Michael L. Freedman

Dosimetry and cost of imaging osseointegrated implants with film-based and computed tomography

Thermoluminescent dosimeters were used to measure radiation doses at craniofacial sites in a tissue-equivalent phantom during film-based multidirectional tomography with the Tomax Ultrascan (Incubation Industries, Ivyland, Pa.) and during computed tomography with the Elscint Excel 2400 (Elscint Corp., Tel Aviv, Israel). Mean absorbed doses for presurgical mandibular and maxillary canine and molar implant assessments were converted to equivalent doses, which were then multiplied by published weighting factors and summed to give effective doses. The computed tomography device consistently delivered higher doses than the Tomax Ultrascan to all anatomic locations; the differences were most pronounced when only one or two implant sites were evaluated. The reasons for the dose disparities are considered both anatomically and procedurally. A survey of examination cost revealed film-based multidirectional tomography to be less expensive than computed tomography.

Dependence of the Mutation Spectrum in a Shuttle Plasmid Replicated in Human Lymphoblasts on Dose of Gamma Radiation

The frequencies and types of mutations induced in the target gene, supF-tRNA, of the shuttle vector pZ189 were analysed following the replication of the gamma-irradiated plasmid in the human lymphoblastoid cell line, GM606. The mutation frequency measured in progeny of unirradiated pZ189 was 1.02 x 10(-4), increasing to 17.5 x 10(-4) at 1000 cGy, and to 63.4 x 10(-4) at 5000 cGy, approximately 17- and 62-fold over background levels, respectively. Simultaneously, the number of plasmids capable of replicating in Escherichia coli decreased with increasing radiation dose to 4% of the control value at 5000 cGy. Electrophoresis of the irradiated DNA showed a correlation between increases in mutation frequency and decreases in plasmid survival, and the formation of open-circular and linear DNA. The majority of the spontaneous (69.8%) and induced mutations (85.7%) at 1000 and 79.4% at 5000 cGy) were base substitutions and were generally of similar types among all groups. However, changes at 2500 (12.7%) and 5000 cGy (13.2%) involving A:T base pairs were greater than those in unirradiated controls (3.4%) or those at 1000 cGy (2.0%). This increase in A:T base pair mutations could be a result of reduced repair fidelity when the DNA is extensively damaged by high doses of ionizing radiation.

Idiopathic isolated unilateral hypoglossal nerve palsy: a diagnosis of exclusion

This report describes the rare case of a 22-year-old male with persistent idiopathic isolated hypoglossal nerve palsy. Thorough history, examination, and investigation were needed to rule out the many causes of such a presentation. The clinical presentation showed unilateral atrophy and fibrillation of the affected side and mild deviation on protrusion to the affected side. The differential diagnosis included neoplasia, trauma, infection, endocrine, autoimmune, neurologic, and vascular causes. Investigations included magnetic resonance imaging, computerized tomography scan, chest x-ray, cerebrospinal fluid culture, and a range of hematologic tests. These led to a diagnosis of persistent idiopathic isolated hypoglossal nerve palsy.

Effect of alveolar bone support on zygomatic implants: a finite element analysis study

The objective of this study was to investigate the influence of maxillary alveolar bone on the stress distribution of zygomatic implants. A three-dimensional finite element model was created of half of a skull. Two zygomatic implants were modelled, placed in the skull supported by the zygomatic bone and the maxillary alveolar bone and connected by a fixed bridge. This model was duplicated, and the area of the maxillary alveolar bone supporting the implants was removed. Occlusal and lateral forces were applied to both models and the maximum von Mises stresses were recorded. Higher maximum stresses were noted in the model with no alveolar support. Occlusal stresses were higher than lateral stresses in the model with no alveolar support. Low stresses were noted in the zygomatic bone in both models. In conclusion, maxillary alveolar bone support is beneficial in the distribution of forces for zygomatic implants.

Genetic Alterations of Streptococcus Mutans’ Virulence

The use of mutants defective in caries-associated traits has enabled the genetic dissociation of agglutination from adhesion, the demonstration of serotype-specific contributions of IPS to virulence, the importance of glucanohydrolase to virulence to a greater degree than to plaque formation, and the apparent lack of importance of agglutination to virulence. We have also been able to demonstrate the ability of plaque formation-defective mutants and other variants both to infect and to emerge, yet not to cause disease. Additional mutants, currently under study in our laboratory include fructanase, invertase, and sucrose permease-defectives. Ultimately, the identification of key, probably surface-associated virulence factors will offer more potent and specific antigens for directed immune responses by the host.

Mutation spectrum in γ-irradiated shuttle vector replicated in ataxia-telangiectasia lymphoblasts

Cells from ataxia-telangiectasia (AT) patients are hypersensitive to the lethal effects of ionizing radiation. To assess radiation mutagenesis in these cells, the SV40-based shuttle vector, pZ189, was used to analyze gamma-ray-induced mutations following the plasmids replication in AT lymphoblasts. Progenies from the AT line GM2783 exposed to 50 Gy showed a mutation frequency of 7.6 x 10(-3), 63-fold over background; surviving plasmids were 3.4% of control. Both values were essentially the same as those of irradiated plasmids replicated in a normal lymphoblast line, GM606. In addition, pZ189 exposed to 25 Gy of gamma radiation and replicated in another normal lymphoblast line and in cells of two additional AT lymphoblast lines showed similar mutation frequencies and percentages of surviving plasmids. Qualitative comparison of plasmid mutations from AT and normal cells showed no significant differences, indicating that the damaged DNA was repaired with similar fidelity in AT and normal cells. These studies suggest that there is no correlation between the enhanced sensitivity of AT cells to killing by ionizing radiation and gamma-radiation-induced mutagenesis of plasmid DNA processed in these cells.

Mutations Caused by γ-radiation-induced Double-strand Breaks in a Shuttle Plasmid Replicated in Human Lymphoblasts

The mutagenicity of open-circular DNA (containing base damage and single-strand breaks) and linear DNA (containing base damage, single-strand breaks, and one double-strand break) produced in vitro by gamma-irradiation of shuttle vector pZ189, was analysed after the plasmids repair and replication in the human lymphoblast line, GM606. By comparing the survival, mutation frequency, and types of mutations in descendants from the two DNA forms, the effects of the double-strand break were determined. The percentage of viable plasmids from linear DNA was two-fold lower than that from open-circular DNA, 7.8 versus 14.0 (compared with unirradiated, control DNA). The mutation frequency in progenies of the open-circular plasmid was 4.2 +/- 1.7 x 10(-3), compared with 7.8 +/- 0.1 x 10(-3) in progenies of the linear DNA, again, nearly a two-fold difference. Approximately 59% of the mutations from the linear DNA were deletions and 34% were base substitutions. In contrast, only 13% of mutations from open-circular DNA were deletions, but 87% were base substitutions. All recoverable deletions were small, ranging from 1 to 205 base pairs, and the majority contained direct repeats at the deletion junctions, indicating non-homologous recombinations. Thus, mutations found among descendants from the linear and open-circular DNAs were qualitatively similar but quantitatively different. The data suggests that producing one double-strand break in DNA by ionizing radiation causes a two-fold increase in both lethality and mutation frequency.