Crown Replacement Research Articles

On the brittleness of enamel and selected dental materials

Although brittle material behavior is often considered undesirable, a quantitative measure of “brittleness” is currently not used in assessing the clinical merits of dental materials. Objective To quantify and compare the brittleness of human enamel and common dental restorative materials used for crown replacement. Methods Specimens of human enamel were prepared from the third molars of “young” (18 ≤ age ≤ 25) and “old” (50 ≤ age) patients. The hardness, elastic modulus and apparent fracture toughness were characterized as a function of distance from the DEJ using indentation approaches. These properties were then used in estimating the brittleness according to a model that accounts for the competing dissipative processes of deformation and fracture. The brittleness of selected porcelain, ceramic and micaceous glass ceramic (MGC) dental materials was estimated and compared with that of the enamel. Results The average brittleness of the young and old enamel increased with distance from the DEJ. For the old enamel the average brittleness increased from approximately 300 μm −1 at the DEJ to nearly 900 μm −1 at the occlusal surface. While there was no significant difference between the two age groups at the DEJ, the brittleness of the old enamel was significantly greater (and up to four times higher) than that of the young enamel near the occlusal surface. The brittleness numbers for the restorative materials were up to 90% lower than that of young occlusal enamel. Significance The brittleness index could serve as a useful scale in the design of materials used for crown replacement, as well as a quantitative tool for characterizing degradation in the mechanical behavior of enamel.

Post-fire epicormic branching in Sierra Nevada Abies concolor (white fir)

In California’s mixed-conifer forest, which historically had a regime of frequent fires, two conifers, Sequoiadendron giganteum and Pseudotsuga menziesii, were previously known to produce epicormic sprouts from branches. We found epicormic branching in a third mixed-conifer species, Abies concolor, 3 and 4 years after a wildfire in the central Sierra Nevada Mountains of California. Sprouting occurred only from the boles. We investigated (1) whether the degree of crown loss and the extent of epicormic branching were independent; and (2) whether epicormic branching differed by tree size. The vertical extent of epicormic foliage increased with increasing severity of crown loss. There was a significantly greater proportion of large diameter-class (>50 cm diameter at breast height [dbh]) trees with epicormic branching than small/medium diameter-class (25–50 cm dbh) trees. These results suggest large diameter Abies concolor may survive high levels of crown loss, aided by crown replacement through epicormic branching, but that reiterative green foliage may not appear for up to 3 years after fire damage. If this response is widespread, it would suggest some ‘dying’ trees logged under current salvage guidelines could survive, and that higher-intensity fire may substantially reduce the density of small post-fire suppression white fir, while retaining many larger overstory trees.

Reasons for the placement and replacement of crowns in general dental practice.

The purpose of the study was to apply established methods to survey reasons for the placement and replacement of crowns in general dental practice in the United Kingdom. One hundred and twenty-eight general dental practitioners were recruited. Participants recorded the principal reason for the provision of each initial and replacement crown they provided over a 12-week period. Overall, data were collected from 92 practitioners in respect of 1714 patients and 2164 crowns, of which 1452 (67%) were initial placements and 712 (33%) replacements. The teeth most frequently crowned were maxillary incisors (33%), with 72% of the crowns surveyed being of the porcelain bonded to metal variety. Overall 64% of the initial placement crowns were provided because of restoration failure (26%) or tooth fracture (38%). The most common reason for crown replacement was crown failure (27%). It is concluded that surveys of the type reported may provide new insights into the reasons for and pattern of provision of initial placement and replacement crowns in clinical practice. In this study the most common reason for the provision of initial placement crowns was tooth fracture. The most common reason for the replacement of crowns, notably porcelain jacket crowns, was crown fracture.

Radial growth of hardwoods following the 1998 ice storm in New Hampshire and Maine

Ice storms and resulting injury to tree crowns occur frequently in North America. Reaction of land managers to injury caused by the regional ice storm of January 1998 had the potential to accelerate the harvesting of northern hardwoods due to concern about the future loss of wood production by injured trees. To assess the effect of this storm on radial stem growth, increment cores were collected from northern hardwood trees categorized by crown injury classes. For a total of 347 surviving canopy dominant and subdominant trees, a radial growth index was calculated (mean annual increment for 1998–2000 divided by the mean annual increment for 1995–1997). Sugar maple (Acer saccharum Marsh.), yellow birch (Betula alleghaniensis Britt.), white ash (Fraxinus americana L.), and red maple (Acer rubrum L.) categorized in injury class A (crown loss of less than one-half) had mean growth index values of approximately 1.0, indicating no loss of mean radial growth after 3 years. For injury class B (crown loss of one-half to three-quarters) and class C (crown loss greater than three-quarters), growth index values significantly decreased for sugar maple, yellow birch, and red maple. For white ash, growth index values of classes B and C were not significantly different from those of class A trees. Growth index values of A. saccharum and A. rubrum in injury class C were the lowest of those measured. These results indicated that the severity of growth loss due to crown injury depends on tree species and crown replacement as well as the extent of crown loss.

Finite element analysis of anterior tooth root stresses developed during endodontic treatment.

Vertical tooth root fractures are diagnostically challenging, frustrating, and an increasingly common cause of failure of tooth restoration. These vertical root fractures have been associated with many causes, including the endodontic process itself. To investigate these endodontic causes, various phases of crown replacement for an anterior tooth were modeled using nonlinear, plane strain finite element (FE) analysis. Stresses developed during obturation, post positioning, crown placement, and masticatory and occlusal loading of the restored tooth were estimated using this analysis method. The minimum (compressive) principal stress was greatest during obturation of cones 1 and 2, and during mastication of the restored tooth. Although these stresses were significant (-150 to -280 MPa), they did not exceed the compressive strength of dentin. The maximum (tensile) principal stresses, 160 to 300 MPa, were also observed during obturation of cones 1 and 2. These peak stresses exceed the dentin tensile strength.

Practical considerations and design of porcelain fused to metal

Certain design principles and precautions must be followed when porcelain fused to metal is used in restorative dentistry. These precautions make this restoration no more difficult to use than other combinations of materials. The design allows permanent cementation of fixed partial dentures, and if breakage occurs, it involves only single units of porcelain. Each pontic and coping is prepared to receive a porcelain jacket crown replacement. Porcelain fused to metal best answers most restorative problems requiring strength and esthetic appearance.