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Biomedical subjects

R G Alexander

Publications and source records attributed to R G Alexander.

At least 19 recordsLinked to original sources
PubMed2005

Long-term stability of combined rapid palatal expansion-lip bumper therapy followed by full fixed appliances.

INTRODUCTION: The purpose of this study was to evaluate the long-term postretention stability of rapid palatal expansion-lip bumper therapy followed by full fixed appliances. METHODS: The sample included 20 treated patients (11 women and 9 men) who were recalled to obtain postretention records. The subjects were out of retention for a minimum of 4 years and an average of 7.9 years. They had begun treatment in the late mixed dentition at a mean age of 11.1 with considerable incisor crowding but, on average, no tooth size-arch length discrepancies. Pretreatment, posttreatment (mean age, 13.6 years), and postretention (mean age, 24.3 years) models were digitized, and the computed measurements were compared with untreated reference data. RESULTS: The majority of treatment increases in maxillary and mandibular arch dimensions were statistically significant (P < .05) and greater than expected for untreated controls. Although many measurements decreased postretention, net gains were maintained for 21 of the 30 measurements evaluated. The notable exception was arch perimeter, which decreased to less than pretreatment values. Postretention incisor irregularity increased 0.5 +/- 1.2 mm in the maxillary arch and 1.1 +/- 1.5 mm in the mandibular arch. CONCLUSIONS: Based on the good long-term stability observed in this study, we concluded that use of rapid palatal expansion-lip bumper expansion therapy in the late mixed dentition followed by full fixed appliances is an effective form of treatment for patients with up to moderate tooth size-arch length discrepancies.

Child↗
PubMed2004

Challenges of assuring crew safety in space shuttle missions with international cargoes.

The top priority in America's manned space flight program is the assurance of crew and vehicle safety. This priority gained greater focus during and after the Space Shuttle return-to-flight mission (STS-26). One of the interesting challenges has been to assure crew safety and adequate protection of the Space Shuttle, as a national resource, from increasingly diverse cargoes and operations. The control of hazards associated with the deployment of complex payloads and cargoes has involved many international participants. These challenges are examined in some detail along with examples of how crew safety has evolved in the manned space program and how the international partners have addressed various scenarios involving control and mitigation of potential hazards to crew and vehicle safety.

Accidents, Aviation↗
PubMed1996

Long-term stability of Class II, Division 1, nonextraction cervical face-bow therapy: II. Cephalometric analysis.

The long-term stability of Class II, Division 1 nonextraction therapy, using cervical face-bows with full fixed orthodontic appliances was evaluated for 42 randomly selected patients. Part 1, a study model analysis, was published in the March 1996 issue of the JOURNAL. Each patient was treated by the same practitioner, with the same techniques, and the treatment goals had been attained for all patients. Pretreatment records were taken at a mean age of 11.5 years; the posttreatment and postretention records were taken 3.0 and 11.6 years later, respectively. The results showed that the ANB angle decreased 2 degrees during treatment, most of which was due to the 1.6 degree decrease of the SNA angle. The mandibular plane angle was not changed significantly during treatment. Although upper incisor inclination was maintained during treatment, the lower incisor was proclined 2.3 degrees and the lower molar was tipped back 4 degrees. Of the 22 cephalometric measures evaluated, only four indicated relapse related with the treatment change. Three of the four measures pertain to lower incisor retroclination subsequent to excessive proclination. The ratio of treatment proclination of incisors to posttreatment retroclination is approximately 5:1. Similarly, for every 3 degrees of molar tip back, there was approximately 1 degree of relapse. It is concluded that nonextraction therapy for Class II malocclusion can be largely stable when the orthodontist ensures proper patient selection and compliance and attains treatment objectives.

Adolescent↗
PubMed1996

Long-term stability of Class II, Division 1, nonextraction cervical face-bow therapy: I. Model analysis.

The long-term stability of Class II, Division 1 nonextraction therapy remains poorly described. This study evaluates the face-bow therapy, in conjunction with full-fixed appliance therapy, of 42 patients (34 females and 8 males) who were treated by the same practitioner. Treatment goals had been attained for all patients. The pretreatment, posttreatment, and postretention records were taken at 11.5, 14.5, and 23.1 years, respectively. The results showed that mandibular and maxillary arch widths were increased significantly during treatment. Mandibular intercanine width decreased 0.3 mm during the postretention period; the remaining width measures increased or remained stable. Arch length, which did not change during treatment, decreased 1.0 mm after treatment. Overjet and overbite decreased 4.4 mm and 2.5 mm, respectively, during treatment. Both overjet (0.5 mm) and overbite (0.4 mm) showed small increases after retention. Mandibular incisor irregularity was decreased 2.7 mm during treatment and increased only 0.4 mm after treatment. Within the limits of this study, it is concluded that, when the described techniques are used, nonextraction therapy for patients with Class II malocclusion is largely stable.

Adolescent↗
PubMed1996

Three-dimensional diagnosis and management of Class II malocclusion in the mixed dentition.

Class II malocclusion is a commonly observed problem, occurring in about one third of the United States population. The numerous treatment approaches that have been advocated to treat this malocclusion presumably produce differing treatment effects within the skeletal, dentoalveolar, and soft tissue components of the face. In the first section of this article, the three-dimensional components of Class II malocclusion are described, with transverse maxillary discrepancy, mandibular skeletal retrusion, and increased lower anterior facial height observed as common findings in a mixed dentition sample of Class II subjects. Second, the literature concerning two seemingly diverse treatment methods (extraoral traction and functional jaw orthopedics) is reviewed in detail. Last, cephalometric data are presented from a retrospective clinical study and is used to evaluate the treatment effects produced by cervical traction and the FR-2 appliance of Fränkel in comparison with an untreated sample of mixed dentition Class II patients. The results of this study indicated that although both skeletal and dentoalveolar components of Class II, Division 1 malocclusion were altered in the Class I direction by either a facebow or a Fränkel appliance, these two appliance systems accomplished the correction in dramatically differing ways. Cervical traction affected the skeletal and dentoalveolar components of the maxilla and mandible, whereas the FR-2 appliance had less of an effect on maxillary and dentoalveolar components and a greater effect on mandibular length. Thus, these two treatment modalities produce decidedly different treatment effects in patients with Class II malocclusions.

Analysis of Variance↗
PubMed1994

Differences in dental arch morphology among adult females with untreated Class I and Class II malocclusion.

The purpose of the study was to evaluate differences in dental arch morphology among an orthodontic sample of 386 untreated adult females between 17 and 68 years of age, categorized by age group (17-25 years, 26-35 years, or 35+ years) and malocclusion (Class I, Class II division 1, or Class II division 2). The results show that both maxillary and mandibular dental arch size were significantly larger for the younger age group. Arch shape was relatively shorter and wider for the oldest age group. Palatal height was greatest for the youngest age group and least for the oldest group. Subjects with Class II malocclusion had significantly smaller arches, greater maxillary incisor irregularity, and less mandibular incisor irregularity than patients with Class I malocclusion. Subjects with Class II division 1 malocclusion had greater palatal heights and relatively longer/narrower maxillary dental arches than subjects with Class II division 2 malocclusion.

Adolescent↗
PubMed1993

Physiologic drift of the mandibular dentition following first premolar extractions.

A retrospective study of two groups of patients was conducted to evaluate the physiologic drift, or "driftodontics", of the mandibular teeth following the extraction of four first premolars. Group 1 included 32 patients who underwent early extraction in the mixed dentition stage at a mean age of 10.4 years; they were followed for approximately 2.5 years postextraction. Group 2 included 20 patients whose premolars were extracted after the permanent dentition had fully erupted. Their mean age at the time of extraction was 14.2 years, followed by a 0.8 year observation period. Except for the extractions, no other mandibular therapy was rendered. The results show no differences in rates of molar movements between groups. The molar apex moved mesially approximately 0.6 mm/yr; the molar cusp moved mesially approximately 1.2 mm/yr. In contrast, there were marked group differences in movements of the mandibular incisors and canines; rates of change were significantly greater in Group 2 than in Group 1. The canines drifted laterally and distally into the extraction sites while the incisors became more upright over basal bone and less crowded. Incisor irregularity decreased 1.3 mm/yr in Group 1 and 5.5 mm/yr in Group 2. The group differences in amounts of tooth movement were accounted for by changes in incisor crowding.

Adolescent↗
PubMed1991

Lip bumper therapy for gaining arch length.

With the use of pretreatment and posttreatment lateral cephalograms and study models, lip bumper therapy for two groups of 20 patients was evaluated. One group was treated with lip bumpers fabricated from stainless steel round wire covered with shrink tubing and activated every 2 to 3 months. The second group was treated with larger prefabricated lip bumpers covered with acrylic shields from canine to canine and activated every 4 to 5 weeks. Yearly rates of treatment change indicate that the type of lip bumper used and the method of clinical manipulation have no effect on mandibular incisor position. Both groups showed similar rates of controlled incisal tipping with the center of rotation at the apex. Dental movements of the posterior segment were significantly different between groups. The second group displayed significantly more molar tipping than the first group. The second group also showed significantly greater transverse expansion of the canines, first premolars, and first molars.

Acrylic Resins↗
PubMed1990

Reversible accumulation of plant suspension cell cultures in g(1) phase and subsequent synchronous traverse of the cell cycle.

The induction of DNA synthesis in Datura innoxia Mill. cell cultures was determined by flow cytometry. A large fraction of the total population of cells traversed the cell cycle in synchrony when exposed to fresh medium. One hour after transfer to fresh medium, 37% of the cells were found in the process of DNA synthesis. After 24 hours of culture, 66% of the cells had accumulated in G(2) phase, and underwent cell division simultaneously. Only 10% of the cells remained in G(0) or G(1). Transfer of cells into a medium, 80% (v/v) of which was conditioned by a sister culture for 2 days, was adequate to inhibit this simultaneous traverse of the cell cycle. A large proportion of dividing cells could be arrested at the G(0) + G(1)/S boundary by exposure to 10 millimolar hydroxyurea (HU) for 12 to 24 hours. Inhibition of DNA synthesis by HU was reversible, and when resuspended into fresh culture medium synchronized cells resumed the cell cycle. Consequently, a large fraction of the cell population could be obtained in the G(2) phase. However, reversal of G(1) arrested cells was not complete and a fraction of cells did not initiate DNA synthesis. Seventy-four percent of the cells simultaneously reached 4C DNA content whereas the frequency of cells which remained in G(0) + G(1) phase was approximately 17%. Incorporation of radioactive precursors into DNA and proteins identified a population of nondividing cells which represents the fraction of cells in G(0). The frequency of cells entering G(0) was 11% at each generation. Our results indicate that almost 100% of the population of dividing cells synchronously traversed the cell cycle following suspension in fresh medium.

Journal Article↗
PubMed1987

A computerized analysis of the shape and stability of mandibular arch form.

To determine whether a particular ideal orthodontic arch form could be identified, the mandibular dental casts of 30 untreated normal cases, 30 Class I nonextraction cases, and 30 Class II nonextraction cases were examined. Following computerized digitizing and the use of a mathematic function called polynomial of the fourth degree, arch forms were generated for each sample and then compared to 17 commercially produced arch forms. Results showed that no particular arch form predominated in any of the three samples. A shape representing a combination of the "Par" and "Vari-Simplex" arch forms approximated to only 50% of the cases in the three samples. The remaining 50% of the cases displayed a wide variety of arch forms. Cases that had changes in arch form during nonextraction treatment frequently were not stable; almost 70% showed significant long-term posttreatment changes. Customizing arch forms appears to be necessary in many cases to obtain optimum long-term stability because of the great individual variability in arch form found in this study.

Adolescent↗
PubMed1987

Nonextraction orthodontic therapy: posttreatment dental and skeletal stability.

To assess the long-term stability of nonextraction orthodontic treatment, the dental cast and cephalometric records of 28 cases were evaluated. Thirty cephalometric and seven cast parameters were examined before treatment, posttreatment, and an average of almost 8 years postretention. Results showed overall long-term stability to be relatively good. Relapse patterns seen were similar in nature, but intermediate in extent, between untreated normals and four first premolar extraction cases. Significant decreases were seen in arch length and intercanine width during the postretention period despite minimal changes during treatment. Incisor irregularly increased slightly postretention; intermolar width, overjet, and overbite displayed considerable long-term stability. Mandibular incisor mesiodistal and faciolingual dimensions were not associated with either pretreatment or posttreatment incisor crowding. Class II malocclusions with large ANB values and shorter mandibular lengths showed increased incisor irregularity, shorter arch lengths, and deeper overbites at the postretention stage, suggesting that the amount and direction of facial growth may have been partially responsible for maturational changes seen during the postretention period.

Adolescent↗
PubMed1986

Differential diagnosis and treatment planning for the adult nonsurgical orthodontic patient.

Increasing numbers of adult patients are seeking orthodontic care and some, despite significant skeletal malocclusions, elect not to have combined orthodontic-surgical treatment. The purpose of this article is to outline some of the diagnostic and therapeutic principles that can be used in the adult nonsurgical orthodontic patient. The importance of realistic goal setting in the face of compromised occlusions is emphasized. Diagnosis should include evaluation of all three dimensions and recognize the limitations of therapy in each dimension for the nongrowing patient. Periodontal considerations, extraction decisions, and retention regimens are of vital importance to the achievement and maintenance of an optimum result. Clinical records will demonstrate four commonly seen problems and their resolution.

Adolescent↗
PubMed1985

Droplet sorting of large particles.

The systematics of droplet formation conditions for orifices with diameters up to 200 micron are described. Sorting recovery experiments indicate that particles up to 44 micron in diameter can be recovered by charged droplet deflection of two drops with at least 75% recovery. By reducing the jet velocity, a deflection of greater than 1 cm was obtained for all droplet sizes.

Cell Separation↗