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Biomedical subjects

W C Shellhart

Publications and source records attributed to W C Shellhart.

At least 19 recordsLinked to original sources
PubMed2001

Bracket bond strength with transillumination of a light-activated orthodontic adhesive.

The literature describes transillumination as a means of curing orthodontic light-cured composite adhesive. The literature also recommends a 2 to 3 times increase in light exposure time when light curing using transillumination. The purpose of this study was to determine the transmittance of the curing light through human enamel and the effect of transillumination on the bond strength of orthodontic brackets. One hundred extracted human maxillary incisors were used in this study. Brackets with orthodontic composite adhesive were placed on the labial surface of the incisors and light cured from either the labial or the lingual (transillumination). The control sample was cured from the labial for a total of 40 seconds of light exposure. Experimental samples were cured from the lingual (transillumination) for 20, 30, 40, or 50 seconds. The shear-peel bond strengths were tested at 30 minutes and 24 hours after light application. The results of this study demonstrated no statistically significant difference between 40 seconds of labial curing and most of the lingually cured groups. The only experimental group that differed statistically from the control group was the 40-second lingual cure group tested at 30 minutes after light application. Actual bond strengths, however, were lower for all experimental samples. The samples tested at 24 hours that received 50 seconds of transillumination were nearly the same as the control values. This study demonstrated that transillumination of maxillary incisors is an acceptable method of curing orthodontic adhesive, particularly if the exposure time is increased from 40 to 50 seconds.

Analysis of Variance↗
PubMed2001

Rapid curing of bonding composite with a xenon plasma arc light.

The use of light-cured orthodontic adhesives is an increasingly popular method for the bonding of orthodontic brackets. However, one of the disadvantages of light-cured adhesives is their long curing times. The xenon plasma arc curing light is purported to dramatically reduce the required curing time. The purpose of this study was to test the efficiency of a xenon plasma arc light versus a conventional tungsten-quartz halogen light in producing effective bond strengths for orthodontic brackets. Standardized brackets were bonded to bovine enamel with 3 different orthodontic bonding materials. The bonding materials were exposed to the tungsten-quartz halogen light for 40 seconds and to the xenon light for 3, 6, and 9 seconds. Bond strength was tested 30 minutes and 24 hours after light-curing. The results showed that bond strength with the application of the xenon light was greater with longer exposures. There were no statistically significant differences between the bond strengths of the brackets exposed to the tungsten-quartz halogen light for 40 seconds and those exposed to the xenon light for 3, 6, or 9 seconds. However, xenon light exposures of 6 or 9 seconds were required to create bond strengths equal to those produced by the tungsten-quartz halogen light. The xenon light produced equivalent bond strengths at very short light exposures.

Analysis of Variance↗
PubMed2001

Enhancing wire-composite bond strength of bonded retainers with wire surface treatment.

Bonded orthodontic retainers with wires embedded in composite resin are commonly used for orthodontic retention. The purpose of this study was to test, in vitro, various wire surface treatments to determine the optimal method of enhancing the wire-composite bond strength. Coaxial wires and stainless steel wires with different surface treatments were bonded to bovine enamel and then pulled along their long axes with an Instron universal testing machine. Wire surface treatments included placing a right-angle bend in the wire, microetching the wire, and treating the wire with adhesion promoters; combinations of treatments were also examined. The results demonstrated a 24-fold increase in the wire-composite bond strength of wire that was microetched (sandblasted), compared with that of untreated straight wire. The difference between the amount of force required to break the bond produced by microetching alone (246.1 +/- 46.0 MPa) and that required for the bonds produced by the retentive bend (87.8 +/- 16.3 MPa), the adhesion promoters (silane, 11.0 +/- 3.1 MPa; Metal Primer, 28.5 +/- 15.8 MPa), or for any combination of surface treatments, was statistically significant. Microetching a stainless steel wire produced a higher wire-composite bond strength than that obtained from a coaxial wire (113.5 +/- 27.5 MPa). The results of this study indicate that microetching or sandblasting a stainless steel wire significantly increases the strength of the wire-composite bond.

Animals↗
PubMed1999

Maxillary midline diastemas: a look at the causes.

BACKGROUND: Maxillary midline diastemas are a common esthetic problem that dentists must treat. Many innovative therapies have been used, varying from restorative procedures to surgery (frenectomies) and orthodontics. At times, these procedures have been performed by the dentist without full appreciation of the factors contributing to the diastemas. CASE DESCRIPTION: Before the practitioner can determine the optimal treatment, he or she must consider the contributing factors. These include normal growth and development, tooth-size discrepancies, excessive incisor vertical overlap of different causes, mesiodistal and labiolingual incisor angulation, generalized spacing and pathological conditions. A carefully developed differential diagnosis allows the practitioner to choose the most effective orthodontic and/or restorative treatment. CLINICAL IMPLICATIONS: The differential diagnosis leads to a treatment approach that most effectively addresses the patient's problem. By treating the cause of the diastema, rather than just the space, the dentist enhances both the patient's dental function and appearance.

Adolescent↗
PubMed1999

Uprighting molars without extrusion.

BACKGROUND: Orthodontic molar uprighting often results in extrusion of the molar, which is not always beneficial. The authors present a strategy for accomplishing molar uprighting without extrusion. DESCRIPTION OF THE PROCEDURE: The authors review and compare the benefits of molar uprighting with and without extrusion. The helical uprighting spring is probably the most popular appliance used for molar uprighting. It exerts an extrusive force on the molar during uprighting. The authors explain why this appliance produces an extrusive force and present a strategy to modify the appliance so that it can be used to upright without extrusion. CLINICAL IMPLICATIONS: The information presented in this article should help the dental practitioner select a suitable appliance, understand how it works and use it appropriately so that molar uprighting can be achieved without extrusion of the molar.

Dental Stress Analysis↗
PubMed1998

The use of bovine enamel in bonding studies.

Bovine enamel is commonly used in enamel bonding studies, therefore, a familiarity with some aspects of bovine enamel bonding are important in order to evaluate the studies. Bovine enamel has the advantages of easy attainability and similar microstructure to human enamel. In this study the strength of the enamel bond using an orthodontic adhesive was compared between deciduous bovine, permanent bovine, and human enamel, as well as, the effect on bond strength of multiple rebonding to bovine enamel. This study found that the bond strength to bovine enamel was 21% to 44% weaker than to human enamel, and the bond strength to deciduous bovine enamel was significantly greater than to permanent bovine enamel. Either all deciduous or all permanent bovine incisors should be used, or permanent and deciduous bovine incisors evenly distributed in sample groups. Bovine enamel was rebonded five times without significantly affecting bond strength, thus, bovine enamel can be reused in bonding studies without significantly affecting the results.

Analysis of Variance↗
PubMed1998

Case report: management of significant incisor root resorption associated with maxillary canine impaction.

Orthodontic and periodontic management of a patient with bilaterally impacted maxillary canines and advanced resorption of the adjacent lateral incisors is presented. In treating the patient, the affected laterals were retained and the canines were surgically exposed, then moved away from the roots of the laterals and into position in the arch. Alternative treatment strategies are discussed. Long-term follow-up evaluation revealed successful maintenance of the dentition and of periodontal health.

Adolescent↗
PubMed1997

A prospective study of lip adaptation during six months of simulated mandibular dental arch expansion.

The stability of dental arch expansion with conventional orthodontic treatments is disappointing. An increase in labial soft tissue pressure resulting from the expansion may contribute to this instability. An 8-month study of lip pressure changes resulting from lip bumper wear has been conducted, but no long-term studies have been conducted on pressure changes resulting from conventional expansion using fixed appliances. The purpose of the current study was to investigate changes in labial soft tissue pressures when conventional expansion was simulated. Twenty-two subjects wore a stent simulating mandibular dental arch expansion. They were instructed to wear the stent full-time. Lip pressure was recorded initially at 1 week, and monthly for 6 months. A repeated measures ANOVA was used for statistical analysis. A statistically significant (P < or = 0.05) increase in pressure was documented after initial insertion of the expansion-simulating stent. The increase was not maintained, suggesting an adaptation of the labial soft tissues.

Adaptation, Physiological↗
PubMed1997

Effective orthodontics: achieving objectives, avoiding misadventures.

The authors outline four principles that should be considered when planning orthodontic tooth movement. They suggest that dentists have clear treatment objectives, know the force requirements to achieve those objectives, choose the best equipment to provide the needed forces and prevent unwanted tooth movement during treatment. To illustrate use of these principles, the authors refer to a sample treatment scenario. They also provide a mnemonic device to help dentists recall these principles.

Dental Stress Analysis↗
PubMed1996

Lip adaptation to simulated dental arch expansion. Part 1: Reliability and precision of two lip pressure measurement mechanisms.

Understanding the influence of lip and tongue pressure on tooth position requires a reliable method of measuring pressure. A transducer with a beam mechanism has been used extensively in the past. A transducer with a diaphragm mechanism has been recently introduced. Comparative in-vivo tests of these transducers have not been published. The purpose of this study was to investigate transducer reliability and precision. Transducers were placed intraorally in 22 subjects, and two lip pressure measurements were recorded. Paired t-tests and interclass correlations were used to evaluate repeatability and reliability. The error of the method was analyzed for each transducer type. Both transducer types produced measurements that were repeatable and reliable. The error was smaller for the diaphragm transducer. The diaphragm transducer is more precise.

Adaptation, Physiological↗
PubMed1996

Lip adaptation to simulated dental arch expansion. Part 2: One week of simulated expansion.

An increase in resting lip pressure and the resulting disruption of the intraoral pressure equilibrium may be responsible for the poor stability found with orthodontically expanded dentitions. Passive expansion strategies seek improved stability by altering lip pressure, thus creating a new equilibrium. One of these strategies has been shown to alter pressure favorably. However, pressure changes associated with conventional expansion need to be studied before conclusions regarding the superiority of passive expansion can be drawn. The purpose of this study was to examine lip pressure changes after 1 week of simulated conventional expansion. Twenty-two subjects agreed to wear a mandibular expansion-simulating stent full-time for 1 week. Resting pressure was measured in the midline and right canine areas. Midline lip pressure decreased significantly after 1 week. Resting pressure was measured in the midline and right canine areas. Midline lip pressure decreased significantly after 1 week while pressure in the canine area did not change significantly. This finding suggests an adaptive response that varies according to anatomic location.

Adaptation, Physiological↗
PubMed1995

Equilibrium clarified.

Understanding the concept of equilibrium is crucial to understanding the mechanics of tooth movement. The purpose of this article is to clarify the concept of equilibrium. This concept is most clearly understood when the clinician focuses on the appliance. Understanding the forces acting to deform the appliance readily translates to knowledge of the forces acting on the teeth. Knowing the forces that act on the teeth allows the clinician to predict tooth movement more accurately.

Acceleration↗
PubMed1995

Reliability of the Bolton tooth-size analysis when applied to crowded dentitions.

The Bolton tooth-size analysis is widely taught and used in orthodontics. However, its reliability has not been documented. The purpose of this study was to evaluate the reliability of the analysis when performed with needle-pointed dividers and a Boley gauge. Four clinicians measured the teeth on 15 sets of casts with two instruments at two sessions. The measurements were used to calculate tooth-size excess. To evaluate the measurement error, the difference between the two analyses made by the same investigator on the same set of casts was calculated. More of the same-investigator analyses were significantly correlated when the Boley gauge was used than when the needle-pointed dividers were used. Between-investigator analyses revealed significant correlations for each measurement session with both instruments. Every investigator was found to have at least one measurement error for each analysis and with each instrument that was as large as a clinically significant result of a Bolton analysis. The results of this study demonstrate that clinically significant measurement errors can occur when the Bolton tooth-size analysis is performed on casts with at least 3 mm of crowding. The Boley gauge demonstrated a higher frequency of significantly correlated repeated measures and thus may be somewhat more reliable for this analysis than the needle-pointed dividers.

Cuspid↗
PubMed1992

Effects of bisphosphate treatment and mechanical loading on bone modeling in the rat tibia.

Bisphosphonate treatment has been shown to decrease endosteal bone formation and increase periosteal bone apposition in the rat tibial diaphysis. This study tested the hypothesis that the increase in periosteal apposition is a compensatory attempt to maintain skeletal mass appropriate for the mechanical load. Twenty-four rats were divided into four groups. In two of the groups, one hindlimb in each rat was immobilized with a sling device to increase the mechanical load on the opposite limb. Daily injections of dichloromethylene bisphosphonate (Cl2MBP) were given at 10 mg/kg to one immobilized group and one mobile group. The other two groups were given daily injections of normal saline. Fluorescent bone labels were administered at two-week intervals. All rats were killed after ten weeks of treatment, and calcified tibial cross sections were prepared for fluorescence microscopy. Bone dimensions and periosteal and endosteal apposition rates were calculated. When compared with saline controls, Cl2MBP treatment decreased endosteal apposition rate in all tibias. Periosteal apposition rate was increased with Cl2MBP treatment in all tibias except the unloaded limb of immobilized rats. The Cl2MBP-induced increase in periosteal apposition rate was greatest in loaded limbs and was proportional to the relative amount of body weight supported.

Animals↗
PubMedDate not supplied

Oral findings in fragile X syndrome.

This study compares the oral findings in fragile X syndrome individuals to those of normal age-matched patients. Sixteen fra(X) males (mean age 22 10/12 years) had a low caries rate (decayed, missing and filled surfaces (DMFS) = 12.3) and minimal intraoral hard or soft tissue disease. Rate of malocclusion, as determined by the first permanent molar classification of Angle, was not significantly different from that of matched subjects. Fra(X) subjects had a significantly higher occurrence of malocclusion as compared to matched subjects using crossbite and openbite as criteria. Palatal dimensions of fra(X) subjects did not differ significantly from those of the matched sample. The fra(X) males also demonstrated significantly more severe occlusal wear of their teeth than the matched sample.

Adolescent↗