, absolute latero-medial and supero-inferior incisal, and condylar intercuspal position (IP)-retruded contact position (RCP) slides between bilateral, unilateral, and no condylar bone change groups. With respect to the types of condylar bone change, there were significant differences found in three-dimensional length, antero-posterior, and absolute latero-medial condylar IP-RCP slides between flattening, erosion and osteophyte groups. These results suggest that large three-dimensional, not only incisal but also condylar, IP-RCP slides might be related to the uni-/bilaterality and kind of TMJ pathosis, which might make such slides useful as clinical indices of TMJ OA.