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Biomedical subjects

L E Johnston

Publications and source records attributed to L E Johnston.

At least 19 recordsLinked to original sources
PubMed1995

A comparison of one-stage and two-stage nonextraction alternatives in matched Class II samples.

In the treatment of Class II malocclusion, an early phase of functional appliance treatment is commonly used to simplify subsequent fixed appliance therapy and to optimize the development of the facial skeleton. Unfortunately, these expectations enjoy little support in the refereed literature. The present study therefore was undertaken to examine the benefits of a two-stage bionator/edgewise regimen in comparison to the more conventional one-stage edgewise alternative. To minimize proficiency bias, we examined the records of the one- and two-stage Class II nonextraction patients who received treatment between 1980 and 1990 by a single experienced clinician. On the basis of descriptive data from 96 sets of initial study models and lateral cephalograms, discriminant analysis was used to identify two subsamples of 36 patients who were relatively similar before treatment and thus equally susceptible to the two treatments. Of these 72 "borderline" patients, 19 subsequently underwent a change in treatment plan (to extraction or surgery), leaving 25 two-stage and 28 single-stage nonextraction patients on whom to base a comparison of treatment effects. Except for a slight posttreatment difference in age (and, hence, size), the two groups underwent skeletal changes that left them essentially indistinguishable at the end of treatment. In both groups, these skeletal changes were largely responsible for molar and overjet corrections that were nearly identical in the two groups. The rates of change, however, differed significantly. As a result, the two-stage treatments started earlier and finished later. Although the present data do not address the relative impact of the two strategies on the 10% to 15% at each tail of the distribution, the early phase of functional appliance treatment conferred no obvious, measurable benefits on the central 75%. Therefore for most nonextraction Class II patients, the choice of treatments may well constitute a practice management, rather than a biologic, decision.

Activator Appliances↗
PubMed1992

A long-term comparison of nonextraction and premolar extraction edgewise therapy in "borderline" Class II patients.

The long-term effects of extraction and nonextraction edgewise treatments were compared in 63 patients with Class II, Division 1 malocclusions who were identified by discriminant analysis as being equally susceptible to the two strategies. A lateral cephalogram, study models, and a self-evaluation of the esthetic impact of treatment were obtained from each of the 33 extraction and 30 nonextraction subjects. The average posttreatment interval was 14.5 years. Although the two strategies produced significant, long-lived differences in the convexity of the profile and the protrusion of the dentition (the nonextraction patients were about 2 mm "fuller"), half of the nonextraction patients and three fourths of the extraction patients ultimately presented with less than 3.5 mm of lower incisor irregularity. The two groups showed an essentially identical pattern of posttreatment relapse/settling that was related more to the differential growth of the jaws than to the posttreatment position and orientation of the denture. Because in the end the various tooth movements tended to cancel one another, excess mandibular growth was also the most important net contributor to the molar and overjet corrections. In the process, both groups showed a marked forward displacement of the mandible, both at the chin and at the condyle. Finally, although it is probable that most of the present sample would today be treated by expansion, the 30 patients who actually received this presumably correct treatment rated their appearance no more highly than did the extraction subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1992

The effect of maxillary first premolar extraction and incisor retraction on mandibular position: testing the central dogma of "functional orthodontics".

It has been argued by a vocal coterie of disaffected dentists that premolar extraction, incisor retraction, and "backward-pulling" mechanics conspire to "distalize" the condyles and, pari passu, to produce craniomandibular dysfunction. Given the gravity of this conjecture, it seemed appropriate to test the predictions it generates in a sample of patients of the type most often said to be at risk: 42 "edgewise" patients with Class II, Division 1 malocclusions, treated in conjunction with the extraction of two maxillary first premolars. Regional and anterior cranial-base cephalometric superimpositions were used to quantify the individual components of the molar and overjet corrections, to measure both at the chin and condyles the mandibular displacement seen during treatment, and to examine the extent to which this displacement is related to the correction of maxillary incisor protrusion. Although the present patients underwent marked upper incisor retraction (on average, about 5 mm), lip retraction was much less pronounced, and 70% of the sample showed a net forward displacement of mandibular basal bone. Significantly, changes in condylar position were not correlated with incisor retraction, as the "functional orthodontists" would have it, but rather with the changes in the buccal occlusion and the growth of the maxilla. Thus, 30% of the patients who showed evidence of distal displacement were generally nongrowing patients who underwent more than average anchorage loss in the mandible and less than average loss in the maxilla.(ABSTRACT TRUNCATED AT 250 WORDS)

Activator Appliances↗
PubMed1989

The effect of cortisone on the eruption rate of root-resected incisors in the rat.

It is generally thought that the periodontal ligament plays a key--perhaps even exclusive--role in tooth eruption. The present study was designed to test the hypothesis that the fundus also may contribute to the process. To this end the effect of cortisone on the eruption rate of root-resected mandibular incisors was examined in male albino rats. Half of the animals received daily injections of cortisone; the other half were given normal saline. Impeded and unimpeded eruption rates were measured at 3-day intervals and tested for between- and among-means differences by repeated-measures analysis of variance (ANOVA). In contrast to the acceleration commonly seen in intact incisors, cortisone produced no change in the eruption rate of root-resected incisors. It was concluded therefore that the accelerative effect of cortisone is mediated by the fundus, rather than the periodontal ligament. Further, it is hypothesized that early eruption is controlled by cellular proliferation within the fundus; however, once the tooth enters the oral cavity and becomes functional, the periodontal ligament matures and becomes the dominant factor in subsequent eruption.

Alveolar Process↗
PubMed1988

Gnathologic assessment of centric slides in postretention orthodontic patients.

After 2 weeks of "deprogramming" with an occlusal splint, 92 subjects with a history of comprehensive orthodontic therapy had an average centric slide that was comparable not only to that of an untreated control group, but also to equivalent estimates reported in the literature. It is suggested that for modern man a CO-CR difference may be a normal, physiologically significant by-product of the nature and pattern of mandibular function and the tooth attachment mechanism. Moderate changes in the magnitude of a given patient's centric slide would therefore be expected to regress, not to the population mean, but rather to the CO-CR difference that was present before treatment.

Adult↗
PubMed1986

Surgical control of mandibular growth: test of a recent biomechanical hypothesis.

It has been suggested that, by changing the orientation (and hence the function) of the condyle/ascending ramus segment, surgical control of mandibular growth rotation is possible in growing children. To test one aspect of this hypothesis--the effect of changes in condylar orientation--one condyle in each of thirty-five young male rats was rotated surgically, either clockwise or counterclockwise, and then wired to the ramus to preserve the new orientation. Amalgam implants and lateral cephalograms were used to measure the subsequent pattern of mandibular growth. The findings revealed that surgical rotation of the condyle and the subsequent pattern of mandibular displacement are poorly correlated (r = 0.1-0.3). It was concluded, therefore, that changes in the pattern of mandibular rotation that may follow early mandibular advancement are the result of factors other than the orientation of the condyle following surgery.

Animals↗
PubMed1985

The control of condylar growth: an experimental evaluation of the role of the lateral pterygoid muscle.

The present study used 21 male albino rats to test the hypothesis that lateral pterygoid traction regulates the growth of the mandibular condyle. The condyles, the rami, and the top of each glenoid fossa were marked with metallic implants, and, following bilateral section of the condylar neck, one lateral pterygoid muscle was extirpated. On the basis of the literal details of Petrovic's cybernetic model, it was assumed that the continued forward growth of the midface and the backward translation of the glenoid fossa would combine to produce a progressive disturbance in the buccal occlusion that would, in turn, generate a reflex contraction of the remaining lateral pterygoid muscle. Initially, however, growth of the isolated condyles would have little impact on the spatial position of the rest of the mandible. As a result, the condyles on the side with the intact lateral pterygoid should grow for a time at a maximal, open-circuit rate, whereas the experimental condyle, deprived of all muscle traction, should show only a minimal "commanded" rate of growth. The serial change in the position of the condylar and ramal implants was assessed cephalometrically for 6 weeks, and between-sides differences were analyzed by randomized block analysis of variance. The presence or absence of the lateral pterygoid muscle had no significant effect on the anteroposterior position of the condylar implants and only a slight, transitory effect on their vertical position. The translation of the ramal implants, however, was greatly affected by the condylotomy. On both control and experimental sides, the mandible collapsed upward and backward until contact between the growing condyle and ramus had been achieved, whereupon a downward and forward pattern of translatory growth was re-established. Although it could not be shown that lateral pterygoid traction per se is a significant factor in the growth of isolated condyles, it was concluded that the condyle is vitally important to the translatory growth of the mandible as a whole. On the basis of these data and the current literature, a simple hypothesis was advanced for the control of condylar growth by the ongoing pattern of functional loading and for the role of this growth in the normal downward and forward displacement of the rest of the mandible.

Animals↗
PubMed1985

The effect of Class II elastic forces on craniofacial growth in rats.

Full-time Class II elastic forces were applied to eight rats in order to evaluate their effects on the growth of the snout and the mandible. Eight animals served as controls. Lateral cephalograms were taken at the start of the experiment and again at the end, 23 days later. The pre- and post-treatment tracings were superimposed on the cranial base to measure alterations in snout growth, and on metallic implants to measure differences in mandibular growth. As judged by t tests, Class II elastic forces caused a significant antero-posterior growth inhibition both in the snout and in the mandible.

Animals↗
PubMed1985

Formation of polychlorinated dibenzofurans and dioxins during combustion, electrical equipment fires and PCB incineration.

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are likely formed by thermal synthesis of a variety of primary precursors. Highest levels of these compounds are expected, however, when the starting material requires only one or two reaction steps for their formation, as is the case with chlorophenols, chlorobenzenes and polychlorinated biphenyls (PCBs). Laboratory pyrolyses have indeed shown that PCBs give significant yields of PCDFs, and chlorobenzenes give both PCDFs and PCDDs. In addition, a variety of other chloroaromatic compounds are formed. From these experiments and from accidents involving PCB fires, it is known that PCDFs are the most important toxic compounds associated with PCBs. Most commercial PCBs contain PCDFs in the low ppm range. PCDF concentration does not increase during normal operation in electrical equipment. Accidents (fires and explosions) involving PCBs can give PCDF levels in soot of up to 1000 ppm and higher. Effective thermal destruction of PCB is possible in modern incineration units, provided high temperatures, excess air and sufficient residence times are used. Exact figures for minimum temperature and residence time cannot be given, since feedstock and incinerator construction greatly influence destruction efficiency. Effluents from EPA-licensed incinerators used for PCB destruction contain only very low levels of PCDDs and PCDFs.

Benzofurans↗
PubMed1985

Decontamination and disposal of PCB wastes.

Decontamination and disposal processes for PCB wastes are reviewed. Processes are classed as incineration, chemical reaction or decontamination. Incineration technologies are not limited to the rigorous high temperature but include those where innovations in use of oxident, heat transfer and residue recycle are made. Chemical processes include the sodium processes, radiant energy processes and low temperature oxidations. Typical processing rates and associated costs are provided where possible.

Chemical Phenomena↗
PubMed1984

The influence of the trigeminal nerve on facial growth and development.

For the past twenty years it has been argued that the growth of the face is governed by the growth of a variety of contiguous soft tissues and functioning spaces (functional matrices). Recently, extensive data have been marshalled to suggest that the growth of the functional matrices is controlled by "trophic" influences from nerves. Accordingly, the present study sought to evaluate the role of the trigeminal nerve in the regulation of facial growth by placing unilateral stereotaxic lesions in the trigeminal nerve (root, ganglion, or major sensory branch) of the 10- to 20-day-old rat. The unoperated side and a number of sham animals served as controls. In all, 323 operations were performed, with 119 animals surviving until the age of 50 days. The location and extent of the lesions were controlled histologically. Cephalometric analysis revealed subtle but statistically significant distortions of the facial skeleton; however, the magnitude of the effect was quite small. Thus, although a modest case can be made for the contention that the trigeminal nerve exerts a trophic influence on the craniofacial complex, the conclusion that trophic effects constitute a major factor in the regulation of craniofacial growth cannot be supported.

Animals↗