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Titanium crowns
Titanium crowns Background.—Titanium has attracted interest as a restorative material for implants that are used to support fixed and removable dental prostheses. It offers biocompatibility, low thermal conductivity, good corrosion resistance, low density, and excellent mechanical properties. The low density and high strength of titanium should allow the design of more functional and comfortable prosthodontic restorations. However, conventional casting methods are challenging because of titanium’s high melting point and extreme chemical reactivity at elevated temperatures. This high temperature produces a reactive layer on the surface of the casting, termed an alpha-case layer. Special casting procedures have been developed, including the use of an inert gas to create a nonreactive casting environment. Crowns can also be machine-milled from prefabricated titanium blocks, using computer-aided design/computer-assisted manufacturing (CAD/CAM) systems so they do not form a reactive surface layer. A crown’s clinical success depends on the size of the marginal gap. A mean marginal gap of 100 mm is considered clinically acceptable. The marginal and internal fit of complete titanium crowns was evaluated with respect to method of fabrication (casting or CAD/CAM) and marginal configuration.
Results.—The cast group’s mean axial internal gap was 67.5 mm, and the CAD/CAM group’s value was 51.0 mm. Mean occlusal internal gap was 109.8 mm in the cast group and 124.6 mm in the CAD/CAM group. The CAD/CAM group had significantly smaller gaps at the axial wall, but the cast group had significantly smaller gaps in the occlusal area.
Methods.—The maxillary first molar was prepared in acrylic resin to have a shoulder (buccal), chamfer (palatal), and a knife edge (proximal) marginal configuration (Fig 1). Forty crowns were made and divided into 20 samples manufactured by casting and 20 samples manufactured by CAD/CAM system. Each crown was luted to the original stone die using zinc phosphate cement. A three-dimensional measuring microscope was used to evaluate the crown’s margin, the axial wall’s center point, and the occlusal area. Results were analyzed statistically.
Discussion.—The fabrication method significantly altered the marginal and internal gap measurements. Marginal discrepancies also occurred depending on the marginal configuration. The amount of internal and marginal discrepancy, however, was within the clinically acceptable range of 100 mm.
The shoulder, chamfer, and knife-edge marginal configurations had mean marginal discrepancies of 55.2, 52.2, and 76.1 mm, respectively, for the cast group. The corresponding values for the CAD/CAM group were 67.0, 59.8, and 80.7 mm, respectively. Both fabrication method and marginal configuration had significant influences on the marginal gap, but these two variables did not interact. Mean marginal gap for the casting group was significantly smaller than that for the CAD/CAM group. There was also a significant difference between the shoulder and knife edge margin and between the chamfer and knife-edge margin, but no significant difference between the shoulder and chamfer margin. Scanning electron microscopy showed better marginal fit for the cast group samples than for the CAD/CAM group samples regardless of marginal design.
Clinical Significance.—Cast titanium crowns had significantly smaller occlusal area marginal
Fig 1.—Marginal configuration of definitive die. Definitive die had shoulder, chamfer, and knife edge margin. (Courtesy of Han H-S, Yang H-S, Lim H-P, et al: Marginal accuracy and internal fit of machine-milled and cast titanium crowns. J Prosthet Dent 106:191-197, 2011.)
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Dental Abstracts
gaps than CAD/CAM titanium crowns. The chamfer and shoulder margins had smaller marginal gaps than the knife-edge margin regardless of fabrication method. Both methods, however, produced gaps within the clinically acceptable range.
Han H-S, Yang H-S, Lim H-P, et al: Marginal accuracy and internal fit of machine-milled and cast titanium crowns. J Prosthet Dent 106:191-197, 2011 Reprints available from H-S Yang, Dept of Prosthodontics, School of Dentistry, Chonnam Natl Univ, Yongbong-ro 77, Buk-gu, Gwangju 500-757, South Korea; fax: þ82-62-530-0130; e-mail: yhsdent@ jnu.ac.kr
EXTRACTS REMEMBER THAT? The brain links new facts experienced at the same time; therefore, recalling one thing can lead to a kaleidoscope of thoughts and emotions that surrounded that information. Scientists have now recorded traces in the brain of this contextual memory from the brains of persons waiting to undergo surgery for epilepsy. The recordings suggest that new memories of even abstract facts are encoded in a brain-cell firing sequence that includes what was happening around the same time, whether it was a daydream or an emotional response. Published in the journal Proceedings of the National Academy of Sciences (PNAS), the study was done by doctors from the University of Pennsylvania and Vanderbilt University. Recordings were made from electrodes implanted in the brains of 69 people with severe epilepsy, which is how doctors locate where the floods of brain activity that cause epileptic seizures are. Patients watched nouns appear on a computer screen, were briefly distracted, and then were asked to recall as many words as possible. Participants tended to remember words in clusters, called the congruity effect. It resembles what happens in the card game Concentration, where players try to identify pairs in a grid of face-down cards. Pairs turned over close in time are often remembered together. A strong neural firing pattern was seen in the temporal lobe of the brain when a word was recalled and appeared identical to the pattern seen when the word was originally seen on the computer screen. The pattern for words occurring just before or after the word recalled was similar. Michael J Kahana, a neuroscientist at the University of Pennsylvania and an author of the study, said ‘‘Here we have shown, in effect, that the word before ‘cat’—let’s say it’s ‘tree’—has colored or influenced the encoding for ‘cat,’ just as ‘cat’ has influenced the encoding of the next word, let’s say ’flower.’’ The process works something like reconstructing a night’s activities after a hangover, with recall of one fact leading to a scene and more facts, such as the people who were there. Those in the study who had the strongest neural updating signals also had the most striking pattern of remembering words in clusters. As Dr Kahana said, ‘‘When you activate one memory, you are reactivating a little bit of what was happening around the time the memory was formed, and this process is what gives you that feeling of time travel.’’ [B Carey: The Then and Now of Memory. New York Times, July 4, 2011]
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2012
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Results.—The cast group’s mean axial internal gap was 67.5 mm, and the CAD/CAM group’s value was 51.0 mm. Mean occlusal internal gap was 109.8 mm in the cast group and 124.6 mm in the CAD/CAM group. The CAD/CAM group had significantly smaller gaps at the axial wall, but the cast group had significantly smaller gaps in the occlusal area.
Methods.—The maxillary first molar was prepared in acrylic resin to have a shoulder (buccal), chamfer (palatal), and a knife edge (proximal) marginal configuration (Fig 1). Forty crowns were made and divided into 20 samples manufactured by casting and 20 samples manufactured by CAD/CAM system. Each crown was luted to the original stone die using zinc phosphate cement. A three-dimensional measuring microscope was used to evaluate the crown’s margin, the axial wall’s center point, and the occlusal area. Results were analyzed statistically.
Discussion.—The fabrication method significantly altered the marginal and internal gap measurements. Marginal discrepancies also occurred depending on the marginal configuration. The amount of internal and marginal discrepancy, however, was within the clinically acceptable range of 100 mm.
The shoulder, chamfer, and knife-edge marginal configurations had mean marginal discrepancies of 55.2, 52.2, and 76.1 mm, respectively, for the cast group. The corresponding values for the CAD/CAM group were 67.0, 59.8, and 80.7 mm, respectively. Both fabrication method and marginal configuration had significant influences on the marginal gap, but these two variables did not interact. Mean marginal gap for the casting group was significantly smaller than that for the CAD/CAM group. There was also a significant difference between the shoulder and knife edge margin and between the chamfer and knife-edge margin, but no significant difference between the shoulder and chamfer margin. Scanning electron microscopy showed better marginal fit for the cast group samples than for the CAD/CAM group samples regardless of marginal design.
Clinical Significance.—Cast titanium crowns had significantly smaller occlusal area marginal
Fig 1.—Marginal configuration of definitive die. Definitive die had shoulder, chamfer, and knife edge margin. (Courtesy of Han H-S, Yang H-S, Lim H-P, et al: Marginal accuracy and internal fit of machine-milled and cast titanium crowns. J Prosthet Dent 106:191-197, 2011.)
264
Dental Abstracts
gaps than CAD/CAM titanium crowns. The chamfer and shoulder margins had smaller marginal gaps than the knife-edge margin regardless of fabrication method. Both methods, however, produced gaps within the clinically acceptable range.
Han H-S, Yang H-S, Lim H-P, et al: Marginal accuracy and internal fit of machine-milled and cast titanium crowns. J Prosthet Dent 106:191-197, 2011 Reprints available from H-S Yang, Dept of Prosthodontics, School of Dentistry, Chonnam Natl Univ, Yongbong-ro 77, Buk-gu, Gwangju 500-757, South Korea; fax: þ82-62-530-0130; e-mail: yhsdent@ jnu.ac.kr
EXTRACTS REMEMBER THAT? The brain links new facts experienced at the same time; therefore, recalling one thing can lead to a kaleidoscope of thoughts and emotions that surrounded that information. Scientists have now recorded traces in the brain of this contextual memory from the brains of persons waiting to undergo surgery for epilepsy. The recordings suggest that new memories of even abstract facts are encoded in a brain-cell firing sequence that includes what was happening around the same time, whether it was a daydream or an emotional response. Published in the journal Proceedings of the National Academy of Sciences (PNAS), the study was done by doctors from the University of Pennsylvania and Vanderbilt University. Recordings were made from electrodes implanted in the brains of 69 people with severe epilepsy, which is how doctors locate where the floods of brain activity that cause epileptic seizures are. Patients watched nouns appear on a computer screen, were briefly distracted, and then were asked to recall as many words as possible. Participants tended to remember words in clusters, called the congruity effect. It resembles what happens in the card game Concentration, where players try to identify pairs in a grid of face-down cards. Pairs turned over close in time are often remembered together. A strong neural firing pattern was seen in the temporal lobe of the brain when a word was recalled and appeared identical to the pattern seen when the word was originally seen on the computer screen. The pattern for words occurring just before or after the word recalled was similar. Michael J Kahana, a neuroscientist at the University of Pennsylvania and an author of the study, said ‘‘Here we have shown, in effect, that the word before ‘cat’—let’s say it’s ‘tree’—has colored or influenced the encoding for ‘cat,’ just as ‘cat’ has influenced the encoding of the next word, let’s say ’flower.’’ The process works something like reconstructing a night’s activities after a hangover, with recall of one fact leading to a scene and more facts, such as the people who were there. Those in the study who had the strongest neural updating signals also had the most striking pattern of remembering words in clusters. As Dr Kahana said, ‘‘When you activate one memory, you are reactivating a little bit of what was happening around the time the memory was formed, and this process is what gives you that feeling of time travel.’’ [B Carey: The Then and Now of Memory. New York Times, July 4, 2011]
Volume 57
Issue 5
2012
265