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Biomedical subjects

A B Carr

Publications and source records attributed to A B Carr.

At least 37 records · Page 2Linked to original sources
PubMed1991

Observations on the motor control of brief teeth clenching in man.

Without artificial feedback control, maximum voluntary isometric contractions were performed for about 1 s by six subjects. Randomly selected surface electromyograms of the anterior temporalis and masseter muscles suggested that, in some cases, the motor control of the entire isometric contraction might have been preprogrammed through the phenomenon of anticipation. In the majority of cases, the control of the initial contraction phase might have been preprogrammed, followed by a phase of servo-controlled motor activity. As a functional basis for the servo-control of isometric force generation, it was suggested that compartmentalized 'extrafusal and intrafusal motor units' were recruited and decruited in an orderly manner, and periods of alpha-motor inhibition were interpreted as signs of switching from one control scheme to another, possibly via a transcortical loop.

Adult↗
PubMed1991

An electrognathographic study of aspects of 'deprogramming' of human jaw muscles.

Three-dimensional electrognathography of an incisor point was used to detect peripheral correlates of deprogramming of the jaw elevator muscles. Putative deprogramming was attempted through the clinically recommended use of a leaf gauge, placed for 10-15 min between the maxillary and mandibular anterior teeth and disoccluding the posterior teeth by about 2 mm. Studied mandibular displacements were those that occurred during voluntary elevation of the mandible from the postural to the intercuspal position. Use of the leaf gauge did not affect the displacement patterns. Within a freeway space of about 0.2 mm3, the incisor point moved about 2 mm vertically, about 1 mm sagittally, and about 0.2 mm laterally.

Adult↗
PubMed1991

Postural contractile activities of human jaw muscles following use of an occlusal splint.

As induced by an occlusal splint over a period of 1 week, this study monitored surface electromyographic changes in the postural contractile activities of jaw elevator and depressor muscles in six healthy adults. The immediate effect of the occlusal splint was to increase the postural contractile activities of the suprahyoid muscles. All postural muscle activities showed wide-ranging biological variation, but the activities induced by the splint tended to stabilize within 1 week, with decreased postural activities in the masseter and anterior temporalis muscles, and increased postural activities in the suprahyoid muscles.

Adult↗
PubMed1991

Comparison of impression techniques for a five-implant mandibular model.

Production of an accurate dental cast framework that meets the implant objective of passive fit demands an understanding of potential processing errors. Working casts fabricated from impressions using two different transfer copings, as provided by a leading implant manufacturer, were investigated for differences in accuracy. A five-implant mandibular model was used to produce seven casts by both the indirect and direct transfer coping techniques. Comparison was made by using a dental cast framework fitted to the master cast. Differences in distances measured between each group and the master cast were analyzed to establish differences between methods. For the model used, the direct technique produced more accurate working casts. A concern for potential impression distortion given the specific clinical situation underscores the method of choice.

Analysis of Variance↗
PubMed1990

An electromyographic study of aspects of 'deprogramming' of human jaw muscles.

Surface electromyograms from the right and left masseter and anterior temporalis muscles were used to detect peripheral correlates of deprogramming, also known as programming and reprogramming, of jaw elevator muscles. Putative deprogramming was attempted through the clinically recommended use of a leaf gauge, placed for 15 min between the maxillary and mandibular anterior teeth and disoccluding the posterior teeth by about 2 mm. Studied contractile activities were those of postural activity (subconscious, semi-isometric, minimal activity) and intercuspal teeth clenching (conscious, isometric, maximal activity). Use of the leaf gauge did not affect normalized postural activity (about 4%), the duration (about 900 ms) and static work efforts of clenching (about 1200 microV.s), the time to peak mean voltage of clenching (about 400 ms), and the peak mean voltage of clenching (about 300 microV). Activity and asymmetry indices showed that the studied motor innervation patterns were not changed by the leaf gauge.

Adult↗
PubMed1988

Adverse reactions to atracurium and alcuronium. A prospective surveillance study.

A multicentre prospective surveillance study was undertaken to compare the incidence and severity of adverse reactions attributed to atracurium and alcuronium. Clinical manifestations were used by the anaesthetist to diagnose an adverse reaction (a cutaneous reaction, a greater than 20% change in arterial pressure or heart rate, and bronchospasm). Of the 1956 patients receiving atracurium, 10.1% had adverse reactions compared with 17.9% of the 1425 patients receiving alcuronium (P less than 0.001). There were no longterm sequelae. The atracurium group had a markedly lower incidence of hypotension (3.4% v. 13.7%; P less than 0.0001), but a higher incidence of cutaneous reactions (4.6% v. 2.3%; P less than 0.005) which were not associated with other adverse reactions. There was a low incidence of bronchospasm in both groups (0.2% v. 0.1%).

Adult↗
PubMed1981

Vitamin C and the common cold: using identical twins as controls.

We analysed self-reported cold data for 95 pairs of identical twins who took part in a double-blind trial of vitamin C tablets. One member of each twin pair took vitamin C and the other took a well matched placebo each day for 100 days. Vitamin C had no significant effect except for shortening the average duration of cold episodes by 19%.

Adolescent↗
PubMed1981

Vitamin C and the common cold: a second MZ Cotwin control study.

Self-reported cold data have been analyzed for 95 pairs of identical twins aged 14-64 who took part in a double-blind trial of vitamin C tablets. One member of each twin pair took a tablet containing 1 g vitamin C and the other took a well-matched placebo each day for 100 days. In the total sample there was no effect of vitamin C in preventing colds. However, subdivision of the data showed a significant preventive effect of the placebo in the 51 pairs living together and an equal and opposite preventive effect of the vitamin C in the 44 pairs living apart. The placebo effect in the pairs living together may be attributed to the large proportion who wrongly perceived which treatment they were taking. The reduction of colds in the vitamin C group of the pairs living apart was about 20%. There were significant correlations between cold symptoms reported and the personality trait of neuroticism. No side effects of substantive changes in serum biochemistry could be attributed to the vitamin C dose.

Adolescent↗
PubMedDate not supplied

Histomorphometric analysis of implant anchorage for 3 types of dental implants following 6 months of healing in baboon jaws.

In an effort to better understand the supporting anatomy for unloaded endosseous dental implants, this study focused on the histomorphometric analysis of 3 different types of implants placed into non-human primate jaws and allowed to heal for 6 months. This report describes data from 24 screw-type dental implants placed in edentulated (2 months healing time) posterior arches of 4 adult female baboons. Three different implants were placed and allowed to heal for 6 months prior to processing for evaluation: commercially pure titanium (n = 8), titanium alloy (n = 8), and titanium plasma-sprayed (n = 8). Circumferential bone-implant interface sampling from 6 regions along the entire length of each implant was obtained for evaluation of percent bone-implant contact (%BIC) and percent bone area (%BA), within 3 mm of the implant. Data were collected (reliability of 1.6% for both parameters) and analyzed by an observer blinded to implant material using IMAGE analysis software for differences between jaws, implant biomaterials, and jaw/biomaterial (analysis of variance, pairwise comparison using least squares method with Bonferroni adjustment). The results indicated that the overall mean %BIC was 55.8 and mean %BA was 48.1. Maxillary and mandibular differences for both parameters were statistically significantly different: %BIC in maxilla 50.8, in mandible 60.8; %BA in maxilla 43.6, in mandible 52.6 (both significant at the P < .05 level). The biomaterial analyses revealed no significant differences between the different implants for %BIC or %BA. The trend observed--that mandibular values were greater than maxillary values for the overall jaw comparisons--was found to be consistent at the jaw/biomaterial level, although the small sample size limited statistical power. These data, along with data from a previous 3-month study, provide insight into baseline supporting anatomy for dental implants.

Alloys↗
PubMedDate not supplied

New high-palladium casting alloys: 1. Overview and initial studies.

High-palladium alloys for metal ceramic restorations, based on the Pd-Cu-Ga and Pd-Ga systems, were introduced to the dental profession during the past decade. These alloys have become increasingly popular because they are much less expensive than the gold-based alloys, and recently marketed high-palladium alloy compositions have excellent mechanical properties. A summary of the important melting and casting considerations for these somewhat technique-sensitive alloys is presented together with an overview of important dental materials science aspects and the results of a dental laboratory survey. A maxillary central incisor coping was chosen as the clinically appropriate specimen shape, and the gas-oxygen torch melting technique was found to produce castings with clinically acceptable levels of marginal sharpness and little evidence of microporosity. The etched as-cast high-palladium alloys exhibited fine-scale multiphase microstructures, and a noteworthy finding was that relative proportions of the microstructural constituents frequently varied for thin versus thick sections of the cast specimens.

Crowns↗
PubMedDate not supplied

The relationship of Periotest values, biomaterial, and torque to failure in adult baboons.

A quantitative measure of implant stability would be beneficial to decision-making regarding the status of the bone anchorage around an implant. The Periotest device has been reported to provide such a measure, but the in vivo data are incomplete and inconclusive in support of this claim. The purpose of this study was to determine in an in vivo model the relationship between Periotest values (PTV), three different implant biomaterials, and torque to failure. A total of 79 screw-shape implants made of commericially pure titanium, titanium alloy, and HA-coated alloy were evaluated in the posterior maxillary and mandibular quadrants of six female baboons following healing for 3 to 4 months. At uncovering, the implants were tested with the Periotest device and torqued to failure using an electronic torque driver. Analysis of the PTV-biomaterial relationship revealed no significant difference among the implants based on the biomaterial (P = .7453). Analysis of PTV-torque revealed a significant relationship (P = .0117), however, the association between PTV and torque was not strong and does not explain 92% of the variation between the parameters (Pearson correlation .2823, R2 = .08).

Analysis of Variance↗
PubMedDate not supplied

Reverse torque failure of screw-shaped implants in baboons: baseline data for abutment torque application.

Torque failure for endosseous implants has been used as a biomechanical measure of anchorage, though the significance of such data is not known. A practical understanding of the resistance to torque failure of implant-tissue interfaces at stage 2 surgery would help in assigning torque levels for implant abutment-screw fastening. The purpose of this study was to measure torque failure levels of commercially pure (CP) titanium, Ti-6Al-4V, hydroxyapatite-coated (HA-coated) screw-shaped implants placed into the maxillae and mandibles of baboons. Implants identical in size were placed into the edentulous posterior maxillae and mandibles of six female baboons (n = 37 each group) using a standardized surgical protocol. Reverse-torque data were collected at postinsertion time intervals ranging from 3 to 4 months using a counterclockwise torque driver and the data were analyzed (repeated measures ANOVA) for torque differences related to time, biomaterial, and jaw. The HA-coated implants exhibited significantly greater torque-removal values compared to both metallic implants (HA: 186.0 Ncm [50.1]; Ti-6Al-4V: 78.6 Ncm [18.1]; CP Ti 74.0 Ncm [24.4]). Analysis of torque interactions with jaw showed no significant difference; however, the mandible was found to be greater than the maxilla in torque resistance for all groups tested. Understanding the risks in inferring animal data to human application, the clinical implications of these data suggest that the recommended torque level of 35 Ncm for abutment fastening may provide a margin of safety for most implants of similar design and material as used in this study.(ABSTRACT TRUNCATED AT 250 WORDS)

Alloys↗
PubMedDate not supplied

New high-palladium casting alloys: Part 2. Effects of heat treatment and burnout temperature.

Castings simulating a maxillary central incisor coping were fabricated from five representative high-palladium alloys, using burnout temperatures of 1,400 degrees F and 1,500 degrees F, with a carbon-free phosphate-bonded investment. Three castings of each alloy were individually prepared at each burnout temperature. Each casting was sectioned into two specimens, and one specimen was subjected to simulated porcelain firing heat treatment. Polished and etched specimens in as-cast and heat-treated conditions were examined with optical and scanning electron microscopes. Mean values of Vickers hardness (1-kg load) were determined for each alloy and condition. It was observed that the simulated porcelain firing cycles caused major changes in as-cast bulk microstructures for the three first-generation alloys, while only subtle alterations were observed for the two second-generation alloys. In addition, complex regions associated with oxidation processes were found near the surfaces of all the heat-treated alloys. While the as-cast microstructure and the hardness of each alloy did not vary appreciably for the two burnout temperatures, the incidence of hot tearing in one first-generation alloy was substantially reduced at the lower burnout temperature. Statistically significant decreases in hardness generally occurred in the high-palladium alloys after the simulated porcelain firing cycles, but the relatively small changes (usually 10% or less) should not have any clinical significance. Other clinically relevant applications are also discussed.

Dental Alloys↗
PubMedDate not supplied

Effects of fabrication, finishing, and polishing procedures on preload in prostheses using conventional "gold' and plastic cylinders.

This study reviews fundamental concepts related to the use of screws and presents data describing the effect of fabrication, finishing, and polishing procedures on as-received preload for implant cylinders. Specifically, this study measured and compared preload produced when using as-received gold cylinders (the reference or gold standard), and cast cylinders produced from premade gold and plastic cylinders in the as-cast condition and following postcast finishing and polishing manipulations. The results reveal that preload in the gold screw-gold cylinder-abutment joint can be affected by the casting process, and that the choice of cylinder type, casting alloy, investment, and finishing/polishing technique may affect the resultant preload as compared to as-received joint conditions. The data from this study indicate that when plastic patterns are used as part of the framework, finishing and polishing of implant cylinder components should provide an increased preload compared to no such manipulations. Also, if maximum preload is desired, the use of premade metal cylinders offers an advantage over plastic patterns in both preload magnitude and precision.

Dental Abutments↗