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R Highton

Publications and source records attributed to R Highton.

9 recordsLinked to original sources
PubMed1991

Molecular phylogeny of plethodonine salamanders and hylid frogs: statistical analysis of protein comparisons.

The bootstrapping method of determining confidence in the topology of phylogenetic trees has been applied to electrophoretic protein data for two groups of amphibians: salamanders of two North American genera (Aneides and Plethodon) of the tribe Plethodontini and Holarctic hylid frogs. Some current methods of phylogenetic reconstruction for electrophoretic protein data have been evaluated by comparing the trees obtained from molecular data sets with available morphological data. Molecular data on the phylogenetic relationships of Aneides and Plethodon, data obtained from electrophoretic and immunological studies, indicate that Aneides probably was derived from western Plethodon subsequent to the separation of eastern and western Plethodon. Thus Plethodon very likely is a paraphyletic genus. The extremely low rate of morphological evolution in Plethodon compared with that in Aneides causes difficulty in indicating their evolutionary relationships taxonomically because there are no synapomorphic morphological characters that define either eastern or western Plethodon, whereas there are several for the genus Aneides. Thus molecular data alone probably indicate the evolutionary relationships of the species in these genera. Highton and Larson's (1979) arrangement of species of Plethodon into eight species groups is supported. The topologies of the unweighted pair-group method using arithmetic means (UPGMA) and distance Wagner trees were compared with independent morphological and molecular data on the relationships of the 28 plethodonine species. It was found that UPGMA trees indicate relationships that are more in agreement with other information than are those provided by distance Wagner trees. The use of the bootstrap technique indicates that the topologies of UPGMA trees are better supported statistically than are the topologies of distance Wagner trees. Moreover, different addition criteria produce a variety of distance Wagner trees with different topologies, each with several groupings that are not supported statistically. It is concluded that considerable caution should be used in interpreting the topology of distance Wagner trees. Very similar results were obtained with a second data set on 30 taxa of Holarctic hylid frogs. Trees obtained by the neighbor-joining method are more in agreement with UPGMA phenograms and other data, so this method of phylogenetic reconstruction may be useful to systematists not willing to assume constant rates of evolution.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1988

Retention magnets in guiding plates of distal-extension removable partial dentures.

This investigation studied vertically placed magnets that act simultaneously as guiding plates and retentive devices. The retention of vertically placed magnets was less than that of horizontally or obliquely placed magnets, but comparable to that of I-bar retainers. Load-induced stresses were lower, transmitted more axially to the abutments and were generally more equitably distributed than stresses produced by magnets in other orientations to the abutment tooth. The force distribution characteristics were less stressful than those of a comparable RPD with distal guiding plates, mesial rests, and I-bar retainers. Although the investigators were initially concerned about the relative capabilities of the vertically placed magnets, in vitro and in vivo studies demonstrated that retention was as good if not better than that of conventional clasp designs.

Dental Stress Analysis↗
PubMed1988

The interaction of a magnetically retained denture with osseointegrated implants.

A replica of an atrophied mandible was constructed in which two Brånemark implants were embedded to simulate osseointegration. Ferromagnetic keepers were then attached to the implants. A complete overdenture was constructed which had magnets positioned directly over the keepers. The denture was then subjected to different loads and the stresses were photographically recorded. The denture was also tested for its retention capabilities. Because of the equitable stress distribution with the absence of high foci of concentration, a magnetically-retained overdenture with osseointegrated supports lends itself well as a prosthetic replacement. In addition, the magnets exhibit sufficient retention without unduly loading the implants or residual ridge. It may be concluded that in a similar in vivo situation, magnets could be an alternative to other attachment systems; however, there are many variables to be considered on an individual patient basis.

Bone Resorption↗
PubMed1987

Force transmission and retentive capabilities utilizing labial and palatal I-bar partial dentures.

A maxillary photoelastic model was constructed to determine the retention and force transmission characteristics of a bilateral distal extension base partial denture. Comparisons were made using palatal and labial I-bars. It was shown that there were no significant differences in retention and force transmission between palatal and labial I-bars. From a stress and retention standpoint, it may be concluded that the use of palatal I-bars is a viable alternative to the labial I-bars, especially where aesthetics is of primary concern.

Dental Stress Analysis↗
PubMed1986

Magnetizable abutment crowns for distal-extension removable partial dentures.

A study of the retention and load-distributing characteristics of bilateral distal-extension dentures with magnetizable abutment crowns revealed the following findings: Comparable retention was obtained with the horizontal and oblique pedestals. Decreased retention was noted when there was space between the magnet and pedestal. The retention values were substantially higher than reported values for the I-bar retainer. For the space and no-space situations, the horizontal pedestal design provided better stress results than the oblique design. Better stress-distributing properties for horizontal and oblique pedestals were obtained with the presence of a space between the magnets and pedestals. Of all the designs tested, the horizontal pedestal with a space demonstrated the best load-transmission characteristics.

Dental Abutments↗
PubMed1986

Retentive characteristics of different magnetic systems for dental applications.

Six magnet-keeper systems were tested to determine the relationship between an air gap and the resulting breakaway force. The maximum retention was obtained when the magnet and keeper were in apposition. However, as the air gap increased in 0.1 mm increments, the breakaway force diminished rapidly initially and, then began to taper off at 0.2 to 0.3 mm. Ideally, the preferred placement of the magnet into the denture is with a carefully controlled system to provide an air gap. Then the magnet is attached with a minimal amount of autopolymerizing acrylic resin.

Denture Design↗
PubMed1986

Retentive and stress characteristics for a magnetically retained partial overdenture.

The retentive and stress characteristics of a magnetically retained partial overdenture were studied. The retentive properties were compared to those of an I-bar and Zest Anchor retained overdenture. The magnetically retained partial denture uniformly distributed stress to the supporting structures and exhibited significantly greater retention than that of the I-bar, but less than that of the Zest Anchor. Based on these observations, the magnetically retained denture should be considered as an option to be used instead of or in conjunction with other types of partial overdentures.

Dental Abutments↗